PIR underfloor insulation

21 Jul 2010
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United Kingdom
I am planning to squeeze under suspended timber floors tomorrow to insulate with PIR boards (celotex). What I am not sure about is whether I should insulate between the joists or under them.

I know kingspan instruction state between joists but there is plenty of advice which states that its better to insulate underneath the joists to avoid condensation on the bottom of the joists e.g.


Any thoughts on which is the right advice?
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Thanks. Any particular reason thats the better way to go?
There is nowhere for condensation to form with this method .............

provided the underfloor void is ventilated! Which it should be whether you have insulation or not. ;)
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Well after all that, the crawlspace is a lot smaller from inside than when looking in from outside. I can crawl in but after dragging myself 4 meters into it, I just need to get out again. I've insulated the easy half today but not sure how I'll ever do the other half without some strong tranquilisers :eek:).

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