Need advice on Insulation - Celotex, Kingspan or Polystyrene

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I'm looking for some help with deciding which insulation type to go for in our sitting room.

It was formerly half a double garage and was converted a few years back - but the subfloor consists of 47 x 100 bearers (mostly cut to 90mm), no cross pieces, and 50mm polystyrene insulation which has simply been chucked in, gaps everywhere, it is too small for the channels.

It is basically a very cold room. The walls have been done properly I believe, soft blocks and well insulated, so I think the concrete floor and poor insulation is the culprit.

I will be replacing the insulation with something thicker, and have 85mm Celotex in mind.

Would this be a good idea, or would I be better with something else? I don't want to spend a fortune, the room is approx 4.2m x 2.4m.

I also want to put down a DPM for belt & braces as there is nothing there, and it suspect this will help the cold problem a little also. Should I just lap it up the walls to the level of the chipboard T&G?

As a final thing I was going to seal around the room with expanding foam, probably after installing the chipboard, to prevent draughts. Is this advisable? Do I need to leave an expansion gap for the chipboard though?

Thanks in advance for any help, much appreciated...
 
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Yes 85mm Celotex would be good, though why not 90mm if that's what the joists are. A dpm will do no harm if it makes you feel better, seal all gaps between any joists and insulation and at chipboard edges and under skirtings, airtight is good with respect to these types of floors.
 
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Yes 85mm Celotex would be good, though why not 90mm if that's what the joists are. A dpm will do no harm if it makes you feel better, seal all gaps between any joists and insulation and at chipboard edges and under skirtings, airtight is good with respect to these types of floors.

Thanks for your help, may go for 90mm yes.

Should I use duct tape to seal the gaps, and how high up the sides should I have my DPM?
 
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Seal any gaps you can with expanding foam and with your insulation being installed foil face up tape over the joints with foil tape if you really want to go to town. DPM can lap up the wall and be sealed against the wall. Presumably if the skirting is still in place you can only lap it up to that.
 
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Seal any gaps you can with expanding foam and with your insulation being installed foil face up tape over the joints with foil tape if you really want to go to town. DPM can lap up the wall and be sealed against the wall. Presumably if the skirting is still in place you can only lap it up to that.

Thanks, the skirting is already off.

Does Celotex compress a little if required? If I go for 90mm then it may protrude above the timber frame a little in a few places because the floor is slightly raised at the edges (the 4x2 bearers have been trimmed to account for this). Would it compress when I screw down my chipboard?

Also, do I need to leave any kind of expansion gap around the perimeter of my timber sub floor frame, and/or around the chipboard flooring ? I assume not, is it ok to fill these gaps with expanding foam?
 
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Do a floating floor if you can i.e. no sub floor frame, but this depends on the levelness of the original garage floor, so you could take it all out and then look at a levelling screed if necessary, then DPM, insulation and chipboard. Thermally this is a better detail as the insulation is continuous (no timber cold bridging) and less cutting as the factory cut edges can butt against each other and be foil taped as opposed to cutting to fit the timber sub floor frame. The Celotex should not compress (obviously it will if you bash it), so I'd go for what works with your floor level.
 
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Whilst I'd agree a floating floor is beter for insulation its not quite as solid feeling.

As mentioned the Celotex won't really compress much, a mm or so but no more.

Just lap the dpm up the walls a little an inch or so pat the chipboard and seal against the wall with some gaffer tape. You can eave a little gap around the perimeter of the chipboard say 5-10mm, seal with sealant. and seal under the skirting too.
 
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I'm reviving this thread as something has come to light that I just want to check.

I completed the subfloor last December, basically it's a timber frame laid onto an extra DPM which is on top of my original concrete garage floor (the room was a garage conversion). It is a solid floor, not a suspended floor.

I first sealed all the gaps around the walls to prevent any drafts, then built my frame and put Celotex in the square timber voids, 90mm thickness. I sealed around the perimeter of the timber frame with expanding foam, then put 22mm chipboard on top of the timber frame, with all joins supported by the frame and well screwed down, and sealed around the outside of it with foil tape (ie lapping it up the walls slightly).

I then laid laminate flooring on top of underlay onto the chipboard, then skirting etc.

The floor is very solid and I'm pleased with it.

So what's the problem? I went out for a beer last night with someone who knows a bit about building regs, and he said that ideally the timber frame subfloor should have been ventilated, using air bricks or whatever. It is supposed to be to reduce the condensation risk I guess.

I suspect he's correct but everything I've found online shows this is required only for SUSPENDED timber frame floors on a ground floor, with a 150mm air gap underneath? I think this could be for older houses or for those built on a slope, where there is a crawl space under the floor. My timber frame isn't suspended in any way, it simply sits on my extra DPM on the concrete floor, as described above.

There is no real air space at all under the chipboard, as the frame has celotex in the voids, and is sealed around the outside using expanding foam. It is sealed off from the room space above pretty well due to the foil tape etc.

The room is centrally heated; I assume that providing we don't use the room as a laundry (!) with the doors and windows shut then I won't have a problem?

The previous subfloor was a similar (but poorly executed and uneven) construction, has been in for 10 years as the garage conversion was done then, there was no evidence of rot or degradation to the timber at all, and it wasn't ventilated from the outside.

Any advice or help would be much appreciated - the last thing I want to do is have to take all of this up again, it would be a nightmare.

Pretty sure I'm OK with the way I've done this, but just want to check.
 
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If the there was a void under the joists then vents would be required, but there isn’t, your joists sit on a dpm, the void is full filled with insulation with a vapour barrier on top, there is no moisture to get in that needs to be let out and so no risk of condensation and consequential timber degradation. Worry ye not!
 

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