Less Risk (LR) roofing

Balancing the scales between thermal performance and condensation risk is child’s play with a new generation of low resistance underlays from Thermic Technology.

PhotonAir, is a Low Resistance (LR) insulating breather membrane, approved by LABC, that upgrades thermal performance without risking condensation.

Made with high density glasswool encased between a perforated reflective lower layer and Klober Permo® air, PhotonAir is installed in the same way as a standard roofing tile underlay.

When synthetic breather membranes first emerged in northern Europe BRE (British Research Establishment) and BSI (British Standards Institution) established a requirement below 0.25MN.s/g for vapour resistance for their use in warm roof and cold non-vented applications. Anything higher carries a risk of interstitial* condensation forming which can deteriorate the fabric of a building through corrosion, rot and decay and also reduce the thermal resistance of materials, affecting energy efficiency.

According to BS 5250:2011 (Code of Practice for Control of Condensation in Buildings) there are two types of roofing underlay:

• HR (High Resistance) – water vapour resistance greater than 0.25 MN.s/g
• LR (Low Resistance) – water vapour resistance not exceeding 0.25 MN.s/g

The Code of Practice 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.” The Standard goes on to recommends an Air and Vapour Control Layer is installed on the warm side of the insulation, and 25mm deep ventilated voids, at high and low level, formed between the underside of the underlay and insulation. This however dramatically reduces any thermal benefit it provides.

In warm pitched roofs, PhotonAir stops the risk of interstitial condensation forming and removes the need to ventilate the underside of the underlay. There may also be no need to provide an AVCL on the warm side if doing so isn’t practical.

Low Resistance underlays are a vital part of any insulation project today as they remove the risk of interstitial condensation without roof void ventilation – turning a complex ventilation job into something much more manageable.

Further information about the Photon range call our helpline on 01752 692 760

*Interstitial condensation is a form of structural damping that occurs when warm, moist air penetrates inside a wall, roof or floor structure, reaches the dew point and condenses into liquid water.