Insulating my suspended floor
Discussion
Hello. I have an Edwardian house and it's bloody freezing.
In the front room (which is about 5m ish wide and 6 ish m long) it's particularly bad. We are about to take up the carpet and renovate the floorboards below. there is a bit air brick on the front elevation to vent the floor. Whilst the log burner that's about to go in will help, my concern is it will be very cold. so I'm thinking of taking the floor up and popping in 25mm pir (which I can get cheap) or possibly 50mm pir (which isn't as cheap).
Is it worth the hassle/cost or won't it make much difference? If it costs a few hundred to do and will make the room warmer then I don't mind. if I'm wasting my time thinking of doing this - tell me now !
Cheers
Jack
In the front room (which is about 5m ish wide and 6 ish m long) it's particularly bad. We are about to take up the carpet and renovate the floorboards below. there is a bit air brick on the front elevation to vent the floor. Whilst the log burner that's about to go in will help, my concern is it will be very cold. so I'm thinking of taking the floor up and popping in 25mm pir (which I can get cheap) or possibly 50mm pir (which isn't as cheap).
Is it worth the hassle/cost or won't it make much difference? If it costs a few hundred to do and will make the room warmer then I don't mind. if I'm wasting my time thinking of doing this - tell me now !
Cheers
Jack
I've done my floors with 100mm pir between the joists. 1 room by taking up half the floor (joist needed replacing) the other room by crawling down a foot square hatch in to the 2 to 3 foot void and sliding around on bits of cardboard. Pir secured with wooden battens screwed in to the joists. All gaps filled with expanding foam.
Horrible job to do but the most worthwhile bit of DIY I've ever done. Made a huge difference to the warmth of the floor and removed all draughts from the rooms.
Horrible job to do but the most worthwhile bit of DIY I've ever done. Made a huge difference to the warmth of the floor and removed all draughts from the rooms.
I'd highly recommend it. I put 100mm Celotex in between the joists in the whole downstairs of my victorian terrace a few years back.
It has made a massive difference. Before the rooms were very cold and draughty in the winter and now they stay very warm.
100mm is probably overkill, but this was before material prices had gone wild, and i had some spare. I'd imagine 50mm will still make a big difference. Make sure to use foil tape on the gaps and joints to get it air tight and as sfella suggests, a bit of expanding foam around the edges.
It has made a massive difference. Before the rooms were very cold and draughty in the winter and now they stay very warm.
100mm is probably overkill, but this was before material prices had gone wild, and i had some spare. I'd imagine 50mm will still make a big difference. Make sure to use foil tape on the gaps and joints to get it air tight and as sfella suggests, a bit of expanding foam around the edges.
jfdi said:
I've done my floors with 100mm pir between the joists. 1 room by taking up half the floor (joist needed replacing) the other room by crawling down a foot square hatch in to the 2 to 3 foot void and sliding around on bits of cardboard. Pir secured with wooden battens screwed in to the joists. All gaps filled with expanding foam.
Horrible job to do but the most worthwhile bit of DIY I've ever done. Made a huge difference to the warmth of the floor and removed all draughts from the rooms.
Same here - I ordered and had delivered 56m2 of 75mm Kingspan in the beginning of September (about £1000) - since then I've been spending spare time at weekends crawling around in well ventilated 2 foot void below the ground floorHorrible job to do but the most worthwhile bit of DIY I've ever done. Made a huge difference to the warmth of the floor and removed all draughts from the rooms.
It is without doubt the worst DIY job I have ever done but the difference is night and day in terms of comfort in the ground floor
The floors were always freezing - the drafts round your feet made you feel cold
While I was down there I also re-did all the insulation on the water pipes under there.
I had to buy one more sheet of 25mm to do above the CH pipework runs because I was never going to get 75mm in there
The cold snap we have just had would have normally resulted in running the boiler at 82 Deg and cranking up the thermostat to max just to keep the house comfortable
I'm running 68 deg flow temp and heating on for 1 hr in the morning and 3 hrs in the evening and the rooms heat up quite quickly and are much slower to lose the temp.
or use the proper tape for the job http://www.gapogroup.com/rather than using foam
Just done two rooms plus the hallway.
We removed the floorboards completely and fitted 100mm PIR with gapotape and foil tape over the top.
Glued and screwed 22mm chipboard floors and even without working radiators it is night and day in terms of heat loss in the downstairs.
