1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

Does this roof need a DPM or not?

Discussion in 'Roofing and Guttering' started by d000hg, 12 Nov 2019.

  1. d000hg

    d000hg

    Joined:
    17 May 2013
    Messages:
    1,335
    Thanks Received:
    19
    Country:
    United Kingdom
    Our building has a solid wooden boarded roof (spruce planks).
    We'd initially planned to nail shingles through 50mm PIR directly onto the roof boards, and had been advised this wouldn't need a DPM since the roof is warm?

    But we are now leaning towards a 'sandwich' design with a frame and outer OSB layer on top.
    upload_2019-11-12_11-19-34.png

    Probably 75-100mm PIR, potentially more. Roof boards are 19mm, OSB specced at 18mm I think.
    No air gap planned.

    I am trying to figure out of we need DPM - is there going to be a condensation risk? Have had conflicting advice. If so, where should it go in my diagram? I thought the point of the insulation is it prevents an abrupt temperature gradient leading to condensation.

    If it's needed is it a fancy (expensive) product or just something bog standard like this: https://www.screwfix.com/p/capital-...amp-proof-membrane-black-1000ga-15-x-4m/22842
     
  2. Notch7

    Notch7

    Joined:
    15 Sep 2017
    Messages:
    17,019
    Thanks Received:
    1,210
    Location:
    Sussex
    Country:
    United Kingdom
    I think you are referring to a VCL vapour control layer.

    Ideally it goes on the warm side to stop vapour getting through to the cold outer material and condensing. (Interstitial condensation).

    You can lay a VCL on top of the osb board, before laying the insulation.

    You can use fancy superfoil ones or cheap plastic ones.....
    This is superfoil

    https://www.amazon.co.uk/SuperFOIL-...H4D2YTNR0XA&psc=1&refRID=EGRHP98WDH4D2YTNR0XA

    To be honest Ive tended to omit the vpl but once insulation is down, foam all gaps and foil tape all joints. Then as belt and braces use foil backed plasterboard on the ceiling.

    Its a compromise but since warm roofs have no gaps there is almost nowhere for moist air to build up -I think nice tight insulation with foamed up gaps and foil tape is a pretty good barrier.

    On my garden cabin / office, I did fit the vapour barrier + taped joints.

    Are your shingles felt or real cedar?
     
    • Thanks Thanks x 1
  3. d000hg

    d000hg

    Joined:
    17 May 2013
    Messages:
    1,335
    Thanks Received:
    19
    Country:
    United Kingdom
    Did you mean on top of the first (inner) wood layer... Osb is only the outer layer as our inner is planks, so I assume that's what you meant: inside-wood-VCL-insulation-wood-shingles?

    Shingles are bitumen type stuff I think
     
  4. Notch7

    Notch7

    Joined:
    15 Sep 2017
    Messages:
    17,019
    Thanks Received:
    1,210
    Location:
    Sussex
    Country:
    United Kingdom
    Yes indeed.

    VCL goes on the inside face of the insulation layer to stop warm moist air reaching the cold side.
     
  5. d000hg

    d000hg

    Joined:
    17 May 2013
    Messages:
    1,335
    Thanks Received:
    19
    Country:
    United Kingdom
    Thanks for confirming.

    So this is literally a waterproof barrier, not breathable - if I want something basic then plastic DPM plastic sheeting like in my original post would be suitable? Because the guy is keen to have as few joins as possible and it comes in 4x15m sheets which would cover one whole side as a single sheet.

    I've been doing a little googling and there are mixed views whether foil-backed PIR, properly taped, would provide the same function. Any thoughts?
     
  6. d000hg

    d000hg

    Joined:
    17 May 2013
    Messages:
    1,335
    Thanks Received:
    19
    Country:
    United Kingdom
    Can anyone offer any advice on spacing of battens in the external roof frame? The roof itself is calculated to support all the load (dead and dynamic) it needs to and PIR itself is a pretty strong material - as I understand it you could walk around on PIR with no sheathing quite safely?

    The fewer the number of battens the better the insulation and the lower the cost of materials and the less labour, but are there any structural implications here to speak of, or BR? PIR comes obviously in 1200x2400 boards so 600mm spacing is an obvious option but what about 1200 spacing given the external OSB will be resting directly on the PIT with no unsupported air-gap to flex in? Or for that matter even 2400 so the battens are just there to hold things in place?
     
  7. Notch7

    Notch7

    Joined:
    15 Sep 2017
    Messages:
    17,019
    Thanks Received:
    1,210
    Location:
    Sussex
    Country:
    United Kingdom
    The OSB deck needs screwing down sufficiently to hold it firmly and around the edges enough so the screws pull the OSB down and the abutting boards are reasonable flush. I would think 600mm centres would be okay -maybe around all the edges and a few down the centre of the board -in a way the fewer the better as every fixing is a thermal bridge
     
  8. d000hg

    d000hg

    Joined:
    17 May 2013
    Messages:
    1,335
    Thanks Received:
    19
    Country:
    United Kingdom
    So that would basically mean each 'cell' is sized for a half-board to snugly fit?

    600 would be fairly typical for a roof for trusses I thought, so I was hoping we could open it up a bit. Though I can see in this scenario it's perhaps less about load and more about holding the roof down/flat.
     
  9. Sponsored Links
  10. So you usually omit the vapour barrier on jobs for others but include it on your own job , that doesn't sound good :!:
     
  11. (Just to confuse the issue :!:) you can use Visqueen 1000 dpm as a vapour barrier/ check/control layer and this always goes on the warm side of the insulation as others stated.
     
  12. d000hg

    d000hg

    Joined:
    17 May 2013
    Messages:
    1,335
    Thanks Received:
    19
    Country:
    United Kingdom
    I don't know the brand but from the price and description this seems similar to the stuff I found - it's essentially like what extra thick bin-bags are made from? No magic to it?
     
  13. Correct , unless you think plastic sheeting is magic :D
     
  14. d000hg

    d000hg

    Joined:
    17 May 2013
    Messages:
    1,335
    Thanks Received:
    19
    Country:
    United Kingdom
    Black magic according to environmentalists
     
  15. catlad

    catlad

    Joined:
    29 Jul 2011
    Messages:
    3,933
    Thanks Received:
    529
    Location:
    Lancashire
    Country:
    United Kingdom
    I'm a bit lost here what are the batons for?
     
  16. d000hg

    d000hg

    Joined:
    17 May 2013
    Messages:
    1,335
    Thanks Received:
    19
    Country:
    United Kingdom
    A frame for the outer OSB layer to attach to
     
Sponsored Links
Loading...

Share This Page