Insulating suspended timber floors from below

[Stuart Roebuck]

Has anyone got any advice / experience of insulating suspended timber floors from below (e.g. without removing the floor boards)?

The

site contains some useful pointers indicating that this is something that can be done and is sensible to do.

They suggest either a rigid insulation (e.g. extruded polystyrene) supported by nails or battens in the joists, or 'loft insulation' supported by netting stapled into the joists.

However, they caution letting the polystyrene come in contact with electrical cables which can go brittle, so that restricts that suggestion, and they say that any insulation needs to be kept in contact with the floor above, so the idea of suspending loft insulation by netting seems practically impossible.

Hence the question - has anyone actually done this? What are the practical solutions?

Cheers,

Stuart.

[John Bramley]

I did this at the start of the year to stop some of the draughts through the floor.

I didn't fancy using polystyrene because of the problems with electrical cables you mentioned, and also I was slightly worried that moisture in the air might condense on the top of it (don't know if that would really have been a problem). I went for rolls of loft insulation instead because I figured it would be able to 'breathe'.

I suppose that is to stop draughts getting around the sides/ends and circulating above the insulation.

Well it was quite easy with rolls of loft insulation that were slightly wider than the distance between joists. I just unrolled lengths of insulation and pushed it between the joists (parallel to them) and it mostly stayed in place by itself. It is quite thick and springy so pushing it up into place meant that it was in contact with the floorboards. I didn't use netting to hold it up, and instead used string stapled to the sides of the joists under the insulation, in a zigzag fashion.

All in all it was remarkably easy - the worst bit was looking up and getting dust falling in my face, and lying on the rubble in the void. The room seems a bit warmer now, although the vast expanse of glass in the bay window is the biggest loss of heat, but that would cost a lot more to do something about, for now thick curtains make it quite cosy at night.

John.

[Stuart Roebuck]

John,

Thanks for that feedback - very helpful. I'll be trying it myself in a month or two's time!

Stuart.

[G&M]

As well as using Rockwool as insulator between the joists, rather than netting we used Airtec from Screwfix pinned to the underside of the joists. This forms a reasonably airtight seal to keep moisture out and adds some insulation, though we also added a small fan to force air around the underside and prevent any condensation forming on the underside of what is now quite a lot of insulation.

[Michael Mcneil]

I thought polystyrene was a fire hazard.

You can put the rockwool/fibreglass in a cover some manufacturers sell it in plastic bags.

Wear old rags to do the job and sling them after. Then shower in cold water to stop the glass getting into your skin as it will with an hot shower. I hate that stuff too bits!

[Stuart Roebuck]

It sounds like in your situation you were trying to protect against a fairly damp cavity below.

Under normal circumstances, wouldn't sealing under the joists go against the normal advice of maintaining ventilation to prevent rot? If I understand you correctly the fan is only increasing the venting below the seal and doing nothing to vent the joists and boards above.

On a different point - are there any advantages/disadvantages in using Airtec instead of rockwool as the insulator, and is there any notable heat insulation loss in pinning (stapling?) this on?

All thoughts and comments much appreciated,

Stuart.

[David]

In article , Michael Mcneil writes

Its treated now with a fire retardant or at least building grade polystyrenes are.

[RichardS]

I asked the manufacturers of Wickes' standard 50mm polystyrene sheet insulation about this (Wickes couldnt' tell me whether or not it was fire retardent). I think that they said that Jablite was another one of their products, they're based near Hull, have lost their details.

For in-wall insulation, roof-space insulation and non-ground floor insulation yes, it does need to be fire retardant.

For ground floor insulation it doesn't, even under timber suspended floors.

Their technical department said that they had been to fire investigations where their products had been installed, and the insulation was found to be intact and untouched by fire (though as someone pointed out when I posted this before, no surprise there, as heat rises...).

I specifically asked them whether the product was suitable for installation under a suspended wooden ground floor (no cellar) and their answer was a definite "yes". If they'd just been trying to make sure I used their products then they would have recommended that I search out one of their fire retardant products, so I think it was sales-neutral advice.

-- Richard Sampson

email me at richard at olifant d-ot co do-t uk

[G&M]

Yes - damp earth. Water table is roughly level with the earth.

The Airtec has been sealed all round so no damp should rise from below, but any damp within the joists can still rise through the Rockwool and up through the natural holes between the floorboards. At least that's the theory :-)

I used both - had already used normal plastic sheet to support Rockwool under the conservatory but then Screwfix had Airtec on offer so thought I'd give it a try with the Rockwool. Agree that the pinning will cause some heat transfer but shouldn't be great. The Rockwool is the prime insulator anyway.

[Rick Dipper]

I have done both

The loft insulation is a nightmare, the dust gets in/on your body and you feel like **** for weeks.

The solid boards are much better. I jamed mine in the gaps real tight, and they are still there just fine.

Rick