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.

Saturday, 3 October 2015

What's in a Y-value?

The Y-value is a figure used in SAP to indicate the amount of heatloss attributable to linear and point thermal bridging for a given dwelling. The Y-value is expressed in units of W/m2.K - the same units as a U-value. It is calculated as follows: first, for each junction the psi-value is multiplied by its length, the resulting figures for all junctions in the building are summed, to give the Htb. The Htb is the thermal bridging heatloss factor in W/K. This is converted to a Y-value by dividing by the surface area of the building. The default value is 0.15 W/m2.K and good practice design can typically reduce this by about half.
For a typical house, crunching the numbers indicates that the heatloss due to default levels of thermal bridging (i.e. a Y-value of 0.15) adds between 25% and 40% to the heatloss - a massive amount! with good practice design, experience indicates that it is usually possible to halve the Y-value; therefore some attention to detail (pun intended), could the FEEs by between 13% and 20%. Under the 2013 version of SAP the FEEs must achieve a mandatory level and this potential saving has a direct impact on that figure. the BER will also be reduced, but at a lower rate because there are additional factors in play such as lighting, hot water demand and renewables. Our research indicates that halving the Y-value would yield a typical saving of 10% on BER.
With such significant savings possible, no-one can afford not to consider thermal bridging very carefully.

I don't need to worry about thermal bridging... do I?

If you read our first post - maybe even if you didn't - you understand what thermal bridging is. So what does it all mean for designers, assessors, inspectors and builders? There are two main problems that thermal bridging causes. The first is increased heat loss, as discussed previously. This increased heat loss increases heating bills and carbon emissions associated with heating, which impacts on the climate change as well as Part L calculations. There may also be an adverse impact on alternative energy assessments such as Passivhaus. Part L
We've undertaken some simple research to quantify the impact of thermal bridging on a typical SAP calculation, and found that moving from the default values to best-practice detail design can reduce the BER by up to 10%. This is clearly significant, and could mean the difference between a pass and a fail, or the need to fit PV’s or the ability to omit them.
The second problem is that internal surface temperatures will be reduced locally around a thermal bridge. If this reduction is significant, it can result in less than optimal thermal comfort, surface condensation and mould. This issue can be visualised in finished buildings using a thermal imaging camera, or at design stage by plotting the isotherms on the 2D simulation.
Minimising thermal bridging is therefore an important aspect of the design of any building, and it is our responsibility as professionals in construction to ensure this is done in order to deliver quality and value to our clients.

What is a Thermal Bridge

The Context

Standards of energy performance in UK buildings have been increasing gradually over the last 50 years, particularly since the update to Part L in 2006, which brought the UK in line with EU legislation (the Energy Performance of Buildings Directive). One of the main features designers have used to reduce heat loss is insulation. As levels of insulation, and the performance of other aspects is increased, features that previously had a small impact on the overall performance of a building increase in importance. To stretch an analogy, when the low hanging fruit has been taken, we have to reach higher for further improvements. Thermal Bridging is the fruit that’s a few branches up.


A thermal bridge could simply be described as a discontinuity in insulation. In places, by design or by issues on site, there are gaps in insulation, filled by materials (including air) that conduct heat better, so that more heat flows through that point than the surrounding insulation. Often, but not always, this is due to the building’s structure, for example the studs of a timber frame, where there is insulation placed between the studs.


There are several types of thermal bridge, which are accounted for in different ways in thermal models such as SAP.


Repeating thermal bridging is generally spread across the area of a wall, roof or floor in a regular way. It is usually accounted for in the U-value calculation. Examples would include wall ties in a cavity masonry wall, and the studs of a timber frame wall. In the first example, the ties conduct heat better than the insulation material so heat passes along them more easily than it does through the insulation, so the presence of the wall ties increases the rate at which heat passes through the wall. U-value calculations done in accordance with the relevant Standards should take such bridging into account using the methods described in the standards.


Non-repeating thermal bridging covers everything else, and can be broken down into sub-categories. The main two categories are linear thermal bridges, and point thermal bridges. Linear thermal bridges are more common and the bulk of the work of Advanced Details.


Linear thermal bridges typically occur where one element of a building joins a different one, for example at the eaves where a wall meets a roof. They can also happen in the middle of an element, for example a steel post in the insulation zone of a wall. The bridge occurs in two dimensions, visualised via an architectural detail drawing.
Point thermal bridges occur where insulation is ‘punctured’, usually by a structural element. A steel beam passing perpendicular through the insulation would be an example. N.B. if the beam was parallel to the insulation this would probably be a linear thermal bridge.

What is a psi value

A psi-value is similar to a U-value: it is a number that represents the rate of heatloss through a linear thermal bridge. ‘Psi’ is the english written version of the greek letter Ψ(uppercase) or ψ (lowercase), pronounced “sigh”. It has the units W/m.K, which is similar to the units for a U-value, W/m2.K. Note that for a U-value, the unit contains m2, a measure of area because a U-value relate to a planar element, whereas a psi-value contains m, a measure of length because it is linear. Both units contain W for Watts, which is the rate of heat transfer, and K, which is Kelvin, representing the difference in temperature between inside and outside.
Much like a U-value, a high psi-value is generally a bad thing, because it means more heat loss, and a low psi value is good because it means less heat loss. A Ψ-value should not be confused with the Y-value, (the characters can sometimes look similar) which is a number that SAP uses to represent all of the heatloss due to thermal bridges in one particular building – more on that later.
The other role of psi values is to account for the difference between our simplified thermal model consisting of simple planar representations (Euclidian planes) and real building elements that have a thickness. In the middle of a wall, assuming the wall is much longer and taller than it is thick, it is an acceptable simplification to assume it is a Euclidian plane with a given U-value, because the heat flow can be represented in one dimension, perpendicular to the plane. However at the junctions with other walls, the roof, floor and windows, the heat flow is more complex and not perpendicular to the wall or whatever it is joined to; the heat flow becomes two-dimensional. Instead of a one dimensional calculation (a U-value) it is necessary to undertake a two-dimensional calculation to evaluate the psi-value. As a result, these calculations are much more laborious than a U-value calculation.