The links between airtightness, thermal bridging and IEQ
We spend a significant part of our lives inside. Studies have shown that the average adult in Britain spends between 85-92% of their time indoors on a weekly basis.1,2 A poll commissioned in 2016 put these statistics in a different light and described how a typical prison inmate spends more time outside than three-quarters of the children in the UK.
As we accept that our professions and lifestyles are keeping us indoors for longerperiods of time, an awareness of the importance of indoor environmental quality(IEQ) is also growing. The US-based National Institute for Occupational Safety and Health (NIOSH) defines IEQ as “the quality of a building’s environment in relation to the health and well-being of those who occupy space within it”. Key to this definition is the role that IEQ factors like air quality and thermal comfort play in our productivity and enjoyment of life, as well as our physical and psychological health.
Improvements to UK building standards over the years have helped to ensure that an appropriate level of insulation is incorporated into building work. The availability of higher-quality insulation materials and a better understanding of modern building physics work together to improve overall building thermal performance, reduce energy demands and carbon dioxide emissions, and support the thermal comfort of building occupants. However, insulation is only part of the solution to providing excellent IEQ.
Air leakage and thermal bridging
Tests confirm that poor airtightness can be responsible for up to 40% of heat lossfrom buildings because cracks, gaps and holes in the building envelope allows airto bypass the thermal insulation.5 Energy efficient heating systems are also rendered less effective if warmed air simply escapes through a draughty structure while cold air is allowed to flow in.
While adding more insulation should improve thermal performance in theory, thebenefits of thicker insulation may not be fully realised if the envelope’s airtightness is not also addressed – the heat lost through air leakage will simply increase proportionally.
To illustrate this concept, Section 6.2.0 of Building Standards technical handbook2017: domestic buildings (Scotland) describes a typical 1960’s house with poorly fitted windows. In the example, air leakage represents approximately 20% of the total heat lost in the house’s base condition. If the house is then upgraded to modern insulation standards but the leaky windows are not remedied at the same time, the proportion of the total heat lost via air infiltration will increase to over 40%.
Thermal bridges also undermine efforts to insulate properly by providing pathways through the insulation for heat to escape. The most commonly thoughtof thermal bridges are studwork, mechanical fixings and other penetrations through the insulation layer, but thermal bridges can also include larger elements like party cavity walls. As described in Section 3 of Approved Document L1A: Conservation of fuel and power:
“Where outside air flows into the party wall cavity, a cold zone is created which causes heat loss through the wall sections on either side. The air movements involved can be significant and ... the resulting heat losses can be large.”
Unaddressed thermal bridging can result in the formation of condensation within building structures. Interstitial condensation can then cause mold and mildew growth on interior surfaces, compromising indoor air quality and potentially occupants’ health
Clearly, as we continue to spend the majority of our time indoors, the building industry must keep IEQ considerations at the forefront of their designs. Strategies to support IEQ should include appropriate levels of insulation, an uninterrupted airtightness layer, and measures taken to identify and control thermal bridging in order to prevent interstitial condensation.
Wraptherm®: Combining airtightness with insulation
Wraptherm is a composite material comprised of 10mm Spacetherm® aerogel insulation blanket bonded to the face of Wraptite® vapour permeable air barrier. It is ideal for the refurbishment of existing buildings.
Wraptherm is self-adhering and can be applied to practically any substrate without the need for tapes or mechanical fixings. By being installed on the internal face of an existing façade, Wraptherm provides an air barrier on the external side of the ‘services zone’. This means there is less risk for the airtight layer to be accidentally punctured. As well, there is no requirement for expensivespecialist components such as airtight junction boxes or light switches.
Having a nominal thickness of only 11.5mm, Wraptherm also greatly reduces thermal bridging as new framing can be installed directly over it. Additional insulation can then be added within the frame to bring the construction up to therequired U-value.
By combining the airtight and insulating properties of Wraptite and Spacetherm, Wraptherm helps by both improving a building’s thermal performance and supporting IEQ by mitigating the risk of interstitial condensation.