INSULATION And R-values

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1 INSULATION And R-values The key to an energy efficient house is insulation, and plenty of it. There s not much social kudos in this you can hardly show off the thickness of your loft insulation to visitors but it will insulate you from ever escalating fuel bills as well as the cold. And you can sleep easy knowing that you have done your best for future generations by curtailing your carbon emissions. Heat is lost from buildings mostly by conduction through the fabric, though convection and radiation also have small parts to play and ventilation, too, of course. Indeed, in a super-insulated house, heat losses due to ventilation are comparable to those due to conduction. Last month we were looking at CE marking. From July 1, insulation materials will have to show their thermal resistance and thermal conductivity in a Declaration of Performance. (It's somewhat odd that both values have to be declared, since, given the thickness, one figure can easily be derived from the other.) Heat loss by conduction The symbol usually given to a material's conductivity is λ (lambda). For material with a cross section of one square metre: Rate of heat transfer by conduction = λ x ΔT / d (watts) where λ: Conductivity of the material (W / m.ºc); ΔT (Delta T) : Temperature difference across the material (ºC); d: Thickness of the material (m). (Note: the symbol K, for kelvin, is sometimes used instead of ºC.) Of course, ever increasing thicknesses of insulation lead to ever diminishing returns, so the question is: where to stop? Some say there is a fairly simple answer to this: when you have designed out the need for a central heating system. Money saved by not installing central heating can pay for lots of insulation (and good airtightness). At present, most selfbuilders play it safe and install some central heating. But within a decade, fitting central heating systems may well have become passé. (A MVHR system can distribute small amounts of heat around a house.) Air based insulation Still air is a remarkably good insulator, with a conductivity of only W / m.ºc. But if convection takes place, heat transfer shoots up. So the basis of most insulation materials is the trapping of tiny pockets of still air tiny enough to prevent mini convection currents occurring. As a result, these diverse insulation materials have surprisingly similar conductivities, generally within the range 0.03 to Examples (with typical values for λ): λ (W / m.ºc) Straw, along the stalk 0.08,, across the stalk 0.05 Cork 0.04 INSULATION 1 MAY 2013.

2 Recycled newspaper (eg, Warmcell) 0.04 Foamed glass 0.04 Hemp wool 0.04 Sheep s wool Wood fibre Air filled polyurethane (eg, Icynene) Rock wool slab Glass wool slab Expanded polystyrene (EPS) Extruded polystyrene (XPS) Gas based insulation Some gases have lower conductivities than air, so trapping such a gas instead of air allows materials with lower conductivities to be made: λ (W / m.ºc) Polyurethane foam (PUR) Polyisocyanurate foam (PIR) Phenolic foam No longer are environmentally unacceptable HCFC s used as blowing agents, and nowadays the trapped gases are usually carbon dioxide and/or pentane. (Pentane is a gas with a mild environmental impact.) Over time, carbon dioxide (and to a lesser extent, pentane) can gradually diffuse out from these materials, to be replaced with air. So there is a tendency for the conductivity to increase, and eventually the material could downgrade to have a conductivity similar to air based insulation. However, to prevent such gas migration, insulation boards are often faced with aluminium foil. This has an extra benign effect in that the shiny foil reduces heat losses by radiation. The British Standards for these foams require the conductivity and resistance values to refer to the aged values after 25 years. (For example, the standard for PUR insulation, BS EN 13165, requires the conductivity and resistance values to be derived from foam samples which have undergone an ageing treatment being stored at 70ºC for six months.) Despite its nasty sounding name, polyisocyanurate chemically resembles polyurethane. The big advantage of PIR is that it behaves much better than PUR in fire. Phenolic foam, too, shows excellent behaviour in fire. (By the way, polyurethane can be produced with air inside - Icynene - and as such it appears on the earlier list, too.) Other conductivities For general interest here are typical conductivities for some other building materials: W / m.ºc Aluminium 200 Carbon steel 45 Concrete 1.6 Brickwork 0.8 Glass 0.8 INSULATION 2 MAY 2013.

