installing underfloor insulation

[Harry Davis]

What's the best type of underfloor insulation to use beneath vile chipboard floorboards, with about 60cm crawlspace?

Harry

[The Natural Philosopher]

celotex/kingspan and fioanm to sel any cracks

[Harry Davis]

The Natural Philosopher wrote in news:jn37f7$udp$ [e-mail]:

Hi - many thanks - this is what I like - a straight answer!

What's the best kind of foam?

Does it matter that I will be going in from the top, i.e. taking the floorboards up and fixing the insulation in?

Also would you recommend putting Celotex or similar on the walls of the crawlspace? The place ain't half draughty! I have seen some people recommend turning the crawlspace into a 'conditioned' space by putting waterproof and vapour-proof sheeting on all of its surfaces except on the 'ceiling' of the crawlspace, and then putting Celotex on its walls.

But perhaps this is an alternative to putting Celotex under the floorboards?

Thanks again! Harry

[geraldthehamster]

news.albasani.net:

I'm assuming that your underfloor space is ventilated to the outside by air bricks, in which case it's very important, for the dispersal of damp, that this ventilation is maintained.

You need to fix the Celotex between the joists. I'd foam where they abut the joists, and also where they abut the wall. It's also OK to fix it to the underside of the joists (support on offcuts then foam, and the foam will hold it there). This way you create a vapour-tight seal between the warm space above, and the ventilated space below.

But it is important to maintain the ventilation below. In my house, the air bricks were level with the joists, so I had to build ducts from Celotex to vent them below the level of the insulation that I was installing.

Regards Richard

[Harry Davis]

geraldthehamster wrote in news: [e-mail]:

I don't quite understand you here. If I put the Celotex beneath the floorboards, should I fill the whole of the space between it and the floorboards with foam? Would it matter that there was no air space above the top layer of foil?

What do you think of some people's recommendation of turning the whole of the crawlspace into a conditioned area by lining it with plastic sheeting, apart from on its top surface, and then lining its walls with Celotex, including on top of the air bricks? The only reason I'm even thinking about this is because it's very draughty down there, and it might possibly be difficult to create the vapour-tight seal.

Also this may be a silly question, but if I did put the Celotex beneath the floorboards, what materials would the ventilation need to ventilate? There'd only be masonry and foil down there (?)

Harry

[geraldthehamster]

No, I didn't mean that. You just need to run a bead of foam where the Celotex abuts the joists and the wall, to fix it there and seal any gaps. It wouldn't matter if there was no air space above, but it's easier to fit the Celotex towards the bottom of the joists - you only need a couple of inches, though I went for overkill with 2 inches below and 2 inches between.

I can't see the point. It's not at all difficult to seal it up with foam, as above. If you did get the odd pinhole gap it wouldn't matter much.

Masonry.

Be careful of wiring, it shouldn't be buried in insulation because it could, theoretically, overheat. There's also a school of thought that expanding foam is bad for its PVC coating. If you have any runs of wiring across the room they are best clipped to the side of joists, above the Celotex. While the floor is up you might think whether you want to run any phone, ethernet, hifi, etc. cables under there as well. I suppose there's no reason why they can't hang under the insulation, if it avoids having to drill too many joists. Some of mine do.

It's well worth doing, by the way, insulating under the floor like this. Eliminates draughts and a certain amount of heat loss (assuming you've already attended to cavity wall insualtion, loft insulation, double-glazed windows and substantial curtains ;-)

Regards Richard