Insulating suspended timber floor

[paulb33]

Any tips on insulating a suspended timber floor? It's single layer Douglas fir 2 inch boards above a 1.5m void/basement. The basement floor is at exterior ground level and is pretty dry. With just rugs on the suspended floor above the basement, it gets pretty cold and draughty during the winter. Is there a recommended breathable insulating material that can be used to line the underside of the floor, perhaps?
Cheers
Paul

 

[paulb33]

The basement floor being at exterior ground level means the void is very draughty (wind whistling through the air bricks and up through the boards.....)

 

[plasticpigeon]

Wow, 1.5 metres, that is good, almost a stand space for some, but definitely a crawl space. I have a foot or maybe 18 inches in my house. There is some spongy rope that goes between floor boards to stop the drafts. I've never used them mind but plan too.

 

[MdB]

Rockwool batts are the obvious answer. If you aren't using the basement and beauty isn't important then you can hold it in place with netting though it should be fitted tight enough that it stays in place by itself - at least for a while.

Don't do anything regarding the through draught though. You want as much air circulation under the floor as you can get. You just want to stop it coming through the floor.

 

[paulb33]

Indeed: we have an access hatch and stairs down into the void. This was a later addition and allowed access to a bomb shelter under the floor. The under floor area was reinforced with structural steel work for this purpose...

 

[paulb33]

you can hold it in place with netting though it should be fitted tight enough that it stays in place by itself - at least for a while.

Thanks. And, might you expand on this? What sort of netting and how does this hold the Rockwool? Keen to get this done as a relatively simple DIY job before winter sets in. (And, one eye on removing double glazing.....)

 

[MdB]

The netting just stops the rockwool from falling out from between the joists, it is nothing clever. Anything will do that will last. Others have used string around nails I believe but a stapler and nylon net will do - in an looking awful non traditional sort of way.

I guess it doesn't really matter how you hold in place as long as it fits well. Any gaps down the sides of the joists will greatly reduce its performance. As there will be differences in air pressure between room and basement, you want to fit it so it can't move around a lot.

(oh and expect to pay x10 for rockwool batts compared to rockwool rolls :-( )

 

[paulb33]

(oh and expect to pay x10 for rockwool batts compared to rockwool rolls :-( )

So I need the rigid stuff, rather than the floppy stuff that'll just drop down? Yes....I'm a noobie.

 

[MdB]

(oh and expect to pay x10 for rockwool batts compared to rockwool rolls :-( )

So I need the rigid stuff, rather than the floppy stuff that'll just drop down? Yes....I'm a noobie.

The rigid stuff will be a lot easier to fit but if cost is an issue, you may be able to use the cheap stuff (which is heavily subsidised which is why it is cheap). Spend £5 on a roll and give it a go - it won't be a nice job though. I strongly suggest a good mask and rubber gloves - I hate rockwool even though the current stuff is better than what you used to get.

 

[Feltwell]

I think you may be getting confused on the "rigid" and "floppy". Stop that sniggering at the back, we can all hear you...... :wink:

You can fit rigid foam boards, such as kingspan or celotex. These offer a very high thermal performance for their thickness, but you have to cut them to fit exactly - any gaps will compromise their performance to a large degree. Some on here have used them, but if you've got the full depth of the joists to use then I wouldn't bother. They are a pain to fit well, I had to use them on my roof as the thickness was critical for that application.

You can fit "batts" of rockwool insulation, which are "floppy" and which is I think what MdB is talking about. These are flat pieces of rockwool, you cut them slightly oversize and squish them into place, so you don't get the problem of any gaps round the edge.

Lastly you can use rolls of ordinary loft insulation, which is exactly what I've done. You can use rockwool, but you can also get insulation made from recycled plastic bottles which is only a bit more expensive and is much nicer stuff to handle - no need for dust masks or keeping your skin covered whilst using it. You can also be very green indeed and use sheeps wool insulation, which is more expensive again but very nice stuff indeed to use.

If you wait a couple of months for the colder weather then the DIY stores will all start having deals on insulation, B&Q sell the recycled plastic stuff much cheaper in winter.

I also have a gap underneath my floors - not as big as yours, about 3' in my case, but enough to crawl about in and insulate from underneath. I stuffed the loft insulation up from underneath - you don't give the age of your house, but in most Victorian houses the floor joists are very handily at the same spacings as ceiling joists, so the loft insulation was already at the right width in my case. I then stapled chicken wire to the bottom of the joists to stop the insulation falling out - except that I ran out of chicken wire, so the last bit hasn't been done, and that was 3 years ago......

However, the insulation actually grips quite well on it's own, so it's still there!

Don't forget to do the narrow bit round the edge of the wall, or you'll get problems with drafts. You don't need to worry about the insulation being breathable, it doesn't have to be. What you want is a good degree of ventilation beneath your insulation, make sure you've got enough air bricks to give a good cross-flow or air and none have been blocked up, buried or covered by later extensions.

 

[AMc]

What Feltwell said - and...
The cost of insulation is usually very quickly recouped in fuel savings so don't panic about getting a roll for £10 instead of £5, Been and Queued had their stuff out on display last week when I visited.
If you have a draughty void under your floor with wind whistling through the boards then I would get on with it now and enjoy warm feet into the autumn.