SFS White Paper sheds fresh light on window position

Callum Thornton, the Fenestration Product Manager for SFS UK, explains why social housing specifiers and housing managers should consider continuity of insulation around window openings as a priority for tackling thermal-bridging and mould growth.

The tragic death of a young boy whose family lived in unacceptably sub-standard conditions led to the Social Housing Regulation Bill being amended in 2023 to include ‘Awaab’s Law’ compelling landlords across the country to investigate and rectify damp and mould within a strict timeframe. These all-too-common problems also serve to underline the need to understand the key causes of mould growth, including the crucial positioning of windows within the outer walls.

This is a highly complex technical challenge, that is just as important to address during refurbishment and improvement contracts, as it is when trying to comply with the latest requirements of Approved Document L for new build projects. And this April saw the New Build Heat Standard implemented in England (and Wales) while Scotland is tightening requirements ready for the enactment of its own PassivHaus standard in two years’ time; with windows having to achieve a U-value of 0.80 W/m2k .

A chequered history
The size, shape and location of window openings have been a key feature in architectural styles down the ages, but current day concerns over energy performance mean that the actual position of fenestration units within the wall section has now taken on a far greater significance. Getting it wrong will not only be reflected in higher heating costs but will almost inevitably impact on occupants’ health, as well as their comfort.

In order to assist building professionals, including social housing providers, to better understand the conundrum of maintaining continuity of thermal performance across the building envelope, SFS has added to its library of White Papers with the latest entitled: “Optimising window placement.”

Windows are an essential component of the building envelope, serving multiple functions such as allowing natural light to enter, providing security, offering panoramic views, and facilitating ventilation while minimizing heat loss. Because of this, windows are unfortuntely usually the weakest point in the thermal envelope.

However, with the introduction of revisions to Approved Document Part L in 2022 and new SAP software, the maximum permitted U-values for a window unit were lowered and, as we look ahead to the Future Homes Standard 2025, it is evident that compliance requirements will become even more demanding. With this in mind, it is crucial to carefully consider the impact of window position on the overall efficiency of the glazed unit.

It is more than three decades since the BRE first identified that the floor/wall and other junctions across the building envelope were just as important as the elevations themselves. We waited until 2008 for the guidance on cold bridging to be introduced into Building Regulations, but the subject is comprehensively addressed in the current Approved Document L, Volume 1 section on ‘Continuity of Insulation,’ which emphasises how windows should be “contiguous with the insulation layer of the external wall.”

The approach taken towards compliance with Approved Document L, Volume 1 can vary depending on the design of the overall wall build-up chosen. Typical approaches in the UK vary from cavity wall construction, solid masonry wrapped with an insulated render or an EWI system or, for existing properties – especially in conservation areas – Internal Wall Insulation (IWI) is often preferred.

The principle of continuity is important in all cases, but the SFS White Paper on Optimising Window Placement’ explains the challenges, technical requirements, and potential pitfalls of falling short.

One part of the document utilises thermal modelling – in the form of software generated heat flux section drawings – to illustrate how the different window positions can have a marked effect on the thermal performance of the overall element.

The examples include a solid wall construction where an equal thickness of insulation has been affixed to the external face, but contrasts locating the window frame within the masonry, and shifted forward so it is fully aligned with the insulation zone, which reduces heat transfer through the frame itself. Other examples show how the PSI values, or thermal bridging heat losses, can also vary between the top and bottom of a window frame.

While window cills, heads or jambs are only some of the thermal bridges occurring around a building, their high frequency across the envelope mean their significance is magnified, particularly for high-rise and HMOs.

In addition, as with any thermal bridge, the loss of heat around window perimeters will lead to condensation as moist air cools rapidly as the dew point is exceeded. At best, the surface condensation is a nuisance and can stain plasterwork; at worst it causes mould growth, allowing spores to be released into the atmosphere, precipitating or exacerbating respiratory problems.

Planning for thermal improvement

The question of timing, as well as alignment, also regularly arises in retrofit and refurbishment situations where EWI (a favourite recourse for both high-rise and low-rise improvements) is counted as the former, and window replacement as the latter with property owners, particularly social housing landlords, generally wishing to maximise the service life of existing fenestration. Inevitably, though, many contracts constitute urgent upgrades and ‘quick fixes’, often involving complications such as having to decant tenants. Accordingly, at the FutureBuild event in March, Savills reported that evaluating initial waves of the SHDF showed that joined up thinking for such interventions was essential to minimise visits and avoid ‘tenant fatigue’.

In f act, social housing providers frequently find themselves aiming to invoke measures attracting defined funding, such as ECO schemes, rather than striving to achieve best outcomes. Unfortunately, fitting new windows some years after EWI rarely sees the frames aligned contiguously with the wall insulation, and problems of thermal bridging are simply exacerbated, storing up problems for the future. Such failures can lead to higher heating bills, degradation of the building fabric and poorer health outcomes for residents.

Ideally, coordinated and comprehensive upgrades across our building stock will save lives and a key strategy for avoiding such issues should see building managers and other specifiers seeking to have high performance windows installed on special cantilever brackets, beyond the building structure, including supporting much heavier, triple-glazed windows. Inevitably, aligning these higher performing window frames with wider insulation zones, outside the main structure, demands longer cantilevers and the sustaining of greater lever-arm forces in order achieve the ultimate goal of thermal continuity.

As well as manufacturing appropriately designed and tested brackets and offering holistic technical support to all parties seeking to optimise the benefits of energy upgrades in our buildings, SFS has published a series of White Papers on the subject of external wall insulation, rainscreen systems and fenestration.

For further information, call 0330 0555888 or visit https://uk.sfs.com/