EXTERIOR DOORS And doorsets

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1 EXTERIOR DOORS And doorsets The front door of your home gives visitors an impression of the style to be found within. Is your aspiration modern, post-modern, olde worlde, or what? For many selfbuilders, a front door that combines elegance with sturdiness is ideal. Construction Timber has been the traditional material for doors, and that has enforced its own discipline on the design of doors, whether in softwood or hardwood. It is not practical, for example, to make a door from one wide board. So a four panel, solid timber door has a vertical 'stile' at each side, a horizontal 'rail' at the top, bottom, and part way up, and there are vertical 'muntins' midway between the stiles to help hold the panels. Exterior doors are also becoming available in modified wood such as 'accoya' this is less likely to warp, shrink or rot than natural wood, and paintwork lasts longer. Composite doors are common. They have a framework made from softwood and/or plastic, the front and back faces are formed of skins of GRP (fibreglass), upvc, or steel, and the inside is filled with a high density plastic foam usually polyurethane (an excellent insulator) but sometimes polystyrene. The skins are usually moulded to imitate the contours of a timber door, and coloured to appear as paintwork. Who can spot the difference? There is, though, a big difference in U-values. The U-value of a composite door can be half that of a solid timber door, or even less see later. 1. Door frame: upvc with steel reinforcement. 2. Subframe: Composite PVC. 3. Inner frame reinforcing: Hardwood. 4. Core: Injected PU foam. 5. Edge banding: 2mm PVC. 6. Skin: 2mm GRP with through colour. 7. Threshold: Part M compliant wheel chair access. 8. Cill: Options 85mm, 150mm, no cill. Section of a composite door, with a door frame and threshold (Diagram by Composite Door Shop.) There is another big difference between a solid timber door and a composite one, a difference where this time solid timber has the advantage: strength. (The video clip listed in Further Info shows what happens when a sledge hammer meets a composite EXTERIOR DOORS 1 OCTOBER 2015.

2 door.) So the core of some composite doors is a timber panel rather than foam. The panel gives strength, but loses the superior thermal benefits of foam. Give some thought to the security of your back door, too. Would not a would-be intruder be more likely attack the back door, rather than a front door visible from the street? There is a wide variety of styles available for front doors, but less variety for back doors. Glazing is an option, to let light in and to let you see out. If a door's glazed area is within 1.5m of the floor, then building regulations require safety glass laminated glass is better than tempered glass in this situation, as laminated glass remains more of a barrier after being hit. Thermal performance Like windows, exterior doors need low U-values. (But with good house design, an exterior door might open into an unheated porch or lobby, with less heat being wasted through the door than might be expected.) Let's find the U-value of an imaginary oak door with a uniform thickness of 44mm. The thermal conductivity of oak is 0.16 W / m C. So U-value = 0.17 / = 3.9 W / m2 C. (ie, its thermal transmittance) That's nearly twice worse than the limiting U-value of the building regulations, 2 W / m2 C (see below). A solid oak door would be strong but thermally unacceptable. Composite doors with a core of injected polyurethane foam are much better insulators. IG Doors produce composite doors with fibreglass or steel skins. For their GRP Fibrecolour doors they claim an amazing U-value of only But this value applies only to the core of the door it does not take into account its subframe, nor the door frame. [See Footnote 1.] As we see below, U-values for 'doors' are a tricky subject. The reader may like to look more closely at the diagram given above that shows a typical composite door. The core is polyurethane foam which is a very good insulator. Surrounding that is a subframe of PVC and hardwood indifferent insulators. But worst is the door frame, of PVC with steel reinforcement. We all know that steel is a poor insulator, so in effect the door frame is one long thermal bridge! (A timber door frame would be more thermally efficient.) The official door Question: When is a door not a door? Answer: When the door is an official door. Let me explain: When the word 'door' is used within the context of building regulations, British Standards, etc, it usually means a doorset, with a door leaf fitted into a frame, and with door furniture. This eccentric usage of the word can lead to much confusion. A few door manufacturers give U-values for their doors, or more precisely for their door leafs. Most give U-values for 'doors' that are, in fact, doorsets. The U-values of 'doors' referred to in Approved Document L1A are for doorsets, not door leafs. And there can be a big difference. As mentioned previously, IG Doors claim an outstandingly good U-value for the core of their Fibrecolour composite door: They also sell this door as part of a doorset, and its CE Declaration of Performance shows that the U-value of the doorset is 2.1 (ie, not worse than 2.1). That's a EXTERIOR DOORS 2 OCTOBER 2015.

