Insulating a suspended floor (downstairs)
Discussion
Hi all,
We have a detached 1920s house, cavity walls (not insulated). Downstairs flooring is floor boards on top of joists - with soil about 50cm below. A few air bricks spread around the building.
Downstairs always feels much colder than upstairs. I know this is probably partly explained by hot air rising, but it always feels particularly cold at floor level.
Just wondered if anyone had a similar house, and had insulated under the floors? If so, did it make a big difference? Or, is it probably not really worth doing unless part of a bigger project?
In terms of how to do it - I heard that you put battening along the bottom of the joists, and cut some form of rigid insulation board to sit on top of the battening. Is that right? Are you supposed to leave a gap between the bottom of the floor boards, and the insulation boards? Any potential pitfalls with damp or condensation?
Any tips welcome - even if it's "don't bother"
Steve.
We have a detached 1920s house, cavity walls (not insulated). Downstairs flooring is floor boards on top of joists - with soil about 50cm below. A few air bricks spread around the building.
Downstairs always feels much colder than upstairs. I know this is probably partly explained by hot air rising, but it always feels particularly cold at floor level.
Just wondered if anyone had a similar house, and had insulated under the floors? If so, did it make a big difference? Or, is it probably not really worth doing unless part of a bigger project?
In terms of how to do it - I heard that you put battening along the bottom of the joists, and cut some form of rigid insulation board to sit on top of the battening. Is that right? Are you supposed to leave a gap between the bottom of the floor boards, and the insulation boards? Any potential pitfalls with damp or condensation?
Any tips welcome - even if it's "don't bother"
Steve.
I don’t know the name of the product but can find out, architect specified it.
As you can see the insulation effectively hangs in that breathable material. Allows air to circulate still but keeps the cold air away from the floorboards. Hard to tell how much difference it made as was part of bigger project. HTH !
As you can see the insulation effectively hangs in that breathable material. Allows air to circulate still but keeps the cold air away from the floorboards. Hard to tell how much difference it made as was part of bigger project. HTH !
I'm currently following the approach described in the below link;
https://www.ecologicalbuildingsystems.com/post/bes...
Completed a small office last year and the change was very noticable so I'm now working round the other downstairs rooms.
A far as I have found, the company in the link is good on price for the hemp insulation but other companies are vastly cheaper for the tape, membrane, etc.
I I recall correctly there are at least two threads that cover this - and I think they also concluded that it was well worth the effort.
https://www.ecologicalbuildingsystems.com/post/bes...
Completed a small office last year and the change was very noticable so I'm now working round the other downstairs rooms.
A far as I have found, the company in the link is good on price for the hemp insulation but other companies are vastly cheaper for the tape, membrane, etc.
I I recall correctly there are at least two threads that cover this - and I think they also concluded that it was well worth the effort.
stevesuk said:
Hi all,
We have a detached 1920s house, cavity walls (not insulated). Downstairs flooring is floor boards on top of joists - with soil about 50cm below. A few air bricks spread around the building.
Downstairs always feels much colder than upstairs. I know this is probably partly explained by hot air rising, but it always feels particularly cold at floor level.
Just wondered if anyone had a similar house, and had insulated under the floors? If so, did it make a big difference? Or, is it probably not really worth doing unless part of a bigger project?
In terms of how to do it - I heard that you put battening along the bottom of the joists, and cut some form of rigid insulation board to sit on top of the battening. Is that right? Are you supposed to leave a gap between the bottom of the floor boards, and the insulation boards? Any potential pitfalls with damp or condensation?
Any tips welcome - even if it's "don't bother"
Steve.
Sounds like you have a decent space between the soil and the joist s and if you don't have any problems with condensation/damp now you shouldn't have in the future - as long as you don't block the airbricks in any wayWe have a detached 1920s house, cavity walls (not insulated). Downstairs flooring is floor boards on top of joists - with soil about 50cm below. A few air bricks spread around the building.
Downstairs always feels much colder than upstairs. I know this is probably partly explained by hot air rising, but it always feels particularly cold at floor level.
Just wondered if anyone had a similar house, and had insulated under the floors? If so, did it make a big difference? Or, is it probably not really worth doing unless part of a bigger project?
In terms of how to do it - I heard that you put battening along the bottom of the joists, and cut some form of rigid insulation board to sit on top of the battening. Is that right? Are you supposed to leave a gap between the bottom of the floor boards, and the insulation boards? Any potential pitfalls with damp or condensation?
Any tips welcome - even if it's "don't bother"
Steve.
Putting in noggins/battens on the bottom of the joists is a painful task, you'd be better off using a decent vapour membrane such as
https://insulationmerchant.com/products/pro-clima-...
