foundations for timber framed building with gas membrane and void former

[Lars P]

Hi diyers and pros

Does anybody have experience in with this topic. 

[Redbeard]

Not 100% sure what the question is. I have helped to build extensions with Radon barriers and sumps. The footings were conventional strips/underground blocks.

Edited October 9 by Redbeard

[Lars P]

Great - thanks Redbeard - let me try pick your brain. 
First of all - the main house has no gas protection whatsoever - only a not very tight fitting DPM over the block and beam floor, - so seems somewhat pointless having to go through the effort of protecting the extension as it will be one big room. Anyway.
Below is the design so far but I seem to have an issue with the anchor bolts penetrating the membrane.

Also - I'm unclear how the membrane should be finished off where it meets the outside wall of the main house. Any ideas ?

 

 

[Redbeard]

I know what you mean about the apparent lack of logic re old and new but we had to do it to get BC approval, so that was it. If you do major works to the existing house you may be asked to incorporate a radon sump, and that, at first sight, seems even less logical/practical.

 

First things first: Have you looked at the https://www.ukradon.org/ site? Loads of useful info. It helps a great deal to know what risk area you are in. At a guess it's 'High'? Are the air-bricks 225 x 150, and are they all the way round 3 sides? I know little about the technicalities but I understand that in lower-risk areas radon may be kept at safe levels just by air movement below the building. That's not to say a radon barrier makes no sense; it gives you 'belt and braces'. Ours were solid cast concrete floors. so the sump just sat below the membrane with a 100mm pipe out to atmosphere and, theoretically at least, post-build tests dictated whether it needed a fan attached or if natural ventilation was enough. 

[Lars P]

It's actually not Radon driving this palaver, but the fact that there is an old landfill 245.5 meters away from my front door - so my house is in the 250m 'buffer zone' - even though the extension in the back garden will be by 1m2 or so. Talking about measuring elastic band with a caliper  

So did you do something along these lines - just with the insulation and the slab swapped around - and then 100mm out though the foundation - and the membrane through the foundation as well ?

[saveasteading]

Radon barrier is just a slightly more expensive type of plastic dpm. 

The bolts through are of no consequence as the radon will flow out of the airbricks.

The drawing saying radon barrier by specialists is far OTT. It is a roll of plastic.