Passive Solar Design. Passive solar design 1 New South Wales Technical and Further Education Commission 2014 (TAFE NSW - WSI)

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1 Passive Solar Design The overall design concept of designing a house to make the most use of ambient energy in the environment rather than imported energy from fossil fuels, is Passive Solar Design. Passive solar design is a design technique that includes the effective management of solar radiation and natural heating/cooling sources impacting on the building envelope to minimise heating, cooling and lighting energy requirements for occupant comfort. It is achieved through the use of strategies such as: appropriate orientation managing direct heat gain and lighting through windows shading managing indirect gains (thermal control in opaque systems) ventilation thermal mass as a heat sink or other techniques to provide indoor comfort. A passive solar design building works as an integrated system that includes solar energy collection, distribution and storage. This means that the sun s daily and seasonal cycles are considered when the building is designed, which aids natural heating, cooling, lighting and ventilation without the larger costs of undertaking these activities mechanically. As low-energy buildings seek to regulate heating and cooling by natural means, the design principles are simply concerned with admitting and storing the sun s energy when it is needed and excluding and removing heat from the building when it is not needed. Passive solar design has a number of environmental advantages as every opportunity to capitalize on natural radiation and daylight will minimise reliance on fossil-fuel generated electricity, which in turn reduces carbon- dioxide emissions. There is also the added advantage of significant savings in energy costs. Good design for thermal comfort in the various climatic conditions described in the previous chapter are based on the following six principles: Orientation of frequently used areas towards the north, to allow maximum sunshine when it is needed for warmth, and to more easily exclude the sun s heat when it is not. Glazing used to trap the sun s warmth inside a space when it is needed, with adequate shading and protection of the building from unwanted heat gain or heat loss. Thermal mass to store the heat from the sun when required, and provide a heat sink when we need to be cooler. Insulation to reduce unwanted heat loss or heat gain through the roof, walls, and floors. Passive solar design 1

2 Ventilation to provide fresh air and capture cooling breezes; and Zoning to allow different thermal requirements to be compartmentalised in winter. The way a building uses energy can best be understood as the interaction of several related energy systems that continually respond to changing climatic conditions and the comfort requirements of the occupant. The following list of design factors should be considered when attempting to passively design an energy efficient house. Site Orientation Internal room placement Foundation, walls, roofs Heating, Ventilation and Air Conditioning (including passive solar design) Thermal mass Insulation Lighting and Appliances Landscaping All of the above factors will be considered in turn in the following sections. Site When designing in the natural environment, it is important to direct effort into preserving the site, to look at the natural forms of the land and the way nature has shaped it through movement of water and erosion, and to allow the design to follow and respond to these influences. A detailed survey of the site and surrounding areas, including its topography, geology, hydrology, wildlife, vegetation, carrying capacity, local culture, tradition and knowledge, is needed before decisions on location and orientation are made. Plant types and planting locations also have important effects on perceived comfort. Any trees to the immediate north should allow the sun into the building in winter and provide shade in summer. Deciduous trees are useful for this purpose as they can provide shade to the eastern and western facades. Shading of the surrounding land is important to reduce glare caused by reflection from dry, exposed ground cover. However, there are few native deciduous trees in Australia so a decision needs to be made between planting deciduous trees for the benefits of passive solar design, or planting native trees which may contribute to the biodiversity of the site. As was discussed in the previous chapter, the micro-climate of the site can play a major part in determining the design strategies taken, and it is the topographical and other natural features of the site that determine the micro-climate. Design Guidelines [for site] Analyse site microclimate including-solar access + overshadowing- Prevailing winds in winter and summer Analyse topography, slope and orientation of site Take advantage of the above through the siting of the house. Eg to take advantage of access to cooling breezes, or screening of cold winter winds. 2 Passive solar design

3 Minimise impact on site by-minimise removal of vegetation it may be helpful to remove some to improve solar access.-minimise site ground and stormwater flow disturbance Analyse site for any surface and/or subsurface drainage areas upslope and downslope of site. Building orientation The ideal building orientation depends on the climate zone within which the house is sited. In hot humid climates and hot dry climates with no winter heating requirements, orientation should aim to exclude sun year round and maximise exposure to cooling breezes. In all other climates a combination of passive solar heating and passive cooling is required. The optimum degree of solar access and the need to capture cooling breezes will vary with climate. Where ideal orientation is not possible, as is often the case in higher density urban areas, an energy efficient home can still be achieved with careful attention to design. In Australia, to make the best use of solar access, entertaining and living areas, which are used during the day should be on the north side of the house, with rooms which are used less often such as bathrooms, and bedrooms facing south or west. As very few blocks are orientated along compass direction, a number of different plan layouts for the differing block orientations are available. These are shown in the diagram below. Figure 25. Optimal living area locations in a variety of block orientations. Source: Amcord Passive solar design 3

