Insulation under suspended ground floor

1 Mar 2008
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United Kingdom
I've been trying to decide what the best cost/benefit insulation is for insulating under the ground floor of my c1900 house. Suspended wood, joist 170mm deep, width between joists 230-370mm, 2' crawl space, but only small(2'sqr) access.

Glass mineral wool seemed the best option as its cheaper and thickness is not an issue. However a couple of doubts...

1 Does 'encapsulated Space Blanket' have an advantage over standard roll (other than comfort while installing). Since the void is very well ventilated does mineral wool lose value if against moving air (as opposed to a nice 'still' loft). Or would using garden permeable membrane to hold up the insulation have similar effect. I have read a post that suggested insulating over the joist as well (but reckon this might risk damp)

2 Is Branded mineral wool really better than say, Wickes own brand (as they don't give a U,R value)..

3 I have read that you should not place mineral wool against the end brickwork (due to moisture). Is this true and what is best to use if it is?

4 I might decide to put Hot Water UFH in one of the rooms (so would be fitting from above for that room). Is solid insulation preferred in these situations, or is solid only used when wool depth required becomes impossible.

5 I also read that solid blown insulation properties degrade more quickly than mineral wool ( due to gas escaping).

Cost is a factor, but I only want to do it once and well!!?

All comments welcome.
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Go for rigid stuff, the limited working space will make the job a bitch if you use fluffy. 100mm polystyrene between joists will do the trick, and it's fairly cheap.

Floors do lose heat but nowhere near as much as walls/roof/windows. So no need to go too mental.
i have just done a section of the ground floor in my 1900 house.
the floor was freezing before and a lot of draughts through gaps between the floorboards. i lifted all the floorboards and replaced them. (i have used the old ones in my shed). it does make a big difference. i haven't even turned on the UFH yet.

for UFH solid insulation is used to fix the pipes to. expanded polystyrene is not used unless under a concrete slab i think due to fire regs. also it is less dense to get a good fixing for the pipe clips. someone correct me if i'm wrong. i got my kingspan phenolic insulation from seconds and co.

i wouldn't fancy fixing solid board insulation from below. there is a bit of messing around cutting it to the right size, and cutting it again, filling the gaps with foam... i wouldn't fancy it at all but i have a feeling that it could me easier with mineral wool as you could squash it between the joists easier. you will need to isolate it within a draught proof membrane and yes - keep it away from any possible moisture sources like external solid walls or sleeper walls. you could use a bit of kingspan in those areas.

any other advice you want from the experience of my project just ask.
Thanks for the feedback. For information, what type of UFH did you put in. (insul. +screed, metal plates, foilcoated insul.) The many systems make cost/benefit had to calculate.
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Thanks for the feedback. For information, what type of UFH did you put in. (insul. +screed, metal plates, foilcoated insul.) The many systems make cost/benefit had to calculate.

120mm kingspan kooltherm with pipes at approx 100mm centres (4 between each joist) set in 25-30mm screed. i was told that foilcoated insulation only helps when used facing an unventilated air gap.

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