Underfloor insulation?

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sooby, Aug 2, 5:50pm
My 2c:

You want Polyester insulation with the highest R value possible, R1.8 and up is good.

If possible, order insulation that will friction fit between your joists, this means much less cutting to fit.

I used Mammoth R1.8 Multi polyester insulation and can't recommend it enough - made the installation easy and now we are all cosy!
dinx, Aug 2, 6:44pm
The professional installers did use 50mm tape, it was very warm and dry, it was the week before Xmas about 18 months ago, so not cold at all. We didn't think there was any damp under the house but seeing it was funded anyhow we did it, and are surprised how much drier the room nearby is. Much of it stuck down well at the time but started lifting within a couple of weeks. I think the tape was pulled a little tight when applied even but it also contours down the bank.

When we get around to it, DP is going to get 4" wide tape, the same stuff his family use for silage pits, it handles damp conditions well.
jaffa77, Aug 3, 3:41am
I am wondering how the vapour barrier can make a difference if you have underfloor insulation as I would have thought that this would mean there were no gaps for cold air to get through. I have the sitauation described in #17 in that there is a layer of cold air at floor level even though I have expol under floor insulation. I always thought I didn't need a vapour barrier as the ground under my house is dry (or at least I think it is). I would much prefer to live in a new build that is designed for warmth. In the meantime I am stuck with trying to make a 1940's cottage as warm as I can. Has anyone put in the polythene after they did the underfloor insulation? If yes, did it make a noticeable difference? Thanks.
vanderbel, Aug 9, 10:11pm
Thanks for all the opinions on this, I have gone with Earthwool R 1.8 glasswool and polythene (250mu) vapour barrier, the glasswool was much cheaper than the polyester. Have had this installed for a few days and seems much warmer, hopefully the power bill will show a difference as well.
tahnasha, Aug 9, 10:25pm
You still need one, moisture migrates up through the dirt and makes the subfloor damp. Just make sure you don't cover your air vents though.
tweake, Aug 9, 10:47pm
pay to make sure there is drainage holes in the polythene. i had rain blow in the other day and formed a puddle under the house.
a few pegs in the low points makes drainage points.
jaffa77, Aug 24, 3:18am
Just bumping this thread to see if anyone else has an opinion as to whether a vapour barrier would solve my problem of cold air at ground level even though I have expol under floor insulation. There is a definite draft at ground level but I haven't been able to work out where it is coming from. I don't really want to pay the money for a vapour barrier if it is unlikely to solve my problem. Thanks.
tweake, Aug 24, 4:01am
whats it like on a very windy day/night?
what the floor like?
expol won't stop air coming up as air will get around all the little gaps. really hard to get a perfect fit with polystyrene.
tweake, Aug 24, 4:05am
opps i forgot.
vapor barrier won't stop air going through the floor, if thats the problem.
what it will do is reduce the moisture that the air can carry by stopping the water in the ground from evaporating under the house.
jaffa77, Aug 24, 4:17am
It's a 1940's house. I have carpet on the floors and I presume there are probably nice wooden floor boards under that. I installed the expol about 8 years after I moved here and it made a huge difference in terms of the house retaining heat. Before I installed it, the temperature would drop as soon as the fire died down and it would be so cold in the morning that it would take your breath away. The fire still pumps out lots of heat but there is definitely a cold layer at floor level. The house is on a slope so the amount of space under the house varies from about 1.2 meters ath the front to about half a metre at the back. I am surprised that there would be gaps around the expol given that it is a pressure fit and was generally a really tight fit when it was installed. So if a vapour barrier won't help what else should I do given that I already have underfloor insulation. The problem doesn't seem to be affected by wind. Thanks.
jonners2013, Aug 24, 4:37am
I'd be trying to find the source of the draught.

What are the windows like? Original 1940s single glazed timber? If so, there will be draughts coming in the gaps in the window framing which would fall to the floor. Draught could be coming down the chimney, under doors or similar.
jaffa77, Aug 24, 4:59am
The windows are the original ones and are very drafty to the extent that the curtains flutter a bit when it is windy. I am planning on replacing them before next winter. I hadn't thought of them being responsible for the cold air at floor level.
jonners2013, Oct 18, 7:45pm
I certainly find that with my old place, air comes in through the window gaps, hits the curtains and then drops to the floor.