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Poly sheet inside shed walls?

Discussion in 'In the Garden' started by andrewukr6, 9 Oct 2019 at 1:38 PM.

  1. andrewukr6

    andrewukr6

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    Hi all. Trying to lower the humidity in a large old overlap shed. Back wall takes a real battering from the elements and even after weather coating it, it can get quite wet. I have concreted the floor.

    I planned to tyvek the inside (can't redo the outside yet to big a job), however I've read this is pointless (should be on the outside).

    Is it a bad idea to use polythene sheeting tacked to the inside? As a vapour / water barrier? I was also planning to fit 2" EPS polystyrene as a basic insulation in between.

    I'm not looking to live in it. Its' not heated. I just want to stop my tools from rusting! Thanks
     
    Last edited: 9 Oct 2019 at 2:01 PM
  2. JohnD

    JohnD

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    breathable membrane will be better. Poly sheet is liable to be running with water inside and out and will prevent the water from ventilating away.

    Shiplap is not proof against driving rain unless you hammer tarry oakum into all the joints. For best results you should fix breathable membrane to the frame before nailing on the cladding. Too late now.

    You could clad the inner surface of the frame with ply and accept that the cavity between will be damp. I suppose that would mean treating it for rot and applying a water-resistanf breathing stain, such as a fence stain.
     
  3. andrewukr6

    andrewukr6

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    Yes the shed was built before I was even born I believe! Also I made a typo its overlap cladding, even worse. Annoying about the poly sheet, I'd hoped I could fit it to the worst wall to stop water ingress. Perhaps I'd be better to pull of all the cladding and start again.
     
  4. JohnD

    JohnD

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    "breathable membrane" is the stuff roofers use under tiles. It replaced the old roofing felt. I don't know how much you'd need for a shed, but maybe start with the exposed end.

    I clad my shed with WBP ply, which is rainproof but you need extra paint or other protection on all the cut edges. I've treated it with fence stain, and put a metal kickplate along the bottom to prevent rainsplash or earth touching the wood. With a good rainproof cladding, you can then add insulation and a lining if you want.

    Have you got eaves and gutters to reduce the amount of water hitting the walls?
     
  5. andrewukr6

    andrewukr6

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    Yes I brought 50m of Tyvek originally, but returned it as I don't think it should be used on the inside of the wall. I put up guttering, and I dug a french drain along the back wall. As you say it's the driving rain on one wall which soaks it, and then I end up with 90% humidity as it evaporates into the shed over time.

    I am thinking I will rip all the cladding down, install tyvek or similar, reclad, polystyrene between the shed batons and then use the plastic poly sheeting as a VCL on the inside wall possibly.

    I should note the shed is not heated, and I'm not in it very much.
     
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  6. scbk

    scbk

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    What is the cladding treated/painted with?
    Does the shed have much ventilation?
     
  7. andrewukr6

    andrewukr6

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    the overlap was tanalized from new (some of its old) and it was just coated with creo-cote. Even after this, one wall (with worst of weather) gets wet patches after heavy rain...

    Ventilation wise well it's really not very air tight! However I did fit a large extractor fan on a humditiy control, it runs most of the time apart from summer days.
     
  8. scbk

    scbk

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    I would look at painting it with something more water repellent
    Depends what your budget is and what you want it to look like?
    Creocote isn't as good as the proper stuff (coal tar creosote)
     
  9. andrewukr6

    andrewukr6

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    Yes I heard the original was better but hard to find! Painting the back wall could be an idea. To be honest I am thinking about ripping the back wall cladding off, putting tyvek on, then recladding. Its more likely to be watertight with a membrane underneath? I'm already fitting polystyrene as insulation on the inside. Still not sure if its a good idea to fit plastic sheet on the inside of a non heatred shed.
     
  10. Notch7

    Notch7

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    If want a proper weatherproof job, remove cladding, fit tyvek or similar breathable membrane, then fit some battens say 38 x 19. Then the cladding.

    It is the same as timber frame house construction. The membrane makes the building weathertight, the battens create a cavity so water getting through the cladding drains out and the cavity allows air flow behind the cladding increasing the lifespan of the cladding.

    I would avoid a VPL on the inside, it is used in house construction where the inside is almost always warmer than the outside -not necessarily true in a shed -if you get a frosty night with a sunny morning you could get condensation on the polythene
     
  11. andrewukr6

    andrewukr6

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    I think this is what I'm going to do on the backwall. Just do the job properly. If I use the tyvek am I ok keeping with the feather edge board?
     
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