Should I add vapour barrier to the ceiling of an old house before blowing in fibreglass?
I have a 1920s wood frame house with asphalt shingle roof above an unfinished attic in Victoria BC (similar climate to US Pacific Northwest, so 4C but with less rain).
I removed the blown in cellulose above the ceiling, which is drywall + lathe and plaster + wood joists, and plan to get a contractor to blow in R50 fiberglass (there’s a rebate for adding significant insulation).
There was no vapour barrier originally, and no mould that I saw. Multiple contractors told me not to bother installing a vapour barrier above the studs + new wiring, but their reasons seemed vague to me. It seems to me the dew point will be within the insulation in winter, and the building being old doesn’t change that.
I have some 10 mil poly, and was thinking of laying it over the joists + wiring + LED pot lights, taping the joins, and blowing in over that. What do you think?
Thanks,
Chris
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Replies
Chris,
What you want to do is as much air-sealing as possible, not add a vapour-barrier. Seal the tops of wall plates, and any penetrations.
Thank you. Just to clarify - you're saying use something like tuck tape and/or expanding foam to seal gaps around the top plates to keep warm air trapped within the walls?
My original concern was that moisture would go through the ceiling materials and condense within the new insulation, leading to mould. If I understand correctly, new buildings have vapour barrier between interior wall materials and insulation to prevent this. If I understand that right, why would an old building not need the same thing?
Chris,
Almost all the moisture that moves though walls and ceilings piggybacks on air-leaks. That is primarily what the poly we still use up here in Canada is designed to stop. So it's mostly an air-barrier - but by custom, builders often still just call it a vapour-barrier.
The painted ceiling already provides you with a vapour-retarder, which will effectively deal with interior moisture moving through the ceiling by diffusion. So yes, sealing air-leaks is what you want to concentrate on. That will do a few things:
- Stop moist interior air making it's way into the attic.
- Reduce energy losses through air-leakage
- Reduce the stack effect - meaning less air will be drawn into the house at the lower levels to replace the air that is being lost.
Makes sense - thanks Malcom!
Chris,
Cheers. I'm just up the coast from you in Shirley.
If you have a vented attic, any moisture that gets up there should go right through the insulation and get exhausted to the outdoors by the attic vents and airflow. You'll have condensation WITHIN the batts if the batts are contained in an assembly. In a typical attic, where you're insulating the floor (which is probably what you're doing here), moisture will move through the insulation and condense on the underside of the roof sheathing, not within the insulation itself.
All that said, I agree completely with Malcolm: do a good job of air sealing the attic floor prior to insulating and you'll have far less risk of any problems occuring down the road. You don't need a vapor barrier here.
Bill
Chris,
Bill makes a good point I missed in my reply. The dew point of air moving though walls or roofs is often in the permeable insulation. That doesn't matter until it reachs what is termed The First Condensing Surface of Interest, which is usually the exterior sheathing, and where it will accumulate. In a vented attic with insulation on the floor, the moisture just continues to move though it.