Insulation between floor joists of a shed


19 Jun 2004
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United Kingdom
I'm going to build my own shed. The floor will be framed using 2x4 pressure treated timbers with an 18mm WPB ply as the deck. I plan to insulate the walls and roof using rockwool.

The wall cavity will only be 4in deep between the plasterboard on the inside and the exterior sidings. Is this enough insulation to be worth it?

I also intend to insulate between the floor joists, this time using XEPS foam insulation. Is it simply a case of cutting and wedging/gluing the foam between the joists? Is the foam durable enough to be left exposed on the underside of the floor (the floor will be raised another 4in on skids sitting on a concrete slab patio). Is having the foam in contact with the joists and ply likely to create moisture problems with the floor joists or ply?
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I'm going to build my own shed.
It won’t meet habitable space requirements but that's hardly just a shed :LOL: Why are you going to that degree of damp proofing & insulating, do you intend living in there or maybe renting it out?

My only comment would be don’t use plasterboard on the inside, even MR. Gypsum PB hates damp & unless you control the humidity, it will eventually buckle & disintegrate.
For what it's worth, I'd be inclined to build a rough floor surface from WBP or OSB on top of 2 or 3 inch timbers, lay kingspan on this and overboard this with a t&g board, or half decent laminate.
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Richard, as you said, i dont want it to be habitable, however I do want it to be comfortable and long lasting. Its for the kids to play in and when they are too big it'll be for me to play in ;).

How could i control the humidity indside? Or can you suggest an alternative to plastering? I guess i could go for pine panelling, but i'd assume this would be a lot more expensive and i'm not sure i'd like the look - it would feel a bit like being in a sauna!

The inside finnish is something i've got a while to mull over though, my most important question relates to the base.

Cantaloup63, i had thought of putting the insulation on top of the ply floor and then floating a laminate floor on top (I got a load of laminate in the garage that i lifted carefully from the dining room :), however this would loose a few inches from the inside (i dont want to raise the height of the shed any more, plus i'm over 6ft tall so i dont want to reduce the internal headroom either), hence my thoughts about putting it underneath the ply on the outside.

So, can XEPS foam insulation be glued underneath a ply floor? Is it durable enough to be used outside (it wont see any sunlight) and is it likely to cause moisture issues to the joists/ply floor?
Er, since you are building your own shed, you could use 2 by 2 instead of 4 by 4 timbers on the floor. Also, you can adjust the height as you see fit. A couple of inches up and a couple down gives you 4" which is more than plenty for floor insulation in this situation.
Children at play will produce a lot of water vapour, and this needs to go somewhere. However, leaving the door open will solve this problem.
When my children were younger they liked to sleep in their shed, with their friends from time to time, so it needs to be warm and dry.
Your best bet is to cover the walls, ceiling and floor with two inch thick sheet polystyrene, cover the polystyrene on the walls with at least 6mm plywood, anything less will be damaged sooner or later, a fully floating floor of oriented strand board, will provide a nice dry warm floor.
With this set up you won't need to think about heating, their bodies will supply that. Your roof can go to 8 feet, plenty of height for you, plus floor, plus base.
I suggest cutting the ground away and lowering it about six inches all round, then filling the space with clean stones, this will stop the rain from bouncing and making the shed wet.
thank you perry.....i'll probably go down the ply route to clad the interior of the shed, i'll have a good 4in of insulation inside the frame of the shed so this should be enough to keep it cool in the summer and warm in the spring/autumn (i dont think we'll be letting them run in and out during the winter!) - its a shame that plasterboard is not suitable as i was thinking of plastering it, i wanted the interior finish to feel like thier own little house (plastered, skirting, blinds, etc).....

i know a few people now have suggested putting the insulation on top of the ply floor and then floating a floor on top of that - however noone has answered my question about whether or not polystyrene is suitable to be used underneath the floor, between the joists. This is still my preferred option as it wont lose any height in the shed and it wont raise the height of the step into the shed (I dont want to go from 4in joists to 2in joists as suggested by cantaloup63, i want to ensure that floor 'feels' solid).
Of course you can fit polystyrene sheet between the joists, it works very well, just cut one side at a slight angle and push it in place.
Because joists are not always straight and parallel, I find it best to fill every second space and then go back and fill the remainder. Also if you are a diy fanatic, knocking up a hot wire to cut the polystyrene saves an awful lot of mess, I find fitting 4 foot lengths is easy to handle.

Note: More than a quarter of your floor will be wood. Wood is not a good insulator (compared to polystyrene) If you are filling the spaces between the joists keep in mind that you will have a solid wood route out for your heat via the floor and the joists. (the ends being open to the cold outside)It is best to fit sheets of polystyrene below the joists to stop this. Condensation never forms on a warm surface.

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