Thermal Bridging in SAP 10.0 - Part Deux

Last month, I explored the changes to thermal bridging calculations that will be introduced in SAP 10.0 and set these updates in the context of a case study based on a typical cavity-masonry house by a mid-size house building company. In this part, I have applied a similar analysis to a flat within a typical mid-rise building consisting of a concrete frame with SFS infill. If you’ve not read part 1, I recommend you do so before reading this.

The mid-rise, concrete and SFS flat considered here is a very different sort of building to the detached ‘traditional’ build house discussed in the previous post – in almost every respect including thermal bridging. A lot of this comes down to one issue: form factor. This term is bandied around Passivhaus circles more regularly than one hears it elsewhere, but the concept is universal. How much exposed area does each dwelling have compared to the useful space inside? A small detached house with lots of ‘interesting’ architecture (overhangs, sticky-out bits, funky roof profiles, dormer windows, recesses etc) will have a worse form factor than a boring, boxy, larger, semi- or terraced house because the latter group will generally have less surface area relative to the internal space available. Flats are even better, because they share more of their surface with adjoining flats. This can raise other problems to do with natural ventilation and daylighting, but that’s another blog… When a colony (yes that is the right collective noun!) of penguins squeeze together to keep warm, they are improving the whole group’s heat loss form factor. Remember the penguins, we’ll come back to them.

The following case study is one of the more extreme from our archive, selected to illustrate the point rather than to be representative. The internal floor area is 52m2, and the gross exposed wall area is around 16m2. Given the floor to ceiling of 2.4m, that’s less than seven linear metres of exposed wall. Of that wall area, almost half is window, at 7.5m2. The walls and roof have a pretty respectable U-value of 0.15 W/m2.K

We have a handful of details to which linear thermal bridges must be applied, including jambs, lintel/head, sill/threshold, party wall, party floor, and balcony. The following graph shows how the thermal bridges stack up in terms of the Y-value.

For almost all the details, the calculated values represent a significant improvement relative to the default values. The exception in this case is the door threshold/sill, where SAP is arguably under-egging it and this particular design left room for improvement – thresholds are one of the hardest junctions to design from a heat loss point of view in our experience. However, the most striking thing about this graph is just how good the notional building appears. It is of course the notional building against which we must compete, and in this example the target heat losses from thermal bridges, set out in Appendix R, are very good, making it more difficult to match the notional and achieve compliance.

I’ve also highlighted the default Y-value on this graph (the red line), but in case you were hoping that this represents a loop hole, think again. If you go with the default Y-value, the notional also gets a default, but to discourage this approach it gets a much lower Y-value of 0.05 W/m2.K There appears to be a typo in Appendix R, which mentions the older (SAP 2012) default Y-value of 0.15, but is probably safe to assume this will get corrected soon.

What does this mean for your projects?

As with the detached house example, the new incarnation of SAP penalises thermal bridging more severely than it currently does. The example analysed here suggests that not only are the days of default values over, it may no longer even be sufficient just to run bespoke calculations on basic details - we as an industry must up our game when it comes to designing out thermal bridging. We may have to re-think many of our design practices and standard details to avoid thermal bridging more actively. For example, as popular as SFS systems are, they often introduce a number of thermal bridging challenges which could be entirely avoided by a different system of build, such as casting the walls in concrete along with the floor slabs (an approach popular on the continent).

The benefits of designing out thermal bridging are many. Great detail design can make it easier to achieve excellent airtightness and eliminate the risk of surface mould and condensation. The savings in CO2 on the SAP calculation can reduce the need for complicated renewables systems and make it easier to comply with planning conditions such as the London Plan. The occupants of buildings with no thermal bridging will have greater thermal comfort and be less likely to suffer from the annoyance and health impacts of mould.

At Greengauge, we have a wealth of knowledge and experience in not just assessing thermal bridging but designing to avoid it. This latter skill will become increasingly important as the new edition of SAP kicks in, so get in touch and let us help you make the right decisions to ensure an easy path through Part L and efficient, healthy, buildable buildings.