Green Building Forum - Air tightness and electricians in masonry build

Air tightness and electricians in masonry build

Greenfish — Thu, 01 Aug 2013 10:12:25 +0100

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.

Air tightness and electricians in masonry build

tony — Thu, 01 Aug 2013 16:01:27 +0100

I would ask for parge behind all metal boxes on outside walls, seal all cappings conduit and penetrations with acrylic sealant similar to part P requirement for ceiling roses. And seal all wire entries to boxes and all redundant holes to boxes. It is possible to not use capping or conduit with t&e cables but most plasterers don't like that but it would be more airtight. It is unusual to chase wires into walls these days so are you sure, if you do be careful not to weaken the wall and if you do this you will either need the parge or seal the box entries.

Be very careful with outside lights etc

Air tightness and electricians in masonry build

TimSmall — Thu, 01 Aug 2013 19:24:30 +0100

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

Air tightness and electricians in masonry build

davidfreeborough — Fri, 02 Aug 2013 08:30:26 +0100

Try to avoid chasing. If you need it for back boxes then fit all boxes on a parge coat or continuous bed of adhesive.

For cables, instead of chalking up the cable positions on the wall, run airtight tape down the wall from ceiling void to back box. Fit cables & cappings as normal & incorporate the lot into wet plaster.

If you do both of the above then you shouldn't need to seal unused back box holes.

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.

David

Air tightness and electricians in masonry build

woodgnome — Fri, 02 Aug 2013 16:28:20 +0100

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

Air tightness and electricians in masonry build

tony — Fri, 02 Aug 2013 16:31:17 +0100

yes but air tightness still needs to be maintained.

Air tightness and electricians in masonry build

Greenfish — Sat, 03 Aug 2013 09:22:56 +0100

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?

Air tightness and electricians in masonry build

davidfreeborough — Sat, 03 Aug 2013 11:00:50 +0100

Posted By: GreenfishI'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.

He might change his mind if you ask him to parge coat all the chasing.

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

Agreed. I would use tape if not chasing & a parge coat if chasing.

Posted By: Greenfishc)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.

Is it too late to stand-off the plasterboard from the VCL to minimise VCL penetrations?

David

Air tightness and electricians in masonry build

Mackers — Sat, 03 Aug 2013 11:20:46 +0100

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.

Air tightness and electricians in masonry build

davidfreeborough — Sat, 03 Aug 2013 12:13:29 +0100

This approach is just as good. It's not been a problem because you're doing it properly & taking care to fill all the voids. This is not how the average builder does it!

It also not common to use conduit, most electricians would use capping for domestic installations under wet plaster. This creates an air movement cavity which is open to all the gaps in the masonry. If you're using capping then you need to apply a tape or parge coat before fixing the capping.

David

Air tightness and electricians in masonry build

Greenfish — Sat, 03 Aug 2013 12:59:42 +0100

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.

Air tightness and electricians in masonry build

tony — Sat, 03 Aug 2013 16:42:32 +0100

it 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.

Air tightness and electricians in masonry build

Greenfish — Mon, 05 Aug 2013 13:14:50 +0100

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?

Air tightness and electricians in masonry build

RobinB — Mon, 05 Aug 2013 13:38:39 +0100

At the Denby Dale Passivhaus they got round this by not having sockets on external walls and not having ceiling light on the top floor. I though it was quite a clever idea to simplify things in this way where possible.

Air tightness and electricians in masonry build

tony — Mon, 05 Aug 2013 13:41:01 +0100

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

Air tightness and electricians in masonry build

RobinB — Mon, 05 Aug 2013 16:02:43 +0100

Do you think it will be OK in the long term? I used Orcon F in a few areas.

Air tightness and electricians in masonry build

joe90 — Mon, 05 Aug 2013 16:27:09 +0100

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

Air tightness and electricians in masonry build

tony — Mon, 05 Aug 2013 16:51:06 +0100

almost anything! from lime and sand or cement and sand through all basecoat plasters to clay plaster grout, etc all it does is blocks up all the small holes and cracks.

Air tightness and electricians in masonry build

Gotanewlife — Mon, 05 Aug 2013 21:50:18 +0100

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!

Air tightness and electricians in masonry build

tony — Mon, 12 Aug 2013 16:58:08 +0100

I have very recently discovered that parge coat is available in bags from plaster manufacturers though the enchants often only want to sell it in full pallet quantities.

I am now wondering how much parge coating is being done.

Air tightness and electricians in masonry build

Paul_B — Mon, 12 Aug 2013 20:26:38 +0100

Posted By: joe90Can someone confirm exactly what a parge coat consists of as I am not sure.

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