Yet another shed insulation question

[timboh]

Having trawled through dozens of similar questions, I imagine regulars here could probably live without another confused post about shed insulation, so my apologies, but…I’m confused.

Shed is a log cabin, 44mm T+G, with a pent roof. Should this work: batten out with 25mm deep timber, then foil-backed celotex over that - i.e. on top of battens, not between, so there’s a 25mm airgap - and foam/tape all joins. Then ply on top, fixed through celotex to battens.

Similar for roof, but with vents into the airgap at either end.

Should there also be a breathable membrane on walls between T+G and battens? I’m wondering if that’s more likely to trap any moisture against the inside of the T+G than just leaving the airgap.

thanks
TimH

 

[Newboy]

Are you planning to heat the shed?

 

[John D v2.0]

Yes ok but the battens and wood on the outside should be treated, and the void created needs to be drained and ventilated.

 

[timboh]

Yes ok but the battens and wood on the outside should be treated, and the void created needs to be drained and ventilated.

By drained, do you literally mean a row of holes in the floor within the airgap? (Pardon the naive question!)

 

[timboh]

Are you planning to heat the shed?

Yes - probably an oil-filled electric radiator.

 

[John D v2.0]

By drained, do you literally mean a row of holes in the floor within the airgap? (Pardon the naive question!)

As long as it doesn't damage anything or pool anywhere, that would be fine.
Basically any unheated void can get condensation in, unless you can somehow seal it perfectly with dry air inside (eg a double glazed window unit meets that criteria)
So given that, you need a way for it to escape without harming anything.

 

[timboh]

Many thanks John. I forgot one other question: I was planning to put electric cabling in the void and poke it through the insulation where needed, sealing round it with foam. I think, from further trawling of the archives, that this shouldn't be an issue in terms of cables overheating for such relatively thin insulation, though might mean using slightly heavier cable than otherwise to compensate for the de-rating(?) effect. Does that sound about right?

 

[^woody^]

It's a timber shed. It don't need vapour barriers, membranes or vented cavities. Timber is permeable and breathes on it's own, and is not massively prone to condensation.