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Insulating under floorboards - has anyone done it?

(34 Posts)
daisie4 Wed 08-Jun-11 13:43:27

Hi, We'd like to insulate under the floorboards in our sitting room as it tends to get really cold in there but am having real trouble finding out how.

We've got no access from underneath as there's a cellar below with a lathe and plaster ceiling, so we'd have to lift the boards. Is there a way of doing it so we only have to lift every few for example with loose filling? Or is something like Kingspan the way to go, in which case is the only way to get it in by lifting the whole floor and removing skirting? And should we get someone in to do it?

Any help would be much appreciated!

Blu Wed 08-Jun-11 13:46:58

I'll be watching this with interest, because I was looking to do something similiar, and the lifting and replacing of the floor boards and filling the spaces between joists with some sort of insulating granules seemed to become a v expensive job! And I have started exploring putting a floor down on top instead. We have a 'crawl space' under our boards.

7to25 Wed 08-Jun-11 13:56:44

surely it would be better to remove the lath and plaster ceiling which must have a limited life span, and insulate with solid insulating material between the joist, then cover with plasterboard

daisie4 Wed 08-Jun-11 14:00:32

Hi 7to25, That is one option, but it was hoping not to do that as dc use room below as a playroom in the summer. But it may well be the cheapest way to do and we could just cover in plaster board as you suggest and paint white. Have you done this sort of thing?

ThisIsANiceCage Wed 08-Jun-11 14:16:44

As you have an existing ceiling underneath, you could just take up a few boards and shove rockwool in with a broom handle. If you have full-length boards, you might have to cut through them in the middle of the room, so you can get the ends out from under the skirting. But you can probably do this job with only two boards up down the middle of the room. Alternatively, you could try cutting down a strip of ceiling, but I suspect this will go badly with lathe and plaster.

My place is like Blu's, with only a (euphemistically named) crawl space, in which I had to lie stapling garden netting to the bottom of my joists. No fun at all.

Generally the method is slightly problematic, as if there are gaps in the wrong places, and a gap between rockwool and boards, cold air will get above the insulation and circulate freely. But a flexible, stuffable material like rockwool will have fewer problems than a rigid material, which will need spray-foaming at the sides to seal to the joists.

One also has to worry about damp, as one is drastically reducing the ventilation to timbers. But both of these problems are probably vastly reduced by having a ceiling and well-ventilated cellar underneath.

ThisIsANiceCage Wed 08-Jun-11 14:20:06

Actually you've reminded me there's a patch with a definite draft which needs restuffing - fortunately under a screwed board.

daisie4 Wed 08-Jun-11 14:27:09

Oh thank you cage - that sounds encouraging, as we could do this ourselves and it shouldn't cost too much. I don't think there is any ventilation as I can't see any airbricks on the outside of our house - so do we need to worry about this?

Have you done this pushing rockwool method? If so did it make much difference?

ThisIsANiceCage Wed 08-Jun-11 14:42:08

Yes, sorry I didn't actually say: the netting was to support rockwool.

Hard to know exactly what it makes, as I spent the first winter with boards up, no central heating downstairs and holes in the walls. So not really a direct comparison!

But yes, it's got to be better than nothing, and in your situation would be (comparatively) cheap and easy. I've only done it under the room with no carpet, where those boards can breathe upwards. And it's definitely slowed the infiltration of cold air through the board gaps if nothing else. (I may fill the gaps, but actually the plan is to use those as the house's necessary but slow ventilation, rather than seal them up completely and need trickle vents in windows, etc)

teta Wed 08-Jun-11 14:44:51

I have this problem.I have overlaid the old pine floor in my study with engineered wood.This stops all the draughts- but it looks synthetic [and modern rustic] in my house and the original floor boards are much better.Now i am thinking of getting that special tape that you push down between the boards thats meant to stop draughts.There was an article in one of the antique magazines a few months ago comparing the two different types on the market.Finally if you have no vents and a wooden floor it will rot and you need to put some in.We seem to have several enormous vents for each room [an over-built Edwardian house] and consequently have gale-force winds under the floorboards.

conculainey Wed 08-Jun-11 14:45:53

Check for external vents at floor level before installing anything in the joist voids, the vents are there to allow the floor area to breath and prevent rot. If there are any vents do not put anything in the joist area.

ThisIsANiceCage Wed 08-Jun-11 14:55:36

Damp in cellars... Don't know too much about the different dynamics. But if it's an unheated room you do have to think about how much moisture is down, the routes it comes in (inc condensation from warm, moist air in house) and how it will get out.

