Insulating floor joists - quick question

[Deluks]

Your original plan is fine. Just make sure the membrane is taught between the joists, and staple as low down as you can to ensure you have the full depth of insulation at the edges.

 

[catlad]

James I cant see anything wrong either with your proposal.

 

[catlad]

In my opinion floors are the last thing to insulate in a house because you get so little gain.

 

[hllns]

Other than not feeling like you're walking on a slab of ice in some properties?

 

[robinbanks]

Plan is to lay it over the joists and creating pockets between each joist.

That'll use twice as much as fixing it to the underside, clamping it between a screwed batten and the joist

If netting is best, what would you recommend?

That stuff that stops debris/people falling off scaffold. Can be free too if you can find some scaffolders taking down and wazzing large amounts of it in a skip

Assume 100mm is plenty?

Oof, that's a bit skinny. If that's how deep the joists are I'd use PIR between and over

PIR is vastly superior

Hmm.. depends on the quality of fit and future shrinkage. Wool self expands to fill gaps and is a lot faster to fit well than PIR, to the extent that if a bad PIR install ends up with a draught blowing round the boards, it can become useless and the wool, though it's only hlf-to-two-thirds as performant thermally, does a better job

In my opinion floors are the last thing to insulate in a house because you get so little gain.

10%+ heat loss through a floor isn't what I'd call "little", but I get what you're saying - if there was a certain amount of cash in the for-insulation pot I would spend it on the floor last

the insulation has made zero difference to the warmth of the room

Heating the room is what makes a difference to the warmth of the room. The insulation just means it leaks away at a lower rate. If it's cold on that thermostat setting before, it'll be just as cold after, but the boiler will fire up less and the gas bill will be lower. If you want the floor to feel warmer, make it the heat emitter for the room; insulate it and put UFH in

then a breather membrane on the joists

Not a breather membrane; a vapour control (impermeable) membrane. You want the breathable bit to be on the world side and the impervious bit to be on the room side

Wouldn't the concrete just act as a massive heat sink and take ages to get warm?

It works both ways; heat stored in a large sink evens out temperature swings in the house when the temperature falls and the warmed block of concrete starts emitting it instead

It is not the draught that makes it cold but the heat loss because rockwool is **** and thermal bridging

Draughts are one of the most effective things at making a human feel cold even if the ambient temperature is acceptable. As well as carrying away your paid-for heat and cooling building materials theyre exposed to, if your skin is exposed to a draught it strips away the layer of warmed air around your skin, making you feel colder than you otherwise would; google "windchill"

Rockwool isn't ****; it has a lambda value between 0.032 and 0.044 on PIR's ~0.022 meaning it flows between 1.5 and 2 times as much heat as PIR does, but rockwool is a better gap filler around irregular or solid objects and when installed under slight compression will always try to expand and keep gaps closed. PIR is rigid and predominantly incompressible and needs careful foaming in. It is also known to shrink over time, which means that your well fitted PIR now may one day be letting draughts in. On a cost basis you can usually buy enough of either wool or PIR for a given amount of money that you can achieve the same insulated result if you have the space for the thicker amount of wool required

Overall the difference between PIR and wool isn't as stark as you think, especially when you consider that wool will generally do better to provide ongoing draught resistance in awkward spots. While it's commendable that the building regulations have been tightening/improving insulation requirements for a number of years, they have long remained particularly awful with regards to draughts/air tightness. When one looks at passivhaus systems, where no space heating system is required at all, the spec that they're built to with regards to insulation is about the same as what the regs now demand, but the draughtproofing that passivhaus requires has long been around 10 to 15 times better than the regs.

All the insulation in the world can't solve the problem of cold air blowing through a gap in it, and cold air permeation needs to be kept on the world side of insulation, rather than allowed through to the room side/the back side of the plasterboard.

 

[Ivor Windybottom]

I've got concrete here, with 50mm of celotex on top and 22mm chipboard on top. It's a floating floor, no joists, just held up by foam and held down by gravity. The celotex makes a vast difference in comparison to the other rooms that are just concrete. No numb toes, and the room stays warmer for longer.

I frequently deal with boxes of stock for my business, which are stapled. I find that in just a few years in a dry environment the staples turn brown and crusty. Under a floor, relying on standard staples could be risky. Eventually the whole lot will fall off the underside of the joists.

