Thermal Bridging at Junctions
There’s a potential design versus as-built issue in how heat loss at junctions is considered for existing buildings energy assessments.
SAP and RDSAP assume a global thermal bridging factor, a y-value (as defined in SAP Appendix K), based on the age band of the dwelling, which applies an adjustment to the dwellings calculated heat losses from its exposed area including main dwelling, extensions and non separated conservatory.
A value of 0.15W/m²K is added to the overall heat loss for calculations for age bands A to I (Pre 1900 through to 2002 for England). Improved levels are assumed for age bands J and K (more recent dwellings). New build energy assessments allow for better values to be inputted, based on detail lengths and calculated Psi values for the junctions, or a default of y=0.15 can be taken if no information is available. Existing dwellings just take the default. The actual pre-improvement y-value for existing dwellings might well be worse than these values, if all the actual Psi values and detail lengths were known.
This means heat losses for thermal bridging may be being underestimated in calculations for existing dwellings.
From work we’ve undertaken previously, even new build dwellings constructed in accordance with the Psi values for the Accredited Construction Details (ACD’s), don’t achieve the previously assumed y-value of 0.08W/m²K (which was assumed under 2006 Approved Documents for buildings constructed following the ACD’s). In most instances, it’s generally around 0.01 to 0.04 worse, although some dwellings have come fairly close to that level. There are a lot more junctions considered now, than was previously the case.
If all junctions were assumed as SAP default worst case Psi values, with actual detail lengths entered, then you are in many cases looking at a y-value closer to 0.20W/m²K. If all the actual Psi values for construction junctions were calculated for existing dwellings, rather than a value just assumed, then the overall thermal bridging allowance might be even worse than that.
Installing Internal Wall Insulation (such as Kingspan Kooltherm K17 or K18) or External Wall Insulation (such as Kingspan Kooltherm K5 External Wall Board) could give significantly better overall thermal bridging performance alongside better U-values than an uninsulated existing dwelling might achieve, but calculated Psi values might still be worse than the RDSAP defaults.
Thermal modelling is still of benefit as a determination of a potential indication of risk of condensation and mould growth.
However, there is currently no benefit for anyone to model them from a thermal viewpoint, as there is no avenue in RDSAP to enter the calculated values and in any case, the default position is probably better than would be achieved if all junction lengths and Psi values were entered anyway.
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