Heat loss and underfloor ventilation

[Martin Pentreath]

Hi all,

Our living room is the typical sort of thing you find nowadays in a victorian terrace - two rooms knocked through into one. We have the original boards stripped and varnished with a few rugs around. It looks nice (to my simple tastes), but being on the ground floor with plentiful ventilation underneath it's all outrageously wasteful in winter, with lots of heat vanishing in drafts blasting up from between the gaps. The obvious solution is to put fitted carpets in. But I'm wondering a few things.

Is the ventilation underneath the floor so important when there is no floor covering on top? The air circulating around in the room will ventilate the underfloor void through the gaps (in much the same way that the freezing air under the floor is currently ventilating the living room through the same gaps!). So could I block up the vents?

If this logic is flawed, I'm tempted to get onto my back in the wriggle space under the floor with some of the outrageously cheap fibre glass insulation currently available and to pin it to the underside of the joists, but to leave the vents open so a gale could still howl beneath the insulation. Would the permeability of the insulation be able to provide enough ventilation to the underside of the floor without there being major drafts?

[The Natural Philosopher]

Underfloor vents ate there to dry the joist out. from moisture from above

Go under and use CELOTEX twixt the joists and seal every last crack with a sealant gun.

That will stop 99% of the damp coming fro the hot steamy rom.

fibreglass is USELESS in a stiff breeze.

[Tabby]

If you fit insultion you need a VB too, and that must be on the warm side. So you'll need to take the floor up, fit insulation then a VB and re-lay.

NT

[The Natural Philosopher]

Not necessarily. Foil backed celotex is nearly as good, pushed up from underneath.

[Martin Pentreath]

Thanks for the advice. I'm buggered if I'm attempting this in the current arctic conditions, it's a job for next summer. Presumably the foil-backed celotex needs to go foil side upwards (ie foil on the warm side of the insulation)?

[dennis]

IIRC celotex is closed cell so it makes no difference which way it goes as it is impermeable to moisture, but you need to check the data sheet as the foil may be there for other reasons (besides heat radiation effects).

If you have lots of fibre glass available what's to stop you pushing that in the gaps and finishing it with a breathable membrane?

[Martin Bonner]

Can I reiterate previous poster's points about *fill all the gaps*. We had the same construction but decided to lift the floor, fit celotex, and lay a new wooden floor.

The floor was laid, and it didn't feel much warmer.

The builder went round all the edges with expanding foam (before fitting skirting), and it was suddenly all toasty.

In fact, I suspect I could have got at least 3/4 of the insulation effect by:

- lift floor

- fit 8x4 sheets of thin hardboard (or even, just a big polythene sheet)

- relay floor

- expanding foam round the edges

[John Stumbles]

OK but - and I'll admit this isn't my area of hexpertise - what the OP seem to be suggesting is making the sub-floor space (relatively) warm, so you won't get condensation from moisture from above.