3 remarkably unimpressive claim, especially as the Building Regulations' limiting U-value is 2. In fact, the value 2.1 is the U-value of IG Doors' most thermally inefficient doorset. For a particular doorset, IG Doors will calculate an actual U-value see Footnote 1. In fact, many manufacturers of composite doorsets will supply you with a bespoke energy certificate. The use of ' ' in CE marking has been a contentious issue. Such a claim is of little value to purchasers its use is more about a manufacturer conforming to the letter of the law. Doors in SAP SAP is encouraging builders to fit a doorset, rather than a separate frame and door leaf. According to the SAP documentation: When available, the manufacturer's certified U-values for windows or doors should be used in preference to the data in this table. Remember, the word 'door' in the above sentence really refers to a doorset. The SAP table gives default U-values for 'doors' and windows. For a solid wooden 'door' (doorset) the default U-value is 3.0 very poor. For the best possible SAP rating, fit a prefabricated doorset with a certified U-value, so that a good U-value figure can be entered into the calculation. Nonetheless, it is still a common practice to fit door leafs into separate frames. Even if you know the U-value of a door leaf sometimes called a 'slab' you can't really calculate the U-value of the doorset, so your SAP assessor should enter the poor default U-value given above for the 'door'. (But remember: the SAP rating is only a theoretical figure, not the reality itself. It is the reality that will keep your home cosy, not a rating number.) Limiting and reference U-values Approved Document L1A of the building regulations gives the limiting U-value for exterior doors: 2 W/m2 C. (Again, in this context 'door' means doorset: door leaf, door frame and furniture.) This does not mean that an individual door with a U-value worse than 2 is not permitted. The limiting value applies to the area weighted U-value of all the exterior doors. (As we saw last month in my article, Windows and the Building Regulations, the limiting U-value for windows is also 2 W/m2 C, and it has a similar application.) Of more interest are the reference U-values for the exterior doors of SAP's notional house: Glazed area (% of total area) Reference U value (W/m 2 C) <30% ('Opaque') 1 30% 60% ('Semiglazed') 1.2 >60% ('Glazed') 1.4 SAP's reference U-values for 'doors' (ie, doorsets) For a so-called 'opaque' doorset (up to 30% glazed!) the reference U-value is 1.0 W/m2 C. This is the figure that you would want an opaque doorset to beat. (See EXTERIOR DOORS 3 OCTOBER 2015.

4 Footnote 2.) Likewise, for 'semi-glazed' and 'glazed' doorsets the figures to beat are 1.2 and 1.4, respectively. In the course of a SAP calculation, the solar gain that a 'glazed door' contributes is taken into account. But for other doorsets (with glazing area less than 60%), no solar heat gain is to be included in the SAP calculation. Jeld-Wen offer the 'Castle' range of doorsets with 44mm thick composite doors that have fibreglass skins over a core of polyurethane foam. The U-values of these doorsets range from 1.0 (no glazing) to 1.8 (generous glazing). They also offer the 'Knightsbridge' range with timber doors incorporating an oak veneer U-values from 1.9 to 2.2. (Only the unglazed Castle doorsets, which have a U-value of 1, meet SAP's reference U- value.) Door (Set) Energy Ratings Jeld-Wen's Castle Dunster doorset (U-value of doorset a composite door with wooden frame is 1.0 W/m 2 C.) Window Energy Ratings by the British Fenestration Registration Council (BFRC) were the subject of my February article. The BFRC introduced a similar scheme for doors in 2011: the Door Energy Rating (DER) scheme. Many doors have glazing in them, and so have a similarity with windows. For a 'glazed door', the formula for the Energy Index is the same as that for windows: where: Energy Index = x g 68.5 (U + Air Leakage Factor) g is the Solar Factor for the doorset an index (0 to 1) of the heat gain due to solar energy transmitted through the doorset. This depends on the relative area that is glazed, as well as the nature of the glazing. (g=1 for a doorset which is perfectly transparent to all solar radiation no such doorset exists, of course.) U is the thermal transmittance of the doorset. The Air Leakage Factor is obtained by using the volumetric specific heat of air to convert the Air Leakage Rate into an equivalent heat loss rate with the same units as U. EXTERIOR DOORS 4 OCTOBER 2015.