4 As a rule of thumb for houses in middle latitudes in Australia, optimum energy efficiency is obtained when north and south facing walls are between 1.5 and 2 times longer than the east and west facing walls. Buildings should be planned in such a way that benefit is obtained from shaded indoor and outdoor living areas when the weather is hot and protected, sunny indoor and outdoor areas when the weather is cold. Figure 26. On a north facing slope the potential for access to northern sun is ideal for higher housing densities. Well-designed buildings in the Southern Hemisphere should be oriented, and the spaces arranged in such a way, that the majority of rooms face towards true north. In this way the eastern and western sides are exposed to the low-angle summer sun in the morning and afternoon. The high angle of the sun in the northern sky in summer makes it easy to shade windows using only a roof overhang. The longer northern side of the building benefits from the low sun in winter. The roof overhang on the north should allow the sun to shine into the building when its warmth is required in winter and provide shade from high angle sunlight in summer. If the majority of windows are designed into the north wall sun penetration into the building will be maximised. Living areas should be sited to gain maximum benefit from cooling breezes in hot weather and shelter from undesirable winds. This does not mean that the orientation of the building should be varied from north towards prevailing breezes as it does not have to face directly into the breeze to achieve good cross-ventilation. Within the internal planning, rooms such as dining and recreation rooms that require more heat during the winter months should be placed on the northern side of the building. Rooms that are used for short periods of time during the day can be placed in southern areas (for example bathrooms, laundry, ensuite, entry corridors, stairs, bedrooms, bars). Whilst windows facing north are ideal, windows which are less than 20 degrees east or west of north will still allow the Sun to enter through them in winter, whilst excluding much of the summer Sun if the eaves are well designed. 4 Passive solar design

5 Windows facing east or west are more difficult to shade during the summer months without the use of landscaping. South facing windows will lose heat during winter and gain some heat in the late afternoon and early evening during the summer months. To calculate the ideal overhang required for eaves, multiply the distance from the eaves line to the base of the window by 0.4 for shading from October to February and 0.7 for shade from September to March. Remember that the eaves width needs to be worked out considering a range of factors along with the above role of thumb. Sun s angles - Design Guidelines In cooler climates north-facing walls and windows should be kept well back from large obstructions to the north such as buildings, trees or fences, as they cast shadows two to three times their height in mid winter. A distance of at least 5.5 m from a single-storey obstruction to the north, or at least ten metres from a double-storey obstruction, is recommended. In cooler climates if solar access is poor, alternative methods to gain northerly winter sunlight into the home should be considered, such as using high level windows, or more complex means such as light deflectors. In warmer climates it is still good to orientate a house with the majority of the windows facing north because this is the easiest elevation to shade effectively with an eave or overhang. Minimising windows to the east and west in warm climates reduces unwanted heat gain from low angled morning and afternoon sun. Such low angled sun is difficult to shade against. In cooler climates, building on the south, east or west boundaries should be preferred options in order to maximise the potential for solar gain from the north. Garages, carports and other buildings should not be placed on the northern side of the block, to prevent them from overshadowing the house or outdoor living areas. Sited to the east or west garages can form a barrier to low angled sun in summer. Shared walls with neighbours, particularly on the east or west boundaries should also be considered. Heat loss to an adjoining neighbour is negligible compared to that lost to the air through an external wall of a house. Zoning - Internal planning and room placement Rooms are used for different purposes at different times of the day and their location will influence energy efficiency and comfort levels. Avoid large, open plan living areas which have to be heated at the one time when only small areas may be in use. Creating zones by grouping rooms with similar uses and closing off unheated rooms reduces heating and cooling needs. Grouping together rooms which use hot water also improves the efficiency of hot water usage. Daytime living zones (family rooms) with northerly aspects are warm and bright during winter and can be easily protected in summer, improving energy efficiency and making them comfortable all year-round. Stairwells and high ceilings can increase heating requirements by more than 40%. Passive solar design 5

6 They allow heated air to rise, leaving cooler air at the lowest floor level, increasing the volume of air which has to be heated. Correctly-placed windows and doors with short distances between them are essential to encourage cross-ventilation to help cool the residence on summer evenings. Design guidelines For cooler areas of the country, rooms with similar uses should be grouped together and doors used to separate the various areas of the house into zones that may be efficiently heated Glass doors or bi-fold doors can be used to retain the open-plan aesthetic and improve daylight penetration where necessary. Doors should also be used to separate formal living areas from other living areas, and heated areas from unheated areas. Daytime living areas such as kitchens, family and rumpus rooms should be placed to the north in cooler climates. Other zones can be arranged around the family area depending on their use. A northerly aspect for formal living and dining areas though not essential, is desirable. A westerly aspect should be avoided. Westerly aspect for bedrooms should also be avoided particularly in warm climates. An easterly or northerly aspect is desirable for children s rooms and playrooms. Rooms with a southerly aspect will be cooler all year-round. Areas that use hot water should be grouped to minimise plumbing costs, heat loss from pipes, and water wastage. Utility areas such as bathrooms, laundries and toilets can be used as buffer zones on the west and south sides of the building. Rooms or garages should not be placed where they will overshadow northern windows during winter mornings or afternoons. Avoid deep north- facing courtyards. In warmer climates garages and carports on the east, west or south sides can be used to protect the rest of the home from summer sun and winter winds. Airlocks at external doors can be used to limit the escape of heated air when the external doors are opened in cool climates, or the ingress of hot air in warm climates. In cooler climates ceiling heights should be kept low, preferably no higher than 2.7 m to minimise the volume of air to be heated Avoids voids and cathedral ceilings. Doors at the base of stairwells will prevent heated air being lost via open stairways. In warmer climates higher ceilings can be advantageous as they allow the use of ceiling fans, and they allow warm air to rise up and away from the occupants. Ceiling vents or high level clerestory windows are a good strategy as a means of evacuating hot air from the building at a high level. 6 Passive solar design

