Vapor seal for insulating old attic
I’m looking into insulating the attic of a very old house that has not been lived in through the winter in several decades, but will be this winter. We unfortunately only have oil heat blown into the house through venting, which is not only inefficient but very expensive. Hoping to insulate the attic to cut heating costs but I was told that this could create vapor that can potential dampen and rot the old dry wood. Is there a technique or certain kind of insulation I can use? Do I need lay a sheet of plastic underneath insulation? I was originally planning on simply using R-38 fiberglass on top of the attic floor, though the eves on the side of the attic are open and I could potentially use blow-in insulation underneath as well. Thanks in advance for the help!
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James,
There is no need for a vapor retarder or a plastic vapor barrier in your attic. The two main sources of attic moisture are roof leaks and air leaks through a leaky ceiling. (When warm, humid indoor air leaks into an attic, the escaping air carries moisture.) Vapor diffusion is NEVER responsible for attic moisture problems.
Air leaks are a serious issue, however, so it is vital that you or your contractor do air sealing work before you install a speck of insulation. For more information, see Air Sealing an Attic.
After your air sealing work is complete, you can install the insulation of your choice. Most experts recommend that the best attic insulation, by far, is cellulose.
+1 on the air-sealing and blown cellulose! Cellulose is very air-retardent, and can adsorb & buffer a good deal of moisture without damage or losing it's insulating function, which is protective of the structural wood. But you still need to air-seal- the buffering capacity isn't infinite, and air leaks can move a LOT of moisture.
In parts of Canada some vapor retardency at the ceiling is required by code even in vented attics, but in the US that isn't usually necessary. Air sealing is by far the most critical aspect of keeping moisture from the conditioned space from accumulating in the attic over a cold winter.
Oil heat is expensive, and oil heat delivered via leaky ducts is even more expensive due to the low system efficiency. Depending on the climate and the overall floor layout, offsetting some or all of that oil use with ductless mini-split heat pumps is usually VERY cost effective, paying for itself in reduced oil use in 3-5 years. The operational cost of heating with mini-splits varies with climate (which affects raw efficiency), and your actual electricity rates. (Diesel powered island grids with 50 cent/kwh electricity won't be a net-winner, but even 25 cent electricity in a mini-spit would be a significant savings over oil.)