Linings. Introduction

Transcription

1 Linings Introduction In previous units in this subject, building materials were divided according to their nature of origin (e.g. clay products). Because both wall and ceiling linings and insulation materials can comprise any number of different base materials or combinations of materials, it seems more logical, in this case, to approach this unit differently according to the function which the materials perform rather than the nature of the raw material. This unit, therefore, is divided into two sections: the first deals with wall and ceiling linings and the second with insulation. Linings in details, Unit CPCCBS6001, Ed 1 1

2 Wall and ceiling linings The terms wall lining and ceiling lining refer to the internal wall and ceiling covering of the building as opposed to cladding which refers to the external wall covering or, sometimes, roof covering. Additionally, in this unit wall and ceiling lining are defined as being distinct from finishes (such as ceramic tiles, wallpapers and paints) which are usually applied to the wall or ceiling lining. The most common forms of wall lining used in Australia are gypsum plasterboard, fibrous cement, timber or composite lining boards or sheets, plastic coated wall sheeting and solid plaster. Timber and composite lining boards and sheets are covered in Unit 2. Timber and plastic coated wall sheeting is mentioned in Unit 9. In this unit, we will concentrate on the other alternatives. Plaster The term plaster refers to a jointless and usually smooth lining applied to the base wall or ceiling structure. Solid plaster was one of the first lining materials to be used in buildings. The plaster which was made of lime and sand, often with hair included, was applied in situ to the masonry wall or, in the case of a timber stud wall or ceiling, to timber laths which are thin battens fixed close together to provide a base. Today, solid or in situ plaster is reserved for solid masonry walls; timber stud walls are lined with plasterboard. However, in situ plastering is a wet and messy process and often internal masonry is left unplastered (face brickwork, for example). Composition Plaster comprises a binder, clean sand and fresh water, which sets to a comparatively hard, dense layer. The properties of the final product depend largely on the type and quantity of the binder used. The binders most commonly used in Australia are gypsum plaster, Portland cement and lime or organic binders. Gypsum plaster Calcium sulphate or gypsum plaster can be used for undercoats and finishing coats. (Plaster of Paris is one type of gypsum plaster.) It is derived from naturally occurring gypsum rock which has been pulverised and heated to drive off most of the chemically combined water, resulting in a white, pink or grey powder. When water is added to gypsum plaster it sets and hardens into a crystalline solid, giving off heat and expanding slightly in setting. Two other similar binders are derived from gypsum plaster: hard wall plasters which provide a harder finish and Keene s cement, which is the hardest of the gypsum plaster mixes. Portland cement Portland cement is sometimes used as a binder in undercoats and finishing coats where an exceptionally hard surface is required. Too rapid drying increases the likelihood of cracking, and shrinkage must be substantially complete before a further coat is applied. 2 Linings in details, Unit CPCCBS6001, Ed 1

3 Limes Plasters in which limes are the only binders are rarely used today as the final strength is very low. Workability agents or plasticisers, based on non-hydraulic lime or organic materials, are used to improve the workability of the mix and distribute shrinkage stresses, thus reducing visible cracking. These mixes, which contain lime and cement, shrink on drying out hence each coat must be allowed to shrink before further coats can be applied. This lengthy procedure delays completion of the work and lime plasters have been replaced almost entirely by calcium sulphate or gypsum plasters. These plasters set within a few hours, produce a harder finish and only expand slightly on setting. Lime is used now only as a workability agent, and as an accelerator for some gypsum plasters. Process The process of applying solid plaster to a base structure is known as rendering. Solid plasters are usually applied in two coats. The undercoat is often referred to as the scratch coat and the finishing coat as the set coat. If the base is particularly smooth and the suction uniform, a single coat only may be required; alternatively, a particularly irregular base may require three coats. In some applications the coats may not be of the same composition but it is important that each coat be well matured before another coat is applied, especially if cement is used. A general principle to be followed is that each successive coat should be weaker than the preceding one. However, the current trend is to apply render in single coats. The choice of a plastering system depends upon the base to which the plaster is to be applied, the performance of the required finish and the texture desired. Cement-sand or cement-lime plasters are moisture-resistant plasters, while gypsum-based plasters should be used internally in dry situations only. Mixes containing Portland cement make the hardest plasters, and have the greatest resistance to impact damage. Keene s plaster is the hardest of the gypsum plasters, while lime plaster is the softest. Tables 1 and 2 indicate suitable plaster mixes for two- and threecoat internal plasterwork. Table 1 - Mixes for undercoats for internal two-coat and three-coat work Finishing coat Cement setting Cement: lime: sand Gypsum plasters Gypsum plasters Undercoats (by volume) 1 cement 4 to 5 sand 0.10 lime 1 cement 5 to 7 sand 0.10 lime 1 plaster 2 to 3 sand (or 1: 3 to 1: 4.5 by weight) 1 gypsum plaster: 1.5 sand: 0.10 lime (or 1: 2 by weight, plus lime 5% of Linings in details, Unit CPCCBS6001, Ed 1 3

