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Vapour barriers?? I'm confused!


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I think I have finally out-thought myself when it comes to vapour barriers for self build observatories. I am now totally confused! There are breathable, non breathable and just plain waterproof sheet products that can be used under the timber cladding of an observatory. Simple question - which is the best to use??

I've been through the DIY observatory section and the green plastic waterproof stuff (Polythene sheet?) seems to be the favourite..... luckily it also seems to be quite cheap!

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I used the thickest waterproof builder's plastic sheeting I could find at B&Q. It was about £15 I think but you could pretty much shoot it with a gun and I reckon it would still be fine. I tucked the edges up over the floor and under the base of the walls for good measure.

Two years of appalling wet weather at the base of a hill in a flood plain and there has not been the slightest hint of damp or water from the floor.

Cheers

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I am just in the process of putting plywood on the walls of my observatory and was told it's best not to put a dpm on the walls as air needs to circulate around the cladding to let it dry out if it becomes damp or wet. Interested to know anyone else's take on this.

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I'd be inclined to use a proper breather membrane ( Tyvek or similar ) and an extra batten to allow an air gap between the back of the cladding and the membrane .

This will prevent moisture/condensation getting trapped on the back face of the shiplap and rotting it from the inside out.

Not as cheap as a roll of DPM I know but a much better long term result.

You'll probably be able to pick up some short roll ends from a friendly local roofer if you have one handy.

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I don't work in the building trade, so those who do may feel free to correct me :)

When a house roof is constructed it is important that damp air doesn't get trapped in the attic and make the roofing timbers susceptible to rot. So much so that I believe it's now part of the building regs that the roof space must be adequately ventilated. (I've not built a roof, so I've not read the regs covering roof-building that closely.) By using a permeable membrane under the tiles or slates damp air can escape from the attic.

If you think there's a chance that your obsy may become something of a moisture trap then as Steve says I think there's a fair bit of sense in using a permeable membrane to allow moist air to escape into the outside world. If you're going to run a dehumidifier in the obsy it may not be such an issue. If the inner walls are skinned such that moisture is unlikely to be able to escape through the walls then it may not be such an issue either. Nor if the obsy is directly ventilated to the outside world.

James

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Just an observation. The human body gives off a lot of water vapour, so while are you trying to keep the moisture out you are a moisture generator.

OK once the roof is off/open then moisture from inside can escape. But the interior of a building will change more slowly than the outside when shut up.

But I would go with a breathing barrier, it works inside out while keeping the outside out.

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I am wondering how the obsvy will be built?If there is any insulation installed then you need vapour barrier-or if the "shed" wil be heated.Not too sure of your temp differences over there,but moisture,mold must be an issue.JamesF is right about roof venting-here there are specific codes for vapor barrier,venting,etc.Basically in my house the attic is supposed to be the same temp as outside....say -30'c with the house inside at 21'c(or whatever),& the methods to do this are used everywhere.Here if not done right there will be an inch or more of frost on the roof sheeting in the attic.....We use treated lumber for the floor structure-floor joists,treated plywood for the floor itself,works well.Our gasoline shed at the family's cabin-built in 1963-is all open,very simple and has lasted well-this is where my Dob will be stored-it wil be around a lot longer too.The gas shed uses open(screened) small vents,so it can temp equalize itself....no condensation,mold or rot.I wouldn't seal it right up,unless you've insulated or are heating.....

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Great topic as I plan to build an observatory/ice fishing shack that will be heated AT TIMES,but then not for when I go out to use my telescope,if not done right it won't last a couple years....I've see ice shacks disintegrate as they are being dragged across the ice!! :grin:

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Sorry Gerry, but i have no intention of building an ice shack!!!! :eek: Although to be fair to you, we did have around -15°C or so last winter, but that is very cold (and rare) for the UK.

I have bought myself this stuff: http://www.screwfix.com/p/roofers-choice-lightweight-breathable-membrane-1-x-25m/21912?_requestid=246954 I'll report back when it has been in use for a while.

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Good construction principles are used universally-or should be :smiley: .For some reason I didn't think you would be building an ice shack,but that you were just trying to protect a shed with a roll off roof or whatever.Your construction technique will tell you if you were right or not in the end.....no mold or rot then perfect.

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Any decent timber construction should be constructed with a Breather Membrane (BM) and Vapour Control Layer (VCL).

The BM goes behind the cladding and the VCL goes on the warm side.

The VCL's job is to manage the passage of water vapour. The BM will repel water from the outside whilst letting moisture, from the inside pass through.

Definitely do not use a DPM type material.

I hope that helps.

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Thanks to all for your advice - I think the breathable mambrane under the cladding and 6mm ply inner cladding should do the trick. There shouldn't be any (significant) temperature differentials to worry about and the humidity level should also be pretty much the same inside and out.

Experience with my metal (single skin) shed is that condensation is minimal.

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In all honesty, I think in obsy construction you could get away with not using either, especially if there is no insulation. Generally with DIY obsy's we are not constructing an air tight structure like a house or other building. That is to say there are plenty of gaps and holes which will allow the whole structure to breathe and will prevent moisture buildup, i.e moisture build up from a person being inside will escape through these gaps and holes (the biggest one being with the roof open :grin: )

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I wonder if the whole thing is just another case of "amateur over-engineering". I will have a "warm room" but it will only be a partitioned off little "snug" and certainly won't be heated or insulated - just somewhere to keep the computer etc whilst working in there.

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True enough in the 'scope-house' but a very different matter in the warm-room .

That depends on construction method, detailing and workmanship. Having said that if you have insulation between studs in your warm room I would use a BM and VCL.

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I wonder if the whole thing is just another case of "amateur over-engineering". I will have a "warm room" but it will only be a partitioned off little "snug" and certainly won't be heated or insulated - just somewhere to keep the computer etc whilst working in there.

I reckon so.

If there's no insulation then there is no need IMO for a BM or VCL.

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