Insulation advice

It is sometimes possible to push insulation along between plasterboard and floorboards, if you could lift some boards in the centre.

What's underneath the floor boards? An air gap? Does it connect to the cold loft above or is it sealed?

Putting insulation on top of an air gap is like putting a coat on top of a baggy jumper. If the jumper is loose fitting at the top and bottom and allows cold draughts to flow behind it, the coat isn't going to make much improvement because the heat is already gone before reaching the coat.

I would look at how you can seal off this space in an airtight way before thinking about any insulation on top. Blown fibre insulation is one option, but another is taking up sections of boards and stuffing fibreglass insulation laterally from each entry hole. With some appropriate poking tools you could maybe reach a metre around each hole.

Foil backing does two things:

- rejects radiation. Most of the sun's energy is received as radiation (no conduction or convection in the vacuum of space), so foil will reject it - keeping the room underneath cooler in summer. Also, in the winter a lesser proportion of heat is lost radiatively and foil will reflect that back into the room.

- acts as a vapour barrier, preventing room humidity from travelling into a wall but also preventing a wet wall from drying (in that direction). As long as you have a an existing vapour barrier (eg plasterboard ceiling) and your joists have a path to dry into the loft air then you should be ok.

However you may find it harder to buy non-foil-faced, in which case there's nothing wrong with using it in this case.

OK. There's a law of diminishing returns so any insulation is better than none.

PIR is pretty strong but it's not good at point loads - think high heels, furniture feet and similar. More or less any covering that's able to spread the load will suffice. If it's only occasional use and you're careful about what you put on it and choice of footwear then you might be ok without any covering. The thinner the PIR is the more vulnerable it is.

How big are the areas? Could spray foam be useful here? (thoughtfully applied). Or rockwool stuffed with tools?

Theo

B> would be helped by the boards, any old insulation that's under the

You > - The attic is used for storage so gets walked around, and there are

The best answer is bite the bullet and rip up the boards and do it properly. Before re-boarding the attic you can either add to the joists to bring you above insulation level or use loft legs

If you go the PIR route you can over board it and create a floating floor but it would have to T&G chipboard again and glued together into one contiguous floor. I have this in my garden office.

You would have to constrain the floor surface and the big disadvantage would be losing all access to what is below eg wiring, plumbing etc.

Richard

The main part of the attic is about 60ftx20ft - that's quite a lot of 4" floorboard to lift! Having said that, there's no doubt that it would be the best way to do it. The need for risers has been one thing that's put me off, but with a 2:1 gain from mineral felt to PIR I suppose I could cut strips of 100mm+ PIR.

Because this is an attic, with infrequent and light traffic, I was thinking of just laying sheets of hardboard or 9mm OSB over the PIR to stop it from being dented. Tape (single and double sided) would do what was necessary.

Access shouldn't be too bad with 2.4x0.6, or 1.2x1.2 panels and a loose cover ... plus, the only thing in the attic floor is for the upstairs lighting. The bathroom and shower room downlighters would have holes around them.

Might be possible, but the ceilings are lath and plaster, so it would need care. Also it's a large area with (at a guess) fifty joists gaps to insulate, so it would still need a lot of boards lifting.

Just a thought, but have you considered putting PIR underneath the ceiling instead? You'd obviously lose room height, but if it's an old house with high ceilings it may not be so bad. You'd get a much better seal that way, and it would make a new ceiling over eg old Artex, which could be more appealing if the ceilings are tired. Not a good idea if you have ornate plasterwork though!

You can get PIR backed plasterboard but you could also dot and dab regular PB + insulation screws underneath regular PIR.

Theo

Not a solution that would be accepted by SWMBA - she's recently painted several of the upstairs rooms.

Thanks for that, it sounds similar to what I'm considering, except for using 2 layers (which sounds like a good idea). OOI why did you need to screw it down? I'm thinking of a mix of adhesive tapes and (possibly) foam.

Or you could lift the floorboards, fill up the gap between joists to be flush with their top with 'rockwool', then add a complete layer of PIR and just relay the old floorboards at 90 degrees to what they were before using longer screws.

Unless you store a lot of heavy stuff the PIR will not crush but if you are worried then use a timber core cutter to make a 2 inch diameter hole where each floorboard crosses a joist and push a bit of 2x2 timber cut to be the same length as the depth of the PIR and fix through this into the joist. You dont need to fix every crossing point if the original floorboards are nice and flat.

