Insulating a suspended floor

[Welderpaul]

I need to insulate a suspended floor, joists are 4" x 2". I'd rather not use solid board, am I ok to use standard 'loft insulation' on a roll?
It's pretty dry under there and has an air flow. I'd need to use some form of fabric mesh to support it.

Thanks

 

[JOEPRO]

I would use PIR. Say 80mm or 100mm thick.

Nail small ledgers on so it stays between joists and tight below boards. I would additionally foam seal the edges of the PIR boards like this:

 

[Onoff]

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Make sure your air bricks are present / clear as in letting airflow under the floor for cross ventilation sounds like that's OK though.

Chicken mesh or breathable felt if you go fluffy stuff / rockwool.

 

[JimmyS]

As long as you have sufficient ventilation you can use whatever insulation you want

 

[brightspark]

best done with kingspan or similar realy. and easier than fitting mesh and stuffing loose on a roll in

 

[Gareth J]

It intuitively sounds wrong but a lot of good arguments have been put forward for filling such floors with polystyrene beads. I'll try to find the discussion I read that just about convinced me it wasn't insanity if of interest.

The risk with filling the gaps with foam boards is that you cool the lower/outer edges of the timbers and make them more prone to condensation. That said, it's what I did recently with a similar floor + membrane for airtightness.

 

[gaz1]

dont use normal insulation roll for that floor area as too low

string it with stainless wire or plastic netting your choice its only there as support

fit knauf 32 37 slab as water proof



dont use kingspan and certainly not near the walls as it dosnt breathe

 

[brightspark]

dont use normal insulation roll for that floor area as too low

string it with stainless wire or plastic netting your choice its only there as support

fit knauf 32 37 slab as water proof

View attachment 471428

dont use kingspan and certainly not near the walls as it dosnt breathe

application dritherm cavity slab masonary cavity walls

 

[gaz1]

application dritherm cavity slab masonary cavity walls

it has many other uses due to being water proof so dosnt hold water like normal insulation does

can be used with wood sheds as it allows the wall to breath and water dosnt hold within the insulation

also can be used in this application for the same reason

 

[Onoff]

I think from memory polystyrene beads touching pvc cable are a no no. You'd have to duct/protect them.

 

[brightspark]

I think from memory polystyrene beads touching pvc cable are a no no. You'd have to duct/protect them.

yep iv seen it it rots the pvc and well sticks to it

 

[gaz1]

I would use PIR. Say 80mm or 100mm thick.

Nail small ledgers on so it stays between joists and tight below boards. I would additionally foam seal the edges of the PIR boards like this:

View attachment 471421

with expanding foam once it has set and it is left uncut it is waterproof but once it is cut into in will absorb water.

best to use a proper foam gun

 

[GRW]

Not same application but I did my campervan floor (quite a popular method) with a combination of a 25mm thick timber lattice, with 25mm close-fitting PIR board in between.
Instead of expanding foam, the edges between timber & PIR is covered with adhesive Foil Tape.
Above this, 12mm Ply floor, and H/D vinyl floor covering (mastic sealed around edges) to finish.

 

[Onoff]

Not same application but I did my campervan floor (quite a popular method) with a combination of a 25mm thick timber lattice, with 25mm close-fitting PIR board in between.
Instead of expanding foam, the edges between timber & PIR is covered with adhesive Foil Tape.
Above this, 12mm Ply floor, and H/D vinyl floor covering (mastic sealed around edges) to finish.
View attachment 471453

Farts will linger I bet.

 

[GRW]

Farts will linger I bet.

Lighter than air though Onoff - here the Extractor Fan is your friend .
(Think of Farts as "recycled cooking smells )

 

[JOEPRO]

with expanding foam once it has set and it is left uncut it is waterproof but once it is cut into in will absorb water.

best to use a proper foam gun

Not sure what your point is Gaz!

I didn't want or require water proof foam when sealing the edges of PIR insulation in a stud wall. I wanted the foam to hold and stop and draughts around the PIR so I get the full performance from it.

 

[gaz1]

my point is to say not to be used in this setting where damp and water will be present

to be advised that cutting expanding foam makes it not waterproof anymore once trimed is fair warning

 

[Misterg]

I need to insulate a suspended floor, joists are 4" x 2". I'd rather not use solid board, am I ok to use standard 'loft insulation' on a roll?
It's pretty dry under there and has an air flow. I'd need to use some form of fabric mesh to support it.

I did the underside of our lounge floor (slightly deeper joists) with rockwool on a roll. I stapled nylon string across the bottom of the joists to keep it up (zig-zagged between joists at about 6" pitch). I was installing it from underneath though, on my back in a crawl space that tapered to ~12" high. One of the most hateful, dirty, jobs that I've done - worse than dropping a lath and plaster ceiling!

It did make the room warmer, but also highlighted the draughts coming up around the edges of the floor and under the skirtings.

I'd also be nervous about using PIR there - I think the wood needs to 'breathe'.

 

[123hotchef]

I would be using solid boards like celotex for that floor cut to a tight fit

 

[123hotchef]

I did the underside of our lounge floor (slightly deeper joists) with rockwool on a roll. I stapled nylon string across the bottom of the joists to keep it up (zig-zagged between joists at about 6" pitch). I was installing it from underneath though, on my back in a crawl space that tapered to ~12" high. One of the most hateful, dirty, jobs that I've done - worse than dropping a lath and plaster ceiling!

It did make the room warmer, but also highlighted the draughts coming up around the edges of the floor and under the skirtings.

I'd also be nervous about using PIR there - I think the wood needs to 'breathe'.

it would still breath through the top and bottom