4 weight of plaster) Table 2 - Mixes for finishing coats for internal work Background or undercoat Brick, block, or concrete Cement: sand Cement: lime: sand Concrete background Cement: lime: sand (undercoat) Gypsum plaster Finishing coats (by volume) 1 cement 4 sand 0.10 lime 1 cement 1 lime 5 sand 1 cement 1 to 2 lime 6 to 9 sand 1 lime 0.25 to 4 gypsum plaster Preparation Porous bases, such as clay bricks and concrete blocks, which have a comparatively high suction rarely require much preparation other than raking of the joints and the removal of loose material. Smooth, dense materials, such as concrete, have little suction and offer no mechanical key and are either hacked or else treated with a spatter-dish, sand-cement mix, often including a PVA adhesive, to provide a key. Rough textured surfaces, such as rough concrete, provide a good mechanical key and require little preparation. Fibrous plaster Fibrous plaster is made of gypsum plaster reinforced with sisal hemp fibre. Nowadays it has been replaced by plasterboard for sheeting applications but is still used for the more complicated decorative mouldings. Fibrous plaster is dimensionally stable and easily decorated but is not satisfactory in moist conditions. Gypsum plasterboard Plasterboard is the most commonly used lining for timber-framed construction and brick veneer. It comprises a core of gypsum plaster reinforced with two outside layers of kraft paper, one on each face. Some are available with an aluminium foil on the back which improves thermal insulation performance. Plasterboards are easily decorated and are reasonably tough and strong in normal grades but are not satisfactory in damp situations. A water-resistant board is available which is designed to be used in areas where high humidity persists and in wet situations where they are protected with tiles or a similar impervious material. Sizes: Sheets are available in a broad range of sizes. Thicknesses commonly used in domestic applications are 10 mm for walls and 13 mm for ceilings. However, a 10 mm thick board is now available for ceilings also. 4 Linings in details, Unit CPCCBS6001, Ed 1

5 Fixing: The boards are fixed to the studs or ceiling joists by gluing or nailing with special flatheaded nails. Boards are available with either square or recessed edges, the latter being used where a flush surface is required. For a flush joint, a strip of perforated reinforcing paper is embedded in bedding compound in the recess and the area is covered with a topping cement (see Figure 1). Figure 1 - Fixing General properties of plaster and plasterboards Thermal insulation: Plaster linings are relatively thin and make a correspondingly small contribution to the thermal insulation of a building. Fire resistance: Normal plasters are non-combustible, have no spread of flame and do not produce smoke. Special fire-rated plasterboards are available for applications which require a fire rating. Often, the addition of a specified thickness of plaster or render on internal masonry walls is used to achieve a required fire rating according to the Building Code of Australia. Sound absorption: Ordinary plasters have fairly low sound absorption values but special acoustic plasters and plasterboards are available. Sound insulation: As plaster linings are relatively thin, they contribute significant sound insulation to lightweight components only. However, plaster can improve sound insulation by sealing the surface to porous base structures. Hardness: In housing, a fairly soft finish may be preferred but harder surfaces are often required in public buildings and the choice of system should take this into account. Metal angles are used to protect vulnerable corners and provide a line for the plasterer to work to (see Figure 2). Figure 2 - Expanded metal reinforcement used at external plasterboard corners Durability: Gypsum-based products are not usually waterproof and the durability of the finish depends largely on the composition of the plaster. Texture: Smooth-trowelled surfaces comprising either neat gypsum or gypsum with admixtures are most common but texture can be provided by special trowelling or by including sand in the finish. Bagged finishes are popular on masonry walls. These comprise a thin sand-cement mix which is wiped over the wall surface with a piece of hessian. The resultant thin coat allows the form of the masonry units to show through. Linings in details, Unit CPCCBS6001, Ed 1 5

