Attic Vapor Barrier for Insulation in Cold Climates?
Insulation details change about as often as the weather here in New England. One question that seems to come up is whether or not to use a vapor barrier with attic insulation in cold climates? The answer to this question seems to have changed in recent years and may surprise you.
For this article, I’m strictly focusing on vented attics in a cold climate. The details will be different for un-vented attics and/or warm climates.
Air & Water Vapor Movement
The key to this discussion is understanding the movement of air and water vapor. In a perfect world none of the conditioned air (from the house) would enter the attic and none of the unconditioned air (from the attic) would enter the house. In addition, ideally all water vapor would be managed with mechanical systems. Unfortunately we do not live in a perfect world and building construction is FAR from ideal.
Water vapor wants to move from warmer to colder air. That means in the winter time water vapor inside the home wants to move to the colder attic space. In the summer the warm humid air will want to move to the interior of a cooler home.
Ideally ceilings in homes would be perfectly sealed with drywall and good quality latex which would drastically reduce the movement of air and water vapor. However, in reality most ceilings today look like swiss cheese. Ceilings are riddled with holes from lights, mechanical vents and access hatches. Each one of those “penetrations” typically allows a significant amount of air and water vapor to move back and forth from the conditioned to unconditioned spaces.
Attic Vapor Barrier Detail for Cold Climates
Even though we apply vapor barriers to exterior insulated walls in cold climates it’s no longer recommended for ceilings (floor of the attic). Because it’s nearly impossible to seal penetrations in the ceiling, the reality is water vapor will get into the attic. Due to this inevitable fact it’s actually better to omit a vapor barrier between the insulation and drywall of the ceiling below the attic.
The best approach for a vented attic, in a cold climate, is installing a layer of drywall with a good coat of latex paint (the paint creates a semi-permeable vapor barrier). The insulation (fiberglass or cellulose) is then placed on top of the drywall with no vapor barrier above or below.
The latex paint allows the ceiling to breath a bit but still help prevent significant movement of air and water vapor. This really helps prevent trapped moisture between a traditional vapor barrier and the drywall.
Should A Vapor Barrier be Used Over Attic Insulation? The answer is no. Either install un-faced fiberglass, loose fill fiberglass or cellulose directly on top of the drywall.