3 Basalt (used in Teplo wall ties) 0.7 Water 0.6 (don t let insulation become wet!) Hardwood timber 0.18 Softwood,, 0.13 Gypsum plaster 0.16 Silica aerogel Notice that extremely low, last figure we ll have a look at aerogels next month. Lambda 90/90 We ve seen that lambda refers to a material s conductivity, but increasingly a value is being given for λ90/90. (This is the value that will appear on a CE Declaration of Performance.) What is λ90/90 all about? In short, it s a sophisticated value for λ. Simply treat it as the value for the conductivity and you won t go far wrong. An explanation for the curious: In the past, when a value for λ was quoted it would have been the average value obtained by measurements on many test samples. Now, the academic thinking is: Discard the outlying test results, otherwise freak samples may have too much effect on the analysis. So the outlying 10% of results are discarded, ie, only 90% of the test results are used λ90. From the retained test results, calculate statistically the value such that 90% of sampled values would have a lower conductivity λ90/90. This figure is a safe, pessimistic figure for the conductivity. In practice, it is 90% likely that the insulation actually used will have a lower (better) conductivity than λ90/90. So λ90/90 is higher than the mean value, λ. (In old BBA certificates, it is the mean value, λ, that is given. The BBA say that adding to this figure for λ will give a good estimate of λ90/90.) Calculating U-values and R-values A value for thermal conductivity, λ, is only applicable for the conduction of heat in a uniform material. For a building element composed of several materials, eg, a wall, a different figure, the U-value, can be used. So for a unit area of a wall: Rate of heat transfer = U-value x ΔT (watts) where ΔT is the temperature difference across the wall. That is simple enough, and most readers of this magazine will be familiar with U-values (also called 'coefficients of thermal transmittance'.) How many, though, can work them out in practice? Here s the calculation for a traditional wall with cavity insulation: INSULATION 3 MAY 2013.

4 Thickness (m) λ (W / m.ºc.) Thermal Resistance R = Thickness / λ (m 2.ºC / W) Outside surface 0.04 Brickwork Full-fill insulation Dense blockwork Gypsum plaster Inside surface 0.13 TOTAL THERMAL RESISTANCE: Example: Calculation of thermal rsistance, R, of a cavity wall filled with insulation. (The Thermal Resistance, R, is sometimes called the R-value.) The U-value of the example wall is the reciprocal of its R-value: U-value = 1 / R-value = 1 / = 0.58 (W / m 2.ºC). As can be seen from the table, the layers of still air at the inside and outside wall surfaces have some thermal resistance. (The thermal resistance of the outside surface is lower mostly because of the effect of wind. The figures for surface resistances have been taken from the BRE publication, BR 443: Conventions for U value calculations. This publication also gives the thermal resistance of the cavity within a cavity wall as 0.18 m2.ºc / W, irrespective of the thickness of the cavity.) Why R-values are easier to work with than U-values Say we intend to apply 10 cm of external insulation (with λ = 0.04) to a wall with a U-value of 0.5 (ie, with an R-value of 2). What improvement would result? The R-value of 10 cm of the external insulation is 2.5 (= 0.1 / 0.04). So the R-value for the improved wall is simply 4.5 (= ). R-values are added, and this sort of calculation can easily be done in the head. In contrast, evaluating the new U-value requires a calculator: U new = 1 / (1/ ) = 1/ 4.5 = Because they are simpler to work with, R-values are preferable to U-values, and they are widely used in other countries. I first mentioned this in my book Practical House Building, which was published way back in (The book is still in print. By the way, I'll give a copy of this or my other book, All About Selfbuild, to the first person who tells me the origin of the term, 'U-value'. See Further Info.) In the UK, the inertia engendered by familiarity with U-values will eventually be overcome, and the more useful R-values will prevail. But be wary. R-values are widely used in the USA. With selfbuilders doing lots of research on the internet, an unwary enquirer may take an R-value quoted on a USA website as being applicable over here. There are three countries in the world which INSULATION 4 MAY 2013.

5 have not yet officially adopted the SI system (the metric system): Burma, Liberia, and the USA. A USA R-value is 5.7 times greater than the equivalent SI R-value. Best and worst U-values and R-values Under the Building Regulations, the worst U-values allowed, and the corresponding R-values, are: Maximum U-value (W / m 2.ºC) Minimum R-value (m 2.ºC / W) Roof Wall Floor Building regulations: Limiting U-values. (In practice, U-values need to be much better than the above figures by 50% or more.) The R-values of the roof, walls and floor have a great effect on the SAP rating, and so your design R-values will need to reflect the SAP rating you intend to achieve. (If you were to adopt the worst R-values allowed, as in the table above, your SAP rating would almost certainly fail.) K-values No thanks K, k, K-value, and k-value are sometimes used to represent conductivity instead of λ. Their use creates needless confusion. In the UK, the use of the term, 'K-value', is particularly unfortunate as this term is widely used in the USA with values in USA customary units: British Thermal Units per hour per square foot per unit temperature gradient in ºFahrenheit per inch! FURTHER INFO: BR443: Conventions for U value calculations From the BRE Bookshop: A Guide To Sustainable Insulation Materials Free download from: BBA note about Lambda 90/90 SIG 360 SIG is the holding company for Sheffield Insulations nationwide suppliers of insulation. They offers advice on insulation products to housebuilders. The origin of the term 'U-value' INSULATION 5 MAY 2013.

6 I've long been curious about the origin of this term. If you know the answer, please me at A copy of one of my books, Practical House Building or All About Selfbuild, for the first authentic answer! Words: Copyright article by Robert Matthews in SelfBuild & Design magazine, May, INSULATION 6 MAY 2013.