6 Fibrous cement Fibrous cement sheeting has replaced asbestos cement as a lining and cladding material due to the health hazards associated with materials containing asbestos. Composition Fibrous cement is made from a mixture of Portland cement, sand, cellulose fibre and water, compressed into sheets, boards or other shapes. Sizes Sheets are available in a number of sizes. Thicknesses for domestic use are generally as follows: as lining material for eaves, verandas or carports 4.5 mm or 6 mm sheet; for internal wall and ceiling linings 6 mm; compressed fibrous cement for wet area floors is 15 mm or 18 mm thick. Fixing Sheets can be glued or fixed with special galvanised flat-head fibrous cement nails to timber frames; joints can be covered with fibre cement cover moulds or PVC sheet holders (see Figure 3). Figure: 3 - Cover and junction moulds for fibrous cement sheets Exposed internal linings can be flush jointed. Special recessed-edge sheets are taped with a perforated paper reinforcing tape and finished in a similar way to plasterboard sheets, with a topping cement. Uses Externally, fibrous cement products can be used as cladding in the form of boards, sheets or shingles. However, internally, because they are waterproof, fibrous cement sheets are used primarily as a base lining for other finishes (such as tiles) in wet areas. Compressed fibrous cement sheeting is also used as a base floor material for ceramic tile floors in wet areas. General properties Thermal insulation: Fibrous cement sheets are relatively thin and make a correspondingly small contribution to the thermal insulation of the building. Fire resistance: Fibrous cement products will not burn, have a zero spread of flame index and do not produce smoke. Sound absorption: Unless special acoustic material is used, fibrous cement lining contributes little to the sound absorption characteristics of a room. Sound insulation: The sheets have a greater density than plasterboard but are thinner and therefore do not significantly affect sound insulation. Hardness: Care should be taken during handling and storage to prevent edges from chipping since the material is particularly brittle. When painted or otherwise finished, however, a hard surface finish can be obtained. Durability: Fibrous cement sheets are unaffected by sunlight, moisture or termites and should not split or rot. Hence its suitability for external and wet area applications. 6 Linings in details, Unit CPCCBS6001, Ed 1

7 Defects in plaster 1. Blistering this is a movement of the finishing coat caused by expansion, which breaks the bond with the inner faces of the undercoat. It is caused by radiant heat, or in the case of anhydrous plasters, persistent moisture that causes delayed expansion. 2. Bond failure this is the failure of the bond between coats of plaster or background. Its causes are: undercoat that is too smooth weakness of mix dust on the surface before plastering grease or efflorescence a film of unset plaster excessive relative moisture or thermal movement. 3. Cracking movements in the background structure, shrinkage movement. 4. Crazing fine cracks, which are usually related to lime or cement and their tendency to shrink on setting. 5. Efflorescence salts in background, which are brought to the surface by movement. 6. Popping or blowing these are small holes in the plaster caused by small particles of unslaked lime that expand and push out the plaster in front of them. The holes are known as a pop or blow. 7. Re-occurrent surface dampness the presence of deliquescent salts (common) brings about re-occurrent dampness when humidity is high. Linings in details, Unit CPCCBS6001, Ed 1 7

