Party Walls – How are they treated in SAP?
Much like thermal bridging, research has shown that heat loss attributed to party walls is much higher than previously thought. Research carried out by Leeds Metropolitan University has in fact shown that air movement within the cavity of a party wall can create more heat loss than a similar area of external wall. The research shows that a wall which was thought to contribute no heat loss, does in fact have a typical U Value of between 0.50 and 0.63w/m2k.
Back in SAP 2005 the heat loss within party walls was always assumed to be zero and therefore your assessor can discount that wall from the SAP calculations it was not deemed a heat loss wall. Now within SAP Calculations this area of wall is drawn into the equation, and a U Value attributed to it depending on how this air movement is dealt with.
The TER (Target Emission Rate of the dwelling) is now based on party walls achieving a value of zero or 0.0 w/m2k. Therefore any value worse than this will negatively impact the actual DER (Dwelling Emission Rate). It is for this reason that developers should look at the party wall issue as a very first step to achieving effective overall U Values.
So, we are after a value of 0.0w/m2k how do we achieve it? SAP provides four scenarios:
- Unfilled, unsealed cavity 0.50
- Sealed and unfilled cavity 0.20
- Fully sealed and sealed cavity 0.0
- Solid Wall 0.0
MIMA have produced a guide to party walls to demonstrate how these three solutions can be determined by the assessor and specified by the designer.
The guide provides mineral wool solutions which not only meet the SAP calculations requirement but also meet Robust Details standards for acoustic performance something which initially was lacking when Part L 2010 first appeared.
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