gas membrane query

[yorkie789]

Hi all

am hoping for a bit of advice please. Am building an extension and building control have requested that I install a gas membrane (methane / co2) across the entire footprint of the building. I understand that I must fit this & incorporate it as a cavity tray between inner & external cavity wall with weep vents which is no problem, however do I need to fit airbricks in the brick course below the weepvents? My intended floor will be even a concrete slab floor so would air bricks in the external leaf be needed only to allow any gases to escape out of the cavity?

Any advice much appreciated, thanks

 

[freddiemercurystwin]

Presumably the slab is ground supported? You wouldn’t normally fit any additional vents. Your gas tight membrane will keep it out.

Sometimes BC insist on a sump and then a pipe but I have only found they expect these on New-builds rather than extensions.

I believe the ethos behind not having to vent an extension floor is because the existing house will not have any venting either so why bother.

 

[yorkie789]

Many thanks for your reply, much appreciated.

ps - yes the concrete slab will be ground supported

 

[jeds]

I'm curious to know why are they asking for a gas membrane. Is your house built on a landfill site?

 

[freddiemercurystwin]

Some LA's (like Devon) now ask for it even when Radon/gas etc is not present in the immediate area as it is known to occur when there was no evidence of it before ie the map. Besides the additional measures required to create a gas tight membrane rather than just damp proofing is minimal.

 

[jeds]

Fair enough. It just struck me that the op specifically mentioned methane/gas rather than radon.

 

[freddiemercurystwin]

I guess to add to this further, the OP should nevertheless ask the question why he has to make it gas tight, as his LA may not insist on gas membranes per se like Devon, maybe his BCO is just a Berk and being over zealous.

 

[yorkie789]

Hi
I did wonder the same but when I queried it was advised due to close proximity to an old landfill site which was news to me.
Considering the house doesnt have such a membrane kind of feels daft putting it on the extension but hey ho if thats whats required!