How VE Compliance Handles Thermal Bridging Part L2 2010
The Virtual Environment meets the requirements of The National Calculation Methodology modelling guide (for buildings other than dwellings in England and Wales) 2010 Edition with regards to non-repeating Thermal Bridging by following the rules set out in Paragraphs 31 and 73.
31 For SBEM and DSM software, the non-repeating thermal bridge heat losses for each element (including windows etc) must be allowed for by adding 10% to the standard area weighted average U-values, or by an equivalent method that satisfies BS EN ISO 14683, and be consistently applied to both Actual and Notional buildings. Note that the U-values as given in Table 1 and the corresponding construction elements in the database DO NOT include this allowance, so the calculation tool must make the adjustment explicitly.
In a DSM analysis an additional 10% heat loss is added to make an allowance for the extra conduction via non-repeating thermal bridges. For this reason you should observe an Alpha value of 10% in the Technical Data Sheet within the BRUKL document. The differentiation between metal clad and non-metal clad constructions is only made when following the EN ISO 14683 method which is only applicable to an SBEM calculation. Note from the paragraph above that it is not a requirement of the NCM Modelling guide to use a method that satisfies EN ISO 14683.
The use of BS EN ISO 14683 is arguably a more detailed approach to allowing for thermal bridging as it allows individual psi values to be considered and the heat loss based on the linear length of the junction.
The SBEM calculation uses this method however this is not without it’s own issues. For example the Notional buildings glazing will be made up from a single band of glazing. In buildings with similar glazing percentages this can lead to significantly lower linear lengths in the Notional Building thus giving it a lower thermal bridging heat loss.
To give a real life example of this the facade of the Actual Building shown below has the following characteristics:-
Wall Length 40m
Wall Height 3m
Wall Area 120m2
Windows 25 @ 1m x 1.9m
Glazing Percentage 39.6%
In contrast the Notional Building has the following characteristics.
Wall Length 40m
Wall Height 3m
Wall Area 120m2
Windows 1 @ 40m x 1.2m
Glazing Percentage 40%
In order to compare the two let’s assume the Psi values are the same in both cases and concentrate on the glazing non-linear thermal bridging heat loss.
|
|
ACTUAL BUILDING |
NOTIONAL BUILDING |
|
Psi value |
1.27 |
1.27 |
|
Linear Length |
25 x (1+1+1.9+1.9) = 145 |
1 x (40+40+1.2+1.2) = 82.4 |
|
Total Loss (W/K) |
184 |
105 |
From the comparison above you can see that even when the Psi values match the that of the Notional building, there is still 75% more heat loss in the Actual building. As this difference stems from the standardised form of the Notional Building which would be impractical in a real design it could be argued that use of this method does not allow for a fair comparison to be made between the Actual and notional buildings.
While the SBEM calculation method does allow for the Psi value to be included, arguably more detailed than the 10% addition employed by the DSM method, there are many other aspects of the DSM calculation that allow more detailed analysis than SBEM is capable of.
