How to insulate shed kit floor with concrete base

15 Aug 2019
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United Kingdom

I'm installing a shed in my garden for use as an art studio (among other things), and I'm hoping to get some advice on insulation, as I want to use the building year round.

A nice level concrete base has already been installed by a local groundswork tradesman, and a 10x14 44mm log cabin kit is ordered and will hopefully arrive in a few weeks (I'll assemble it myself).

The walls:
The log cabin style kit has 44mm thick walls and double glazed windows, which I believe should provide reasonable insulation for the walls.

The roof:
For the roof, I plan to add some insulation board or foil insulation internally then clad to match the rest of the interior.

The floor:
This probably needs done before the shed is build, as I don't want to lose any more head height.

My assumptions (from internet research):
  • its not a good idea to place insulation directly onto the concrete base, so it needs an air gap to prevent moisture rising up
  • Foam insulation boards look to be a good approach
    • They need to be tight up against the floorboards and bearers, foil side up
    • 50mm thickness would be sufficient.
    • Expanding foam sealant and aluminium tape to cover and gaps.
  • Foil bubble wrap would not be well suited to this, due to moisture forming in gaps.
If the above is correct, then the 58mm x 44mm bearers that will come with the kit won't provide enough height for 50mm foam board + air gap. I could order some 58mm x ~100mm bearers which would provide sufficient height. Making them higher than they are wide means I'd probably need to tie the bearers together by adding box sections, without closing off the air gap.



Does this sound reasonable or is there a better way? Any advice will be very much appreciated :D
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Just my take, yes, perhaps would have been better to put the insulation under the concrete slab.

As you've not got this option, why not lay a damp proof membrane over the whole base and foam insulation directly on top.
You won't need an air gap as the DPM will stop damp and if you use foil backed foam boards, again this is an extra vapour barrier.

If you need to nail flooring down, then you could build a frame work, on top of DPM layer, then batten frame on top, insulate inbetween, then floorboards on top this.
Thanks Mr Chibs, I wasn't aware that no-airgap was a potential solution. I'll need to have a look into that.
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Im not sure a ventilation gap is a benefit or not.

In general terms, a dpm layer is used to stop passage of damp. Then a vapour control layer on inside stops moist warm air getting to cold surfaces -which might lead to condensation.

If a dpm is used, no ventilation gap should be required.

Im not sure how you would deal with the perimeter -you need to seal all gaps.

If the concrete is dead flat, you could lay insulation with no gaps and lay a t and interlocked chipboard floor over -its a floating floor

Or joists with insulation in between. Use 75mm foil tape and go over each joist to connect the foil from board to board for a complete vapour layer. Then use an underlay with a vapour layer -if you are having laminate.

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