8 Thermal insulation The question of thermal insulation really forms part of the problem of energy efficient design of the building as a whole, which includes consideration of the following points: orientation of the building to maximise the use of solar energy (see Figure 4) location in relation to summer breezes (see Figure 5) protection from winter winds (see Figure 6) location and treatment of windows (see Figure 7) use of wide eaves or pergolas which shade windows and walls from summer sun but allow entry of winter sun (see Figure 8) use of solar energy in the design to heat floors or walls (see Figure 9) interior planning (see Figure 10) prevention of heat loss through unnecessary gaps (see Figure 11) design of floors (see Figure 12) the colour of the exterior of the house. 8 Linings in details, Unit CPCCBS6001, Ed 1

9 Figure 4 - Paths of the sun in winter and summer Figure 5 - Location in relation to summer breezes Figure 6 - Protection from winter winds Linings in details, Unit CPCCBS6001, Ed 1 9

10 Figure 7 - Location and treatment of windows Figure 8 - The use of eaves to exclude the sun in summer Figure 9 - The use of solar energy to heat floors or walls Figure 10 - Interior planning Figure 11 - Prevention of heat loss through unnecessary gaps (from 10 Linings in details, Unit CPCCBS6001, Ed 1

11 Figure 12 - Prevention of heat loss through the floor by enclosing the sub-floor space Thermal insulation can assist by improving the thermal efficiency of the structural components of the house by reducing heat loss or gain through the major surfaces, such as the walls and ceilings. Heat transfer Heat is transferred by: conduction heat is led from the side of the material at a higher temperature to the side at a lower temperature convection when air is heated it expands and begins to circulate and heat up colder surfaces by losing some of its heat to them radiation when air comes in contact with a warm object, heat is transferred to the atmosphere. Thermal resistance A material s ability to resist the flow of heat is called its thermal resistance or R-value. The higher the R-value of a material, the greater its ability to resist the flow of heat. The Energy Authority of NSW provides data on recommended R-values for different areas in NSW. For instance, if you live in Coffs Harbour the recommended minimum level of thermal insulation is R1.5 but if you live in Cooma, which is colder, the recommended minimum level is R3.0 (see Figure 13). The heat flow through a wall or ceiling is not reduced in direct proportion to the R-value of any insulation added above the recommended level: in fact the extra benefit to be gained diminishes fairly rapidly beyond this level. Thus, there is not much point in installing insulation to a value beyond the recommended R-value for your area. Linings in details, Unit CPCCBS6001, Ed 1 11

12 Figure 13 - Thermal insulation levels for NSW If you do not live in NSW, contact your local energy authority and inquire about the recommended R values for your state. 12 Linings in details, Unit CPCCBS6001, Ed 1

13 Types of insulation Reflective This type of insulation uses the heat-reflective properties of aluminium foil which prevents heat transfer by radiation. The following types are available: Foil laminated to reinforcing membranes, supplied in rolls of varying widths. This is used for roof sarking and wall sheathing. Laminated foil layers separated by partition strips. When the foil is installed over ceiling joists the partition strips separate the two layers and provide an additional air space to increase the effectiveness by decreasing conduction. Foil laminated to bulk insulation. Foil-backed plasterboard. Solar reflective film which can be applied directly to glass panes. Metal reflective-treated fabrics for blinds, curtains and so on. Bulk This is normally a cellular material with entrapped air bubbles which slow down heat transfer by conduction. Several forms are available. Batts and blankets Insulation batts and blankets are available in the following materials: Mineral wool (fibreglass or rockwool), manufactured from inorganic raw materials that are melted at above 1000 C and spun into fibres which are then bonded together to form flexible sheets. Urethane foam sheet, made from foamed polyurethane. Expanded polystyrene sheet (EPS), made from foamed polystyrene. Loose fill Cellulose fibre, manufactured from waste paper. Exfoliated vermiculite, manufactured from a micaceous material. Mineral wool, manufactured as explained above. In situ foam Urea formaldehyde is pumped in as a mixture of chemicals using special equipment. The mixture foams up in situ and forms a rigid foam filled area. Urethane foam is pumped as fluid foam into the space where it sets chemically to form a rigid insulation. Linings in details, Unit CPCCBS6001, Ed 1 13

