Monday, 16 May 2016

How to improve SAP score at any stage of design .

Just how much difference can calculated PSI values have on your SAP score?


Having PSI values calculated is one of the easiest ways to improve your SAP score because it can occur at any stage of the project, without the need to change the design. Typically, improvements of the SAP score will depend on the type of project and quality of the detailing, however we have prepared this article to test the idea out. Using calculated Psi values is a great way to quickly improve SAP score. But how much of an impact can it have on the Design Emission Rate? To find out we have created a sample two bedroom house and put it through a SAP assessment.

For this example, we have created a two bedroom two storey end of terrace house with internal area of 72m². The software used for SAP assessment was FSAP 2012 vr.: With the following input data listed in the table below:

Input data

Fabric Area [m²] U-value [W/m²K]
External Wall 102 0.188
Ground Floor 36 0.105
Roof 36 0.110
Windows 25.5 0.800
Party Wall 48 -
Ventilation Type Balanced with heat recovery
Mech.Vent Product Paul - Novus 300
Air permeability 2 m³/hm²
Heating group Boiler systems with radiators or under floor heating
Sub group Gas boilers and oil boilers
Heating controls Programmer and at least two room thermostats
Heating fuel Mains gas
Electricity tariff Standard tariff
Main heating efficiency 89.1%
Boiler brand and model Vaillant ecoTEC plus 825 H combi A
Water Heating
Hot Water System Gas boiler/circulator for water heating only
Cylinder Volume 120 litres
Declared loss factor 1.32 kWh/day

Original Results

The above house achieved following results:

SAP calculation results:
TER 20.29
DER 23.21 - fail
SAP result C 80
IE rating B 82


For the project to pass part L the SAP score had to be further improved. Up to recently, the most typical option used to improve the SAP score was to install PV, adding even more insulation to areas susceptible to heat loss, such as the roof or walls and improving air tightness. However, using calculated PSI values we can improve the SAP score, simply by providing more accurate information about the existing design, with no changes to the actual fabric of the building. The scale of this improvement has been found to be so significant, that in many cases we can avoid the need to seek alternative measures altogether.

How much did it improve?

To find out just how much an improvement this is, we re-run the model with calculated psi-values for each SAP junction. There were eleven junctions in total. Using calculated PSI values from the Advanced Details Database for each junction, this resulted in a reduction of our y-value from the SAP default of 0.15 to 0.0379. In terms of heat loss, this resulted in reduced values being input into the FSAP model. To quantify this we calculated total heat loss for the dwelling. Total heat losses through external elements are a combination of the heat loss both through the building fabric and thermal bridges. Whereas the former is well known and addressed, the importance of the latter can be underestimated and default values are typically used. Using calculated values for thermal bridges is the most accurate way to quantify total heat loss, and with well designed junctions, such as those available in AdvancedDetails database, results in significant improvements.

Heat losses
Heat loss through fabric u-values only 73.122 W/K
Total heat loss including thermal bridging :
- default y-value: 114.655 W/K ( +56% )
- calculated details from AD: 79.682 W/K ( +9% )

Improved Results

After inputting calculated psi-values from the house achieved following scores:

SAP calculation results:
TER 20.6
DER 20.27 - pass (1.6% reduction)
SAP result B 81
IE rating B 84

The model achieved satisfactory SAP score and DER score has been reduced from 23.21 to 20.27. Therefore using calculated Psi-Values from AdvancedDetails reduced DER score by 15.7%.

How does this compare to other improvements available?

To put this into perspective we have re-run the model multiple times changing following settings to see how much of impact they have on DER score. We have also estimated the cost of such improvements. This is represented in the table below.

SAP score imporvements:
Improvement DER score reduction Estimated cost Stage / impact on design
Boiler Increasing gas boiler efficiency by 10% (from 80% to 90%) 6% £400 Late stage construction
Windows Reducing u-value of windows from 1.6 to 0.8 15 % £1300 Design /early stage building
Air permeability Reducing air permeability from 3 m³/hm² to 1 m³/hm² 5.3% £625 Early stage design
PV panels Installing 1kWp PV panel array 19% £3000 Post construction
Wind turbine Installing 1kW wind turbine 15% £2500 Post construction

Some of these improvements are easier than others and some can only have limited impact. In the sample house we have already taken advantage of the more common improvements, such as an efficient boiler and double glazing. Further opting in for custom Psi-calculations allowed us to pass the SAP requirement. It is important to note the impact that thermal bridging has on the results, highly depends on the size of the building. The greater the size the bigger difference there will be in the improvement. This is due to the multiplication of Psi-values by the length of the junction. We purposely chose a small two bedroom house to be more conservative with the test and compare results where they are likely to be of smaller magnitude (than in the case of larger dwellings). We will investigate this in a later blog.


To summarise, using can help achieve Part L compliance. Using calculated Psi-values improves the DER score and can eliminate the need for costly alternatives. Using AD to improve your SAP score can be done at any stage of the project. While the greatest benefits would be gained when thermal bridging is considered early on, using calculated Psi-values post-construction will also be highly beneficial. In case where the dwelling has performed badly on the air tightness test or is missing few points to pass SAP assessment, using calculated psi values is an ideal solution as it requires no changes in the design. This is because the improvement relies on inputting more accurate data into the model rather than default figure (y-value). However, we advise to consider thermal bridging as early in the design stage as possible to maximise the benefits, as choosing better performing junctions will save on running cost/energy.

Thursday, 12 May 2016

Making Sense of Appendix K - Part 2

In the last post, we gave a summary of some of the main junction types found in a building and listed with SAP Appendix K. Here we continue down the list!

E10 - Eaves (insulation at ceiling level)

It is easy to design and build a good eaves detail, but it is equally easy to do a bad one. As with all things thermal bridging, the key is continuity of insulation, so make sure the ceiling insulation runs right over the wall insulation, both on the drawing and on site. Deeper rafters with the minimum 'bird's mouth' joint depth help to maintain insulation thickness over the wall plate, and may make installation a bit easier.

E11 - Eaves (insulation at rafter level)

Much like the E10 junction, deeper rafters may help, but will simultaneously make it more tricky to achieve a good U-value as there will be more repeating thermal bridging. Insulation over the rafters is better as it can run right over instead of stopping at the inside surface of the structure, but take care to avoid repeating thermal bridging via fixings of the roof finish.

E12 - Gable (insulation at ceiling level)

This is a classic overlooked thermal bridge, typically in a cavity masonry wall where the inner leaf bridges between the cavity wall insulation and the ceiling. Lightweight block inner leaves are better than heavyweight but its best to use a structural insulation block such as foamed glass.

E13 - Gable (insulation at rafter level)

This junction is easier to design for a minimal thermal bridge than E12, but requires some thought. Where possible, the inner leaf should be stopped short of the rafter insulation, which is then simply run out over the top edge of the wall insulation. If using a barge ladder, sit the 'rungs' on top of the inner leaf rather than building them in.

E14 - Flat Roof

This is generally assumed to mean a Flat roof verge (where the roof over-sails the wall). This presents a similar problem to the E13 junction. Where possible, roof joists should stop on the warm side of the wall insulation but this is often not possible, particularly if there is a deep overhang. Ensure the wall insulation buts right up to the deck (assuming you have a warm timber deck).

E15 - Flat roof parapet

This is another classic, often overlooked thermal bridge. It presents a similar problem to E12, and can be resolved the same way. An alternative is to use only the outer leaf of masonry as the parapet, and stop the inner leaf under the roof insulation which runs right out over the wall insulation.

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.