7 Building Envelope - Foundation, Walls, and Roof Heat loss, comfort and heating costs The graphic below shows the ways that a home gains and loses heat through the building fabric. Figure 27. The variety of ways that a home loses and gains heat Passive solar design 7

8 The table below shows an example of typical house heat losses that may occur and the relative routes of heat loss through the different components of the house. Steady state heat loss calculations are a quick way of indicating the potential of strategies for the different systems in the construction. Thus, in this house, windows account for 47% of total heat loss. If glazed with R 0.3 double-glazing it would be 29% of total heat loss, and if glazed with R 0.7 gas filled low-e glazing it would be 16% of total heat loss. Building Fabric Existing R Area m 2 Rate of Heat Value m 2 K W Loss W/ o K Roof Floor (slab on ground) Wall (BV with RFL) Windows (3mm glass/ metal) Total rate of conduction heat loss through fabric Ventilation heat loss Total rate of heat loss What is R-value? The R-value is a materials resistance to transferring energy or heat. The value is expressed in m2k/w (which is, watts per metre squared kelvin), and is the rate of transfer of heat (in watts) through one square metre of a material divided by the difference in temperature across the material. The higher the number the better the materials effectiveness. Please Note: Steady state calculations are indicative. They do not take account of human behaviour, heat loss to orientation, and all the other complex interactions, which can lead to variations of ±300% between apparently similar households. The envelope of the building has a great impact on how much energy is required to heat and cool the building. The challenge in designing the footing, walls, and roof is to minimize conductive heat loss or heat gain, depending on the outside air temperature, while minimizing the uncontrolled movement of air into the building. The comfort of the building during the heating season will depend in large part on interior surface temperatures of the floors, walls, and roof as well as the amount of cold outside air entering through leaks in the envelope. The opposite also holds true for those times of the year when cooling is required. 8 Passive solar design

9 The roof and ceiling Roof lights or overhead glazing should be double glazed to reduce heat loss, especially if they are over living areas. They should also be screened to prevent direct summer sun penetrating the room as this can cause over- heating and glare. To install an acrylic roof light, the walls of the roof light should be insulated and the roof light should not be permanently ventilated. An acrylic diffuser at the ceiling level can provide additional insulation. A cathedral ceiling should not have the insulation between the roofing material and the ceiling lining compressed as the airspaces in the batts are necessary for the insulation to perform effectively. Exhaust fans should be vented to the outside and not into the roof cavity as the vapour in the exhaust air may condense in the insulation reducing its effectiveness. Roof/Ceiling Construction Type Pitched Roof tiles with sarking, roof cavity and 10mm plasterboard ceiling Pitched Roof tiles, sarking, roof cavity + 25mm timber ceiling Pitched Roof metal sheet, roof cavity and 10mm plasterboard ceiling Flat Roof 150 concrete slab with 75mm screed and asphalt Overall Construction R-Value (with no added insulation) R 0.39 R 0.52 R 0.39 R 0.56 Walls Brick or Block Cavity Walls Brick or block masonry walls should always have some form of insulation. If the living space faces north masonry internal walls can also act as a heat store for heat gained through windows. The cavity of the external masonry walls should be insulated with a polystyrene board. If living spaces face south, then lightweight insulating masonry such as Hebel blocks should be used for the internal skin. Brick-veneer external walls A properly insulated brick or block-veneer wall in climate zone 6 or 7 should have R2.0 insulation within the studs and a layer of reflective foil on the outsides of the studs, properly taped at the joints to trap the air within the insulation. Lightweight stud walls Like brick-veneer external walls a stud framed external wall will not provide any thermal mass for the storage of heat and are therefore more appropriate for south facing blocks. As with brick-veneer R2.0 insulation and reflective foil should be used. Timber studs are better than metal studs as metal provides a cold bridge between the internal and external lining. Passive solar design 9

