There are many residential projects that include great energy efficient junction detailing, however SAP Calculations usually does not benefit from this energy efficiency. Many projects choose to apply generic nominal thermal junction heat loss values (accredited Construction Details) rather than accurately model the specific heat lost through project thermal junctions. This leads to an increased pressure and over specification of ground floor, walls, roof and window U Values. This need not be the case. By modelling the specific energy efficiency performance of thermal junctions an accurate representation of the heat lost through these linear lengths can be achieved. This permits a reduction in specification in both building fabric as well as fixed building services.

Whilst working with Galliford try on a new residential development comprising 3, 4 & 5 bedroom homes we discussed the benefit of modelling certain psi values to more accurately describe the energy efficiency standards to which these junctions had been designed. Through collaborating together we identified a suite of 4 psi values to calculate and use across the site of 210 units. The design of the thermal junctions enabled us to achieve great psi value results

Junction Name

SAP Ref

Comments

ACD

Results

% reduction

Window Sill

E3

2D

0.04

0.004

90%

Window Jamb

E4

2D

0.05

0.006

88%

Gable

E12

2D

0.24

0.103

57%

Corner

E16

2D

0.09

0.057

37%

These 4 psi value results achieve an average improvement over Accredited Construction Detail (ACD) values of 68%. This performance enabled Part L calculations to comply easily with both TFEE & TER Part L thresholds without a need to over specify in project U Values or increase specification from fixed building services. Modelling these thermal junctions ensured that the energy efficient design on the project was incorporated to its fullest and was not wasted through the application of generic (less energy efficient values).

Given the number of units on the site, the cost of each junction was £1.19 per plot. This cost efficiency will further reduce as these details can be used on future projects where these details are employed again.

If we are consulted on the performance of your thermal junctions early enough we can also provide guidance as to how to further improve a junctions energy efficiency. 

Simon from Galliford Try said the following about the service that we provided “Working with Darren Evans to identify and develop psi values for critical junctions has delivered significant benefits to SAP and our financial model. These have been a huge benefit and one that is to be repeated on new projects. Their technical expertise has assisted us to maintain a fabric first approach to compliance with ADL1a”.

Working to support the construction industry to achieve a more energy efficient housing stock is something that we are passionate about.

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