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.

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