We are knocking a wall down at the moment, to merge kitchen and diner into one room. It gave me a chance to look in the void under the boards, where I found no sign of insulation. Is it worth me diving under there and fitting some, while we are a mes anyway? I guess it would be fluffy stuff held between the joists with netting? Any thoughts gratefully received, would be up for it if it is going to make the room feel cosier!

Definitely worth doing if you’ve got the floor up anyway. Use Celotex ( foil faced rigid foam) – 100mm of this is approx equivalent to 200mm f/glass. Make sure the void below the insulation is well ventilated via air bricks.

Cheers, what would be the best way to secure the celotex? I’m hoping to leave most of the boards in place by crawling in the void, not looking forward to it!

WE looked at this but as I wasn’t going to do it myself the price was astronomical. Apparently very few builders want to crawl around under the floor fitting insulation. It is pretty grim.

They were going to cut it to size and wedge it in. You could probably put in simple wooden cross beams to make sure it doesn’t fall out.

If you’re leaving the boards in place, the Celotex will friction fit between the joists, assuming you’ve got room to manoeuvre (good luck), as long as you are accurate in cutting the sheets down. If you’re lifting the boards & installing from above, likewise, but you can also pin battens into the joist sides or even just nails to support the sheets, if they are not a good friction fit (which they should really be to get the full benefit).

It might make the floor fell warmer but remember there is almost no heat loss through the floor; cold air sinks + hot air rises (which is why you have 10″ of glass wool in the loft!)

That’s what I was wondering- hassle vs benefit, would be interested in other’s experiences.

I contemplated doing this in the last house – mid Victorian – but the cost:benefit put me off.
Ended up with rugs covering most of the floor area, and then used a mix of PVA glue & sawdust to fill the gaps between boards & stop the draughts where detected.

If you must, here is a good example, with nail/screw bearers for the Celotex

It might make the floor fell warmer but remember there is almost no heat loss through the floor; cold air sinks + hot air rises (which is why you have 10″ of glass wool in the loft!)

Incorrect.

Yes warm air rises but thermal imaging has proven significant heat loss through floors, more so in a suspended floor that’s not insulated – if you have air leakage you have heat loss

I’ve just done this job this weekend – having done similar to you (wall knocked down).

I disagree with chicken man somewhat – based on my research on the internet. Heat loss through a suspended floor was quoted as being in the region of 15-20%, so hardly insignificant. In my case, Victorian House, without lining the walls (inside or out) there is virtually nothing i can do to improve the insulation (double glazing already fitted)

i lifted the boards and hung insulation netting from the joists, then packed with traditional loft insulation.
Celotex sheets would probably have been better, but I’m mid project & hadn’t planned to do it till late in the project so wanted a quick solution. Got it done in just over 1/2 a day.

NB – if you want any insulation netting I can send some to you – I had to buy a huge quantity as i bought it locally – couldn’t wait for a delivery from the net (where smaller quantities are available).

If you’re going to have carpets down is it worth doing?

If you’re going to have carpets down is it worth doing?

Yes. I CBA to look up the K value of carpet…but Im sure its not good.

If your carpet is 100mm thick, no. Otherwise yes.

Ours has just all been done – the builder made sort of hammocks between the joists out of a membrane of some sort and then filled them with rockwool (or something that sounded like rockwool when he said it?!). V pleased, but as entire inside of house had insulation added no idea how much of the benefit we currently enjoying is as a result of the floor!

TKID: Let me know when your heating bills drop by 20%…
Heat loss and draughts are different things; of course letting cold air through gaps between boards and under skirtings is going to make the house colder!

I used 12cm thick recycled polyester. I don’t like Celotex because working with it in restricted places is difficult, it needs cutting accurately to be effective, doesn’t breathe and is less effective in stopping draughts through gaps in and around the boards. If the rest of the house is well insulated you won’t be losing 15% through the floors it will be much higher.

I’m currently walking around barefoot in a house at 20°C that hasn’t been heated since March. It’s 7°C and raining outside.

Just looking at educators pic… While your down there, lag any pipe work to rads etc etc …

No central heating needed, those are the waste pipes from a sink and the overflow from the solar hot water tank.

I used 12cm thick recycled polyester.

recycled polyester what? is it like fibreglass ie a big roll of fluffy duvet?

Yes, it comes in rolls. It’s A+ from a health point of view so much nicer than fiberglass or rock wool to work with.

Edukator,

Like the others posting to this thread I have been thinking about installing sub floor insulation. Had been thinking Celotex or similar. How did your polyester stuff compare price wise, do you know? And where did you get it?

where did you get it?

B&Q sell recycled polyester insulation. I’ve got it in the loft; it cost about the same as the itchy stuff.

Thanks Rio

If you’re going to use rolls, SpaceBlanket is handy as it’s easier to manoeuvre into the gaps and you can staple the sheath to the joists so you don’t need extra netting or timberwork to keep it in. Then a good carpet underlay like Cloud9 11mm has made a decent difference to our cold dining room.

It might make the floor fell warmer but remember there is almost no heat loss through the floor

Single biggest loss of heat in our house is suspended floors. If you put your hand down at about 1″ above the floor you can feel a layer of really cold air.