Can i use PIR board between loft joists?

[NHW]

Not looking to do a loft conversion or to turn the loft into a habitable room. Just want to put xmas decorations and boxes of clothes up into the loft.

There is currently old wool insulation between the Joists and i would like to put something rigid and solid down so that i can use the loft as storage aswell as having a decent amount of insulation in the roof.

Question is, can i just put PIR insulation boards inbetween the joists (foil facing down) and then tongue & groove loft chipboard over that, so that then the loft will be a fully flat surface along with having some insulation. The depth of the joists are 75mm with 350mm centres (1970s house).

My concern is mainly around moisture and/or air gaps, if an airgap is needed the only thing i can think of is to put a 20mm baton first and then 50mm PIR board and then that still leaves 5mm space so that the insulation doesnt protrude past the height of the joist.

if a moisture barrier is needed also, then would i need to add this on the top side of the insulation, as this would make it 10x easier as opposed to having it inside the joists.

e.g. from top to bottom would look like

sorry for the crappy quality, i just quickly made it in MS Paint lol

any advice would me much appreciated. Thank you

 

[RandomGrinch]

PIR insulation is good stuff, but it is expensive. It has its place where you want to minimise the thickness of the insulation, but maintain performance.
This is why rockwool/fibreglass is still used extensively in lofts; the thickness of the fill doesn't matter hugely, the cost is less, and fitting is generally simpler.

The recommended thickness of loft insulation is currently 300mm of rockwool/fibreglass. This equates to about 150mm of PIR.
I'm afraid your 50mm may be a little underwhelming!

Cold bridging should also be considered; this is why full-fill insulation is recommended between the joists, and a top-up layer is rolled out perpendicularly to the joists.

Moisture barriers aren't generally needed in the loft, as the best place for a barrier would be behind the plasterboard of the ceiling. Although any downlighters cut into the upstairs ceiling can be problematic, especially if in a bathroom!

After I topped-up my insulation, I installed a loft leg system over the top. It was easy to install, is rigid enough - even on slim joists, and has an air gap between insulation and deck to maintain ventilation.

Maybe something to consider?

StoreFloor Raised Loft Boarding | LoftZone UK

Loftzone Storefloor is an award-winning, fully patented loft boarding system designed especially for installing loft boarding over insulation.

www.loftzone.co.uk

 

[NHW]

PIR insulation is good stuff, but it is expensive. It has its place where you want to minimise the thickness of the insulation, but maintain performance.
This is why rockwool/fibreglass is still used extensively in lofts; the thickness of the fill doesn't matter hugely, the cost is less, and fitting is generally simpler.

The recommended thickness of loft insulation is currently 300mm of rockwool/fibreglass. This equates to about 150mm of PIR.
I'm afraid your 50mm may be a little underwhelming!

Cold bridging should also be considered; this is why full-fill insulation is recommended between the joists, and a top-up layer is rolled out perpendicularly to the joists.

Moisture barriers aren't generally needed in the loft, as the best place for a barrier would be behind the plasterboard of the ceiling. Although any downlighters cut into the upstairs ceiling can be problematic, especially if in a bathroom!

After I topped-up my insulation, I installed a loft leg system over the top. It was easy to install, is rigid enough - even on slim joists, and has an air gap between insulation and deck to maintain ventilation.

Maybe something to consider?

StoreFloor Raised Loft Boarding | LoftZone UK

Loftzone Storefloor is an award-winning, fully patented loft boarding system designed especially for installing loft boarding over insulation.

www.loftzone.co.uk

Sorry for the late reply, yeah i used to have loftzone in my previous house but in that house the roof was abit higher so i could still roughly stand up inside, but in this house i am already on knees when going into the loft so i didnt want to raise the height of the floor any further.

and yeah i think 50mm of PIR would be insufficient as it only equates to 100mm of fibre insulation.

I actually have managed to pick up some PIR 50mm boards for really cheap from leftover jobs from people on ebay, i am only about 50% there in terms of the quantity i need but even if i have to pay full price for the other 50% then its still abit more cost effective than expected.

So now i am thinking to cross-batten the existing joists to raise it by roughly 60-70mm. So that this then gives me 2 channels for insulation, for which i will either put fibreglass down first and then the PIR boards (crossing over the battens so that the PIR isnt resting on top of the wool or even touching it) and this would then give me 50mm PIR and 70mm fibre.

Cannot think of which way to do it best, do i have the foil facing down on the PIR boards first as i was told that the foil also then provides a sort of moisture barrier which is facing the warm side (towards the house) and then the wool on top, or do i lay down a vapour barrier down first, over the bare ceiling (which will then also cover the 1st joists) then the wool, cross-batten and then the PIR boards. although ive heard that putting down a vapour barrier takes quite a while and effort to do as it has to be pocketed inside each joist, taped, stapled etc.

so it would essentially look like this from the top to bottom:

18mm OSB BOARD

60-70mm cross batten (with either wool or the PIR inbetween)

70mm original Joists (with either wool or the PIR inbetween)

Vapour barrier

Ceiling plasterboard