(Ignore the fact that this is listed for roofing purposes - it will do the same job here).
Staple to the top of the first joist, run the membrane down the side of the & staple fix near the lower edge.
Pull across to the next joist, staple fix to the side of the joist near the bottom, then run it over the top of the joist and down the other side - stapling on the top and lower side each time - repeat across the floor.
You are basically creating a "hammock" between the joists to hold the insulation which is easier and quicker than cutting/fixing battens.
Seal the edges of the membrane to the walls with a suitable air-tight tape.
Put the insulation in - ideally breathable mat rather rather than foam board so it cab butt up to uneven joists unlike foam which might leave gaps after cutting - something like:
https://www.sisalwool.com/collections/all
Before you put the boards back, put an airtight vapour control membrane over the top of the joists & insulation, again sealed to the walls at the edges with a suitable air-tight tape i.e.
https://www.bpcventilation.com/gerband-sd2-control...
Hopefully some food for thought.
ETA - Basically what Chieb showed above
We did exactly what you are asking about,
I used rigid Kingspan insulation and aluminium foil to seal the joints/gaps. It has definitely made a difference. We also sealed under the skirting boards as best we could.
Are you planning to try and relay the old floorboards or put down some new flooring?
If you are putting new then get yourself a pallet breaker it is the perfect tool to lift the old floorboards., one of these.
I used rigid Kingspan insulation and aluminium foil to seal the joints/gaps. It has definitely made a difference. We also sealed under the skirting boards as best we could.
Are you planning to try and relay the old floorboards or put down some new flooring?
If you are putting new then get yourself a pallet breaker it is the perfect tool to lift the old floorboards., one of these.
I've done the insulation blocks underfloor as shown in the post above. I had to cut every strip custom as my joists are not even nearly equally spaced, so it took ages and was quite hard going with one of those flat blade panel saws.
It's important that the blocks are fairly tight and no gaps - I ended up putting a bit of expanding foam in the few remaining gaps so that the whole thing is air tight at the end (wont be passivhaus type air tight, but close enough).
I screwed in steel L brackets for the insulation boards to sit on - usually 4 per piece of board. You can get big packs of those brackets very cheap online.
In the end it did help, although didn't make a huge difference to the room being a bit cold. The bigger difference was eliminating the drafts that crept in at the edges of the room, and the near elimination of spiders in the house as there's now no easy path in from the outside.
Before fitting I went mad on underfloor cabling and a plastic gutter pipe between two parts of the room to be able to pull through more cables later. Once you do this there's no quick lifting floorboards to do stuff. So I fully re-wired all power sockets (and added a few) and hard cabled in various ethernet cables to wall mounted sockets.
It's important that the blocks are fairly tight and no gaps - I ended up putting a bit of expanding foam in the few remaining gaps so that the whole thing is air tight at the end (wont be passivhaus type air tight, but close enough).
I screwed in steel L brackets for the insulation boards to sit on - usually 4 per piece of board. You can get big packs of those brackets very cheap online.
In the end it did help, although didn't make a huge difference to the room being a bit cold. The bigger difference was eliminating the drafts that crept in at the edges of the room, and the near elimination of spiders in the house as there's now no easy path in from the outside.
Before fitting I went mad on underfloor cabling and a plastic gutter pipe between two parts of the room to be able to pull through more cables later. Once you do this there's no quick lifting floorboards to do stuff. So I fully re-wired all power sockets (and added a few) and hard cabled in various ethernet cables to wall mounted sockets.
Check out some of the existing threads on this topic.
e.g.
https://www.pistonheads.com/gassing/topic.asp?h=0&...
Gapotape is the answer to the wonky joists and will give you a much much better performing floor than either no tape / rockwool / expanding foam.
I used 100mm celotex and gapotape and it is night and day in terms of performance. If you are going to the trouble of lifting the floors, then spend the few £ extra and do the job properly (imho).
e.g.
https://www.pistonheads.com/gassing/topic.asp?h=0&...
Gapotape is the answer to the wonky joists and will give you a much much better performing floor than either no tape / rockwool / expanding foam.
I used 100mm celotex and gapotape and it is night and day in terms of performance. If you are going to the trouble of lifting the floors, then spend the few £ extra and do the job properly (imho).
1930's house with a big space underneath, max of about 10-12ft
When we moved in it was freezing downstairs, managed to get some expanded urethane type insulation incased in thin metal. Stripped it and chopped the insulation to fit, as the joists were not straight they were cut to size.
I'm planning on ripping it out and from underneath again so 8 don't have to lift the boards, sheep's wool insulation (it absorbs moisture when cold and damp and releases it when warm)
Then put a breathable membrane over the whole area
When we moved in it was freezing downstairs, managed to get some expanded urethane type insulation incased in thin metal. Stripped it and chopped the insulation to fit, as the joists were not straight they were cut to size.
