Insulation questions for suspended floor

TonyS82 · 13 Jul 2020

Hi all

I am having to replace the floorboards in our front bedroom due to woodworm so will be taking the opportunity to also add some insulation inbetween the joists as its the coldest room in the house.
I am planning on using 100mm thick mineral or sheeps wool on netting to fill between the 4" joists and then 22mm P5 chipboard flooring, mainly to keep the costs down. The joints will be glued so I am told no vapour layer is required.
This room has three uninsulated external solid brick walls and it has been prone to condensation/mould in the past during the cold weather, possibly because the vents were partially blocked which I have since taken care off.

So I have a few of questions:

Will adding insulation cause any issues with regards to condensation?

Can this insulation be pushed up against the exposed brickwork between the outermost joists and wall?

Is it ok to push the insulation around central heating pipes running along the joists?

Thanks

Tony

JP_ · 13 Jul 2020

I've been doing this, and insulating my walls.

I put 50mm insulation board on the lower part of the joist (held in place by supporting walls, and tile battens under the joists), with 50mm rockwool on top. The boards include vapour barrier.

I also put the same boards on the walls (fixed with everbuild pinkgrip foam), with plasterboard then stuck on too, with a few mechanical fixings (just in case).

I have also been cleaning out the floor dust as this holds moisture, contains mould and fungal spores etc. And improving lagging on radiator pipes.

Lovely and warm. If you don't insulate the walls you will still get condensation damp and mould.

Some photos - this was room 3. Have almost finished the bathroom (just need to tank and tile the shower), and now also on room 4!

Vapour barrier on insulation boards under the floor and on wall prevent living moisture getting to cold spaces and causing more damp and mould problems. I also cleaned out all air bricks and added new ones (here one I made last weekend):

Did first room 2 1/2 years ago - all rooms had black mould on walls and rotten joists and/or woodworm.

Touch wood, all OK so far.

mikeey84 · 13 Jul 2020

Shouldn't the vapour barrier go on the warm side, ie between the floor and the joists?

Having it on the bottom basically means that the joists will get damp and have no way to dry? Or am I missing something

TonyS82 · 13 Jul 2020

mikeey84 said:
Shouldn't the vapour barrier go on the warm side, ie between the floor and the joists?

Having it on the bottom basically means that the joists will get damp and have no way to dry? Or am I missing something

My understanding is that the chipboard flooring acts as a vapour barrier. Happy to be corrected though

mikeey84 · 13 Jul 2020

TonyS82 said:
My understanding is that the chipboard flooring acts as a vapour barrier. Happy to be corrected though

Sorry Tony, my question was meant for Jon.

If you have the board's up, and for the sake of a few quid, I'd put a breather membrane down.

In response to your questions, it should help, yes and yes.

TonyS82 · 13 Jul 2020

jonbey said:
If you don't insulate the walls you will still get condensation damp and mould.
.

I understand that but at the moment funds dont allow it.
Out of interest, did you install the wall insulation below the finished floor level?

I would prefer not to have to take the floor back up to do the wall insulation.

TonyS82 · 13 Jul 2020

mikeey84 said:
Sorry Tony, my question was meant for Jon.

If you have the board's up, and for the sake of a few quid, I'd put a breather membrane down.

In response to your questions, it should help, yes and yes.

Thats great thanks. I assume the membrane should be folded up behind the new skirting and trimmed?

mikeey84 · 13 Jul 2020

TonyS82 said:
Thats great thanks. I assume the membrane should be folded up behind the new skirting and trimmed?

I would think that would be a good idea

JP_ · 13 Jul 2020

TonyS82 said:
I understand that but at the moment funds dont allow it.
Out of interest, did you install the wall insulation below the finished floor level?

I would prefer not to have to take the floor back up to do the wall insulation.

No, wall insulation started at for level. 2400x1200mm boards are about £22, plasterboard £10, can of Pinkgrip £9, roll of silver tape £10. So a 2.4m high by 2.4m long wall will cost about £100

Biggest cost is if you get it skimmed. Oh, skirting board. Paint.

£150 ish

Bargain Bucket · 13 Jul 2020

Why 50mm insulation board and 50mm rockwool? Why not 100mm insulation board with no rockwool?

JP_ · 14 Jul 2020

Bargain Bucket said:
Why 50mm insulation board and 50mm rockwool? Why not 100mm insulation board with no rockwool?

Cosr, and easier if you have anything, stuff that needs to run top half.

oldbutnotdead · 14 Jul 2020

Is your bedroom above a heated room? 4" joists sound a bit skinny (unless the room is tiny). And what is above the bedroom- if its a roof space have you maxed the insulation up there?

TonyS82 · 14 Jul 2020

oldbutnotdead said:
Is your bedroom above a heated room? 4" joists sound a bit skinny (unless the room is tiny).

Its a bungalow with suspended floors throughout. Sleeper walls approx 12" high, 4x2 wall plate and 4" joist. The room is 4mx3.5m

oldbutnotdead · 14 Jul 2020

Ah fair dos. To achieve target U value for a refurb you'd usually need 100mm PIR in the floor but your plan will be easier to get airtight.
Being a bungalow, have you looked at external wall insulation- your surface area is large but access should be easy (no scaff required), that would keep heat in the walls and reduce your condensation problems

TonyS82 · 14 Jul 2020

oldbutnotdead said:
Ah fair dos. To achieve target U value for a refurb you'd usually need 100mm PIR in the floor but your plan will be easier to get airtight.
Being a bungalow, have you looked at external wall insulation- your surface area is large but access should be easy (no scaff required), that would keep heat in the walls and reduce your condensation problems

Unfortunately external insulation is not workable on one of the long walls as the front door frame is installed perpendicular to the wall and does not allow enough of a reveal to install insulation at the required thickness.

My thinking was any insulation is better than none at all

diynot

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Insulation questions for suspended floor

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