Suspended Ground Floor Insulation Advice Please

eidolon · 9 Jul 2017 at 17:46

I'm in the process of renovating my house and it's now time to start on the ground floor. I'm going to rip up the floorboards and replace with Caberfloor which will have engineered wooden flooring on top.
I saw this thread https://forums.overclockers.co.uk/t...nded-floor-insulation.18771152/#post-30623640 and decided that I might as well insulate under the floor while I've got the opportunity.

Should I use batts, rolls or PIR slabs? And what size? Is damp a problem when you insulate a ground floor?

robj20 · 9 Jul 2017 at 20:04

Pir is too expensive for suspended floor, use the rolls of stuff. Or maybe Polystyrene.

thezappa · 9 Jul 2017 at 20:09

Why are you remove the existing floorboards?
For the insulation you can create a 'hammock' (two battens with fine mesh attached all the way along the length of the joists) and have cellulose insulation blown in, or use rolls of fibreglass. Just cellulose is better for the environment.

ANDARIAL · 9 Jul 2017 at 21:11

For the insulation you can create a 'hammock' (two battens with fine mesh attached all the way along the length of the joists)

This is what we did about 20 years ago to the ground floor of our extension
We used fibreglass rolls

cadmunkey · 9 Jul 2017 at 21:31

PIR is the better solution, mount flush with the bottom of your existing joists and then counterbatten them so they cant slip.

robj20 · 10 Jul 2017 at 07:47

cadmunkey said:
PIR is the better solution, mount flush with the bottom of your existing joists and then counterbatten them so they cant slip.

You will never make your money back using PIR. You also have the issue of vapor barrier. Something breathable is much better, you loose so little heat through a suspended timber floor its almost not worth it at all, most cold comes from draughts so seal these up and you will be fine.

eidolon · 10 Jul 2017 at 09:03

robj20 said:
You will never make your money back using PIR. You also have the issue of vapor barrier. Something breathable is much better, you loose so little heat through a suspended timber floor its almost not worth it at all, most cold comes from draughts so seal these up and you will be fine.

With regards to the vapour barrier, would the underlay for the hardwood flooring cause the same problem? (it's the silver reflective type)

robj20 · 10 Jul 2017 at 09:13

eidolon said:
With regards to the vapour barrier, would the underlay for the hardwood flooring cause the same problem? (it's the silver reflective type)

No only an issue if you block the bottom of the floor boards and joists. It's designed to breath. Above isn't an issue, nice thick underlay good carpet and you have your insulation.

Felix · 10 Jul 2017 at 10:55

I am still yet to do mine. It's a chipboard floor so going to fill with earthwool the high of the joists (150mm) held in place with http://www.screwfix.com/p/barrier-fencing-orange/2080f#product_additional_details_container which will hopefully allow airflow. I'll be working from underneath!

eidolon · 10 Jul 2017 at 18:33

It looks like the fibreglass rolls are going to have the best price/performance ratio. Is there any particular type to go for or will any 150mm stuff do the job?

Derek W · 10 Jul 2017 at 19:34

eidolon said:
It looks like the fibreglass rolls are going to have the best price/performance ratio. Is there any particular type to go for or will any 150mm stuff do the job?

Any type of fibreglass rolls will work fine. The main thing to remember is to leave a minimum of 150 mm from the underside of your insulation / floor joists depending which will hang lower and the solum (ground level below your floor) and ensure that any of your subfloor vents are cleaned out to allow the building to breath and prevent any possible damp forming under your floor.

Felix · 11 Jul 2017 at 09:02

eidolon said:
It looks like the fibreglass rolls are going to have the best price/performance ratio. Is there any particular type to go for or will any 150mm stuff do the job?

Earthwool 44 is what I was looking at.

eidolon · 11 Jul 2017 at 10:00

Pringle W said:
Any type of fibreglass rolls will work fine. The main thing to remember is to leave a minimum of 150 mm from the underside of your insulation / floor joists depending which will hang lower and the solum (ground level below your floor) and ensure that any of your subfloor vents are cleaned out to allow the building to breath and prevent any possible damp forming under your floor.

My joists are 160mm deep (I think) so would this be ok?

http://www.wickes.co.uk/Knauf-170mm-Standard-Top-Up-Loft-Roll-Insulation-6-47m2/p/109450

supported by

http://www.screwfix.com/p/barrier-fencing-orange/2080f#product_additional_details_container

Felix · 11 Jul 2017 at 11:01

For others that have done this, did you use a vapour control layer?

robj20 · 11 Jul 2017 at 11:16

Felix said:
For others that have done this, did you use a vapour control layer?

You don't need any. You want the floor to breath not hold in moisture.

Felix · 11 Jul 2017 at 11:38

robj20 said:
You don't need any. You want the floor to breath not hold in moisture.

OK, from what I have read it should be placed over the joists to still allow air flow but stop moisture from warm rooms above.

robj20 · 11 Jul 2017 at 12:02

I wouldnt use it at all. You want the floor boards to breath as well as the joists. If you really want to use a barrier put it ontop of the floor boards.

Derek W · 11 Jul 2017 at 13:27

Felix said:
OK, from what I have read it should be placed over the joists to still allow air flow but stop moisture from warm rooms above.

Depending on where you are, the stuff you're referring to falls onto the detailing for the Accredited Scottish Details for new houses. There is a vapour control layer / air tightness barrier placed below the floor boards on the joists with the insulation below this. Not sure if the detailing is similar to what the regs require in England / Wales / Elsewhere. But this is aimed at new build houses rather than remedial work.

eidolon · 11 Jul 2017 at 14:39

I'm getting confused now.
My floor will look like this from top to bottom

Engineered hardwood flooring
Underlay (silver faced)
22mm T+G caberfloor
Joist (with 170mm insulation inbetween supported by netting)

Is this ok?

robj20 · 12 Jul 2017 at 06:06

eidolon said:
I'm getting confused now.
My floor will look like this from top to bottom

Engineered hardwood flooring
Underlay (silver faced)
22mm T+G caberfloor
Joist (with 170mm insulation inbetween supported by netting)

Is this ok?

Yes.