It’s a big old job and my learning is that I would not want to do it from underneath. I can’t imagine doing anywhere near as good a job from the bottom and it would be horrendous being down there. Our joists were not straight at all and roughly 30-35cm apart so each PIR piece needs to be accurately cut.
I’d also say deciding to bin the old floorboards is also a good choice. They are a pain to get up - I used a pallet breaker which at least made it doable without having to cut the tongues off every board.
Pipework is a pain. I had Ch pipes to be insulated around and used a spoon to cut channels in the 100mm PIR for the pipework and insulated around any pipes that were below the void.
And that the gapotape is b expensive but well worth it in terms of ease of use / allowing rougher cutting of the PIR. I can totally see how they claim a 5x better performance. It is great stuff.
We removed the floorboards completely and fitted 100mm PIR with gapotape and foil tape over the top.
Glued and screwed 22mm chipboard floors and even without working radiators it is night and day in terms of heat loss in the downstairs.
It’s a big old job and my learning is that I would not want to do it from underneath. I can’t imagine doing anywhere near as good a job from the bottom and it would be horrendous being down there. Our joists were not straight at all and roughly 30-35cm apart so each PIR piece needs to be accurately cut.
I’d also say deciding to bin the old floorboards is also a good choice. They are a pain to get up - I used a pallet breaker which at least made it doable without having to cut the tongues off every board.
Pipework is a pain. I had Ch pipes to be insulated around and used a spoon to cut channels in the 100mm PIR for the pipework and insulated around any pipes that were below the void.
And that the gapotape is b expensive but well worth it in terms of ease of use / allowing rougher cutting of the PIR. I can totally see how they claim a 5x better performance. It is great stuff.
fat80b said:
Just done two rooms plus the hallway.
We removed the floorboards completely and fitted 100mm PIR with gapotape and foil tape over the top.
Glued and screwed 22mm chipboard floors and even without working radiators it is night and day in terms of heat loss in the downstairs.
It’s a big old job and my learning is that I would not want to do it from underneath. I can’t imagine doing anywhere near as good a job from the bottom and it would be horrendous being down there. Our joists were not straight at all and roughly 30-35cm apart so each PIR piece needs to be accurately cut.
I’d also say deciding to bin the old floorboards is also a good choice. They are a pain to get up - I used a pallet breaker which at least made it doable without having to cut the tongues off every board.
Pipework is a pain. I had Ch pipes to be insulated around and used a spoon to cut channels in the 100mm PIR for the pipework and insulated around any pipes that were below the void.
And that the gapotape is b expensive but well worth it in terms of ease of use / allowing rougher cutting of the PIR. I can totally see how they claim a 5x better performance. It is great stuff.
I don't think I'd have tried to do your floors from underneath looks like a dirt floor??We removed the floorboards completely and fitted 100mm PIR with gapotape and foil tape over the top.
Glued and screwed 22mm chipboard floors and even without working radiators it is night and day in terms of heat loss in the downstairs.
It’s a big old job and my learning is that I would not want to do it from underneath. I can’t imagine doing anywhere near as good a job from the bottom and it would be horrendous being down there. Our joists were not straight at all and roughly 30-35cm apart so each PIR piece needs to be accurately cut.
I’d also say deciding to bin the old floorboards is also a good choice. They are a pain to get up - I used a pallet breaker which at least made it doable without having to cut the tongues off every board.
Pipework is a pain. I had Ch pipes to be insulated around and used a spoon to cut channels in the 100mm PIR for the pipework and insulated around any pipes that were below the void.
And that the gapotape is b expensive but well worth it in terms of ease of use / allowing rougher cutting of the PIR. I can totally see how they claim a 5x better performance. It is great stuff.
