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Insulation under the house (crawl space)

(12 Posts)
whiskyowl Thu 26-Oct-17 09:27:24

Has anyone insulated their underfloor area with spray foam?

My house is at the top of a hill on a slope. At the front, the crawl space is small - probably about 45cm. At the back, however, you can stand up in it. My floors used to be really, really cold. I put down some high quality insulation under a wood floor, and this really improved matters a lot. However, I'm guessing that further insulation might also help?

I'm just wondering if there are any downsides to using open cell spray foam underneath the house?

wowfudge Thu 26-Oct-17 10:01:30

I'm no expert, but you need the airbricks to be clear and air circulation under the floors I believe.

johnd2 Thu 26-Oct-17 10:06:27

It could work well, just make sure there is no issue with high ground levels or damp anywhere in the floor first.
At the moment it might be evaporating quickly enough to prevent rot, but as soon as you change the balance of air flow it could be getting more and more damp.
Insulation will make a massive difference to your feet though!

whiskyowl Thu 26-Oct-17 10:10:15

@johnd2 My concern is that there's not that much free space under the front of the house - from memory, only about 45cm. This is where we had problems with air circulation before.

I might ask them to do a thinner coating at the front, and go all guns blazing at the back. I know insulation is only as good as its weakest point, but a bit of stuff under there has to be better than nothing.

I'm hesitating a bit because I won't be able to get back under the floor very easily at all once the extension is done. This is literally the only chance I have to insulate that floor without ripping up the engineered wood above it, which I had hoped would last some years.

johnd2 Thu 26-Oct-17 11:32:35

45 cm is plenty as long as you have enough air bricks. Insulation is as good as the overall average, not the weakest point. However condensation and mould are most likely at the weakest point. Floor is the lowest risk surface and the level wouldn't be too bad.

whiskyowl Thu 26-Oct-17 11:34:36

Thanks John, that's really reassuring. I think I'm going to go for it!

PigletJohn Thu 26-Oct-17 13:51:05

spray foam is sometimes used in the inside of roods, and is deeply hated, as it traps damp causing rot, and is no much extra work if you need to replace damaged tiles that roofers usually refuse unless you rip it all off and lay anew.

Floors don't suffer much from rain, but they do get leaks from plumbing.

What attracted you to spray foam? It is much more usual to add mineral wool quilt, or sometimes slab insulation.

Floors don't lose much heat by conduction, but bare boards allow cold draughts through the cracks, and cold air under the skirting boards is very common. Mineral wool is good for this, as it can be stuffed into irregular gaps to block draughts, as well as laying it between the joists. The rigid slabs are harder to fit perfectly. Blocking draughts will do more good than actual insulation.

It is easiest if the floor is coming up, but feasible if you have adequate crawl space or a cellar.

PigletJohn Thu 26-Oct-17 13:53:04

"inside of rooFs, "

"and is So much extra work "

whiskyowl Thu 26-Oct-17 14:13:49

pigletjohn - The spray foam is mainly for speed. It can be done in a couple of hours without holding up the build, whereas other kinds of insulation require hanging or cutting that is more time-consuming. It's really an afterthought - I'm having an extension done, they can get into the crawl space at the moment without extra work.

A small part of the existing floor (a 2 x 2 m utility) will have wet underfloor heating fitted into it (UFH is going into all of the new area, which is much larger) so also thinking a bit about heat escape underneath that part (though it is small). Not sure if that makes a difference?

We did have terrible draughts under the floor a few years ago, but these were really mitigated when we put a decent underlay under an engineered wood floor!

PigletJohn Thu 26-Oct-17 14:21:48

the section that is having UFH will (must) include insulation, but this is included in the usual installation.

Heat losses through a floor that is carpeted or otherwise draught-proof are quite small. It's really only bare boards that are bad.

OOI, what is the price of spraying a floor with foam? Do they protect electrical cables?

whiskyowl Thu 26-Oct-17 14:31:42

I've never seen an UFH installation, so that is super-useful to know pigletjohn.

The cost of doing the whole lower ground floor under the joists was £850. I'm not sure about the electrical cables - I suspect they don't.

It sounds like it's really not worth it! Thanks so much for helping me to reach that decision, I really appreciate it!

The reason I thought about it is that I just had some spray foam fitted as part of a timber-frame extension. It is impressive in that context: a couple of days of cutting bits of Kingspan/Celotex to the right size that they stay between roof joists/wall uprights is reduced to 3 hours of spraying. But in that case, the spray is being added before any electrical/plumbing work!! As you probably know, there are 2 types of the foam - breathable open cell and non-breathable closed cell. Out of interest, I got a bit of the open cell and dunked it in some water, and it was a like a sponge - you could wring it out! - so I can see how it could trap moisture in a loft cavity. The bloke doing it showed me a bit of the closed-cell stuff and it's hard as nails. He'd been installing it in a boat, where I imagine it would be ideal - it's very light and waterproof.

johnd2 Thu 26-Oct-17 23:34:55

Yes heat loss itself through a draught proof timber floor is small, but your feet will be surprisingly cold because of it. Half the reason heat loss is small is because all the cold air hangs around at floor level once the heat has been lost, so if you insulate better, you'll be much more comfortable.

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