Cold, drafty floors in an old house
First off just want to thank everyone in advance. I live in an old balloon framed house in Northeast Vermont. There is a full basement under this house, the foundation is a combination of brick, concrete, and stone. Drafty and moist, my plan is to use closed cell spray foam on the walls..eventually. Unfortunately it isn’t in the budget yet. My question is, would it be safe and effective to staple polyethylene sheeting to the bottom of my first floor joists (attempting to stop the drafts) and to fill the bays with blown cellulose or fiberglass insulation? We heat our home with a pellet stove located on the first floor, so the basement is essentially unheated, and while it does house our oil fired water heater and our oil furnace it is otherwise unused 99 % of the time.
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Chris,
I don't recommend that you install a layer of polyethylene on the underside of your joists. It may become a condensing surface under some conditions, and it might trap moisture near your floor joists.
The best way to insulate floor joists above a damp basement is with a continuous layer of foil-faced polyisocyanurate insulation installed on the underside of the joists. The polyiso seams should be taped with a foil tape.
Of course, if your home has log joists, this will be tricky. But if your house has joists from a sawmill, and not too many ducts or pipes in the way, this is the way to go.
You can supplement the R-value of a foam-under with trimmed batts between the joists too, but leave 2-3" of air above the fiber layer to limit the amount of temperature striping on the floor from the thermally bridging joists.
If it's insulated to the level that the basement could drop below freezing without some heating from above you can potentially frost-heave the slab in a damp basement. That would usually require an air leak the size of an open door, and no incidental heat sources such as a boiler or furnace in the basement, but if you're heating 100% with the pellet stove you may be in that boat. It's not clear just how much jacket & stack loss you'll get off the oil-fired water heater. Air-sealing the foundation walls is always a good idea to limit that risk (and the heat loss), even if you won't get around to insulating them until layer.
IRC code min for a floor over an unheated basement or crawl space in climate zone 6 is R30 total, or whatever is sufficient to fill the joist bays fully (R19 minimum.) If you put 1.5" polyiso under the joists and 5.5" deep R21 or R23 batts above the polyiso you'd meet/beat the R30 requirement, and would have sufficient room above the batts to avoid the temperature striping issues, assuming the joists are at least 7" deep.
If it's log joists of non standard spacing that won't easily accommodate 4'x 8' sheets you can install 1 x 4 furring 24' o.c. perpendicular to the joists and mount the foam to the furring.