Insulating floor joists - quick question

[0Floz0]

Hi,

Had a look round but couldn't find an answer specific to my query. Going with rockwool insulation for a suspended timber floor. I've got a load of house wrap that I could use to hold the insulation between the joists.

However, is this the best route or would I be better off going with netting? I can sell the remainder of the roll for £60 or so as there is still loads left but wondering if i'd need to slit it to enhance that breathability a bit.

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

If netting is best, what would you recommend?

Assume 100mm is plenty?

Thanks

 

[BlueLoo]

A very old, cold and draughty solution.

Better to follow best practice (See your building regs).

 

[0Floz0]

A very old, cold and draughty solution.

Better to follow best practice (See your building regs).

Yeah but PIR is such a pain. Happy to go to 150mm insulation as thats what regs say. Plus its far more expensive and time consuming doing PIR imo. Happy to be proven wrong..

 

[Ivor Windybottom]

If you're laying it over the joists then the pockets won't be rectangular, they'll be some sort of U-shape, but with the sides pulling inwards away from the edges of the joists so you may as well not bother.

PIR is vastly superior. As with most things, doing it properly takes a bit longer.

 

[BlueLoo]

Yeah but PIR is such a pain. Happy to go to 150mm insulation as thats what regs say. Plus its far more expensive and time consuming doing PIR imo. Happy to be proven wrong..

I followed the advice of my company architect when i converted a room doing this.
Probably my worst DIY mistake.

Should have used rigid boards. The floor is freezing.

 

[0Floz0]

I followed the advice of my company architect when i converted a room doing this.
Probably my worst DIY mistake.

Should have used rigid boards. The floor is freezing.

For context, its more the cost thats an issue for me. I'm refurbishing the house to sell on and replacing the laminate as its knackered. So figured I'd help the next owners out with insulating the floor because its freezing now. 150mm should have the same performance as 70mm PIR shouldn't it?

If you're laying it over the joists then the pockets won't be rectangular, they'll be some sort of U-shape, but with the sides pulling inwards away from the edges of the joists so you may as well not bother.

PIR is vastly superior. As with most things, doing it properly takes a bit longer.

Fair point, hadn't considered the U shape issue. Might try and staple it underneath then. Doesn't PIR cause some issues with moisture and breathability?

 

[BlueLoo]

I used chicken wire....

 

[0Floz0]

I used chicken wire....

Yeah, was tempted by that but get the impression its a bit unwieldly? I guess once its on you know it isn't going anywhere. What fixing did you use?

 

[BlueLoo]

Yeah, was tempted by that but get the impression its a bit unwieldly? I guess once its on you know it isn't going anywhere. What fixing did you use?

Staple gun.

 

[StephenStephen]

I used plastic netting fixed tight between the bottom of the joists, then loft insulation between joists, then a breather membrane on the joists, over the whole floor.
It's worked really well, (though I suspect the draft proofing of the membrane has had as much if not more more effect than the insulation)

 

[BlueLoo]

I used plastic netting fixed tight between the bottom of the joists, then loft insulation between joists, then a breather membrane on the joists, over the whole floor.
It's worked really well, (though I suspect the draft proofing of the membrane has had as much if not more more effect than the insulation)

Good idea on the membrane.
My floor was then laid with tongue and groove flooring.
Since then, it's been lino'd.

It is not the draught that makes it cold but the heat loss because rockwool is **** and thermal bridging etc.
The floor is fine and the ventilation under it seems ok

Were to do it again, i think i'd concrete the void.

 

[0Floz0]

I used plastic netting fixed tight between the bottom of the joists, then loft insulation between joists, then a breather membrane on the joists, over the whole floor.
It's worked really well, (though I suspect the draft proofing of the membrane has had as much if not more more effect than the insulation)

What did you use for the membrane? I've got some tyvek house wrap so thinking thats probably a good solution.

Good idea on the membrane.
My floor was then laid with tongue and groove flooring.
Since then, it's been lino'd.

It is not the draught that makes it cold but the heat loss because rockwool is **** and thermal bridging etc.
The floor is fine and the ventilation under it seems ok

Were to do it again, i think i'd concrete the void.

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

 

[BlueLoo]

What did you use for the membrane? I've got some tyvek house wrap so thinking thats probably a good solution.


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

My house is sat on large concrete slab so i'm sure it's ok.

 

[Lower]

I followed the advice of my company architect when i converted a room doing this.
Probably my worst DIY mistake.

Should have used rigid boards. The floor is freezing.

I had our lounge floor insulated with PIR last year. The floor is still freezing. I think the draft proofing was worth it, but the insulation has made zero difference to the warmth of the room.

 

[0Floz0]

I had our lounge floor insulated with PIR last year. The floor is still freezing. I think the draft proofing was worth it, but the insulation has made zero difference to the warmth of the room.

That’s annoying. You would hope you’d see some benefit. To be honest I’m going to be using sonic gold underlay for the laminate which I will tape at the seams so that’ll stop the draughts meaning the membrane is probably excessive in that situation.