Hi,

I am putting some insulation under a chipboard floor, and have two options in mind based on (a) what I have in the shed and (b) what is available locally (not much choice - I live on Skye).

The subfloor is a concrete slab, with a DPM underneath. The DPM doesn't quite reach the walls in all places - the last incumbent of the house installed it, not very well, but that is another story. The relevance here is that the slab is dry, but can't be regarded as 100% damp-proofed. Experience in other rooms implies that this is OK as long as the slab is ventilated. I am not regarding the time/expense/resources in digging it out as an option.

Options are:

1) 2"x1" treated battens on the slab, thus creating an air gap. On top of this lay some of that foil coated bubble wrap. Add another layer of 2" x 1" battens, ensuring that the space that results above is unventilated so the foiled bubble wrap works properly as a radiant barrier, and finally 22mm chipboard.

All these materials I have already in sufficient quantities, either in the room already or in the shed, as the last occupier left, among other things, a big roll of the foiled bubble wrap. My main concern and question is that I will get condensation on the foil (though it would be sitting between treated battens, so perhaps not a big deal).

2) 2"x1" battens (again), topped by a layer of 30mm Recticel PIR, with the chipboard on top. The Recticel layer would have battens in between the Recticel boards so no load is going through the insulation (as its thin and sitting on the lower battens, not a flat floor). I have everything for this, except the insulation itself. I'd prefer this option, and its well within budget, but I only want to buy not-very-green PIR if option (1) is not wise from the point of view of condensation.

To put this in further context, its a downstairs 'spare' bedroom, for visitors. Not surprisingly most of these visit us on Skye in summer, though it would be nice to have it able to retain heat a little better in winter for odd occasions, and I suspect it will end up being used for storage in winter to some extent. There is no point busting a gut on these improvements however as the number of days per year when it will be occupied overnight and requiring heating is low (if it wasn't for having to extract some rotting timber then I wouldn't be bothering in this room).

Other options considered and dropped include (a) expanded polystyrene insulation as it doesn't seem as bad enviromentally as PIR. No one stocks it locally anymore though. Delivery could be an option from the south, but to be honest I am reluctant to bother with this as getting anything bulky delivered here is often expensive and/or hassle. Certainly financially if not environmentally, PIR from the local Jewsons would be more efficient.

Also considered was (b) thicker insulation which could be fitted in within the height (subfloor to ceiling is 2.24m) but I'd end up with a low-ish ceiling and probably more to the point I'm not sure if its worth it for this room.

Lastly the defeatist (or maybe realist...) option of (c) no floor insulation was considered. Its a 1½ storey stone house with 600mm walls, including two such walls running across the house internally (they were originally external, but the house has a c 1804 extension plus a c. 1988 extension). Upstairs is crudely but surprisingly efficiently insulated between the rafters with rockwool, and retains heat overnight quite well, but downstairs is never going to be that good anyway without going down the route of externally insulating.

All comments welcome!

thanks,
-- Tim.

Hi Willie,

Its roughly 3.2m x 4.1m. Problem with Inverness is that by the time I've paid to get there and back never mind factored in my time (6hrs round trip from north Skye), I'm inclined to just go with 'local' PIR. Wickes do now deliver here but at £85 for the delivery charge I think I'll rule that out.

re: floor heights. Most rooms in the ground floor are at different heights because the concrete subfloors are at quite different heights in each room. Don't ask why. We just have a step at most doorways, you get used to it. In fact I'm slightly reducing the difference between this floor and the hallway.

re: rot. The problem was partly that the edges of the slabs in two rooms don't always quite reach the stone wall, and when they do, the DPM doesn't always get that far - so we have a damp perimeter. That problem solved by extracting rotted timber and relaying the floors so that they don't reach this damp perimeter and covering the resulting gap with t&g half lining, the space behind being ventilated. The other problem - though not in this room - was that the slabs had been poured *around* some of the internal stud wall timbers i.e. these timbers went through the slab and if the DPM didn't quite reach that point were sitting in damp.

-- Tim.