5 For a 'semi-glazed door' the questionable decision has been made to ignore solar gain in calculating the Energy Index so that the rating is in accord with the SAP convention. The Solar Factor, g, is set to zero, and so the following formula applies to both semi-glazed and opaque doorsets: Energy Index = 68.5 (U + Air Leakage Factor) A rating is given according to which band of values its energy index fits. The 'A' band corresponds to an energy index which is greater than -70. Currently, the bands B to F each cover a range of 15 in the Energy Index. But at the time of writing, the bands are about to be changed to bring them into line with the band width of Window Energy Ratings, which have a range of 10. So by the time you read this, the bands will be: Rating A B C D E F Energy Index (Minimum) Bands for Door(set) Energy Ratings Note that you can't directly compare window and doorset ratings. For a Window Energy Rating the minimum Energy Index for 'A' is 0, not -70 as above. An Energy Index of -70 gives an 'A' rating as a doorset, but only an 'F' rating as a window. (Over the course of a year, an 'A' rated doorset is likely to lose as much heat an an 'F' rated window of the same area.) Since its introduction, the name of the scheme has been changed to the more accurate Doorset Energy Rating (DSER) a good move. And in the future, a DSER of A+ or A++ will theoretically be possible, mimicking the newly added bands for Window Energy Ratings. Next month: More about doors. FOOTNOTE 1: U-value of IG Door From the IG Door website: GRP Fibrecolour doors have a U-value through the doors of 0.55W/m2K, equivalent to approximately four solid timber doors. As mentioned previously, IG Doors ed me to say that this U-value applies to the core of the door, without the subframe. FOOTNOTE 2: Reference U-values A reminder about a simple way to ensure that your house will satisfy the SAP criteria: if the U-values of its walls, windows, doorsets, floors, and roof are all as good as SAP's reference U-values (and airtightness, too, meets the reference value of 5 m3/m2h), then the Fabric Energy Efficiency (FEE) and the Dwelling CO2 Emission Rate (DER) of your house are likely to be satisfactory. EXTERIOR DOORS 5 OCTOBER 2015.

6 But just to be clear: if some of your actual U-values are worse than the reference U- values, the FEE and DER may still be satisfactory. And there are other factors to be taken into account; but it is the U-values of the shell that are of paramount importance. FURTHER INFO: Abbey Windows video clip A sledge hammer shows the weakness of many foam filled doors. BFRC (The BFRC has been run by the Glass and Glazing Federation since 2005.) They promote the DSER and WER schemes Doorset Energy Ratings and Window Energy Ratings. Composite Door Shop IG Doors Jeld-Wen Wide ranges of doors, windows and stairs. Door-Stop International Manufacturers of composite doors with GRP skins. Solidor composite doors 48mm thick, composite doors with a solid timber core. Endurance composite doors 48mm thick, composite doors with a laminated timber core. Secured by Design. Words: Copyright article by Robert Matthews in SelfBuild & Design magazine, October, EXTERIOR DOORS 6 OCTOBER 2015.