10 Super Insulation Colder parts of Australia require higher levels of insulation. Super- Insulation refers to a standard of insulation above R 3. 0; a level which would be appropriate in these cases. The best detail is to use 150 deep timber studs to take the R 2.5 insulation and provide a moisture barrier between the plasterboard lining and the insulation to prevent condensation in the insulation. Reflective foil is still required on the outside face of the studwork, make sure it is properly lapped and sealed behind service holes and socket outlets. Reverse brick or block-veneer This system allows a higher level of insulation around the masonry inner skin where it is acting as a heat store. An insulation level of R2.5 or higher can be provided outside of the masonry wall by using 100 mm or 150 mm studs and R2.0 or R2.5 insulation. If a single masonry skin with attached battens and insulation is to be used, then the wall lengths and stability and heights should be checked by a structural engineer for stability. Insulation levels of external walls Many people would be surprised to know that an uninsulated brick or block cavity wall provides the same insulation as an uninsulated stud wall with a weatherboard lining. The following table gives the thermal insulation levels or R-values for some commonly used forms of external wall construction. The higher the R-value the better the insulation. External Wall Construction Insulation Level Cavity brick or block, wall R Cavity brick or block, painted on the inside and with R mm polystyrene sheet in the cavity Brick or block veneer with 10mm plasterboard sheet internal lining Brick or block veneer with 10mm plasterboard sheet lining and R2 insulation Weatherboards on 90mm studs with 10mm plasterboard sheet internal lining Weatherboards on 90mm studs with 10mm plasterboard lining and R2 insulation 200mm thick single skin insulated block wall, rendered on the outside and 10mm plasterboard on the inside External brick or block wall with cavity and 125mm insulating block internal wall R 0.51 R 2.50 R 0.54 R 2.78 R 1.75 R Passive solar design

11 Floors A concrete slab should be insulated at the slab edge with a polystyrene panel faced with cement sheet. If the slab is to be used as a heat store then it should be insulated underneath the slab adjacent to the window. Timber floors should be insulated, especially if they are exposed to cold draughts. Simple but effective method of providing basic floor insulation is to use bulk insulation then drape reflective foil over the floor joists, making sure that all joints in the floor are taped. The sagging of the foil creates an air gap between the foil and the flooring, and insulation which adds extra insulation value. Floor Construction Type Overall Construction R-Value (with no added insulation) 150mm reinforced conc. slab + 25mm screed R mm lightweight conc. slab + 25mm screed R mm tongue and groove boarding R mm particleboard with linoleum or vinyl R 0.40 Thermal Mass After siting a house to take advantage of the sun, insulating it to prevent heat loss, and sealing it to prevent air leakage, the next design criteria to consider is thermal mass. Thermal mass can be used to heat or cool internal temperatures as material in the house. Thermal mass can perform the vital role of storing heat during the day (directly via solar radiation or by active means e.g. electric, hydronic etc.) and releasing it at night to help prevent the house from getting too cold. This thermal storage capacity can also be used to advantage in warm climates where the thermal mass is kept cool (i.e. no/little direct solar radiation) which in turn helps keep internal temperatures lower than outside. Figure 28. Timber framed houses can be easily adapted to ensure thermal mass is in the right place to utilise solar gain. FrameDesign.asp Passive solar design 11

12 In cold and cold climates the primary role of thermal mass is to capture heat from direct solar radiation. Exposing thermal mass to direct solar radiation can cause the thermal mass to be heated. However the sun needs to be strong enough and the thermal mass exposed for long enough to heat up. So in winter thermal mass needs many hours of exposure to warm up. If it does not warm up it can reduce internal temperatures. If sufficiently heated it then re-radiates that heat as long wave radiation into the living spaces of the house. A key strategy then is to ensure that the thermal mass is well insulated from the outside air so that as much of the heat as possible is re- radiated into the house rather than being lost to the outside. Thermal mass is most useful in locations that have large swings in temperature between day and night, such as desert climates. Thermal mass can substantially slow the heat flow. In climates that are constantly hot or cold, thermal mass can be detrimental, because all surfaces of the mass will tend towards average daily temperature. If the average temperature is above or below comfort range Note: Thermal mass with insufficient heating can remain cool and therefore acts as a heat sink ( drain heat from a source ). Therefore thermal mass intended to be used to warm a house of an evening must be exposed to many hour of direct solar radiation in a day especially in winter when the intensity of the sun (heating capacity) is very low. Thermal mass is provided by any dense material capable of storing heat. Those most commonly used in building are concrete, bricks, blocks, stone and water. It is important to distinguish between insulation and thermal mass. Insulation is measured by an R-value (thickness/conductivity) which is a measure of the resistance to heat flow, and thermal mass by a Capacitance or C-value (density x specific heat x thickness) which is a measure of the amount of heat it can store. Dense materials are poor insulators and good capacitors as indicated by the R and C values for various building materials given in the table below. Buildings with little thermal mass are unable to store much heat for night time use and thus cool down and heat up rapidly. A sufficiently heated large thermal mass will slowly release heat over a much longer time span. Too much thermal mass or insufficiently solar heated, acts as a heat sink and in a cool climate can result in a cool to cold building that needs excessive supplementary heating to maintain a desirable comfort level. Thermal capacitance has to be balanced against the seasonal solar gain. 12 Passive solar design

