FAQs

λ valueLambda value. Thermal conductivity (W/m.K) demonstrates the thermal conductivity of a material. The lower the Lambda value the better. May also be expressed as k.
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λ90/90 Lambda 90/90 is a statistical process applied to many European Standards that ensures that the declared thermal conductivity can be obtained for 90% of production within a 90% confidence level. The aim is to improve the robustness of design values so that conductivities used in calculations are more realistic and are met most of the time; as a conservative rule of thumb a 90/90 value is 0.005 W/m.K higher (worse) than the typical or mean value.
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Air Closed A type of breathable membrane which comprises a vapour permeable micro porous film which allows vapour transfer by diffusion. Whilst allowing water vapour to pass through the micro pores in the film are too small to allow the passage of air.
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Air tightness The resistance of the building envelope to inward or outward air leakage. Excessive air leakage results in increased energy consumption and a draughty, cold building.
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Air Open There are two categories of Low Resistance (LR) underlays: Air Tight, which comprise a vapour permeable film which allows vapour transfer by diffusion, and Air Open, which have higher vapour permeability with the additional condensation controlling air permeability. Being Air Open allows water vapour to pass more easily than by diffusion alone.
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AVCL Air and vapour control layers (AVCLs) are air and vapour tight. Used on the warm side of a structure, often behind the plasterboard, they help to reduce convective heat loss and provide effective condensation control.
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BS EN5250:2011 Code of practice for control of condensation in buildings
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BS EN16012:2012 'Thermal insulation for buildings - Reflective insulation products - Determination of the declared thermal performance': This is the newly published British and European Standard for determination of the thermal performance of reflective insulation products.
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Cold roofA roofspace where insulation is fitted at ceiling level.
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EmissivityEmissivity is a measure of the reflectivity of a surface to infra red radiation, expressed as e or ε. The more reflective a surface is, the lower its emissivity.
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HR (High Resistance) underlay Defined by BS 5250:2011 as having a water vapour resistance greater than 0.25 MN.s/g. BS 5250:2011 states: "In roofs with an HR underlay.., whatever form of external covering or ceiling is provided, there is a risk of interstitial condensation forming on the underside of the HR underlay; to avoid that risk, an AVCL should be provided on the warm side of the insulation, and ventilated voids should be formed between the underside of the underlay and the insulation. Each void should be at least 25 mm deep and be vented at both high and low level." The requirement to ventilate below a high resistance (HR) insulating underlay dramatically reduces any thermal benefit it gives to the roof structure.
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Interstitial condensationOccurs when warm, moist air (generally, from inside a building) penetrates into a wall, roof or floor or permeable forms of insulation, e.g. mineral wool. If it reaches dew point, condensation will appear within the structure.
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LABC Registered DetailsLABC Registered Details allows manufacturers to register specific solutions so they have national acceptance by all Local Authority Building Control across England and Wales.
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LR (Low Resistance) underlay Defined by BS 5250:2011 as having a water vapour resistance not exceeding 0.25 MN.s/g. BS 5250:2011 states: for warm pitched roofs constructed with LR underlay there is no requirement to ventilate the underside of the underlay and there may also not be the requirement to provide an AVCL on the warm side: PhotonAir low resistance (LR) insulating underlay upgrades thermal performance without the risk of condensation.
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New thermal elementAddition of a new structure to an existing building e.g. a house extension.
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NHBC 2011 NHBC Standards for pitched roofs now includes the requirement for the provision of ridge or high level ventilation equivalent to a continuous opening of 5mm when using vapour permeable underlays. This followed an increasing number of calls concerning roof condensation during cold weather to the NHBC. This requirement does not apply to vapour permeable underlays that are third party assessed as being both vapour and air permeable.
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Part LPart L (Approved Document L) of schedule 1 of the Building Regulations covers the requirements with respect to Conservation of fuel and power.
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PFPhenolic foam insulation.
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PIRPolyisocyanurate insulation.
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PURPolyurethane insulation.
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Retained thermal elementAdding a new layer or completely replacing an existing thermal element e.g. re-roofing.
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R valueThermal resistance (m2.K/W), is the rate at which heat is transferred through a structure. The higher the R value the better.
R = Thickness/Lambda
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SAP The Standard Assessment Procedure (SAP) is the UK Government's recommended method system for measuring the energy rating of residential dwellings.
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Thermal elementA Thermal Element is a roof, wall or floor which separates a thermally conditioned (heated or cooled) space from: the outside, an unheated part of the same building, a structure exempt from the building regulations (such as porch or conservatory) or part of the same building heated or cooled to a different temperature.
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U-valueThermal transmittance (W/m2.K) i.e. how much heat is lost through a structure. It is the inverse of the sum of the R-values of the components and the lower the U-value the better.
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VCLVapour Control Layer. Used on the warm side of a structure, often behind the plasterboard, to restrict the passage of water vapour.
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Water vapour resistance The vapour resistance of a material is a measure of the material's resistance to let water vapour pass through. The vapour resistance takes into account the material's thickness, so can only be quoted for a particular thickness of material. It is measured in MNs/g (Mega-Newton seconds per gram).
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Wind uplift The principle purpose of the pitched roof underlay is to substantially reduce the effect of wind loading on the slate or tile roof covering. Roof underlays have a tendency to "balloon" with the risk of dislodging the slates or tiles. This issue has been addressed in the updated BS5534 Code of Practice for Slating and Tiling. A wind speed map of the UK has been produced and manufacturers are required to provide information on packaging to show the zonal limit suitability.
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Warm roofA roofspace where insulation is fitted at rafter level.
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