Manufacturers of Multi Foil Insulation and Reflective Breathable Membranes, loft conversion, re-roofing, timber frame walls, offsite construction of volumetric modular units, A1 non-combustible to BS EN 13501-1
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Insulating roof tile underlays

There is a new category of composite insulating roof tile underlays that:
PhotonAir is the ONLY low resistance (LR) insulating underlay amongst these four LABC registered products. (Data as at 1st February 2017)

BS 5250 (Code of practice for control of condensation in buildings)

States that in warm pitched roofs only LR underlays remove the risk of interstitial condensation forming without the need for any associated ventilation to roof voids.

BS 5250 defines two types of roofing underlay as:

HR underlay
BS 5250 further 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 negates the thermal benefit it gives to the roof structure.

LR underlay
However, for warm pitched roofs constructed with a LR underlay, BS 5250 has 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.