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I am a DIY'er working on a cottage in a remote part of the UK.

I don't really have access to a professional roofer so I've just been having a go at it myself with some labourers.

So we have constructed a pitched roof and it is stopping any bad weather getting in.

The construction of the roof is:

  1. Concrete roof tiles
  2. Klober Permo Forte underlay
  3. Sarking timber

Picture for reference:

enter image description here

Now I am about to do the insulation and framing inside the roof, do I need to put a vapor control layer inside the rafters?

If I need a vapor control layer, does it get stapled to the sarking or does it sit under the gyproc (dry sheet)?

The trusses are 250mm depth (2'x10') and I will be putting a gyproc sheet with 150mm PIR boards, making it a cold roof.

Picture for reference

(note the roof rests on 800mm thick stone walls)

enter image description here

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Vapor barrier against the inside of the building envelope is to prevent interior moisture, caused by normal human activity, from condensing at the dew point. That usually occurs somewhere in the insulation layer and can result in very wet conditions or frost buildup.

So yes, you still want a vapor barrier inside the insulation. What you put on your roof is largely irrelevant.

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  • Thanks for the swift response, is the VPC the first thing to put on the inside of the roof or does it come before the insulation boards?
    – tomaytotomato
    Commented Mar 8, 2021 at 16:29
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    No, no, no. You don’t want a vapor barrier “inside the insulation “. Vapor barriers are on the warm side of the insulation. In your area, that’s on the room side.
    – Lee Sam
    Commented Mar 8, 2021 at 16:56
  • Isn't the warm side the inside? You aren't making sense. I did specify the building envelope, and not the attic or the roof.
    – isherwood
    Commented Mar 8, 2021 at 17:28
  • @tomaytotomato, let's clear up terms here. You aren't working on your roof when you insulate. You're working on the ceiling. The vapor barrier goes directly against the ceiling, with the insulation outside of that.
    – isherwood
    Commented Mar 8, 2021 at 17:30
  • Thanks for clearing up, the cottage is 1.5 storeys So from my understanding I do this 1. Put insulation boards inside rafters 2. Put VPC membrane over (staple to trusses) 3. Put Gyproc / Drywall on
    – tomaytotomato
    Commented Mar 8, 2021 at 22:21

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