Do I need a breathable membrane?

Discussion in 'Builders' Talk' started by james316, Apr 4, 2020.

  1. james316

    james316 Member

    I've started building my workshop and I'm trying to do it on a budget but i want it to be solid and last. It's a workshop not a home office so it doesnt need to be cosy and warn 12 months a year but somewhere inbetween that an a garden shed. The original wall construction i had planned was from outside:

    -12mm treated cladding
    -Breathable membrane
    -4x2 C24 treated timber with 85mm cavity insulation slabs between studs
    -Plasterbord or 12mm ply internal skin.

    Problem is, I've just made 1 and a half walls and nailed the cladding on and forgot to staple the breathable membrane between them (se pic). As the claddings nailed on I'd probably ruin them if I tried to take them off.

    So I'm thinking I either staple the membrane to the inside (before the insulation and electrics are put in) or I leave it out all together?
     

    Attached Files:

  2. The need for a membrane is really down to how far you want to go. If it’s going to be more of a glorified shed, then no major requirement for one but be prepared for areas of damp/mould. An external membrane would help and in addition to that, a vented external gravity via battens would also be beneficial. But as that starts to go a bit into the BR’s, just do what you can. As you’ve only erected one wall without, there isn’t any harm in making sure the other three do include the external membrane.
     
    james316 likes this.
  3. james316

    james316 Member

    Ok thanks, I'll add the membrane between the cladding and the studs for the remaining walls. For the wall thats up already should I staple the membrane straight onto the inside of the wall before I insulate it?
     
  4. You wouldn’t normally have a breather membrane internally but a vapour control layer. Breather membranes are used on the external (cold) side of the wall in order to prevent moisture, snow, wind and dust from passing through.

    Here is an example of a BR complaint weatherboard timber frame build up. As you’re not going down the formal BR route, you can therefore relax a few of those requirements.
     

    Attached Files:

  5. james316

    james316 Member

    ok cheers, I don't have any batterns and the suppliers have shut now so wont be able to leave space for ventilation between cladding and membrane so guessing I should probably leave it out altogether
     
  6. That’s right and it’s basically a glorified shed so you can only do the best you can without having to get into the usual rules and regulations.
     
  7. james316

    james316 Member

    Am I still ok to fit cavity insulation between studs and cover with ply or will the insulation get damaged by moisture seeping in through cladding?
     
  8. james316

    james316 Member

    Think I'd rather that than millions of deaths
     
  9. There is a chance it could get wet especially as the insulation would normally go towards the outside and leaving the clear cavity on the inside of the studs. A more rigid form of insulation, e.g. Celotex would be better than a mineral wool, e.g. Rockwool as the latter would absorb the moisture a lot more.
     
  10. james316

    james316 Member

    Yea I think you're right, think its going to get expensive!
     
  11. Abrickie

    Abrickie Screwfix Select

    You could of course just staple the membrane to the uprights leaving enough slack to accommodate the insulation ;)
     
  12. james316

    james316 Member

    Do you think I should use PIR board or can I get away with using expanded polystyrene like this https://www.championtimber.com/2400-x-1200-x-50mm-polystyrene-eps70-grade-50poly
     
  13. james316

    james316 Member

    Thats what I was thinking of doing, but was worried the moisture was going to hit the membrane and drip down and pool internally and rot the floor?
     
  14. wiggy

    wiggy Screwfix Select

    You can buy a recycled plastic insulation that doesn't hold moisture, it's used quite a lot in camper conversions.
     
  15. Abrickie

    Abrickie Screwfix Select

    Does that back on the your back fence? If so you’re not going to get that much moisture in on that elevation
     
  16. james316

    james316 Member

    Yea it does, about 200mm gap. Good point, think I'll staple the membrane and add rigid insulation
     
  17. james316

    james316 Member

    Not heard of this before, having a look at it now - thanks!
     
    wiggy likes this.
  18. Rain will find it’s way in and through and this also means it’s an element that will not benefit from exposure to the sun to dry it out (due to its close proximity to the fence).

    If you have a membrane on the inside, it’s not going to prevent moisture from entering as the damage would have already been done to the main structure as well as the insulation. To help prevent this from happening, look to see what external waterproofing paints are on the market for that particular application.
     
    james316 likes this.
  19. james316

    james316 Member


    Think I'm going to leave the membrane all together in that case and just use 50mm ecotherm PIR throughout. This way it will be ventilated between insulation and cladding and the foil will act as a moisture barrier between the cladding and the internal skin. That makes sense in my head, solid plan?
     
  20. Sounds fine for a ‘shed’ :D
     

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