suspended floor insulation

[TomatoBob]

Afternoon....

Imagine a terraced house with an alleyway in place of a ground floor, and a flat taking up the space of the first floor - so all the living spare is above a passageway that has shared access to the back for the neighbouring properties.

Insulating the suspended floor has been on my list of jobs for about 20yrs, but being in the trade [general all round handyman type of bloke] it's the last thing I want to do in my spare time. But every winter's the same "I must sort this out", then summer comes and, "ah, it wasn't so bad".

Getting to the point - rather than insulating between the joists from above [some of which need replacing anyway, so the floor will be coming up regardless] would there be an issue just covering the floor joists with celotex from underneath, followed by some kind of fire board - rather like a warm flat roof in reverse? It's very accessible and adding extra depth wouldn't be a problem. It would save me messing about cutting the celotex to go between the joists, and I could always put some fluff between them subsequently.

 

[Tigercubrider]

I don't quite understand ?
You seem to be suggesting doing the same thing.

Heat rises so any heat gain will be minimal
Presumably you can't add insulation from underneath - externally ?

 

[TomatoBob]

I don't quite understand ?
Presumably you can't add insulation from underneath - externally ?

Err...yeah sorry for the lack of clarity, but that is what I was asking. I have the option of adding insulation from underneath over then whole span of the floor, as in reassembling a warm flat roof upside down, so to speak.

 

[tomfe]

What the juice?
Yeah put some insulation and do it the easiest way. No point in doing it the hard way.
You can always net the floor and put in rockwool.

 

[jabuzzard]

Heat rises so any heat gain will be minimal

That is complete and total rubbish. Heat is unaffected by gravity. Warm air rises but this is NOT the same thing as heat. As such the heat in an area above an unheated space will firstly conduct through any flooring, and will then radiate into the cold area. The rate of conduction is dependant on the temperature difference and the rate of radiation on the temperature difference and the emissivity of the the surface (aka. silver white not much, black loads).

Put another simpler way if heat only rises then there would be no point in insulating any vertical surface aka. walls because the heat would not be able to travel horizontally through the wall.

 

[JohnD]

you may have cold air blowing around under the floor. Packing the space with mineral wool will stop this movement, and also muffle cold draughts, which are common round the edges.

uninsulated floors don't chill the room much if they are carpeted, but bare floorboards are always cold, and draughty in the gaps between boards.

 

[Tigercubrider]

ok I was oversimplifying as i was banging it out on an iPhone.

what i meant was that insulating the floor would be a minimal gain for the effort

 

[tomfe]

what i meant was that insulating the floor would be a minimal gain for the effort

If you have easy access to the under side, then this is not true, you'll get a lot of gain for minimal effort.

 

[endecotp]

Direction of heat flow does change the effect of insulation, but the effect is small in most cases, and it gets smaller as the amount of insulation increases.

When heat flow is upwards, convection on both sides increases heat flow; when heat flow is downwards it reduces heat flow.
One way of modelling this is by imagining an extra layer of material each side of the actual structure with thermal properties equivalent to the effect of convection. Typical conductance values for this imaginary layer are:

Still air, heat flow upwards: 9.3 W/m2K
Still air, heat flow downwards: 6.1 W/m2K
Moving air (i.e. outside): 17 W/m2K

Some examples (in each case with the layers from inside to outside):

A single-glazed glass skylight:
Still air, heat flow upwards: 9.3 W/m2K
Glass: 4.8 W/m2K
Moving air: 17 W/m2K
Total: 2.67 W/m2K

A single-glazed glass "floorlight":
Still air, heat flow downwards: 6.1 W/m2K
Glass: 4.8 W/m2K
Moving air: 17 W/m2K
Total: 2.32 W/m2K

So the heat flow upwards through glass is 15% greater than downwards.

A 200mm celotex roof:
Still air, heat flow upwards: 9.3 W/m2K
200mm celotex: 0.1 W/m2K
Moving air: 17 W/m2K
Total: 0.0984 W/m2K

A 200mm celotex floor with outside air below:
Still air, heat flow downwards: 6.1 W/m2K
200mm celotex: 0.1 W/m2K
Moving air: 17 W/m2K
Total: 0.0978 W/m2K

So the heat flow upwards through 200mm of celotex is only 0.6% greater than downwards.

Conclusion: it is definitely worth insulating in this case!

 

[TomatoBob]

ok thanks forum members......appreciate the replies, I just wanted to make sure it wasn't a bad thing to do, no so much for insulation purposes [odd as that sounds, because that's my objective!] but for condensation problems. If a warm roof works by bring the ceiling joists into the building so to speak [away from warm air meets cold air], then the same must be true what I'm looking to do here. Now I've just got to get some motivation sorted!

 

[tomfe]

Why are you trying to make your floor into a roof? Are you going to put a waterproof membrane below the floor joists?