Wednesday, 14 October 2015

Making Sense of SAP Appendix K

SAP Appendix K contains a list of 41 junction types with default psi values for each and ‘approved’, i.e. accredited* values for 16 of the most common. The table of junctions is divided into three parts: Junctions with an External Wall; with a party wall; and within a roof or room-in-a-roof. Some of these are fairly self-explanatory, some much less so. There are a few situation you might come across where there is no classification for the situation, and exactly how you deal with this will depend on which SPA software you are using, and the view of your Building Control Officer.

In the first of a mini-series, here’s our take on what’s what, and a few tips on how to apply psi values in SAP.

E1 - Steel Lintel with perforated steel base plate

Generally, we strongly recommend against ever using this type of junction. Even with the perforations and the extra length of the “Top Hat” section if there is a non-metal base plate, you’re basically connecting the inside of your wall to the outside with a material (steel) which is typically over 1500 times more heat-conductive. Unfortunately filling the space inside the top hat section doesn’t help much. On top of the heat loss you’ll also often find it’s not acceptable under IP1 06 and therefore presents a condensation risk. Simply specify separate masonry or angle-steel lintels, i.e. make it an E2 Junction. You won’t find any E1 junctions in our database!

E2 - Other Lintels (including other steel lintels)

This one is easy enough to understand - it represents the junction between a wall, and a door or window. One question we’ve been asked is where to measure this - the width of the window, or the width of the whole lintel including the pillar, padstone, or overlap length. Generally its fine to take the width of the window or door, unless you have an E1 junction in which case take the length of the steel. Or don’t do an E1…

E3 and E4 - Sills and Jambs

Again these are pretty self explanatory. Under BRE 497, Conventions for Calculating Linear Thermal Transmittance, pages 26 and 27 suggest that the window is disregarded when calculating the psi-value. We disagree with this; although marginally more complicated to calculate, it is much more realistic to account for the frame as there is a significant amount of lateral heat flow. The upshot is that the psi value comes out worse this way, but we have decided to use this approach in all our modelling in order, in a small way, to close the performance gap.

E5 - Ground Floor Normal

Last time we checked, a ground floor was the bit you stand on, but this is what SAP Appendix P calls the wall to ground floor junction. This is also sometimes referred to as the foundation, as the thermal analysis includes the strip footings, raft edge or whatever is in the ground holding the wall up.

E6 - Intermediate floor within a dwelling

This one is fairly self-explanatory, but don’t confuse it with E7, a party floor to wall junction. It’s pretty easy to get a zero, or at least very low psi value on this one, so well worth finding something better than the default. Watch out for airtightness problems though!

E7 - Party floor between dwellings

This junction is often a bit different to equivalent E6 junctions for acoustic reasons, and is often a bit worse as a result. An important distinction here is that the value gets applied separately to both dwellings either side of the floor. Therefore, if we hypothetically had an identical E6 and E7 detail design, the E7 psi value would be half that of the E6, even though the overall heat flow is the same. This is one difference between SAP and Passivhaus (more on that another time…)

E8 - Balcony between dwellings, wall insulation continuous

It is likely that the wall insulation will only be completely continuous if the balcony is free-standing on an external structure. If not, you have probably got an E9.

E9 - Balcony between dwellings, wall insulation not continuous

The very name of this junction type gives us the heebie jeebies. If at all possible, like an E1, just don’t go there - can you design a different way? This subject is deep enough to warrant its own blog article, which we’ll do very soon.

We’ll give an overview of more typical junctions in a later blog article, so check back soon.

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