13 Figure 29. Graph shows the relationship between construction air temperature for various construction types related to the average temperature at any particular time of day. sive-design/thermal-mass Material selection to capitalise on thermal mass is an important design consideration. For instance, heavyweight internal construction (high thermal mass) such as brick, solid concrete, stone, or earth can store the sun s heat during winter days, releasing the warmth to the rooms in the night. Lightweight materials such as plasterboard are not high mass materials and will act as insulators to the thermal mass, reducing its effectiveness. Lightweight construction responds to temperature changes more rapidly. It is therefore suitable for rooms that need to cool quickly in the evening. The following table presents many common building materials and other materials such as water that are used for thermal mass in buildings. The last two columns represent the material s resistance to heat flow, R, and capacity to hold heat, C. It can be seen that water has by far the highest capacity to hold heat. By comparing the C values of the various materials in the table above it is easy to see why special storage systems such as concrete trombe walls, rock piles and water walls use those particular materials for heat storage. Passive solar design 13

14 Figure 30: Thermal Properties of Some Building materials used for thermal mass in buildings and their capacity to hold heat. Other factors influencing the efficiency of thermal mass are its surface colour and texture. Darker colours absorb more energy than lighter colours, shiny surfaces reflect more energy than dull ones and textured surfaces provide a greater surface area than smooth ones. Thus dark colours, dull surfaces and coarse textures represent high admittance and light colours, shiny surfaces and smooth textures represent low admittance. In a cool climate the most effective place for thermal mass is in those areas that experience direct sunshine. Thermal mass outside this area is only effective if there is a free convective exchange with air from the heated zone. Thermal mass must be an inner core enclosed in an outer shell of insulation so that heat is freely transferred between that mass and the heated space and not lost to the outside. The simplest and most cost-effective solar heating uses the building fabric itself as the system. For maximum energy efficiency, in a cool climate thermal mass should be maximised in the north facing rooms of a building. Any heat gained through the day can be lost through ventilation at night. In using this technique, the thermal mass is often referred to as a heat bank and acts as a heat distributor, delaying the flow of heat out of the building by as much as hours. 14 Passive solar design

15 Passive solar design 15

16 In summary transferred between that mass and the heated space and not lost to the outside. The simplest and most cost-effective solar heating uses the building fabric itself as the system. For maximum energy efficiency, in a cool climate thermal mass should be maximised in the north facing rooms of a building. Any heat gained through the day can be lost through ventilation at night. In using this technique, the thermal mass is often referred to as a heat bank and acts as a heat distributor, delaying the flow of heat out of the building by as much as hours. In summary thermal mass affects the temperature within a building by: Stabilising or moderating internal temperatures by providing heat source and heat sink surfaces for radiative, conductive and convective heat exchange processes Providing a time-lag in the equalisation of external and internal temperatures. This is often termed capacitive insulation, as it is the capacity of the mass that slows the heat transfer into or out of a building, rather than the material s resistance to heat flow/ Providing a temperature reduction across an external wall. Design Guidelines Where mass is used for warmth, it should be exposed to incident solar radiation. Where mass is required for cooling, it should be placed in a shaded zone of the building. A building s mass may be pre-heated using electric or hot water tubing embedded in the mass (usually done in concrete floors). Buildings may be pre-cooled by using night time cool outside air, although this requires significant amounts of exposed mass, and may be necessary only at certain times of the year in most Australian locations where little thermal mass is used. This method removes warm indoor air and replaces it with cool air from outside. It is sometimes termed night flushing, or night purging. It is important to note that in buildings with extended hours of use, thermal mass heated during the day can cause discomfort during the night when the heat is released. 16 Passive solar design

17 Thermal merits of different construction methods Stud Frame Stud frame walling has the ability to be designed with very good insulation value, but it will have almost no capacitance. It is excellent where thermal mass is not required such as an intermittently heated space where it gives quick warm up and low heat loss. As the stud itself only has an R-value of 0.8 (0.5 for hardwood), this represents a cold bridge over about 7% of the wall surface (20% for older buildings). When combined with mass partitions or freestanding thermal mass inside the DIRECT GAIN building, the recommended configuration for passive design is achieved. Figure 31: Typical insulated stud wall detail. Perforated reflective foil is located over the outside of the frame. Make sure that the air space between reflective surfaces is at least 25mm. Source: Passive solar design 17

18 Brick Veneer Brick veneer is easy to insulate to high values. It is lightweight construction giving quick warm up and low heat loss. The external brickwork is a no- maintenance decorative, veneer hung off the studs and whilst it delays the time taken for heat to pass through the wall, it does not contribute heat directly to the inside spaces. Thermal mass can be provided by concrete floors, or masonry partitions to achieve the recommended configuration for passive design. Figure 32: Insulated brick veneer wall. Source: 18 Passive solar design