I used the foil/bubblewrap/foil stuff (Airtec Double) here (other end of Skye). Significant factors in choice were:

a) I was fitting it from below and access was extremely limited in places (see attached).
b) Screwfix were happy to deliver. :)

The way I used it, the insulation goes 50mm up the side of a joist, across to the next joist then down to the bottom again, i.e. an upside down U shape. The airtec comes on rolls 1m wide. As the joists are regularly spaced I could pre-cut a number of sheets and feed them through the trap door before each session down the hole. Where there are joins I overlapped by ~100mm. Where it was particularly tight I found that spreading out bits of cardboard boxes on the floor of crawl space (just bare earth) made it easier to slide about (and less messy).
Once that was done I underdrew the whole thing by stapling landscape fabric across the joists. Over most of the area the cross section is now floor, air gap, Airtec Double, air gap, breatheable fabric, crawl space.

The insulation material itself is impermeable, but should be able to 'breathe' via the overlaps. In the event of a serious spillage 'above' the upside down U should allow water to drain through.

I'm sure that well fitted solid insulation would be better, but this was possible, has been there for 2 winters now, and has had a distinctly positive effect.

Posted By: TimGSSkyewright: I suspect this is the same or very similar stuff. Good to know it does work, though I am leaning towards solid boards now.

Solid is what I'd have used if lifting the floors to gain access had been a practical possibility...

I am about to renovate my 1950s ex council house which has a concrete slab floor and is partly tiled (= very very cold) and have been looking for some kind of insulation which is reasonably rigid over which I can either put a laminate floor and/or lino. At Ecobuild I came across Marmox boards and these look like a possible option, although I need to research a bit further. They are used on concrete slabs below underfloor heating and can be tiled/plastered. Has anyone had any experience of this product? I realise I am not in the market for an amazing reduction in floor u-value, but anything has to be better than the tiles currently on top of the slab, and I need a cleanable surface (rather than carpet) in this area especially as I have a muddy hound.

I am also conscious of water issues in the kitchen area, having never lived anywhere which didn't have a leaking washing machine or a ruptured main at some point.

I agree with Colin, you'd need something above the tilebacker boards before laying Lino. The boards will take direct tiling of course.
The foam used is XPS, extruded polystyrene, Wedi uses the Dow Styrofoam product made in Germany whilst Marmox's XPS is made in an Egyptian XPS plant. Just becasue its blue doesn't mean its Styrofoam. Incidentally the thermal performance is worse than quoted, when XPS is cut to less than 20mm thickness the lambda increases markedly due to the more rapid escape of blowing agent. It cannot possibly be 0.027W/mk as shown on the data sheets (nor even 0.029W/mK)

Sorry, I see the Celotex spec link under the previous comment. But basically there is no way that I can apply that spec because the skim on top has to be min 650mm.

Hi,
Sue Green - An alternative is the chipboard bonded to insulation.

The Celotex site doesn’t offer the chipboard bonded to insulation that is available from others. The Kingspan version is “TF” boards 2400 x 600 which is the same as usual flooring size its 18mm chipboard. For example 25mm + 18mm board = 43 thick, 50mm + 18mm board = 68 thick and so forth.

The problem seems to be that for most only a full pallet quantity is available which is £££ the 43mm is £24/sheet and 68mm is £34/sheet from Kingspan etc.

Another possible is FloorMate which has the Dow Styrofoam (blue) bonded on to it. Panel Systems are selling the 43mm version for £14 with a min of 6 sheets, and £20 for the 68mm sheets. So it’s a good £10 cheaper that the Kingspan equivalent. Styrofoam being what the marmox is in the above post.

I’ll let the others deal with the exact difference between the PUR (standard Kingspan) and Styrofoam insulants but for small adhoc jobs with small quantities the Floormate looks a useful product and although the 25mm of insulation isn’t anywhere near enough it will take the edge off of the freezing floor without raising the level too much. The 68mm with 50mm of insulant would make a significant difference to the comfort under foot. You don’t say whether this is something you are just doing or something under building control.

No connection to any of these products but often these small one off projects are bounded by what’s do-able (with all the usual constraints) rather than what’s best tech spec.

Cheers

Mike up North

Don't use chipboard. You are quite right to be (more than) 'kind of worried'.