I used this as part of my underfloor insulation after digging out the screed many years ago. Most of the GF area had screed deep enough to allow 70mm PIR between 3x2 battens (on edge) but the slab was uneven, so parts had to be done with that pink Knauf stuff. (It took ages to laboriously cut all the 3x2 battens to follow the uneven slab along the bottom while leaving a flat surface on top).

I did notice that it was extruded polystyrene which I do believe would burn horribly if it got the chance (unlike PIR). Not sure if I would be happy with it anywhere where it could not be contained within a 60 minute class O skin.

It's not that clear on the videos - does it start really thin (1mm ish) and then slowly expand to fit?

Adds about £15 per 50mm sheet? That's quite the hit.

I've just done a floor, with reasonable access via a cellar.

I've got a lot better at it with practice. I have got a Bahco saw, which is pretty good, but for 25mm at least, deep score with a stanley knife and 'snap'

- like plasterboard - is far and away the quickest and cleanest. All the joists are unevenly spaced, with several overlaps where old joists have been reinforced . . .

. . . and so far from perfect. I've used foam on the gaps. You're quite right about access. But does the memory foam readily recompress? I suppose come the day you'll find out!

I'd hope PIR doesn't crush - at all. Recently helped someone with a solid floor. He just chucked 75mm (I think) PIR on top of a fresh concrete bed, then covered the whole lot with another 6 inches or so of concrete.

If it did compress, that floor would be pretty unstable.

No, it's not vacuum packed like a memory foam mattress, it starts expanded on a roll. When you shove the PIR into place it compresses to fit the gap. OK, it doesn't compress to zero but it does make it quicker without having to keep tweaking your trim to wonky joists.

I'm using the 100mm version which works out at about £2 a metre. Actually mine was about half that as I got a part box on ebay.

Were I doing more of it, I'd probably get some cheaper Class O (fireproof) foam, cut it into strips and then stick it on with foil tape. But given my confined space I wanted to minimise the messing about.

I have about 0.8m height, and to carry anything in or out is a 20m crawl (10m each way).

It's trickier with the thicker sizes - I'm using 100mm. That doesn't have enough give that you can just hammer something into place. The first say

I'm cutting with a long bladed Stanley knife and then snapping, but often it leaves an uneven core that needs shaving. The Gapotape adds a bit of leeway so that edge can be ropey but still be good enough.

I'm assuming that the foam may need attention if it has to be removed in many years time, but it makes it completely reversible - unlike the spray foam horror stories where you have a roof covered in the foam and the timbers are basically scrap. Spray foamed PIR is not so bad - you can just cut down the joists and take out whole pieces - but I didn't want to go there. I might use it in a few places where it's probably going to fit better than the alternative.

Theo

PIR will crush under high pressure - high point loads which will sink in. That's how it's easy to cut with a knife. If you've put a concrete bed over it, the weight of the concrete and the other loads are spread over the whole of the PIR so the pressure it feels is not more than it can bear. If you put a nail through the floor the PIR would crush around the nail.

I've put minor damage into PIR by kneeling on it, but while there are visible indentations they're not more than a couple of mm and I suspect don't compromise the integrity. If you have PIR resting on joists I'd think worst case the crushing would be like that.

Theo

If you wanted to support really heavy loads like racks of frozen food and forklift trucks then there is a special blue variety which goes under a thicker slab. Thats how cold rooms are built.

Walking on it, even bare foot does make some indentations in my experience. For a loft that has minimal 'visits' then fixing the floorboards with gaps of up to 12 inches between them is probably all he needs for storage and walking about up there.

I think that's EPS or XPS. It's cheaper, where space isn't so critical you can put a good thickness in, and has a higher compressive strength:

If it's thin (say 25mm) I would be more worried about it snapping, which it tends to do if there's enough bending to cause a fracture. If it's thick (say

Theo

I guess my thinking was that, if only taped, the hardboard was not likely to stay in place when stuff was being slid around, which is usual, due to low headroom.

Also, if I had gone that route, the subsequent bowing of the sheets would probably have pulled the tape apart.

I probably also, without running the numbers, felt that some screws were cheaper than quite a bit of foam, and would be firmly in place immediately.

Chris

Ah OK, gotcha.

Thanks - partially(!) convinced by the Gapaotape, and I do wish my roofer had either used it or made a better job of installing PIR. Imaging camera shows some quite obvious gaps.

Worst is a bay flat roof. He charged me £200 for using PIR over rockwool. There are either some serious gaps or he left insulation out altogether.