What do I need to ask the electricians to do in my new build bungalow to ensure good air tightness? Using MVHR and aiming for below 1 ACH air test is not something they are familiar with contributing towards, so will have to give clear instructions. Have masonary walls throughout, with wet plaster as the air seal layer. Wires will be chased into the blockwork and then plastered over, does this need a parge coat under the wires too? Are some kind of gaskets or sealants still needed? Particularly thinking of where the wires pass into the roof space, or where the "tails" come up out the slab. Just not sure what I need or how best to specify it.

Since I've been up to my neck in the Eurocodes this week (I can see why no one in the UK bothers to do lean design for domestic work)...

http://www.eurocodestandards.info/masonry-structures-eurocode-6/verification-of-unreinforced-masonry-walls.html

has a table entitled "Sizes of vertical chases and recesses in masonry, allowed without calculation".

There is also 'effect of chases' here:

http://www.eurocode6.org/Published%20support%20material/Final%2014496%20(11841)%20Masonry%203%20Lateral%2019%20Jan.pdf

I assume you still have to chase the sockets into the wall to get a flush fit with the plaster finish?

I'm a bit surprised about the concerns over chasing, thought it was a standard approach for masonry walls?? Back boxes will have to be recessed a little for flush fit sockets, and my builder channels the long wire runs a little to avoid the plaster over it being too shallow and cracking.

Thanks for the references Tim, I didn't think a 2cm deep narrow channel in the inner leaf of a 350mm cavity wall was going to be a problem, but good to have a technical source.

David, I think I prefer the parge coat idea to sticking tape in a rough cut groove, and chalking would be 2nd choice.

Remember that you'll need to continue the wet plaster or a parge coat into the ceiling/floor/joist void. Its easiest to do this with a parge coat as this can be done before the electrician/plumber are on site.

Yes, have already parge coated this void in areas where it will not be protected by a VCL plastered down onto the wall.

The rules will be:
a) Parge coat any blockwork that is not in direct contact with wet plaster or above a VCL e.g. any chasing for conduits or wires covered by capping, or back boxes.
b)Any penetrations of the external walls or conduits opening into the roof space are to be sealed with flexible silicone sealant or duct sealing putty (rather than acrylic chalk or PU foam which could crack over time).
c)Where wires or conduit pass through a VCL membrane e.g. lighting pendants at the ceiling, the penetration will be sealed with suitable long life tape or adhesive patches.

Not sure the most practical thing to specify for c), that is both effective, affordable and easily obtainable. Thinking some kind of tape rather than gaskets, but which one? Have I forgotten anything?

I`ve never seen it as a problem. When I would be setting the the back boxes I always set them in a bed of mortar, then when hardened the conduits would be piped into them.

After that I would go back and fill the chases in and run a length of plasterers scrim cloth down the chase and bed it into the mortar to stop the chase from cracking.

Never had a problem in the past. This method ensures the chase is fully filled and no cracks appear.

Posted By: davidfreeboroughIs it too late to stand-off the plasterboard from the VCL to minimise VCL penetrations?

All that extra battening didn't seem worth it for a single light pendant per room. MVHR vents will also penetrate the ceiling VCL but the ducting wouldn't fit in such a space anyway.

Posted By: MackersThis method ensures the chase is fully filled and no cracks appear.

That makes sense. What I understand can cause cracking is not chasing but just fixing wire to the surface of the blockwork, with our without capping or conduit, and then plastering over it. No chasing means that the plaster is too shallow and can crack.

Posted By: davidfreeboroughIf you're using capping then you need to apply a tape or parge coat before fixing the capping.

Yes, I have amended a) to mention capping. Wire directly wet plastered in place, encased in plaster, is not going to form an air leak.

Posted By: tonyit is not so much the depth of the chase that is a problem though 10mm should be enough it is how it is done, it is easy to introduce a vertical weakness into the wall, especially by hammering at the wall.

I am requesting they use a purpose made chasing tool, a double disc power grinder, rather than flail away with a hammer!

So parge coat and silicone sealant... my only indecision is over what tape or gaskets to use on lighting cable and MVHR vents as it passes through the ceiling level VCL. What is effective, affordable and easily obtainable? Any suggestions or recommendations?

I used acrylic sealer (decorators caulk) round MHRV and wiring penetrations

Can someone confirm exactly what a parge coat consists of as I am not sure.

Posted By: joe90Can someone confirm exactly what a parge coat consists of

thanks Joe, just one of those questions I never asked because I figured everybody knew except me!

<blockquote><cite>Posted By: joe90</cite>Can someone confirm exactly what a parge coat consists of as I am not sure.</blockquote>

Funny how often parge coats are mentioned on here yet very little information exists on what it is and how it is done. Anyway a search on the internet is likely to lead you straight back here, have a look at this thread with photo's to get some ideas - http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=2345&page=1