I didn't use gapotape because I found I could get a nice snug fit from underneath
Copied from elesewhere on PH
I did my research on the insulation and compared thicknesses with regard to my joist depth (which varies around the house - original floors are 100mm depth (probably old money size) new extension joists are much deeper
| Thickness | Pack Qty | Cost £ (inc Vat) | m2 | Pack m2 | m2 Req | Packs Req | Total Cost £ | Board Cost Ea £ |
| 20mm | 15 | 405 | 2.88 | 43.2 | 58.64 | 2 | £810 | £27.00 |
| 25mm | 12 | 274.8 | 2.88 | 34.56 | 58.64 | 2 | £550 | £22.90 |
| 30mm | 10 | 336.74 | 2.88 | 28.8 | 58.64 | 2 | £673 | £33.67 |
| 40mm | 8 | 316.04 | 2.88 | 23.04 | 58.64 | 3 | £948 | £39.51 |
| 50mm | 6 | 209.93 | 2.88 | 17.28 | 58.64 | 4 | £840 | £34.99 |
| 60mm | 5 | 247.5 | 2.88 | 14.4 | 58.64 | 4 | £990 | £49.50 |
| 70mm | 4 | 225 | 2.88 | 11.52 | 58.64 | 5 | £1,125 | £56.25 |
| 75mm | 4 | 204.76 | 2.88 | 11.52 | 58.64 | 5 | £1,024 | £51.19 |
| 80mm | 4 | 281.24 | 2.88 | 11.52 | 58.64 | 5 | £1,406 | £70.31 |
| 90mm | 3 | 232.66 | 2.88 | 8.64 | 58.64 | 7 | £1,629 | £77.55 |
| 100mm | 3 | 193.5 | 2.88 | 8.64 | 58.64 | 7 | £1,355 | £64.50 |
Sweet spot for me was the 75mm cheaper cost than both 80mm (£382 more) and 70mm (£101 more) and only fractionallymore (£34 than 60mm)
Other considerations were full pack only orders so thinner sheets would have some excess in the cost
So 75mm was the winner and I ordered 57.6 m2
Well started with 57.6 of 75mm board (20 Boards)
As weeks progressed the huge stack in my garden got smaller
Basically this is where I'm working
This is with insulation cut to size and fitted
Different view
Once finished I can drop the last piece of the jigsaw in and re-instate the flooring
Sorry for picture orientation - bloody iphones
We’ve used this outfit a few times on retrofit projects: https://q-bot.co/
Costs about £3k for a standard size house but they need enough room to drive the buggy around under the floor. There’s little disruption, they cut a hole in the floor boards to get the buggy in then drive it round under the floor spraying insulation.
Costs about £3k for a standard size house but they need enough room to drive the buggy around under the floor. There’s little disruption, they cut a hole in the floor boards to get the buggy in then drive it round under the floor spraying insulation.
RammyMP said:
We’ve used this outfit a few times on retrofit projects: https://q-bot.co/
Costs about £3k for a standard size house but they need enough room to drive the buggy around under the floor. There’s little disruption, they cut a hole in the floor boards to get the buggy in then drive it round under the floor spraying insulation.
Surely Same issues with spray foaming roofs? The moisture is held and rots the joistCosts about £3k for a standard size house but they need enough room to drive the buggy around under the floor. There’s little disruption, they cut a hole in the floor boards to get the buggy in then drive it round under the floor spraying insulation.
princeperch said:
Wasn't expecting this level of detail in response but thank you all!
I can actually get cheap pir 50mm or 75mm from b&q - looks like it's non foil polystyrene type stuff. It's clearly not top notch stuff like celotex but I think it would hopefully do the required job.
Pir and eps have different insulation properties hence the cost difference.I can actually get cheap pir 50mm or 75mm from b&q - looks like it's non foil polystyrene type stuff. It's clearly not top notch stuff like celotex but I think it would hopefully do the required job.
50+mm of pir will perform much better than 50mm of eps.
All the examples in the thread so far use pir.
Just done similar myself, half the downstairs done now and will be fully done in a few weeks time before the plasterer comes
Only difference with me is that instead of foil taping over it all I laid 100/120mm insulation between joists, a non-permeable membrane over it all then 18mm P5 flooring
Edited by kiethton on Monday 12th December 22:09
bsc8180 said:
Pir and eps have different insulation properties hence the cost difference.
50+mm of pir will perform much better than 50mm of eps.
All the examples in the thread so far use pir.
That's interesting- didn't realise.50+mm of pir will perform much better than 50mm of eps.
All the examples in the thread so far use pir.
I cannot spend a fortune on this project as I have spent a 6 figure sum on the house in the last 6 months.
https://www.diy.com/departments/jablite-polystyren...
This is less than half the price of PIR insulation of the same thickness.
princeperch said:
That's interesting- didn't realise.
I cannot spend a fortune on this project as I have spent a 6 figure sum on the house in the last 6 months.
https://www.diy.com/departments/jablite-polystyren...
