Why You Need to Vent the Attic
A key benefit of venting the attic is that the approach is the same regardless of how creative your architect got with the roof. Because the roof isn’t in play here, it doesn’t matter how many hips, valleys, dormers, or gables there are. It’s also easier and often less expensive to pile on fiberglass or cellulose insulation at the attic floor to hit target R-values than it is to achieve a comparable R-value in the roof plane. The success of this approach hinges on the ceiling of the top level of the house being absolutely airtight before any insulation is installed. It’s also important to ensure that there isn’t anything in the attic except lots of insulation and air—not the Christmas decorations, not the tuxedo you wore on your wedding day, nothing. Attic space can be used for storage, but only if you build an elevated platform above the insulation. Otherwise, the insulation gets compressed or kicked around, which diminishes its R value. Also, attic-access hatches are notoriously leaky. You can build an airtight entry to the attic, but you should know that the more it is used, the leakier it gets. How do people get this simple approach wrong? They don’t follow the rules. They punch a bunch of holes in the ceiling, they fill the holes with recessed lights that leak air, and they stuff mechanical systems with air handlers and a serpentine array of ductwork in the attic. The air leakage from these holes and systems is a major cause of ice dams in cold climates and a major cause of humidity problems in hot climates. It’s also an unbelievable energy waste no matter where you live. Don’t think you can get away with putting ductwork in an unconditioned attic just because you sealed and insulated it. Duct sealing is faith-based work. You can only hope you’re doing a good-enough job. Even when you’re really diligent about air sealing, you can take a system with 20% leakage and bring it down to maybe 5% leakage, and that’s still not good enough. With regard to recessed lights and other ceiling penetrations, it would be great if we could rely on the builder to air-seal all these areas. Unfortunately, we can’t be sure the builder will air-seal well or even air-seal at all. So we have to take some of the responsibility out of the builder’s hands and think of other options. In a situation where mechanical systems or ductwork has to be in the attic space or when there are lots of penetrations in the ceiling below the attic, it’s best to bring the entire attic area inside the thermal envelope. This way, it’s not as big a deal if the ceiling leaks air or if the ducts are leaky.