19 Cavity Brick Bricks are poor insulators and a cavity brick wall needs a layer of insulation between the brick skins if it is to insulate to the same standard as a brick veneer wall. It is difficult to insulate to a high standard. Many modern clay bricks and concrete blocks have a relatively low density and insulation can be slightly improved if these are used on the outside skin. The internal skin will provide thermal mass, and dark coloured dense bricks would improve its heat absorption. the mass is unlikely to receive direct sunshine and where there are other thermal mass elements, care must be taken to ensure total capacitance balances solar gain. Figure 33: Insulated Cavity Brick wall Source: Passive solar design 19

20 Reverse Brick Veneer Reverse brick veneer is a wall for lateral thinkers that uses the same quantity of materials as the brick veneer wall yet improves on both the insulation and capacitance values in a lesser width. The battens provide room for the mortar to squeeze out as well as creating a still air cavity faced with reflective foil (also acting as a vapour barrier) and reduce the cold bridging effect of continuous studs against the thermal mass. It follows the recommended configuration for thermal mass as an inner core in an outer shell of insulation. Figure 34: Insulation detail for Reverse Brick veneer wall. Source: 20 Passive solar design

21 Mass storage wall systems A storage wall is a variation of the direct gain system which places the thermal mass behind the window. A 300 mm thick concrete wall has a time lag of eight hours and seems to be the optimum thickness. Thinner walls store a lower fraction of useful heat and peak too early in the evening. Heat input and output are in the same direction so there is no reversal of flow as in the floor. Although the glass is opaque to long wave radiation emitted by the wall, there will be a significant convective heat loss overnight. Measures that improve the performance are double-glazing or a selective surface wall facing. A trombe wall is another variation of a storage wall, which has small ventilation openings (about 6% of wall surface), which can be operated to develop convective currents between the room and the wall face. This results in a heat input into the room during the daytime and reduced heat storage and time-lag for evening use. A water wall is simply a storage wall that uses water as the thermal mass. Double glazing or selective surface improves the performance. Figure 35: Shows the detail of a solar (trombe) wall. The sun heats the outside of the solar wall (glass). The heat passes through the trombe wall (mass) and arrives at the inside surface of the walls several hours later to provide heating in the evening. Source: g/solarwall/solarwall.htm Passive solar design 21

22 Direct Heat Gain Systems Direct gains systems comprise a North-facing window admitting solar radiation directly on to the thermal mass, usually the floor, which absorbs and stores the heat. During this solar input the surface is warmest and a downward temperature gradient is developed in the slab. After solar input, the surface must cool (warming the room) before an upward temperature gradient is developed and the stored heat is released into the room. By thickening the slab to mm where it receives direct sunlight, it is possible to provide carry-over heat storage for one or two cloudy days, Heat loss through a slab is greatest at the edge, particularly where it is above ground level. Insulating the slab edges to a depth of 600 mm optimises the performance. Where depth is limited, the insulation can be placed horizontally to achieve the same effect. Partitions, walls and freestanding elements such as water filled drums can add substantially to the thermal mass of a building. Floors often get covered up with furniture and rugs, or are fully carpeted which results in reduced or nil heat storage for night time use. Thin partitions allow heat to transfer through both surfaces. Internal masonry walls or freestanding mass combined with lightweight external walls follows the recommended configuration of thermal mass as an inner core in an outer shell of insulation. Windows and Glazing Glass and many plastic films have the property of transmitting nearly all solar radiation. Solar radiation is commonly classified according to its wavelength. Ultraviolet (UV) radiation, which we can t see, comprises about 3% of the solar spectrum, and is a major cause of melanoma and deterioration of materials. Visible light, the light we see, accounts for about 44% of the solar spectrum. Infrared, which we can t see, but feel in the form of heat, makes up most of the remaining 53% of the spectrum. Different glazings can be selective in the wavelengths they transmit, absorb or reflect. Exploiting these properties so that we can let in all the visible light, keep out the ultraviolet and control the infra-red according to wavelength can considerably enhance our comfort. For example, laminated glass absorbs 99% of the UV wavelength which limits fading of furnishings to the effects of visible light and infra-red radiation only, and greatly reduces the need to draw curtains or blinds for protection. Perhaps the greatest effect on human comfort is determined by infra-red transfer. Clear glass has the property of transmitting nearly all the short- wave solar radiation it intercepts, while at the same time reflecting the much longer thermal waves re-radiated by heated surfaces. This principle has long been used in cold climates to grow plants in greenhouses, hence the expression greenhouse effect. Low-emissivity glass also performs this way, but because its emissivity is in the range 3 %-19% compared to 84% for float glass, the greenhouse effect is improved five-fold. Emissivity is the measure of penetration of long wave infrared radiation through a material; the lower its emissivity the more heat it- reflects. 22 Passive solar design