This is less than half the price of PIR insulation of the same thickness.
Might be half the price, but you will need more to achieve the same insulation value.I cannot spend a fortune on this project as I have spent a 6 figure sum on the house in the last 6 months.
https://www.diy.com/departments/jablite-polystyren...
This is less than half the price of PIR insulation of the same thickness.
As others have said, unless you are pulling up the floor boards already, it is not the most fun to be moving around in a crawl space. That means you won't want to be doing it again in the near future. Yes, the polystyrene might do an OK job but I'd be paying a bit extra now to ensure you can get the materials for a decent job first time and that will pay back more in the long run. If you are paying someone to do it for you, I'd guess labour will be way more than materials anyway.
B'stard Child said:
I don't think I'd have tried to do your floors from underneath looks like a dirt floor??
I didn't use gapotape because I found I could get a nice snug fit from underneath
Copied from elesewhere on PH
I did my research on the insulation and compared thicknesses with regard to my joist depth (which varies around the house - original floors are 100mm depth (probably old money size) new extension joists are much deeper
Sweet spot for me was the 75mm cheaper cost than both 80mm (£382 more) and 70mm (£101 more) and only fractionallymore (£34 than 60mm)
Other considerations were full pack only orders so thinner sheets would have some excess in the cost
So 75mm was the winner and I ordered 57.6 m2
Well started with 57.6 of 75mm board (20 Boards)
As weeks progressed the huge stack in my garden got smaller
Basically this is where I'm working
This is with insulation cut to size and fitted
Different view
Once finished I can drop the last piece of the jigsaw in and re-instate the flooring
Sorry for picture orientation - bloody iphones
thats a better job than all of the construction sites i have been onI didn't use gapotape because I found I could get a nice snug fit from underneath
Copied from elesewhere on PH
I did my research on the insulation and compared thicknesses with regard to my joist depth (which varies around the house - original floors are 100mm depth (probably old money size) new extension joists are much deeper
| Thickness | Pack Qty | Cost £ (inc Vat) | m2 | Pack m2 | m2 Req | Packs Req | Total Cost £ | Board Cost Ea £ |
| 20mm | 15 | 405 | 2.88 | 43.2 | 58.64 | 2 | £810 | £27.00 |
| 25mm | 12 | 274.8 | 2.88 | 34.56 | 58.64 | 2 | £550 | £22.90 |
| 30mm | 10 | 336.74 | 2.88 | 28.8 | 58.64 | 2 | £673 | £33.67 |
| 40mm | 8 | 316.04 | 2.88 | 23.04 | 58.64 | 3 | £948 | £39.51 |
| 50mm | 6 | 209.93 | 2.88 | 17.28 | 58.64 | 4 | £840 | £34.99 |
| 60mm | 5 | 247.5 | 2.88 | 14.4 | 58.64 | 4 | £990 | £49.50 |
| 70mm | 4 | 225 | 2.88 | 11.52 | 58.64 | 5 | £1,125 | £56.25 |
| 75mm | 4 | 204.76 | 2.88 | 11.52 | 58.64 | 5 | £1,024 | £51.19 |
| 80mm | 4 | 281.24 | 2.88 | 11.52 | 58.64 | 5 | £1,406 | £70.31 |
| 90mm | 3 | 232.66 | 2.88 | 8.64 | 58.64 | 7 | £1,629 | £77.55 |
| 100mm | 3 | 193.5 | 2.88 | 8.64 | 58.64 | 7 | £1,355 | £64.50 |
Sweet spot for me was the 75mm cheaper cost than both 80mm (£382 more) and 70mm (£101 more) and only fractionallymore (£34 than 60mm)
Other considerations were full pack only orders so thinner sheets would have some excess in the cost
So 75mm was the winner and I ordered 57.6 m2
Well started with 57.6 of 75mm board (20 Boards)
As weeks progressed the huge stack in my garden got smaller
Basically this is where I'm working
This is with insulation cut to size and fitted
Different view
Once finished I can drop the last piece of the jigsaw in and re-instate the flooring
Sorry for picture orientation - bloody iphones
turbospud said:
thats a better job than all of the construction sites i have been on
But on a construction site they have to do a job from a timeframe/cost perspective I've been doing this since Sept - I had a full nine days at it initially since then at Weekends only
At the mid point from a floor area perspective it was a real low point and a struggle to justify to myself that this was worth the effort...
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