14 Expanded polystyrene beads are mixed on site with a bonding agent and injected into the cavity. Structural and decorative insulation This type of insulation comprises a complete wall or ceiling lining system combining thermal insulation and often acoustic modification with a decorative lining. Several forms are available: Fibreglass panels laminated with decorative finishes. Wood wool panels decorative boards made from wood straw bonded with a cement-like adhesive. Compressed straw panels, manufactured from pine or straw fibres which are compressed and bonded together. Expanded polystyrene, as above with decorative finishes. 14 Linings in details, Unit CPCCBS6001, Ed 1

15 General properties of insulation materials Thermal performance The type and thickness of the insulation is selected according to the required R-value and the application. Reflective foil as insulation in horizontal applications should be laid face down as settling dust renders the upper face ineffective. The R-value should be marked on the product and manufacturer s product information should comply with SAA Standards and Test Methods. Acoustics Some insulation will also contribute to the acoustic performance of the room, especially in the case of some of the decorative panels. Fire resistance Some insulation materials are combustible. Urethane foam, expanded polystyrene and cellulose fibre insulation must contain fire-retardant chemicals. Combustible insulation should be covered with an appropriate non-combustible lining such as gypsum plasterboard. Safety Most bulk insulation materials should be handled with care to avoid dust formation. Gloves and long clothes should be worn when installing fibreglass to avoid contact with glass fibres, which may irritate the skin. In all cases it is advisable to wear a mask covering the mouth and the nose. Suitability The type of construction will limit your choice of insulation system. For instance, loose-fill insulation is generally only suitable on flat surfaces. In situ insulation may make access to the roof space extremely difficult. Loose-fill insulation is good for difficult corners. Linings in details, Unit CPCCBS6001, Ed 1 15

16 Where to insulate Because heat rises, most heat loss occurs through the ceiling. Figure 14 illustrates the proportion of heat loss through various paths for a typical uninsulated detached brick veneer dwelling in Canberra. (Note that the figures given have been calculated specifically for the Canberra region and may not apply to other areas although the general pattern these figures reveal would apply for this type of construction elsewhere.) Figure 14 - Heat loss through a building Although the percentage figure for heat loss through the walls is the highest, in terms of unit area the diagram suggests that (for this type of construction) the greatest heat losses are in fact through the ceiling and, next, the floor. Consequently, the first place to consider insulating is above the ceiling (see Figure 15). Figure 15 - Insulation above the ceiling If the floor is a raised timber floor the sub-floor space should be enclosed, allowing for the required ventilation and bulk insulation can be supported between the joists or reflective foil can be placed over the joists (see Figure 16). 16 Linings in details, Unit CPCCBS6001, Ed 1

17 Figure 16 - Insulation below the floor In extremely cold climates rigid foam insulation around the edges of the slab is advantageous (see Figure 17). Figure 17 - Insulation around the edges of the slab In timber walls bulk insulation can be placed between studs (see Figure 18). Figure 18 Insulation between the studs Foam in-situ insulation can significantly increase the thermal performance of cavity brick walls (see Figure 19). Figure 19 - Insulation between walls The thermal performance of windows can be increased dramatically with double glazing or even triple glazing in extremely cold climates. Full length drapes with pelmets will also greatly reduce heat loss. Figure 20 - Drapes and pelmets Although materials can be introduced to improve the thermal performance of the building, total energy efficiency requires attention to the design of the building as a whole. Some of Linings in details, Unit CPCCBS6001, Ed 1 17

18 the aspects which deserve attention mainly those which can be easily attended to have been touched upon in this unit. 18 Linings in details, Unit CPCCBS6001, Ed 1