I think I'd be ventilating the cellar the same as an underfloor in winter, and ventilating lots in summer. But I Am Not A Builder, so by all means ignore me.

daisie4 Wed 08-Jun-11 15:09:52

Right, I've just had a look outside and there are definitely no airbricks giving ventilation to the floor, our house is old, about 1860s. Are you saying I need to put ventilation in?

The cellar has very small windows a the front, which don't fit very well, but also don't open. It doesn't feel damp, and floor/ ceiling of cellar is about 45cm above ground level.

daisie4 Wed 08-Jun-11 15:14:16

Hello teta - how is your floor? My limestone still looks grubby, but I got some more advice from a friends brother who sells stone and is very knowledgeable (I hope!) - he said leave it for 6 months to breath and build up a patina, the marks should disappear by themselves, so I'm waiting, they're not getting worse, so we'll see...

ThisIsANiceCage Wed 08-Jun-11 15:19:45

A damp meter from Amazon might be a good start, to check status at the moment - but if you're in a part of the country that has had barely any rain for the last few months, it could be deceptive right now. [bitter experience emoticon]

And the thing is, as soon as you insulate you'll be dropping the temperature of the cellar and the dynamics will change.

Lofts have the same problem, btw. Which is why when you top up your insulation you need to check your loft ventilation is adequate, because the roof timbers/surface will become that bit colder.

montmartre Wed 08-Jun-11 15:21:22

If one fits a floor over the existing floor, how would that work with ventilation?
How have you found it teta?

Our sitting room is so much colder than the other rooms, but unless I open the windows every other day the damp just comes sucking back in.

ThisIsANiceCage Wed 08-Jun-11 15:31:52

If the window's drafty, and maybe the cellar door's drafty too, and there's a nice warm floor heating the cellar air, I'd guess in the winter air flows up the stairs into the house or straight out the top of the cellar window, replaced by cold air through the bottom of the window.

Which will help keep it dry. God knows what will happen when you insulate. May depend how much you insulate. And whether you fix the window!

ThisIsANiceCage Wed 08-Jun-11 15:33:48

Floor over floor: is this a suspended timber floor, montmartre? Is the ventilation underneath good?

conculainey Wed 08-Jun-11 15:37:33

Op, you could lay a good quality laminate floor running the opposite diection of the existing floor placed on top of thin reflective laminate underlay which would stop draughs and provide a bit of insulation, this method would probably be the most cost effective and require less work as well as allowing you floor to remain at a similar level to the rest of the rooms/hallways.

Blu Wed 08-Jun-11 16:16:04

ThisIsANiceCage - so basically I can just get DP to crawl under the boards and attach rockwool between the joists ?

montmartre Wed 08-Jun-11 16:46:53

Ours is like your's and blu's I'm guessing- Victorian, wooden joists, crawl space <shudder> beneath, boards afixed to joists.

There is an airbrick type vent at the front under the window, but only the one.
No drafts from windows, hence me opening them frequently.

Just cold, cold, cold!

ThisIsANiceCage Wed 08-Jun-11 16:55:43

Yep. With the above caveats about no air circulating between insulation & boards and plenty of ventilation circulating below where the bottoms of the joists stick out.

And checking every year for moisture levels in the timber and rot.

Oh, and not blaming me if this is all wrong and your house falls down.grin (But my bare boards seem fine about 7 years on.)

conculainey Wed 08-Jun-11 16:56:56

Mont, you could install insulation but I would strongly suggest that you treat your joists with a good anti wet/dry rot preservative first. Kingspan insulating foam is a much better bet than rockwool or glass fibre insulation as it can be cut to the exact width of the joist gap and installed from above, fibreglass insulation s very cheap but will simply drop out if not supported from below which could make installation very difficult.

ThisIsANiceCage Wed 08-Jun-11 17:05:25

Ideally you should have a through-draft underneath, montmartre, so airbricks front and back, and holes through walls under the floor.

If you have a supporting dwarf wall for a ground-story floor, you will have to
a) do the room in two halves (more boards up), and
b) do something clever at the dwarf wall, cos if you block the space between dwarf wall and underside of floor with rockwool you may lose your through-draft.

ThisIsANiceCage Wed 08-Jun-11 17:12:02

Yes, rockwool will drop out without garden netting. Even rigid insulation can slip out, so I've seen it advised to nail battons along each joist, to make little shelves for the rigid insulation to sit on.

Agree, rigid insulation much better insulating qualities - but harder to get the fit exact so that there are no drafts coming round the side of the insulation (I guess you could shove the insulation right up against the floorboards to avoid circulation above).

conculainey Wed 08-Jun-11 17:30:36

If you are going to lift the floorbords to install the insulation you can cover the entire exposed area with silver sided laminate floor underlay, that is what I have done and it has stopped all the draughts and had the added bonus in that the floorbords no longer creak and it was very cheap to buy.

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