Breathable membrane for garden shed in UK (?)

To have or not to have?

How it ought to be installed?

How much should it cost?

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I have a new fully tanalised shed on order using chunky framework and 20mm T&G planks.
It will have an excellent build quality spec, will be 4 x 12 foot, with double doors on the long side for a primary function as a garden tool-shed, and for storage.

However the shed i’ve ordered doesn’t come with a breathable membrane, this is an optional extra which i declined due to the extra £500 expense. The product they use is Onduline Ondutiss Air 95, which incidentally can be purchased from Wickes for £60 for a 50m x 1m roll, easily enough to do the whole shed.

Other costs installing the membrane would include a staple-gun and labour time fitting but i don’t think this would account for much more on the cost.

The function of a breathable membrane is essentially to keep damp out during cold wintery weather. However for this to work does the membrane need a ventilation air-gap between the outer T&G planks?
It looks like the way shed fitters use the membrane is to staple it to the outside of the framework, and then outer T&G (or featheredge) up against it. This looks good and neat on the inside but there is no air gap so will it serve to hold damp between the membrane and outer wood planks?

This is not a product that can be fitted after the shed has been assembled, so i need to make up my mind as to have it or not bother. I will phone up the shed people on Monday to ask but i can’t help thinking it’s a lot of extra cost.

If you are insulating then yes you need the membrane.

If not and there will be a clearance between the shed wall and any stored items then not needed but will help or stop condensation forming on the inside of the cladding. If you put in a membrane you will have to think about how you are going to protect it from damage from stored items.

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I have had three sheds at different UK properties over the years. Damp has never been a problem, though I just store garden tools, mower, bicycle etc. I don’t store things like paint, though that is mainly because of potential sub-zero temperatures.

If I wanted to store anything more sensitive, I wouldn’t use a shed anyway.

I had never even considered using a membrane.

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As I see things, and having retro-fitted (leftover) roofing membrane when repairing some gapping T&G on my sheds, unless the membrane is tacked on the exterior side of the wooden uprights, then it’s questionable why it’s worth doing. You may turn the shed in to a greenhouse-like environment(?)…and you may trap condensation as ‘breathable’ isn’t the same as having natural airflow/ability of the wood to absorb any moisture on the inside.

I’m with you that the £500 cost is silly, as 50M would do many sheds – and as it’s breathable either by perforations and/or via fabric, I don’t think you need to worry re the rear side of the side planking, as this should remain dry come whatever anyway.

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My wife had a studio/shed constructed last year. She had a breathable membrane put in, then insulation over the top.
It was constructed in September, there’s no sign of any damp.

I’ve included a photo showing the membrane partially fitted.

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That is the correct use for membrane, to protect the insulation from absorbing damp.

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But can the insulation protect the membrane from becoming too wet?
This suggests a need for insulation if using a membrane, especially if the membrane is touching the outer planking (?) …and especially so if the outer planking is horizontal.

On @Dashing’s picture above the outer log style planking is touching the membrane, but the planking is vertical so any wet on the membrane could more easily drain down and escape at the bottom, in theory at least?

I haven’t analysed the process but your theory is correct. The membrane provides an impervious barrier to stop condensate water reaching the insulation. On the other hand the insulation will provide a thermal barrier in both directions to stop the impact of chilled air cooling one side of a surface while warm air condensates on the other side of that surface. They work together.

Breaking this down:

  • it’s likely the internal insulation in @Dashing 's picture is foil-backed foamboard (Cellotex-type) and all this needs is breathable membrane, which prevents water penetration and allows breathability to stop rotting. Obviously, it’s not wise to 100% seal any box structure.

  • the planking (whichever way affixed) is the first line of defence and the membrane is the second in preventing outside water penetration - and crucially allows breathability. The third is the foil backing on the insulation board – and if any water/damp gets this far, it is allowed to breath-out via the membrane.

I’ve seen builders apply breathable membrane over wood (sterling board) upstands in roof structures which have vertical tiling.

Yes, I remember at least one side was silver coloured.

When I get around to cladding my garden room, I’ll staple the membrane to the framework and then nail the cladding onto a series of counter-battens (to allow for airflow):
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Wishing you a lovely weekend :relaxed:

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I don’t think you need the membrane.
Keeping liquid water out of the shed is the job of the timber cladding. A membrane might reduce the ingress if water vapour, but unless the shed was hermetically sealed it would find its way in there eventually in any case.
With respect to breathability, a breathable membrane can potentially allow some vapour to pass through compared to an impermeable plastic sheet, but it will still be considerably less breathable than no membrane at all. Even then, the fact that the membrane is porous does not necessarily mean that any water vapour will decide to travel through it and disperse. In a building it generally requires a heat source to encourage vapour to disperse through a membrane, in much the same way that a Goretex jacket relies on the body heat of the wearer to drive moisture through it. So unless you are planning to move that woodburner in there I think it’s a waste of time.

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Looks good to me. I can recommend the Ali Dymock Garden Room YouTube series, he would be very welcome to visit my garden to sort my shed out :smile: :+1:

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I would be more concerned about the runners under the floor sitting in rain water and damp ground and rotting. I installed plastic runners so battery’s not sitting on floor as runners raise. It off the ground.

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The foundation is block & beam.
Solid concrete 7.3n blocks arranged in 6 pairs with two 3" beams across each pair.

This will place the actual shed about 6" above ground level.
The ground under the shed is a firm mix of stones, gravel, loam, and clay, well compacted but with excellent drainage.

Considering the tool-shed is only a storage shed and not something i will spend much time inhabiting, this is what i want to believe :slightly_smiling_face:

It maybe a good compromise to have a well insulated cabinet inside for the most expensive tools. If it’s very cold outside and very cold inside the dew point; when gas turns to liquid (water) may not happen or be a problem, especially if the shed itself has enough ventilation. It would be easy to install a couple of air vents.

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You must have more expensive tools than me! I find a very light coating of oil or WD40 on any non-stainless tools sufficient to prevent corrosion.

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