Best insulation for suspended floor

[trevorbayliss]

On my extension i am doing a suspended floor. It is on the ground floor and i know you need insulation in there.

What is best, rigid insulation sheets or the loft type insulation suspended between a net?

I am thinking about going for the soft insulation as i thought it would be better at sealing any small gaps between the joists. Also what amount do you need between the joists on a floor ?

I am doing the work so the most cost effective method in terms of labour doesn't need to be factored in. I want the best solution to draughts and heat loss really.

 

[tomfe]

I would really suggest you get this designed it may cost a bit more to start with but it will save you so much in your time and problems you'll face due to lack of experience.

 

[trevorbayliss]

Well i thought it was fill the joist with soft insulation supported by netting. Lay a vapour control layer over the top of the joist and then finish with floorboard. From doing more reading it seems the soft insulation is better at creating a draught seal. I am also wanting to look into the sheep wool stuff. Anyone got any experience of this ?

 

[tomfe]

Depends if your floor joists are deep enough to get the correct u values.

 

[trevorbayliss]

Well 150mm seems to be ok, but a little deeper would be better. I am digging out so if a have to go a bit deeper it is fine. Using a grabber truck and he is so much cheaper than a skip. Done 32T's already the other weeks from my drive alone and the old patio and conservatory bases. Still go foundations to go and the overall footprint height.

 

[tomfe]

Yeah just go with rockwool then it's cheaper.
Remember you need 150mm at least below your joists then 100mm for concrete then 100mm for hardcore.
I'd try by best at using a ground slab if you can.

 

[trevorbayliss]

i thought a 50mm over blind with concrete was fine for a suspended floor. No hardcore needed.

 

[Mr Chibs]

Cracking radio Trevor!

I think I also read, that foam board insulation is twice the spec of rockwool, so 50mm foam = 100mm rock.
One advantage of foil backed foam board is, it has vapour barrier already, can be cut and tightly packed into joists.

I will be doing my front room (old house) with this... was thinking of 100mm but might just use 50mm, as they use no thicker in fridges/freezers.
There is only around 250-300mm from floor joist bottom to ground.

 

[geraldthehamster]

You can fill any gaps between your insulation boards the joists, using expanding foam. Make sure that nothing you do compromises your under-floor ventilation

 

[tomfe]

i thought a 50mm over blind with concrete was fine for a suspended floor. No hardcore needed.

Your over site should be on hardcore, you don't really want to lay concrete straight ontop of earth. A 100mm slab will allow you to build your sleeper walls ontop it is also required by some BCs.
50mm is pointless as it will just crack, you might get a way with 75mm but you'd still want some hardcore under it.

 

[trevorbayliss]

Thanks for that link tomfe.

I will speak to BC but i have mentioned a 50mm screed of concrete to keep the weeds off. Doing 200mm of hc and concrete seems a little excessive and you may as well go to full concrete floor. I decided to go ventilated because i was going to have to put back into the dug area about 12T of material. Just seemed stupid to me when i can carry some joists fromm the delivery truck and build a floor that way. Much easier on my body. Yes i will need 1-2T ballast and mix my own cement, but still not that bad.

Mr Chibs - the rigid stuff is fine to use and i would use the expanding foam BUT longer term more gaps are supposed to open up with ridgid fill and so that is why i am looking at soft fill.

 

[geraldthehamster]

longer term more gaps are supposed to open up with rigid fill and so that is why I am looking at soft fill.

Hmm, supposed by whom? I suppose there is a tiny amount of movement in the floor joists, but I can't see that being much of an issue. You won't notice if it isn't 100% air tight (which it won't be by either method)

 

[^woody^]

As this is new build, I suspect that the joists will be spaced evenly, and so a celotex board can be fitted snugly on to either laths fixed on the bottom of the joist sides, or on the under side, and there will be no gaps. You can even fix some on the underside of the joists to take take of any thermal bridging if that is an issue.