23 Glazing Types Perhaps the greatest effect on human comfort is determined by infra-red transfer. Clear glass has the property of transmitting nearly all the short- wave solar radiation it intercepts, while at the same time reflecting the much longer thermal waves re-radiated by heated surfaces. This principle has long been used in cold climates to grow plants in greenhouses, hence the expression greenhouse effect. Low-emissivity glass also performs this way, but because its emissivity is in the range 3 %-19% compared to 84% for float glass, the greenhouse effect is improved five-fold. Emissivity is the measure of penetration of long wave infrared radiation through a material; the lower its emissivity the more heat it- reflects. 3mm Float Glass transmits 89% of visible light and 77% of infrared radiation. These figures reduce as glass thickness increases. Toughened glass has the same transmission properties but is up to five times stronger, with the added safety property of breaking into relatively harmless granules when broken. Low-iron glass gives improved light (91%) and infrared transmission (88%). Laminated safety glass is made from two (or more) panes of glass (float, toughened, lowe, bent, wired, patterned, tinted or reflective) bonded together with plastic interlayers of polyvinyl butyrate (PVB). This combination of brittle and plastic material gives it a viscoelastic property that dampens sound and other energy impacts, as well as the safety property of remaining adhered to the inter-layer when broken. Additives in the inter-layer give it a UV absorbing property with light and heat transmission the same as float glass of the same thickness. Low-emissivity (low-e) glass or plastic film is coated with a thin layer of metallic oxide that gives clear vision and excludes UV radiation. Whilst physicists cannot control infrared radiation according to season, they can formulate coatings for quite different reflective properties. Low-e glass presently available has high solar (short-wave) transmission and low thermal (16ng-wave) emissivity and is available in hard and soft forms. Soft coatings are sputtered on glass or polyester film using a vacuum deposition process. Glass toughening must take place before coating. These coatings have the lowest emissivity, typically 3%. To prevent the coating from corroding, it is sealed in double glazed units at the factory (e.g. Glaverbell Thermopane Comfort, Southwall Heat Mirror), or laminated between polypropylene film (eg. 3M d-i-y film). Hard coatings (pyrolytic) are fused to the glass during production. This process gives less control over the deposition process and emissivities are in the range of 14%-19%. Some coatings show a slight haze in bright sunlight, but the coatings are corrosion proof, abrasion proof, and can be worked or toughened after coating (e.g. Pilkington K Glass; P.P. G. Sungate 500). Double window systems are made by fixing shutters or adding an extra sash to the window frame; they maybe removable, hinged, pivoted or sliding. To reduce the risk of condensation, inner sashes should be well sealed when closed, with top and bottom slots from the cavity to the outside, and sashes arranged so that all surfaces are accessible for cleaning. Often the only architecturally satisfying solution, particularly with renovation work, is to keep the existing windows. Maintenance of existing windows, as well as saving resources, may also be cheaper and safer than replacement. Shutters can be made of an insulating material such as polystyrene, with a plywood, metal or fabric facing. Removable storm shutters, so widespread in North America, offer a low-cost solution as well as an annual ritual to mark Passive solar design 23

24 the opening and closing of the heating season. Single frame double-glazing is usually provided by fixing a second line of glazing to an existing frame, either by glazing into a rebate or attached channel or bead. If condensation is bad, breather holes (6 mm diam. for each 0.5 sq. m) to the outside; plugged with fibreglass to exclude dust and insects, should be provided. Curtains, to be effective, must be impervious to air flow. Acrylic and reflective metal backings are particularly effective, and quilting, layering or foam in-fills increase the insulation value. Fitting the curtain to prevent the cold draught moving down the face of the window is essential. To obtain a good sea], provide minimum 300 mm overlaps at meetings, otherwise seal the edges with Velcro tape, Raven RP 39 magnetic tapes, or hinged side flaps. Proscenium-style curtains are particularly effective insulators if the amount of light entering the room is not critical. Double glazed units The most common double-glazed unit today is the hermetically sealed, insulated glass unit. This form of double-glazing is usually cheaper than well-sealed, insulative curtains. The guarantee offered by the manufacturer gives a good guide to the quality of the various products. Sealed units consist of two (or more) panes of glass separated by a perimeter spacer, with either a single or double seal of elastomeric sealant. The air in the cavity is kept dry with desiccants in the spacer. The main problem experienced with double glazed units is the breaking down of the edge seal, allowing internal condensation to occur. The edge seal is broken down by UV radiation from sunlight, and water, either from rain or cleaning. It is vital that the seal is protected from both of these. Usually the rebate and frame shades the seal from UV light, and a drained glazing keeps it dry and complies with the terms of the warranty. Fully bedded glazing cannot be guaranteed to keep water out. As well as reducing heat loss, double-glazing increases the surface temperature of the glass facing the room, generally by 4 7 K higher than single glazing. So when it is O C outside and 20 C inside, the inside glass temperature of single glass is 6 C, doubleglazing is 13 C, and low-e double- glazing is 15 C+. This increase in surface temperature reduces radiation loss to the cold window and cold convection currents from the window. Now as well as improving comfort, the reduced heat loss results in an energy saving. In a typical house located in a cool climate windows account for 47% of total heat loss if they are standard single glazed windows. If glazed with R 0.3 doubleglazing the percentage would be reduced to 29% of total heat loss, and if glazed with R 0.7, gas filled low-e glazing it would reduce to 16% of total heat loss. High performance glazing When a low-e coating is used, most of the radiant transfer of heat from one side to the other is eliminated and conduction across the cavity is the main method of heat transfer. By using argon or krypton in the cavity, the thermal resistance further improves to R O. 7. In hotter climates, or perhaps where external shading is not provided, low-e glass combined with reflective or tinted glass is used to reflect solar heat outside, keeping inside spaces cooler. 24 Passive solar design

