Posted By: ringiPersonally I would leave a small gaps between the PIR boards, and between them and the walls, then fill this gap with expanding foam, before taping over it. I trust 70mm of expanding foam to give a better airtightness layer then a plastic sheet that will have nails/screw holes in it.

However I don't think adding a plastic sheet as well below the PIR boards would do any harm.

I'd lay a pound to a penny that this stuff would be far superior to ordinary gun foam in that application:

https://www.abbuildingproducts.co.uk/touch-n-foam-600-spray-foam-kit-pr-5205.php

Just FYI.. the foam that this 2 part kit creates is much better consistency than the pre-mixed-but-uses-a-solvent-to-prevent-it-going-off stuff. Soudal type expanding foam is open cell and air permeable, this stuff ain't..


(why so confident? I've done this exact thing in my build)

Ed,
I much prefer 22mm sarking to the 18mm stuff- way less bouncy midway between rafters (more like 18mm ply would behave...)
Gaps. My spin on this is that if you nail sarking wet, then as you say, the shrinkage will convert a 1mm gap to 3mm+. Far less splits too and less pre- drilling at board ends.

VH, is this the same with IWI also? All the commonly available guides all emphasise VCLs and diffusion, with barely a mention of air-tightness. (Perhaps an item for your website?)

I understand you have many practical examples of where good air tightness means diffusion needn't be worried about, however are there any formal studies to show that in the WORST CASE that that is still the case?

VH,

Read up the thread, you will see a few people talking about foaming and taping all joints on the insulation board as well as taping to the wall. Likewise fixing the VCL to the walls correctly etc. In both cases the air-tightness will be reasonable.

However I don’t trust air-tightness in a building that is every worked on by normal trades people, I have had too many cases case of say a plaster board screw missing the button, but making a hole in the airtight less layer, then the trades person just does not understand why it is a issue. Likewise with the screws holding the button to the joists missing the joist.

Just asking an electrician for example what experience they have working to air tightness standards seems to increase the quotes by 50%! Hence I don’t see it is safe to relay on air-tightness when someone is having an extension built etc. Maybe if someone is having a “self build” by one of the 3 or 4 companies in the UK that have a track record, air tightness will save the day…..

Even when a building is tested and gives 1ACH, lets assume there are 10 “sections” of wall and roof, and that 9 of these are at a 0.7ACH standard, this leaves the last section at 3.7ACH with rotten joists etc….

There is also the risk of leaks in roofs etc. I had to get 4 different roofers before the flushing stop leaking on a simple roof, if I had be better organised I would never have seen the leak, as the area would have been boxed in by the time the leak showed up. It is very possible there is still a small leak that is dry out faster then it is getting in, if it had no where to dry out too.......

The case of the SIP roofs failing in Candia, it may have been prevented just by using standard UK roofing tiles, over breathable felt, even if counter battens were not used. You can’t tell me that homes built with sip panels with NEVER in their 100 year life have a long screw put into the inside of a panel then removed.

Likewise if there is a gap in the insulation allowing convection currents to form, there can be issues even if the building has 0ACH.

Hence unless it costs a lot more I wish any building to be able to breathe to the outside.

Depending on air tightness to keep a building safe from rot, is like a car driver thinking it is save to do 50mph past a school while the kids are leaving an the end of the day because all children have been told to look both ways before crossing the road. Aiming for good air-rightness is like telling children to look both ways before crossing roads, great, but don’t depend on it in real life.