Couple of Lining/Insulation Questions

[Dave1w]

Not a build as such, as I opted for one built for me, but finishing off myself.

I've got some breathable membrane to line the inside before putting in insulation then ply over the top.

I'll be leaving a 50mm gap in the roof, with the eaves protected by mesh and 25, or 40mm PIR. Will that give a decent level of insulation?

I think I also read that in the walls there is less or no need to leave a gap, so does sealing the wood then breathable membrane then king span sound OK?

Cheers
Dave


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[Mike G]

Can you list your proposed build-up, from the outside in, or the inside out, element by element. And what is the use of the shed?

 

[Dave1w]

Hi Mike,
It's tanalised shiplap outside, with 3 x 2 framing. I'm planning to treat the inside wall / paint the outside, then breathable membrane, insulation - I am thinking kingspan/PIR, or rock wool, vapour barrier (as it is cheap enough) and finally plywood. I'm just not sure whether to put some battens and a gap between the wood and the membrane or the membrane and the PIR.
If I don't the insulation can go a bit thicker. In the roof, I'm planning on a 50mm that will be ventilated to the outside, but is it between the membrane and insulation or roof and membrane?

It's going to be a basic man cave, will have some toys in there, some dive equipment (dry stuff) some motorcycle goodies, so dry, and warm to be in ideally.


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[Mike G]

OK, that's a bit confusing! List the build-up, outside to inside (or vice versa).

For instance, my workshop is (in to out):

9mm OSB
95x45 SW framing at max 600 centres, fully filled with minieral wool
building paper
25mm void formed by sw counterbattens at each stud location
SW feather-edge boards

 

[Dave1w]

I was talking outside to inside, with the shiplap being 19mm. So from what you describe a 25 mm void between the outer shiplap to the breathable membrane is best?


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[Mike G]

Absolutely certainly definitely without question.

Think of your cladding as a semi-permeable screen. Water WILL get in behind it. So you need an air flow to dry it off, and you need to keep it separate from the frame. Raising the sole plate off the ground on a plinth, and battening out outside the frame are the two things which differentiate between a cheapo shed lasting 5 or 10 years if you're lucky, and a workshop which will last a lifetime.

 

[Dave1w]

Thanks, would it also be an idea to drill a couple of holes top and bottom in each section for ventilation into the void, or just let it breathe through the shiplap?


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[Mike G]

Insect mesh bottom (and at the top if possible), under the bottom edge. My workshop build is a "sticky" here on the forum, and shows that detail quite clearly. Have a look, although you'll have to find it somewhere in 20 or 30 pages.

 

[Dave1w]

One more quick question, as it's a built up shed, I have bought a heap of 25mm battens, but when I put them in, with the breathable membrane, there isn't going to be a vent top or bottom.

So should I perhaps put a couple of say 10mm holes top and bottom to promote some airflow - means a heap of holes and maybe more chance of getting moisture behind the wood, leave it as it is - with possibly some small air leakage through the shiplap, or finally fill between the studs fully - which I imagine is what lot of people just do.

Probably going to go with celotex, which perhaps will be less impacted than if I am using batts.

So the real question is, is there a benefit of the air gap, if it isn't openly vented, or should I in the least drill a couple of holes per section ?


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[Mike G]

One more quick question, as it's a built up shed, I have bought a heap of 25mm battens, but when I put them in, with the breathable membrane, there isn't going to be a vent top or bottom........

Why not? And when you say "in", what do you mean? The battens should be applied to the outside of the frame, over the membrane, at the stud locations.

That gap should be vented. If you only use vertical battens, and don't put battens horizontally at the top and bottom, you should be able to use an insect mesh to give a continuous open area at the bottom and top. This area will be subject to water ingress. No boarding is every 100% watertight. So you need to allow air flow to dry it off, and you need to allow excess to drain out of the bottom. Just drilling holes will only invite bees and wasps to nest, and won't provide adequate ventilation.

Along with raising the shed onto a masonry plinth, these simple measures will prolong the life of this shed almost indefinitely.

 

[Dave1w]

No, I think you misunderstood the original post, this is a ready made shed / workshop that I am insulating, so I am trying to apply best practice as far as possible in insulating and lining from the inside.

There's no additional cladding to go on the outside.

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[Mike G]

Ah, right. Even re-reading it, that isn't obvious.

Well, you're up against it, then. I think the best you can do is forget the breather membrane, which would be a waste of time and money, leave an inch gap between the insulation and the inside of the boarding, and put some circular soffit vents in the top board under the eaves.