Yes another loft insulation question

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The existing insulation is non existent and will be being removed as i move into the loft space shortly to give it an overhaul. The loft space will be used for storage only and the upstairs rooms will all be fitted with downlighters.

Joists are 150+ deeps between upper floor ceiling and loft floor.

I have used quilt before and whilst it is cheap and easy and gives the results i used it in a loft that was not very accessible. This loft whilst only for storage will be accessed quite a bit so i want a sturdy floor in there to walk about on.

The options as i see them are;

1) Install fire caps to all downlighters (60 ish), install quilt, counter batten over joists and board out
2) Install PIR on top of joists and board over (is this acceptable) leaving the depth of the joist as a void for cables and downlights
3) Install PIR between the rafters and board out the floor leaving the depth of the joist as a void for cables and downlight

I guess option 3 is less energy efficient as i will be heating the loft from the rooms below when there is no need.

What do you think is best.

I am having the roof re finished in the next few weeks so thought if i am going with PIR it would be a good time to order the material as it can be dropped into the loft in full sheets rather than having to cut them down to go through the hatch.
 
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Sounds like a LOT of downlighters, maybe see if there are better options to reduce the number? You don't need / want fire caps, you want sealing caps, which are strangely expensive! You are basically wanting to stop any moisture getting into the loft space, unless you have a well ventilated loft.

150mm seems large for the joists, they are usually closer to 100mm. B&Q are good value for insulation when it's on offer (Autumn time), but only do 100mm, 170mm, and 200mm, none of which are great for you.

For a floor, I used this kit https://www.loftzone.co.uk/storefloor/ - it's not the cheapest, but a lot easier and quicker than putting lots of heavy timber up there. They had a sale 2 years ago in January when I got mine, so 25% off everything.
 
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I must admit i am leaning towards the 150mm PIR laid on top of the joists with a timber deck on top of that.

PIR is about twice the price of the mineralwool but will not need battend or system like the loftzone to allow a deck to be installed so probably cost neutral overall.

Has anyone laid PIR accross the loft joists in this way with pics and advise?
 
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I think it's a fair bit more than double the cost! See below rather crude comparison, based on 100mm rockwool and 50mm PIR:

https://www.diy.com/departments/kna...on-roll-l-7-28m-w-1-14m-t-100mm/182146_BQ.prd - £19, covers 8.3 square metres.

https://www.insulationsuperstore.co...-insulation-board-50mm-x-1200mm-x-2400mm.html - £19.60, covers 2.88 square metres

So, PIR in that example is almost 3x times the cost.

Also, do you have a large opening at present to the loft? The cost of smaller PIR boards tends to be more as well. You will also need some way to prevent draught getting under the PIR, maybe some rockwool, or cut pieces of PIR, foamed to everything around it.

You may regret it I fear.... there is a reason people go for Rockwool type stuff in lofts!
 
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Worked it out based on 300mm Earthwool / 150mm PIR, and the PIR came out almost 3x more expensive that way too.

I guess it depends how much loft you have?
 
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Roughly 100m2. Agreed the initial material cost is far greater but with PIR you remove the need to either batten out or use the "loft legs"?
 

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Why not just put 100mm insulation boards between joists, which will leave a gap for cables, and keep space above down lights clear? Then you can board the loft with boards on the joists. Should think that will provide good insulation and be cheaper still. And you could chuck some loft roll on the areas you don't need for storage.
 
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