Insulating floor joists between two living spaces

[nishnish]

I have the floor up in a bedroom above a kitchen.
I thought of filling the space between the joists with some kind of insulation.
The Victorian joists are about 16-17cm high and 30-36cm apart.
As space blanket is cheap, comes in 37cm width rolls and easy to install, I thought it'd be an economical way to fill the space between joists. Could do the whole room 3.9mx3.2m for 20-30 quid.
Would filling the entire space be fine, or is an air gap preferred?
I certainly don't want to to cause a mould problem.
There should be no condensation problems between two heated floors?
They seem to do 15, 17 & 20cm thick.
Or 15cm thick space blanket be better?
Would squeezing 20cm space blanket in there also provide a tad better sound insulation?
TBH sound insulation is probably more important than thermal, but not that important, so I don't wanna spend that much, hence the space blanket idea.
Thanks for any advice.

 

[Deluks]

For sound insulation the best bet is rockwool slabs, followed closely by rockwool loft roll. You can also get acoustic insulation roll which has a paper backing, this is slightly more expensive though.

You can use space blanket but it would provide better performance if you were to remove all or most of the foil on one side. the 200mm will work.

You will gain additional insulation by adding a second layer of plasterboard (if working from below) or if insulating from above, make sure the floor covering has no gaps. If you have floorboards then laying a sheet material such as chipboard or hardboard over or under the floorboards will help.

If you have kitchen downlighters installed, then these will leak sound, so box over them by making a plasterboard box approx 200mm x200mmx150mm deep. any other light fittings inside the ceiling should also be kept clear of insulation.

 

[nishnish]

For sound insulation the best bet is rockwool slabs, followed closely by rockwool loft roll. You can also get acoustic insulation roll which has a paper backing, this is slightly more expensive though.

Interesting. Pricey tho:

A rough calc using 100mm + 50mm rockwool RWA45 slabs comes to about 150quid for that room. That's just based on area. Never mind all the waste I'd have fitting 600mm wide slabs into 30-37cm joist spaces.

You can use space blanket but it would provide better performance if you were to remove all or most of the foil on one side. the 200mm will work.

So, I take it you mean that completely filling the void is fine between heated floors?

You will gain additional insulation by adding a second layer of plasterboard (if working from below) or if insulating from above, make sure the floor covering has no gaps. If you have floorboards then laying a sheet material such as chipboard or hardboard over or under the floorboards will help.

The room above is being completely redone with a double layer PB ceiling.
I was going with carpets and the thickest underlay, Tredaire Dreamwalk 11mm, to provide draft exclusion plus a bit of acoustic and thermal insulation. Always found floorboards noisy and cold.
Would over-boarding the floorboards be really necessary considering the thick underlay?
I'm concerned that access would be a pain with over-boarded floorboards.

If you have kitchen downlighters installed, then these will leak sound, so box over them by making a plasterboard box approx 200mm x200mmx150mm deep. any other light fittings inside the ceiling should also be kept clear of insulation.

Simple centre light on the kitchen below.
Now I'm thinking, why not do the underfloor properly now too, rockwool slabs

 

[ursamajor]

'Owt is better than n'owt. Whatever you go for, make sure than any cables are kept above the layer of insulation (ideally clipped to the joists but it's not always possible).

I'm not a great one for flooboards under carpets either, as I tend to find that even with underlay their outline eventually shows through. However, hardboard cut into 4' squares soaked thoroughly overnight (kept outside or in plastic) and then nailed down with a cheap electric nail gun works wonders and adds to draft and sound insulation.

 

[Deluks]

If you have the double PB and underlay, then might as well go with space blanket if it's the cheapest. The hardboard trick works well, up to you really, depends on the state of the floor and how uneven it is.

Doing the groundfloor is a good idea if not too much disruption, again use what's cheapest as there are no sound issues to address, bog standard loft roll suspended on garden netting works well.
Will save on bills somewhat.

 

[nishnish]

However, hardboard cut into 4' squares soaked thoroughly overnight (kept outside or in plastic) and then nailed down with a cheap electric nail gun works wonders and adds to draft and sound insulation.

Oh, I have an aversion to nails. Much prefer using screws. I like to be able to take stuff apart
What's the reason you soak them for?

 

[Deluks]

Staples better, from a powered staple gun. Soaking is to help the boards fit the contour of the floor without curling up, plus it will shrink back slightly after the soaking so the joins won't 'erupt' like tectonic plates!

You can't really lift and relay hardboard, if it comes up you replace with a new bit.

 

[1john]

In the past I have laid hardboard with 'traps' which unscrew that bridge floorboards with services beneath them. A permanent marker saying whats where can be useful should a problem ever arise.

 

[ajrobb]

As you have the floor up and have mentioned draughts, take a look where joists go into external cavity walls. Gaps around the joists can be a major source of draughts and should be filled.

 

[nishnish]

As you have the floor up and have mentioned draughts, take a look where joists go into external cavity walls. Gaps around the joists can be a major source of draughts and should be filled.

Solid brick walls. Circa 1890. So I guess not such a likely source of draughts?
There are gaps around the joists though. Just fill them with more of the same lime mortar?

 

[nishnish]

In the past I have laid hardboard with 'traps' which unscrew that bridge floorboards with services beneath them. A permanent marker saying whats where can be useful should a problem ever arise.

Nice tip.
I have hardboard in some rooms screwed down. Have removed and replaced same boards several times.
The marked services is an effective idea. I'll remember it for future. Thanks.

 

[ajrobb]

I'm not sure what happens between ceiling and floorboards in Victorian properties when you add insulation. My concern is that insulation will make the external wall and joist holes colder and possible traps for condensation. This would worry me if there were open holes (downlighters) in the kitchen ceiling allowing lots of moisture into the space. It might be worth a couple of coats of eggshell paint on the kitchen ceiling as a vapour barrier.

Remember, hidden cables and pipes should be at least 50mm above the ceiling and below the floor. Otherwise they should be protected (with steel).