A risk/experiment worth taking?

[Linto]

I'm nearing the end of my forever project of insulating under the suspended floors. But I know best practice is to seal it with a breather membrane to prevent air leakage and wind wash from the air vents. 

To do this is going to be a right pain to get done correctly so I was wondering if I can just place breather membrane over the air vents to stop the breeze and monitor the underfloor atmosphere to see how it goes using automatic temperature and humidity sensors linked to my Home assistant set up, removing the coverings if it doesn't work? 

I have a MVHR system up and running and the subfloor is concrete as its a raft foundation that has never shown any moisture penetration, also it is not a radon gas area. This would be the last bit of the renovation and the house is pretty air tight except for the subfloor letting in air via the vents.

Am I just being lazy or is it a viable strategy? I've seen the airex smart bricks but you can't self install them unfortunately.

 

[SteamyTea]

That is a good question.

 

I would give it a go, as long as you can, if needed, put the airtight, but water vapour open membrane in place if you have to.

In fact, give it a go now and see what actually happens.

How do you intend to monitor the air quality on the void?

[saveasteading]

My immediate thought is no. Air bricks are there for a reason.

I'm confused though.

You say it is a raft, which means ground supported, and which  no need, or scope, for vetilation.. But suspended.

Edited February 29 by saveasteading

[SimonD]

Don't seal up the air bricks. The fact that there is no moisture penetration visible does not mean you don't experience humidity within the space and if what you're referring to as raft foundation is just concrete oversite, it isn't generally moisture impermeable either. Depending on the type of insulation you're using, you don't always have to have a breather membrane but can use a mesh to hold the insulation in place. You then use a membrane over the insulation and floor joists for airtightness and then install your subfloor/flooring.

[Linto]

Sorry haven't explained it very well, it's a suspended timber floor, raft foundation due to poor ground conditions and sloping site so there's a crawl space that I'm using to access the bottom of the joists.

It's a renovation so placing the airtight layer between the joists and floor won't work as I don't want to lift them. Plus having the airtight layer below prevents the air movement from the bricks from washing out the insulation compromising it's usefulness.

 

For monitoring the temp and humidity is was thinking of something like 6 of these with an alert on Home assistant if the moisture level gets too high. 

[OwenF]

I’m currently installing breather membrane for wool insulated suspended floor. I’m using the ‘up and over’ method (only 150mm gap beneath joists) and I can tell you- THAT is a pain in the arse. Especially around the noggins

I’d say if you have access from below, then do it. Will be loads easier. Staple up, tape joins, done

[Super_Paulie]

im just doing the job in my place now but im using PIR, 2 layers of with the lower PIR between battens on nails and foamed in, the top layer on battens. Air-tight foamed where necessary, taped over and then the next layer of PIR foamed and taped. Then foam all around the perimeter. Zero drafts, cant feel any air movement at all but it wasnt an easy or quick job but it looks like it'll do the trick for my UFH. Too late to go back now anyways 😆

When i got down to the last piece it was like a howling gale underneath from the airbricks, all gone now.
 

 

 

Edited February 29 by Super_Paulie