 

[hllns]

I've got concrete here, with 50mm of celotex on top and 22mm chipboard on top. It's a floating floor, no joists, just held up by foam and held down by gravity. The celotex makes a vast difference in comparison to the other rooms that are just concrete. No numb toes, and the room stays warmer for longer.

I frequently deal with boxes of stock for my business, which are stapled. I find that in just a few years in a dry environment the staples turn brown and crusty. Under a floor, relying on standard staples could be risky. Eventually the whole lot will fall off the underside of the joists.

Tacwise do staples in stainless but even the normal ones are galvanized

 

[0Floz0]

That'll use twice as much as fixing it to the underside, clamping it between a screwed batten and the joist

That stuff that stops debris/people falling off scaffold. Can be free too if you can find some scaffolders taking down and wazzing large amounts of it in a skip

Oof, that's a bit skinny. If that's how deep the joists are I'd use PIR between and over


Hmm.. depends on the quality of fit and future shrinkage. Wool self expands to fill gaps and is a lot faster to fit well than PIR, to the extent that if a bad PIR install ends up with a draught blowing round the boards, it can become useless and the wool, though it's only hlf-to-two-thirds as performant thermally, does a better job

10%+ heat loss through a floor isn't what I'd call "little", but I get what you're saying - if there was a certain amount of cash in the for-insulation pot I would spend it on the floor last


Heating the room is what makes a difference to the warmth of the room. The insulation just means it leaks away at a lower rate. If it's cold on that thermostat setting before, it'll be just as cold after, but the boiler will fire up less and the gas bill will be lower. If you want the floor to feel warmer, make it the heat emitter for the room; insulate it and put UFH in

Not a breather membrane; a vapour control (impermeable) membrane. You want the breathable bit to be on the world side and the impervious bit to be on the room side


It works both ways; heat stored in a large sink evens out temperature swings in the house when the temperature falls and the warmed block of concrete starts emitting it instead


Draughts are one of the most effective things at making a human feel cold even if the ambient temperature is acceptable. As well as carrying away your paid-for heat and cooling building materials theyre exposed to, if your skin is exposed to a draught it strips away the layer of warmed air around your skin, making you feel colder than you otherwise would; google "windchill"

Rockwool isn't ****; it has a lambda value between 0.032 and 0.044 on PIR's ~0.022 meaning it flows between 1.5 and 2 times as much heat as PIR does, but rockwool is a better gap filler around irregular or solid objects and when installed under slight compression will always try to expand and keep gaps closed. PIR is rigid and predominantly incompressible and needs careful foaming in. It is also known to shrink over time, which means that your well fitted PIR now may one day be letting draughts in. On a cost basis you can usually buy enough of either wool or PIR for a given amount of money that you can achieve the same insulated result if you have the space for the thicker amount of wool required

Overall the difference between PIR and wool isn't as stark as you think, especially when you consider that wool will generally do better to provide ongoing draught resistance in awkward spots. While it's commendable that the building regulations have been tightening/improving insulation requirements for a number of years, they have long remained particularly awful with regards to draughts/air tightness. When one looks at passivhaus systems, where no space heating system is required at all, the spec that they're built to with regards to insulation is about the same as what the regs now demand, but the draughtproofing that passivhaus requires has long been around 10 to 15 times better than the regs.

All the insulation in the world can't solve the problem of cold air blowing through a gap in it, and cold air permeation needs to be kept on the world side of insulation, rather than allowed through to the room side/the back side of the plasterboard.

Thanks Robin. Really appreciate the detailed response.

The underlay I'll use for the laminate (sonic gold) is essentially a vapour barrier so I'm hoping that itself will eliminate a lot of draughts.

I'll try for the under the joist fixing if possible but wary about crawl space being pretty small.

Happy to go to 150mm but i'm sure the joists are only 100mm.

Is there a best route of fitment, i.e. do you always go perpendicular to the joists or if that's a short run do you go parallel? Guessing you just use some 2x1 treated batten for it?

Given the vapour barrier. Is it worth running the permeable membrane on the bottom of the joist or is it better to just use the netting? I can sell the rest of the membrane for £60 so if I don't need it then I will.

Cheers

 

[Ivor Windybottom]

Tacwise do staples in stainless but even the normal ones are galvanized

Good to know. I wonder how many actually use them?

I wouldn't have thought the zinc plating on a staple is going to last long under a floor. Stainless should work though.