In SAP 2005 all party walls were considered to have no heat loss. This changes in SAP 2009 and party walls, built with a cavity, become heat loss elements with a significant default U value of 0.5W/m2.K.
This default value can be reduced by ‘effective sealing around all exposed edges and in line with insulation layers in abutting elements’. In such cases the U value can be taken as 0.2W/m2.K. Frustratingly so far, there is no CLG guidance on what constitutes ‘effective sealing’ but to qualify as ‘sealed’, air must be prevented from entering or leaving the cavity. The cavity must therefore be sealed at the top, bottom and at all vertical junctions.
A solid party wall, or one with a fully filled cavity plus effective sealing, will have its U value reduced to zero. However, when fully filling a cavity party wall, care must be taken so as not to compromise the requirements of Part E (Sound). With the exception of a solid party wall, there are currently no Robust Details that will satisfy both Part L and Part E.
It is important to note that when calculating the Target Emissions Rate, SAP assumes that party walls will have a U value of zero. Consequently, if this were the only change to SAP (and no, of course it isn’t) a semi detached dwelling built with a cavity party wall will have increased emissions under SAP 2009 and would likely fail due to this one change alone unless savings were made elsewhere to compensate.
The easiest option for designers and builders is to use Robust Details Part E, Solid Dense Block option (E-WM-9). This still leaves the builder with the (lesser) problem of different density blockwork for party walls and external cavity walls. i.e. dense blockwork for the party wall and lightweight blockwork (preferably) for the cavity wall.
Of course this option is not suitable for timber frame construction but a detail where insulation is installed on both sides of an unfilled cavity is being considered. In such cases it is hoped that a U value of 0.20W/m2.K can be used but the Jury is still out on that one.
Party Wall Construction U Value
Unfilled cavity, no edge sealing 0.50W/m2.K
Unfilled cavity with edge sealing at all exposed edges 0.20W/m2.K
Fully filled cavity with edge sealing at all exposed edges 0.0W/m2.K
Solid 0.0W/m2.K
For more information or to book a test, call me on 01484 686649 or visit my website on http://www.hometestingyorkshire.co.uk/
I hope this helps,
Matt
This default value can be reduced by ‘effective sealing around all exposed edges and in line with insulation layers in abutting elements’. In such cases the U value can be taken as 0.2W/m2.K. Frustratingly so far, there is no CLG guidance on what constitutes ‘effective sealing’ but to qualify as ‘sealed’, air must be prevented from entering or leaving the cavity. The cavity must therefore be sealed at the top, bottom and at all vertical junctions.
A solid party wall, or one with a fully filled cavity plus effective sealing, will have its U value reduced to zero. However, when fully filling a cavity party wall, care must be taken so as not to compromise the requirements of Part E (Sound). With the exception of a solid party wall, there are currently no Robust Details that will satisfy both Part L and Part E.
It is important to note that when calculating the Target Emissions Rate, SAP assumes that party walls will have a U value of zero. Consequently, if this were the only change to SAP (and no, of course it isn’t) a semi detached dwelling built with a cavity party wall will have increased emissions under SAP 2009 and would likely fail due to this one change alone unless savings were made elsewhere to compensate.
The easiest option for designers and builders is to use Robust Details Part E, Solid Dense Block option (E-WM-9). This still leaves the builder with the (lesser) problem of different density blockwork for party walls and external cavity walls. i.e. dense blockwork for the party wall and lightweight blockwork (preferably) for the cavity wall.
Of course this option is not suitable for timber frame construction but a detail where insulation is installed on both sides of an unfilled cavity is being considered. In such cases it is hoped that a U value of 0.20W/m2.K can be used but the Jury is still out on that one.
Party Wall Construction U Value
Unfilled cavity, no edge sealing 0.50W/m2.K
Unfilled cavity with edge sealing at all exposed edges 0.20W/m2.K
Fully filled cavity with edge sealing at all exposed edges 0.0W/m2.K
Solid 0.0W/m2.K
For more information or to book a test, call me on 01484 686649 or visit my website on http://www.hometestingyorkshire.co.uk/
I hope this helps,
Matt
Nice post.I like the way you start and then conclude your thoughts.
ReplyDeleteThanks
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