I'm planning on ripping it out and from underneath again so 8 don't have to lift the boards, sheep's wool insulation (it absorbs moisture when cold and damp and releases it when warm)
Then put a breathable membrane over the whole area
It's crucial to understand the ventilation and moisture control situation.
You may need a lot more airbricks (or some other solution) now your floor void isn't breathing through the house.
One facet of the problem is that the ground can be warmer than the void, and it can also be wet.
But indoors will also be warm and with a high absolute humidity compared to the void.
The timbers can almost need protecting from both sides, yet conversely needing to 'breath'.
There have been some interesting cases of adding insulation doing a lot of damage.
You may need a lot more airbricks (or some other solution) now your floor void isn't breathing through the house.
One facet of the problem is that the ground can be warmer than the void, and it can also be wet.
But indoors will also be warm and with a high absolute humidity compared to the void.
The timbers can almost need protecting from both sides, yet conversely needing to 'breath'.
There have been some interesting cases of adding insulation doing a lot of damage.
We have just (sort of) addressed this issue in our dining room - 5mx5m and suspended floor above void as per OP. We didn’t have carpet, just varnished boards and the room has always been absolutely freezing.
We have just had it boarded, underlayed and carpeted (after 20 years!) - the difference in warmth is very noticeable. We can actually leave the door open now without making the rest of the house cold.
We have just had it boarded, underlayed and carpeted (after 20 years!) - the difference in warmth is very noticeable. We can actually leave the door open now without making the rest of the house cold.
119 said:
Interesting subject that is quite popular by the looks of it, but realistically, how much heat is lost through a suspended floor?
Especially one with good quality underlay and carpet.
In our case the answer was "loads" - We had a 3 foot gap underneath the floor - The old floorboards that had been hacked at, decent underlay, and carpet on it. But the void was essentially at outdoor temperature as the airflow from the airbricks makes it so. It made our hallway and front room freezing in a cold spell. Especially one with good quality underlay and carpet.
The difference having insulated, taped, and fitted chipboard floors is vast - we used to struggle to get the front room warm on a cold day even with the radiators on max, and it cooled down in minutes when they went off. Now, it basically stays at the same temp as the rest of the house if we just leave the doors open (and I still haven't got round to re-fitting the radiator as it's not a problem).
119 said:
Interesting subject that is quite popular by the looks of it, but realistically, how much heat is lost through a suspended floor?
Especially one with good quality underlay and carpet.
Especially one with good quality underlay and carpet.
| Year | Gas kWh |
| 2021 | 17,148 |
| 2022 | 11,476 |
| 2023 | 8,085 |
| 2024 | 8,701 |
Take a guess at when I insulated the entire ground floor with 75mm PIR
119 said:
Interesting subject that is quite popular by the looks of it, but realistically, how much heat is lost through a suspended floor?
Especially one with good quality underlay and carpet.
It depends whether you are talking about heat passing through the boards, or air passing through the gaps!Especially one with good quality underlay and carpet.
And of course, just how cold the void is.
I had a house where lots of heat was lost through an upstairs suspended floor due to crazy amounts of airflow through the floor void and into the wall cavities.
Still air OTOH is a pretty good insulator, as it carpet.
If you want a numerical answer to 'how much?', you'll need to nail down the scenario and parameters.
When you get down to numbers, draughts are a big element of heat loss.
OutInTheShed][b said:
It's crucial to understand the ventilation and moisture control situation.
You may need a lot more airbricks (or some other solution) now your floor void isn't breathing through the house.[/b]
One facet of the problem is that the ground can be warmer than the void, and it can also be wet.
But indoors will also be warm and with a high absolute humidity compared to the void.
The timbers can almost need protecting from both sides, yet conversely needing to 'breath'.
There have been some interesting cases of adding insulation doing a lot of damage.
Pay careful attention to this ^You may need a lot more airbricks (or some other solution) now your floor void isn't breathing through the house.[/b]
One facet of the problem is that the ground can be warmer than the void, and it can also be wet.
But indoors will also be warm and with a high absolute humidity compared to the void.
The timbers can almost need protecting from both sides, yet conversely needing to 'breath'.
There have been some interesting cases of adding insulation doing a lot of damage.
Stopping the air movement that was there can potentially lead to higher humidity levels I had some monitoring before I did the underfloor insulation and the change in humidity levels was quite an eyeopener
My issues were relatively easily resolved (fitting a PIV unit that runs for on a schedule (6 hrs overnight and 6 hrs during the day)
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