Kingspan insulation on loft floor - Vapour barrier???

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We are moving house soon and the new house only has very limited insulation in the loft. Was hoping to board some of the loft over for storage. So to achieve a decent level of insulation without having to raise the floor above the joist height i was thinking of putting in 90-100mm kingspan between the joists and then just board over it. (i think the joists are 100mm, its a 50's bungalow so i could be wrong).

Having read a few bits of advice the main issue im worried about is condensation. It would seem the ideal solution is to have a barrier behind the plasterboard on the ceiling below. As im not going to be replacing the ceilings below this is not an option. So the 2 options i have seen are:
1) Cut vapour barrier to go between the joists and stable it well to the sides of the joist and put insulation between.
2) Lay the barrier over the whole roof floor area (ie go over the joists) and put insulation on top of it.
I have attached a pic so you can see what i mean.
MVIMG_20180313_170609.jpg

Option 1 seems the easiest and i would assume stop the majority of any vapour rising into the loft. I cant imagine that the joists would get cold enough for condensation to form on them and cause rot?
If i went for foil backed kingspan would i even need the vapour barrier as the foil would effectively do this job?

Do i need to worry about any kind of air gap as the ceiling below would be 'warm' so dont see why condensation would form.

Was probably going to do half the roof that way and the other half with roll insulation as its cheaper. I assume some sort of vapour barrier would be needed as well?

I could do the whole lot with roll insulation but by the time i have installed a new sub floor or put loftlegs in to allow for >100mm insulation it would work out about the same price as Kingspan.
 
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Personally I would just fit the insulation with no vspour barrier.

You can always buy 75mm or 100mm silver foil tape and tape over each joist connecting the foil faces to become 1.

Ive found cutting the insulation accurately is the key. Mark eack piece, if the joists arent parallel, mark and cut accordingly. Mark the exact dimension but when you cut it with a saw, cut with a slight taper so the underside is slightly narrower. The insulation will go in nice n snug. Any small corners etc -spray in some foam.
 
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Millions of houses do not have a VB and they are just fine. Besides the foil is precisely that. No air gaps required.
 
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I think you've misunderstood the use a vapour barrier; it's designed to stop moisture getting to places that can't then get rid of it. As you're loft should have ventilation down by the eaves, then a VB isn't necessary.
 
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I think you've misunderstood the use a vapour barrier; it's designed to stop moisture getting to places that can't then get rid of it. As you're loft should have ventilation down by the eaves, then a VB isn't necessary.

Yes that is certainly true, although air tightness is good to aim for.

If you have an open topped cold water tank, theres not much point worrying about vapour leaking through!
 
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I think you've misunderstood the use a vapour barrier; it's designed to stop moisture getting to places that can't then get rid of it. As you're loft should have ventilation down by the eaves, then a VB isn't necessary.

Agreed. I dont know what roof ventilation is actually installed but i figured it was best to try and stop as much moisture rising into the roof anyway.

Thanks for the comments everyone, much appreciated. I'll put the insulation and and guess i'll have to wait until next autumn/winter to see if there is a moisture problem up there :)
 
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