I'm looking for ways of insulating a ground floor (bare boards) from the musty cellar beneath it. Stopping the cold air coming up through the boards is the main priority, but I'm wondering how effective Rockwool slabs between joists would be against the earthy smell. Would I need a vapour barrier as well? In other words I suppose, how do smells travel?
I know I could use Celotex, but it does seem like overkill for this application, and it's a big old cellar.
I also know I could spend a few grand on the cellar, but there is a water table issue here that I suspect will just have to be lived with.
If you don't care about looking at the boards. ou can do a very fine sealing job with hardboard tacked over..after banfging down any loose nails. This makes a decent base for fitted carpets, vinyl or laminate over, and will reduce airflow to almost zero.
If you want to preserve the boards visually, you need to do something underneath as far as airproofing goes: I would use something like foiled plasterboard or thin MDF tacked under the floor joists, and sealed again with something like duct tape, with a load of rockwool in between. How you hold that up I don't know: maybe staple a DPM membrane up first...that would obviate the need for foil backing
There was laminate there until recently, but the earthy smell got through somehow
I was thinking about stapling a dpm between the joists but it would be very labour intensive. Maybe I could stick the Rockwool in non bio degradable bin liners.
Sounds like that cellar needs ventilating? And floor needs insulating and being vapour barriered on it's upper (warm) side?
We ran into such a situation a few days ago. An undamproofed earth floor, no ventilation, lots of dampness and insects. Also some signs of incipient rot! Everything down there was damp and chilly and rank! And the seller didn't seem to think there was a problem!
There was insulation between the joists of the wooden floor above but no adequate vapour barrier on the 'warm' side above that (i.e. the flooring above it). Not only that there was some completely useless plastic sheeting on the bottom of the joists (the wrong side) some of which was hanging down because it had collected a weight of moisture!
Not a good situation and we think we will reject buying that property in view of the potential future problems and costs to correct.
One idea for the OPs situation might be to insulate the floor; use some type of floor covering that is a vapour barrier (warm side).
Then use a 'permeable building paper' type product stapled or held up by thin wooden battens, to the bottom of the studs below. Some brand names for such a product, depending what country you are in are; Tyvar, Tyvec ...... etc. Other types are a tarry sort of paper that have many minute holes which do not allow water in but allow the insulated floor to breathe out any moisture that condenses in the insulated space.
The ventilation is fine, and the joists and boards above are nice and dry, so the conditions down there don't seem to be affecting the structure of the house. As I said, there's a water table issue here because the adjoining houses also have water in their cellars at the same time, although actual floods can apparently be years apart. Hence I don't feel inclined to suggest any action re the floor.
I wonder why moisture had collected there.
No, those floorboards are so good, I'd have trouble persuading them to cover them up
Faced with exactly the same problem we settled on 80mm Kingspan cut to size and held in place as needed with 50mm nails driven partly home.
I considered a vapour barrier to eliminate draughts but without lifting the boards it seemed impossible to fit satisfactorily. The Kingspan fits so tightly that draughts are no longer a problem.
The final point in favour of rigid foam is that come an inevitable spill it's most moisture resistant. Can't see soggy Rockwool working too well.
Did you have the musty smell, and did it do the trick? I'd love to fit 80mm foam but I can't see the budget stretching that far. Do you think it'll be cost effective for a ground floor?
but if theres no damp proof membrane under the floorboards don't you get dampness from condensation along the joists about halfway through the kingspan which could rot the wood?
so should one seal any gaps with foam (to stop moist air getting through and condensing at the dew points) or leave gaps for any condensation to trickle down? ~ ~
We have a full sized cellar which is fairly well ventilated and fortunately not too musty so can't comment on that. As far as making the house *seem* warmer it's worked well.
Cost effective? That's a tricky one. So for thermal conductance Kingspan Insulation have very useful information on their website in the product data sheets - you could calculate the U-value of your existing floor (which depends on perimeter to area ratio) and the U- value of your proposed scheme, and thus estimate the improvement in energy loss based on some assumptions about temp differential and how many days the heating is on.
Heat loss tends to be higher upwards than downwards, so the benefits might not be that great. The improvements in air tightness might be worth it though.
In the end we aimed to meet building regs standard for the renovation of thermal elements - it's about doing the best one can I suppose.
Terry says: Sorry to be unclear; meant to say insulate 'under the floor' using some form of insulation that would stay up underneath and between the joists. Over here 'they' say it should be close up underneath against the bottom of the floor etc. That sometimes can be a problem, at least as a retrofit, cos of pipes wires etc. can be
However depending on what the heat loss might be to that presumably ventilated but unheated space below it might/might not be economic? This house has uninsulated main floors over a mainly unheated, unfinished fully in ground basement that is dry, has a full concrete floor (area is a subdivided 40 by 35 feet) and is used for storage and workshops; the basement maintains 50 to 60 deg F summer and winter: And the main floors are not that chilly although doubtless insulation would reduce heat loss. But then one might have to put some ancillary heat in the basement!
I'm coming to the conclusion that, in this case, insulation is less important than stopping draughts, but finding something that's easy to fit between the joists is difficult. 25mm foil backed foam may be the best compromise.
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