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We're just about to get a new wood-burning stove fitted with an external air feed and have a load of squeaky floorboards which need fixing in the same room, so we're planning on taking the floor up.

Since the floor is coming up, it seems sensible to get some insulation in while we're at it.

I've read that a reasonable approach is to nail / staple netting to the underside of the joists and then layer rock-wool on top (without compressing it). Then add a vapour barrier on top of the joists but under the floor boards.

Is that the right way to do this or are there better ways to insulate a raised / joisted floor in the UK? Bear in mind that there's only about 1.5ft of clearance under the floorboards so this will have to be done from above.

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What's beneath this floor? –  The Evil Greebo Oct 20 '13 at 10:07
    
It's just soil underneath. –  Jon Cage Oct 20 '13 at 10:21
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2 Answers 2

This would probably work, but "better" would be closed-cell spray foam or rigid XPS foam boards. Both of these will also act as a vapor barrier and have a higher R value per inch of material than fiberglass or rock wool. They are however more expensive, and spray-foam is not typically something that is DIY-installed.

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How would you install either of those from above? Rigid foam boards from above but presumably the joists would then still be poking through? I've read that insulating below the joists makes enough of a difference to be worth considering. I don't mind how much space the material occupies, but the R value per m^2 seems like a more relevant metric to compare them with (or did you mean coverage vs depth?). –  Jon Cage Oct 20 '13 at 22:03
    
You'd cut pieces to fit between the joists, and seal it against the joist with caulking or expanding spray foam (Great Stuff, etc.). –  Steven Oct 20 '13 at 22:38
    
How would you seal under the joists with this method? I was hoping to add insulation underneath them to avoid a thermal bridge. –  Jon Cage Oct 29 '13 at 8:34
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You can do as you suggest with the netting and install it from above. If you use metal netting (I've seen it done with "chicken" wire fencing), you can shape it so that it goes down the side of the joist, spans the gap, and back up and over the next joist (and so on). You can then put the insulation on top, where it will remain suspended by the netting, and then put the floorboards over the top.

(I've seen an insualtion manufacturer's literature showing this, but can't find an example of it right now. While searching for it though, it seems that there are also propriety insulatuion trays available to do the job).

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