Joists on cavity wall and airtightness

[stavros.london]

Heat recovery is one aspect; controlled ventilation is the other and that's going to suffer a bit.

I'm looking at Zehnder 550 currently. Both the manufacturer and the distributors are advising me that decent airtightness is a must. I'd imagine they could just sell me a unit without a hassle if that wasn't the case.

Thanks

 

[Doggit]

This is something I've always suspected about MVHR system; I've never understood how they can quote 95% heat recovery, but if they are telling you to make the house airtight, then I suppose it's possible to keep recirculating the air, which will allow virtually complete heat recovery. They're working on a Pasivhaus system, and they're venting into each room on to it's far side, and then allowing the air to be transferred via an open door into the (it seems from the video) the bathroom, where it's collected by the unit. But if you shut your bedroom or bathroom door for privacy, then the air has nowhere to go, so you need to open a window, which then cuts down the heat recovery rate.

What is you're reasoning for fitting the unit; will you use it for air conditioning in the summer, or are you in a noisy road where you can't open the windows.

 

[stavros.london]

Hi doggit and thanks for your comments although I feel the thread has been derailed a bit

My original question of whether sealing the joist junctions with the cavity wall would cause any issues got conflicting answers so ideally someone with relevant expertise would help here...

 

[Doggit]

Threads always get derailed here, but if there was anyone with the relevant experience, they'd have commented by now, so you've got the second best (well some) I'm afraid.

Okay, so as you're determined to go ahead with the job as you see it, so going back to the main question, "how do I make the joist to wall junction airtight? I was thinking carlite bonding / parging on the exposed blocks and then airtightness tape around the joist end. Would that suffice? Also, would that create any condensation risk or damp issues or anything else?"

You're proposed method is hard work, but shouldn't cause any condensation or damp issues, as you're going to be using an MVHR system. And as you're using the MHVR, I'd suggest a quicker method would be to clean and wipe round the joist ends, and then seal them with adhesive foam.

 

[John D v2.0]

You basically need something that will stick permanently without shrinkage and will move with the joists as they move relative to the wall.
I think you can get purpose made gaskets from Germany for this, but in terms of diy solutions you could use polythene with airtightness tape or acrylic sealant and make sure the polythene isn't pulled tight.
If you don't want to use acrylic sealant due to difficulty getting it to stick permanently you can get something called orcon f adhesive for less than 10 pounds a tube. You'll need a sealant gun to dispense it.

Basically airtightness on surfaces that can move eg wood need polythene loosely stuck, and hard surfaces you can parge with plaster or cement.

If you want to test things get a powerful fan eg old cooker hood, and a sheet of wood and fit the fan into a door, then you can run around with a smoking incense or something finding leaks to seal

 

[chappers]

Can't believe this has run into two pages, just fire some expanding foam around them, but personally as Doggit says I wouldn't bother.