Timber floor insulation from below

[rangy]

Hi guys
I have a 1960s bungalow with a 4ft void from the floorboards and joists to the floor beneath it.
I'm wanting to put PIR boards between the joists and seal them with requisite insulation tape.
Couple of questions:-
I was going to use 50mm boards, is this sufficient?
Would I push boards firmly up to floorboards or leave an air gap( I'm thinking leave no air gap?)
Do I need any kind of vapour barrier? (it's bone dry under the hse as are the floorboards and joists)
Anything else I need to consider, much appreciated, Ian

 

[Wayners]

I think skill builder covered this on YouTube

 

[Koolpc]

Good info here too:

 

[rangy]

Thanks for the replies guys, appreciated.
Would 50mm suffice( just to be clear I'm not lifting floorboards, I'm accessing from below, many thanks

 

[Notch7]

Thanks for the replies guys, appreciated.
Would 50mm suffice( just to be clear I'm not lifting floorboards, I'm accessing from below, many thanks

I would push up tight to underside of floorboard.

It’s not ideal as really you want a vapour barrier above the insulation.


50mm in between floor joists is probably not enough to satisfy building reg u values for a floor, however I would say 50mm or 70mm would be the sweet spot between best thermal performance and diminishing returns.

If it was me, I’d cut the boards a bit slack, screw a few short bits of batten in place to hold them tight, them use gun grade foam to go around the gaps.

PIR insulation won’t go in if it’s cut too tight - it will just start breaking, so you will need to cut quite accurately, you might find joist spacing are quite tapered

 

[rangy]

Interesting, I like what you've said about diminishing returns as an extra 20mm costs about another £200 for my project and I don't want to put extra in if the impact is not noticeable in terms of warmth in the rooms.
Also, does the silver foil backing on the boards not form some kind of vapour barrier?
Thanks

 

[freddiemercurystwin]

yes

 

[Notch7]

Also, does the silver foil backing on the boards not form some kind of vapour barrier?
Thanks

yes, but vapour barriers need to be continuous, yours will be broken at every joist

 

[Deluks]

There will be a lot of accurate measuring to do in a tight space.
If working from underneath, you might find quilt or slab to be easier to work with, you can cut it slightly oversize to fit, not that easy with rigid insulation. It's also cheaper per sq/m.

 

[aveatry]

I did mine from underneath and used insulation slab but I did not push all the way up I left a small gap and the slab is held inplace by its own friction and I stapled wire across the bottom of the joists to fully hold it. I did it years ago before PIR was the big thing it is now.
It was a horrible job in about 2 foot high space access via a small section of floorboards cut to make a hole, we had sanded and varnished floor boards at the time and did not want to take them all up.

 

[jono_h]

I did pretty much the same as AV. I filled my 7” joists with loft insulation, then stapled breathable membrane underneath to hold it in place.

I’ve just done a large vaulted ceiling with PIR and that was tricky enough to get fitting tightly without the access restrictions.

Edit - How about a hybrid? Fill the joists with rockwool insulation and then 50mm PIR attached underneath the joists to hold it in place?

 

[aveatry]

Edit - How about a hybrid? Fill the joists with rockwool insulation and then 50mm PIR attached underneath the joists to hold it in place?

Now that is a good idea, saves the need to try and spry foam in any gaps in the PIR - even with a foam gun imagine laying on your back underneath with foam dripping in your face.
Or from the top a couple of nails sticking out - lay the best you can cut it PIR on them and then rock wool on top of that. Im thinking its all well and good using spray foam in any gaps but that is only getting you a cm thickness of insulation in the gaps and not even sure of spray foams insulation properties whereas you would be getting 50mm of insulation with rock wool and no draughts