space blanket in cellar?

[stuart noble]

My son's house has a large cellar which, due to the local water table, is often wetter than might be expected. With bare boards on the ground floor there is inevitably a musty smell coming up when it rains, so I'm looking for a way to block the gaps between the boards. I've thought about gap seal type products but, given the good access from below, that doesn't seem to be the best option. Celotex between the joists would be ideal but, as heat loss isn't the main issue, I'm wondering about stapling foil/bubblewrap to the underside of the joists. I'm assuming foil tape would be ok for the long edges, but is it likely to sag and blow about over time? Anyone done anything similar?

[dom]

How about some ventilation in that cellar.

[stuart noble]

Plenty of that thanks. You can't fight the water table, which floods every cellar in the road when it's high.

[Tabby]

The only question I'd ask is whether there's a risk of condensation forming on the top of the plastic.

NT

[chris French]

Unless he wants to use the cellar then there seems no need to do such things. As long as the ventilation is sufficient of course

[stuart noble]

AIUI there should be no cold surface on the upper side of the insulation, and the 4" gap between it and the boards will be ventilated from above, but I'm prepared to be corrected about that

[Hugo Nebula]

You would be trapping the warm moist air from the house in the space between the joists. There will be an increased risk of condensation on the upper surface of the bubblewrap (and at the lower surface of the joists) if it doesn't provide sufficient insulation.

Either underdraw the whole floor with Celotex or a multifoil (Tri-Iso) which will ensure that the air trapped there is close to room temperature. Otherwise, mineral fibre insulation (Rockwool) between the joists and, to stop the musty smell, increase the ventilation to the cellar & seal up the gaps between the floorboards.

[Tabby]

A single layer of bubblewrap struggles to qualify as insulation, so I do think condensation would be a risk. Imagine a nice warm summer day, with a damp wet cellar, its asking for condensation from the warm air. If you kept the wrap spaced off the joists and provided little drain holes, that would work, Hugo's suggestion of real insulation sounds better though.

NT

[stuart noble]

I don't want to reduce the height in the cellar so it sounds like I'll have to go between the joists with something. I don't think the budget will stretch to Celotex though

[stuart noble]

Thanks. I'll look into the multifoil products, but at first glance they seem a bit pricey for the ground floor where heat loss isn't the main issue.

[fred]

Absolute junk IMNSHO, stick with the rockwool or celotex in intimate contact with the boards. The gap under the skirting is a common place for draughts and so smells to permeate. I've used a low modulus sealant to seal off mine, an acrylic one if you want to paint over it (not caulk, it's not flexible enough). If they have carpet then you could just lift it, board out with hardboard, duct tape the joints and seal the skirting gap.

[stuart noble]

Rockwool and netting then maybe

[fred]

Yes but watch for stretching/sagging over time, my view is that the insulation should always be held closely to the underside of the boards. My own floor is insulated in fibreglass supported on hardboard secured by battens nailed side-on into the side of the joists. More faff but for a small area it is pretty easy to do. With fibreglass effectively free these days (with subsidised sales in DIY sheds) you could put in double thickness and flush finish in either net or hardboard. Use of the plastic encased stuff should stop fibres drifting into the basement.

[Tabby]

sounds unsuitable, if what you're after is a largely impermeable layer

NT

[Tabby]

rote:

You cant afford to lose half an inch? I dont see how youre going to put insulation between the joists, plus a membrane, and eliminate risk of condensation. It takes very little insulation under it all to prevent condensation.

NT

[Hugo Nebula]

It may (or may not) be what he wants, but it's not the best thing for the floor joists.

[stuart noble]

What puzzles me is that the musty smell occurs when we have rainfall after a dry spell, and not necessarily when there is standing water on the cellar floor. I'm assuming that something organic must be decomposing to produce that smell, or could it be coming from the brickwork itself? As I say, there is no evidence of dampness at ground floor level.