25 Triple-glazed (or more) windows using the low-e suspended polyester film in krypton gas cavities produce higher R-values than any other approach (R O ). The film mounting system adds to the cost of the window, and the film may creep in hot weather. These windows have been available since 1979 in Switzerland and the U.S.A. where Heat Mirror windows, made by Southwall Technologies, offer a variety of film types with different reflective properties. If the double-glazed window could be sealed to sustain a vacuum, then an R-value of R 2.3 is possible.combined with UV blocking and low-iron glass, an extraordinarily high service performance would be provided. Now the need is to find window frames, spacers and edge seals that match this glass technology. As the table of window R-values shows, it is the overall performance of frame and glass that determines the insulation value of a window. The thermal performance of windows under cooling and heating conditions can be rated. Australia has an industry based window rating scheme called WERS (Window Energy Rating Scheme]. The scheme provides two star ratings for each window type, one for summer or warm climate conditions in which the exterior is warmer than the interior and one for winter or cold climates where the exterior is colder than the interior. A vast selection of windows can be found on the on-line database at The climate, orientation, interior usage, and external shading all have an effect on the type of glazing that should be specified for a specific window opening. Glazing Type Typical R-Value and (U-Value) Clear single glazing 3mm R 0.17 (U 5.9) Clear double glazing - (3/6/3)mm R 0.32 (U 3.1) Low e double glazing - (6/12/6) Up to R 0.5 (U 2.0) mm Argon filled double glazing - Up to R 0.6 (U 1.67) (6/12/6)mm Frame Type Typical R-Value and (U-Value) Aluminium R 0.1 (U 10.0) Timber R 0.36 (U 2.8) Windows as solar collectors Window R-values do not account for solar gain during the day. Sunshine coming through north-facing windows and stored in thermal mass within the building is the essence of passive solar gain. There is obviously a relationship between window area and this mass, with too little mass resulting in overheating and too much resulting in under heating. The rule of thumb for effective glass area for a location in BCA Climate zone 6 or 7,is to have 85% of windows facing north and a minimum glass area to each northerly room of 30% of its floor area (7% for a non-mass building). A folded net curtain or fly screen across a window reduces direct gain by about 45 % and glass area needs to be increased to compensate. Passive solar design 25

26 Sunspaces Sunspace, solar greenhouse, glassed verandah, solarium or conservatory are largely interchangeable terms. Their appeal is in providing very pleasant indoor spaces on cold but sunny days, providing a buffer space to the outside, and maximising the area of solar collector. However, because single glass is such a poor insulator, these spaces often suffer a net heat loss from night sky radiation; and because of the greenhouse effect they overheat on hot days. If the sunspace cannot be isolated whilst it is suffering from these extremes, then the building fabric has to be used to modify the climate by using double glazing, (or Polygal double walled polycarbonate or Okalux insulated glass panel for example), thermal mass within the space, (water drums, dark paving, masonry dividing walls), shading with external louvres or shade cloth and ventilation through roof hatches to dump excess heat. Windows and Door Sealing Preventing warm air leaking out of the gaps around windows and doors in cool climates is another simple and effective way of reducing heat loss. Before fitting the architraves and lining boards gaps should be packed with insulating material. Smaller gaps should be filled between the wall lining and the architraves with a caulking compound. Adhesive backed foam seals or rubber seals can be installed around the edges of window and external door frames as well as seals on the bottom of external doors. If the toilet or bathroom window has a fixed vent in it seals should be placed around the internal doors to these rooms too. Insulation Insulation specifications are another important design feature. A building envelope provides a barrier between the indoor and outdoor environments allowing the thermal comfort levels indoors to be adjusted to suit the occupants. This might require heating or cooling depending on the season and location of the building. The energy required for heating or cooling will be greatly reduced if the building envelope is adequately insulated. This means insulating the ceiling, walls and floor of the building, an easy task during construction, but often more difficult for existing buildings. Insulation reduces the rate that heat can flow through the building elements in which it is installed. It limits heat escaping from a building in winter, and unwanted heat coming into a building in summer. In temperature-controlled buildings, this will result in significant energy savings and thermal comfort. Installing the correct level of insulation should reduce the amount of heat loss in winter and heat gain in summer. Insulation has a second benefit it reduces noise transfer through walls and ceilings. 26 Passive solar design