Floor insulation dilemma

[JP_]

OK, so I ordered about 10m square 50mm insulation board from building merchant to insulate between joists. They called to say due to national shortage (discussed in another thread the other week!) that they had none, but would send out double the 25mm instead.

But, rather than keep their word, they just sent out equal value of 25mm insulation, which does not cover the same area! (50mm was 18 boards 0.45x1.2 = 9.72m, 24mm is 4 boards 1.2x2.4=11.52m, but half the thickness, for the same price!).

Advantage of the local building merchant is its free delivery to my door (not roadside) for orders over £30, so I don't want to cause trouble ....

I was thinking, so I've just ordered 2 packs of 50mm Knauf Earthwool Dritherm 35/37 Cavity Insulation instead, that will cover the whole area.

Now, do I:

1. Just put the earthwool in and save the 25mm board for the walls, that need doing too.
2. Put 25mm board on battens and then lay the 50mm on top for better insulation
3. do a mix of 50mm solid using 2 25mm boards, and 50mm earthwool.
4. Give up and watch more Walking Dead.

I am thinking that option 2 might be best as it will save buying netting for the earthwool - I can just put screws into the joists and lay on those. Maybe use expanding foam to seal before laying earthwool on top?

 

[Mr Chibs]

I presume it's a suspended wooden floor?

If you are after an easy job, measure 75mm down from joist top, drive some screws/nails into joists, then cut the board and drop into joists until it rests on screws. Fill gaps with expanding foam.
Then just slap in the 50mm rockwool. Rockwool is half the efficiency of foam board, so it's like putting another 25mm board in.

 

[JP_]

Yes, suspended wood. And "easy job" is def. what I am after! That sounds like my option 2, so I'll def. go with that option. Also, it means I can get started before the earthwool arrives.

Any suggestions on expanding foam and applicator to use? I read that some "burn" insulation, and some expand too much....

maybe one of these?
https://www.amazon.co.uk/Bond-Professional-Foam-Gun-polyurethene/dp/B0052C2234

 

[Mr Chibs]

Get a cheap foam gun, and go to toolstation/screwfix for the foam, not had problems with any of theirs.

Tape any joints for belt and braces job, or gaffer tape if you are doing it cheap

 

[JohnD]

between the joists, mineral wool is fine. There is no convection of heat through a floor, draughts are the main problem, and you can squeeze the wool tightly into the gaps with no precise cutting needed. Pay extra attention round the edges of the room, where draughts blow up under the skirting.

If you have good access underneath, you can tack your rigid foam sheets to the underside of the joists, like a ceiling, and it will save the effort of netting for your mineral wool.

If you haven't bought the wool yet, get one treated with Ecose which prevents it shedding irritant dust and fibres (it's brown). Made by Knauf but also sold as own-brands, but "Ecose" will always be flashed on the wrappr. I would never use yellow glassfibre again.

 

[John D v2.0]

Try calling them, sounds like they were just confused and made a mistake. Could have been the work experience kid on his first day updating your order sheet.