Hi,

Soon to be building my passive house from Durisol blocks. Where do i place the airtight layer:

1. Inside - using wet coat of plaster. Not found a suitable sealed back box/cable detail. Loads of timber frame examples though
2. Outside - do i need the extra cost of a parge coat and how can i rely on there being no cracks after some years. Also wall plate detail is tricky when trying to link across to roof membrane.

Posted By: neilgorswift1. Inside - using wet coat of plaster. Not found a suitable sealed back box/cable detail. Loads of timber frame examples though

My current preferred solution is to fit standard galvanised electrical back boxes on a continuous bed of flexible gap filling adhesive & to fit cables to external walls on butyl fleece tape secured by standard galvanised steel capping & incorporated into wet plaster.

David

Posted By: neilgorswiftThat sounds good would it be something like illbruck Butyl Tape Fleece? So, as long a you connect the tape to the adhesive below the back box your sorted?

That's the idea. There are other sources of butyl fleece tape, try HS Butyl for an interesting selection of products which could be used to improve airtightness.

Posted By: neilgorswiftI assume you have buildings constructed using this method that have achieved close to passive house levels of airtightness?

I'm afraid not. I'm still at the research stage, but I've studied the airtightness details of lots of certified passivhaus's & most just fill the holes in the backbox with silicone sealant or similar.

This seems a more robust approach as both the sealants (butyl tape & flexible gap filling adhesive) are being held under compression. The only practical difficulty I forsee is that the backboxes may need to be recessed into the blockwork & the chase may need to be vaccumed before it can be used as a substrate for the adhesive. So you'll either need a cordless vaccum or use of a generator.

David

Hi, I have built a large extension using Durisol and i am very happy with the product. For the electrics I cut channels in the inside layer of the blocks - which was quite simple with a small grinder and set the plugs directly onto the concrete fill with small plugs. There is no way that air can escape from the back of the box. The inside thickness of the block wall is about 40mm which will give the electrican plenty of space for the cables - I have never liked the thin boxes and tightly bent cables behind the socket!.

Re airtightness - One of the advantages of the block for me was the ability of them to breath to a degree. I have a render on the outside and lime hemp on the inside and we are very happy with the overall effect.