Thermal Bridging:
One major change to the Building Regulations is that energy consultants are now required to assume the default Y-value of 0.15 W/m2K, unless they have been presented with evidence that the proposed construction details comply with a suitable accreditation system. This default has been introduced as it has been recognised that calculations have often have assumed the previous default Y-value of 0.08 W/(m2K), or more recently 0.04 W/(m2K) when using EST or AECB details, whilst the design details did not actually achieve these standards of performance. The Y-value is derived by dividing the total heat loss through the thermal bridges by the surface area of the building. In turn the total heat loss through the thermal bridges is determined by multiplying the thermal bridging coefficient (psi-value, W/mK) by the total length of the thermal bridge for each thermal bridge and totalling them up.
For instance if a house has a surface area of 184m2, with a total heat loss through thermal bridges of 27.56 W/K then the Y-value is 0.15 W/m2K. The sum of the Y-value and the U-value can therefore be understood to reflect the total fabric heat loss. To gain an understanding of the impact that thermal bridging has upon a building, I will use three houses as an example:
House A – Default thermal bridge Part L (England and Wales), Y-value 0.15 W/(m2K)
House B – Accredited Construction Details Part L (England and Wales), Y-value 0.08 W/(m2K)
House C – AECB Best Practice Details, Y-value 0.04 W/(m2K)
House C results in 75% reduction in heat loss compared to House A and a 55% reduction in heat loss compared to House B. A carefully designed PassivHaus building can offer an 86% improvement upon the default Y-value. If the designer is seeking to use an assumption other than the default of 0.15 W/m2K then Energy Consultants will now have to inspect the drawings and any supporting documentation to ensure that the construction details accord with their targets. Previously this has not been the case.
Another way to consider this is the concept of effective heat loss. If we take a wall (U-value of 0.2 W/m2K) with default thermal bridging, the Effective U-value is therefore 0.2 W/m2K plus 0.15 W/m2K i.e. 0.35 W/m2K. The proportion of fabric heat loss attributable to the thermal bridges can therefore be understood to be 42%. If we were seeking to achieve an Effective U-value of 0.24 W/m2K then one could improve the traditional U-value from 0.2 W/m2K to 0.09 W/m2K, or alternatively one could reduce the thermal bridging from 0.15 W/m2K to 0.04 W/m2K. Across a whole project the cost of supply and labour for installing the additional insulation could be considerable, by comparison design is cheap. In essence the lesson here is to ensure that the best possible construction details are used at all times.
Thermal Bypass:
Part L now includes more detailed consideration for party walls. This reflects research by Leeds Metropolitan University (LMU) highlighting the fact that the heat loss from party walls effectively meant that a mid terrace unit performed like a detached house; this contrasts with the theory in previous editions of the Building Regulations that there was no heat loss from the party wall. The reason for the poor performance was that the cavity of the party wall permitted heat loss via convection. Based upon further studies by LMU a number of strategies are now considered within Part L. The first two approaches are considered to achieve a U-value of zero. The first method is to have a solid a party wall however careful consideration should be given to acoustics. The alternative is to fully fill the cavity with insulation. The final option is to provide a continuous seal within the cavity in line with the surrounding insulation – this is achieved by placing an insulated sock wrapped in polythene, but this less successful method achieves a U-value of 0.2 W/m2K. An unfilled and unsealed cavity will have a U-value of 0.5 W/m2K.
NOTE: This article was originally written when author worked at DEWJO’C Architects (which latterly became Devereux Architects). In 2011 Mark set up his own practice so that he could continue to pursue his interests in developing sustainable, low energy architecture.
Notes on the author:
Mark Siddall, principle at low energy architectural practice LEAP, is an architect and energy consultant specialising in low energy and PassivHaus design. He was project architect for the Racecourse Passivhaus scheme and has a keen interest building performance. In addition to architectural services his practice provides project enabling and education for clients, design teams and constructors.
LEAP website: www.leap4.it