Shed insulation using celotex, cavity required?

15 Mar 2019
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United Kingdom
Im looking to insulate my shed using celotex and just have a couple of questions in the hope of clearing up some contradictions I have discovered during research.

My shed is pretty heavy duty, constructed out of 3x2 framing and 25mm T&G cladding. Ive read conflicting views about the placement of the celotex, that being to fit it tight up to the cladding so no air gap, or to leave an air gap.

Unfortunately the shed was constructed off site and assembled on site by the supplier and no membrane was installed. I do have a little water ingress poss due to a knot or shrinkage in a joint which I will remedy.

I was going to fit the 75mm celotex right up to the cladding, then board the inside with OSB or other. I assumed that the foil backing on the celotex would act as a vapour barrier, along with the OSB (which I would also paint/treat). In my head, the warm air would be contained within the shed and the insulation would prevent the internal warm and cold outer surfaces from forming moisture.

I dont really understand the need for an air gap between the inside of the cladding and the celotex, as this will also be a sealed space with no airflow.

Would appreciate some clarification before I commit!

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Polythene sheet the outer wall, insulation will not prevent moisture entering.
The heat you mention? Are you heating the shed?
No, but missus wants to use it as a gym area with a spinning bike so I will need to ventilate.

Re the sheet,I assume you mean on the inside of the outer wall between the framing hosts directly onto the cladding?

Won't the foil backing of the celotex act as a moisture barrier?

Off the celotex site:
A vapour control layer is a material with a high resistance to the passage of water vapour. This might be a foil faced insulation board (like Celotex) or a polythene membrane
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Would it be advisable to inject a flexible sealant between T&G timbers that have shrunk a little as some do let in a little water. I don't know how else to remedy this.
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You don't need a moisture barrier the foil backing is fine, you just pack it in and plasterboard. Built my own garden room this way and never had any problems. I can heat mine in the winter with a very small fan heater. It only needs to be on for 10 or 15 mins or so..

I dont really understand the need for an air gap between the inside of the cladding and the celotex, as this will also be a sealed space with no airflow

The reason is because a timber frame structure is designed to be completely weathertight before any cladding is fitted. Once fitted the cladding is a decorative covering thst protects the structure from sun, wind and rain. The cladding doesnt have to be fully watertight, any water that gets through just runs down the cavity and out. The cavity allows air movement so the cladding dries out again.

A shed is a bit of a compromise, it has the cladding fitted directly to the framework, sheds arent airtight so water or moisture thst gets in will dissapate.

If you push the celetex tight to the cladding you might increase the risk of damp building up. It would be best if the cladding did have a few gaps to allow air flow a bit. Some people leave a 1/2" gap to help.
Thanks for the additional advice, I understand a lot better now.

Two of the sides of my shed are difficult to maintain and are actually out of view so I was thinking of covering these with corrugated pvc, EPDM or other such material to minimise water ingress. I think I'll go with a 50 or 60mm celotex with a 15 or 25mm air gap.

Should I at least seal some of the gaps I have seen between the t&g?

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Good morning all, happy new year.

Have just joined the forum so please go easy on me.

Sorry for digging up an old thread but I am in a similar position to the original poster and due to receive my prefab shed in a weeks time.

The shed I've ordered doesn't have any vapour barrier/sheething and is just the outer cladding nailed directly to the joists (3×2")

I was debating on whether to staple some vapour barrier to the inside of the cladding, then leave an inch gap and then install 2 inch cellotex and cover with OSB which will affix to the stud work touching the cellotex.

My partners father-in-law was advising me to install the cellotex directly to the cladding and then install vapour barrier to the studwork. To me this seems wrong.

I'm in two frames of mind and was either going to go with the first option of installing the vapour barrier or to do the same as motorbiking had posted earlier and was just going to leave an inch gap between the external cladding and the cellotex and just panel up.

Do I really need to install vapour barrier??

Any advice gratefully appreciated.

Many thanks people and hope 2021 brings you all good health and happiness.

It all depends how much heat you are going to put in and for how long?
Look at the celotex website and it will give you the correct installation guide.
It all depends how much heat you are going to put in and for how long?
Look at the celotex website and it will give you the correct installation guide.
Good afternoon catlad,

Thank you for your prompt reply,

The plan is to put a small panel heater in the shed as I am planning to store my motorbike inside as well as use the shed as a mini workshop.

My concern was that if the bike is wet when getting home from my commute, then whatever water is on the bike when its brought in, would create a huge moisture trap and would create a ton of mould and mildew inside the shed.

Had I had enough time I would have shopped around or looked at building the shed myself and had the vapour barrier installed before installing the cladding but sadly didn't have time.

I appreciate your advice though, many thanks

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