In the November 2011 issue of Master Builder I wrote about some of the issues that we as an industry are still struggling with in respect of the revisions of Part L 2010.
One of the key areas that has seen everyone scratching their heads has been the potential for heat loss through party walls and more specifically, that of edge sealing and fully filling the cavity party wall. I am pleased to say that the Building Control Alliance (BCA), working with the industry, having previously agreed a definition for a fully filled cavity separating wall, has agreed a defi nition for what constitutes effective edge sealing.
SOLUTIONS TO PARTY WALL BYPASS
Currently, Part L1A only specifies three possible values for the effective U-value where no specific, independent scientific field evidence is provided to support a solution:
Zero – Fully filled cavity party wall with effective edge sealing or a solid wall with no cavity
0.20 W/m2K – Clear cavity wall with effective edge sealing
0.50 W/m2K – No party wall solution in place.
FULLY FILLED CAVITY SEPARATING WALL
BCA has previously agreed a definition for what is acceptable as a fully filled cavity separating wall. Copies of this can be downloaded from the BCA website – and I have also covered this in previous editions of Master Builder.
EFFECTIVE EDGE SEALING
Edge sealing is required to restrict air movement through the party wall cavity to the external environment or other cavities in the construction, and vice versa.
In order for the edge sealing to be judged as effective the definition sets the following performance criteria:
- Be impermeable to moisture and the passage of air
- Create an effective seal with both leaves of the party wall
- Have continuous runs of material with no gaps and where there are joints in the material; for them to be effectively sealed
- Be in line with the thermal envelope in all abutting building elements
- Should be flexible
- Should not increase the fire load within the cavity.
- Any joints in the material must give confidence of their durability by:
- Being mechanically fixed
- Providing independent evidence of adhesion and compatibility between the materials being joined where adhesive tapes and mastics are used to form the joint.
In the absence of any other supporting information, the above criteria used in isolation would allow an effective U-value of 0.2W/m2k to be used. Where it is used in conjunction with a fully filled cavity party wall following the definition described above a U-value of zero would be allowable.
A full copy of the definition can be downloaded from the Building Control Alliance website at www.buildingcontrolalliance.org.
There are already a number of solutions being marketed as achieving effective edge sealing but until now there has been nothing to allow specifiers, builders and building control bodies to consistently judge whether the solution achieves it. Hopefully, the issuing of this definition settles the last real unknown areas of Part L 2010, and so onto the next edition…
PART L 2013
I had hoped this month to begin to tell you about the consultation in respect of the proposals for Part L 2013 which was due to be issued at about the same time as I am writing this (December) however the consultation has been delayed until the New Year and so the waiting goes on…