Vapour control for garden room

andyjbb

I have built a garden room with treated timber stud walls. I am using PIR insulation between the studs. I have read that I need an internal vapour control layer. Specifically, 500 Gauge – 125MU Green Tint Roll but I can't find anyone selling this in Ireland. I want to ask what else I can use and whether I definitely need to put in this layer

Calahonda52

what stage is the build at and what is the exact buildup, including thicknesses

is the PIR faced with Alu foil on both sides?

10-10-20

500 gauge poly:

https://www.goodwins.ie/polythene/c-1825.html

Available in every builder's providers.

Calahonda52

https://www.boards.ie/discussion/comment/122604229#Comment_122604229

Not quite it seems!

https://www.greengates.co.uk/vapour-barrier-125mu-500g-245m-x-50m-roll-green

10-10-20

https://www.boards.ie/discussion/comment/122604259#Comment_122604259

Christ on a bike - it must have Nasa engineering in every roll!

Calahonda52

https://www.boards.ie/discussion/comment/122604272#Comment_122604272

more like a leaky xured

andyjbb

From outside in, the wall is made up of cedar cladding, counter battens, breathable membrane, treated timber stud wall with 75mm PIR (alu foil on both sides).

Calahonda52

what is exact spec on breathable membrane,

IMO, you have 10% thermal bridging with the studs, the other 90% is 100% airtight/vapour tight and a VCL would be a waste of time and money.

So my view would be get alu tape and tape over the timber studs, sole plate, wall plate, and have no perforations in it at all.

I mean none.

The other alternative is to sheet the inside with vapour tight OSB, not cheap but will allow u fix to the studs

Fit a RH sensitive extract fan

Is the cavity behind the cedar ventilated

Lumen

What depth are the studs?

andyjbb

4 inches

Lumen

https://www.boards.ie/discussion/comment/122604930#Comment_122604930

Is the PIR bought and fitted yet? Have you figured out electrics yet?

You might want to consider reducing PIR in the wall to 50mm, set back 50mm, then using the other 50mm as a service cavity, then use 25mm or 50mm PIR-backed plasterboard over the studs (assuming this room will be plasterboarded). Solves the cold bridging and saves having to chase into the outer PIR (just through the inner one).

I think (not an electrician) that wiring should be set back at least 50mm behind the finished wall surface unless protected e.g. by metal conduit, which you can achieve if the wiring is clipped/taped to the outer layer of PIR (you'll still have pristine outer foil backing).

andyjbb

Thanks for the comments. I have decided to go with a reflective vapour barrier that I found available in Ireland

Calahonda52

pls share