Suspended floor insulation

Polin_ka

Hi, we're planning to insulate our floor. I have been trying to work out how much insulation we would need as minimum legally. We won't be going for anything like kingspan as it's a bit expensive for us.

Also, what else other than kingspan would you recommend? Mineral, sheep wool? 

grumbler

I'd fill the full height of the joists regardless of any legal minimum if it really exists.

Mineral wool supported by plywood or chicken wire.

BUFF

fwiw Historic Environment Scotland seem to favour sheeps wool over mineral.

FreeBear

grumbler said: I'd fill the full height of the joists regardless of any legal minimum if it really exists.

There are recommendations from Building Control, but this isn't legally binding. If the joists are resting on dwarf walls, one must make sure that cross ventilation is not compromised. If the joists are old 3x2 resting on dwarf walls, I'd suggest no more than 25mm thick Celotex/Kingspan supported from below by 25x50mm battens.

Yes, Kingspan boards are a bit more expensive than fibreglass, but is is twice as effective for the same thickness.

Bendy_House

Two things I'd suggest you want to achieve - one is insulation, and the other is draught-proofing.

Polin, are you lifting the floorboards, or adding this from below (how much access void is there?).

You have a number of solutions, but I'm personally drawn towards what FB suggests - rigid sheet insulation fitted snugly against the underside of the 'boards, and sealed around its perimeter (ie against the joists), and sealed to the next panel, and sealed fully around the floor's perimeter.

Also little need - extra benefit - in going thicker than 25 or 35mm high-quality insulation board (eg Celotex), or perhaps ~50mm of 'Jablite' (expanded polystyrene boards).

If you are lifting the floorboards, then just a serrated knife of extending craft knife to cut the sheets to fit snugly - by 'snugly, I mean so's they stay in position, and don't fall out. Pre-cut a good few rows of this, and have then ready in order of fitting.

Then use a nozzle-spray adhesive foam to apply a thin bead to all four edges, and fit the boards in turn - use a timber batten to tap it down until it's exactly flush with the joists. Add more foam to the perimeter if needed - draught proof around here, under where the skirtings will go.

The floor - being the 'coldest' part of a room - will have the least loss of heat through it, so I would hazard that even 1" of Celotex (or~2" of Jablite) will provide the optimum cost/effectiveness ratio. Most of the joists will still be exposed underneath, so will be kept well ventilated.

I'm not claiming this is the best thing to do, just offering it as what I would do.

I would definitely suggest that stopping draughts (timber T&G leaks like a leaky thing) is equally important as adding insulation.

Chickereeeee

Bendy_House said:

The floor - being the 'coldest' part of a room - will have the least loss of heat through it, so I would hazard that even 1" of Celotex (or~2" of Jablite) will provide the optimum cost/effectiveness ratio. Most of the joists will still be exposed underneath, so will be kept well ventilated.

Yes, I do wonder how much heat is lost through floors. I did contemplate doing a full insulation job, as above, but as we have carpets in the rooms under consideration, opted for thick/high tog underlay. This, combined with sealing around the skirting board, and a simple layer of brown paper to stop drafts from between the boards has made a huge improvement to comfort (although not much to bills). I don't think adding more insulation would make much difference.

I do, however, have tiles in the hall and kitchen area, which are very cold to the feet, so will have to bite the bullet at some stage. I imagine 2.5cm of Celotex would be enough.

FreeBear

Chickereeeee said: I do wonder how much heat is lost through floors.

10-15% is the generally quoted figure.

Bendy_House

T&G boards are, themselves, pretty darned good insulators.

I'm pretty sure that, by far, the biggest cause of heat loss due to suspended T&G floorboards is the draught.

Bendy_House

The cosiest and easiest-to-warm room in our house - an extended 1930's bungalow - is, by a large margin, the one I'm in now - a single-skin garage with a solid concrete floor.

The walls have been lined with 1" Jablite and 1" insulated p'board over this, and the timber floor has around 2" Jablite under it. The ceiling has had 2" of Celotex added.

The 200W of the plasma screen warms the room. Why? No draughts. The rest of the house howls...!

Chickereeeee

FreeBear said:

Chickereeeee said: I do wonder how much heat is lost through floors.

10-15% is the generally quoted figure.

The % figures often used are misleading at best. (100% of heat is always lost from house however good the insulation). Generally the figures are for a house with no insulation at all, so carpets/underlay would greatly reduce this %. But loft and cavity wall insulation would increase it!

FreeBear

Chickereeeee said:

FreeBear said:

Chickereeeee said: I do wonder how much heat is lost through floors.

10-15% is the generally quoted figure.

The % figures often used are misleading at best. (100% of heat is always lost from house however good the insulation). Generally the figures are for a house with no insulation at all, so carpets/underlay would greatly reduce this %. But loft and cavity wall insulation would increase it!

True.

It would be better to state xxKWh of the energy is lost through one particular element of the building. But without a complete & detailed assessment, only generalisations can be made. Reduce the heat loss by insulating and draught proofing as best as possible, and the cost of heating will be reduced.