Roofing – Do You Need a Vapor Control Membrane with Breathable Underlay?

roofroofingvapor-barrier

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

Best Answer

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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