Breathable Membrane Question

[tikroh]

Hi,

When installing a breathable membrane on the back of studwall, are you supposed to come our from behind the studwall and go over brick pillars to seal it fully. If I only apply it to the studwall, the gap between the brick wall and the timber will still have a gap.

Also, at the floor, should I bring it under the timber into the room slightly? Will this help safeguard the timber also? I'm not talking a lot, just fold it under and staple it to the front side of the bottom horizontal timber.

I've tried to explain in an image below, blown up large. So down the studwall, under the beam and tacked to front edge.

Thanks!

 

[tikroh]

Oh and whilst I'm asking..

Is something like this suitable https://www.wickes.co.uk/Easy-Trim-...reathable-Roofing-Membrane---20-x-1m/p/144021

Or shall I play safe with https://www.dupont.co.uk/products/tyvek-housewrap.html?

Thank you again, you guys are seriously getting me through this.

 

[robinbanks]

Breathable membranes typically go on the outside, not the inside

 

[tikroh]

It is sitting between the external wall and the first layer on PIR insulation inside a stud wall. Preventing anything getting to studwall timber or my insulation.

So brick wall > breather membrane > studwall with PIR taped > more PIR taped over top > VCL > Plasterboard.

This is not correct?

Thanks

 

[robinbanks]

This is not correct?

Sorry, your pic was confusing and made it look like you were putting the breathable membrane on the internal

Yea, your buildup is correct. Some would say you don't need a VCL if you're taping your foil faced PIR. Personally I use a VCL and don't bother taping the PIR. I'd also have battens between VCL and plasterboard to form a service void so I'm not bashing holes in the VCL and I'd use an aluminised VCL

Housewrap's advantage is it's light, comes in useful 2.8m widths and can be aluminised on one side for better U values. It was relatively cheap back in the day but is a bit eye watering now. I'd use a roofing type felt instead these days, white side out if it was dark coloured. I wouldn't go for the financial shafting wickes are offering though; 30 quid for 20sqm is high compared to eg picking up a 1.5mx50m roll of Cromar vent3 for around £45 - https://www.ebay.co.uk/itm/222693242050

 

[Deluks]

Do you really need it?
Does the brick wall get any ingress of water? And if it did, it would just run down the inside face, in any case, your pir won't be affected by it.
You'd want it under hung tiles, external cladding, or if you had mineral wool insulation, but your current set up doesn't really require it.

 

[^woody^]

Your floor DPM should go under the frame and up the outer face 200mm or so, and the membrane lap that at least 100mm, but not touch the floor.

 

[tikroh]

I'm not 100% sure Deluks. I have no signs of water being in the cavities currently, and 1 side of the building has another out building with a roof built on to so that wall is 100% sheltered, the other end has a wood store up 50% of the wall, again fully covered with a roof etc, and the back wall has a garage 200mm away from my outer wall, so the rain doesn't really get down there either.

Regarding floor, I was intending on doing the floor next year, put a DPM down, 50mm PIR and OSB on top.

But reading this, I guess I have to start with the floor as I'll never get to the outer walls to put DPM up there once I've done them.

If I already have the original DPM, do I need another? I have 1 2 bricks up that I can see from outside.

The building is likely 15-20 years old already.

Also, could I use a liquid DPM, or are the polythene ones far superior?

If Polyethene, is this OK?

https://www.amazon.co.uk/Membrane-Polythene-Sheeting-B2C-Various/dp/B094YG8KSK/ref=asc_df_B094YFS127&mcid=9faee97fdddb39ba95ae977b88a87af5?tag=bingshoppinga-21&linkCode=df0&hvadid=80401916258594&hvnetw=o&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4584001435755938&th=1

300 MU should be OK right?

Thanks