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Venting the Attic

Time for another mechanical "guts" project--ventilating the attic. You may recall that we've written on this topic before, and now we've come back to apply what we learned.

The Problem
While our house did have a ventilation system when we bought it, it was poorly designed. It also wasn't operating properly, for that matter--our friends the raccoons had snuck in through the vent and chewed on the cables to the vent fan and cut the power. At that time we solved the immediate problem (keeping furry creatures out) but waited until now to repair the fan.

With the ceilings removed we've taken the opportunity to fix the fan but also to enhance the overall circulation by adding vents in new locations.

A poorly ventilated attic can cause several problems. In the summer, without proper exhaust heat from the sun can warm an attic to over 150 degrees. This can drive up energy costs--especially since we've installed the central duct work for our central air conditioning up there. In the winter the problem is more about moisture--any moist air that gets in the attic and can't escape will condense, damaging insulation or rotting wood. This can be a problem in summer months, too.

Our Solution
First we dealt with the attic fan. The attic fan is used in the summer when proper ventilation requires a more rapid circulation of air (this article suggests at lest one air change per minute in warm months and one per ten minutes in cold ones).

Fixing the fan was a simple matter of buying a new one. Home Depot carries attic fans in several sizes and we picked up one appropriate for the square footage of our attic--a 750 CFM (cubic feet per minute) model. It has a thermostat switch which turns it on when the temperature exceeds 120 degrees.

With the fan installed we turned to the venting system itself. The current system had three vents on the roof--a large one for exhaust (that had the fan) and two small vents for intake. The problem is that they're all at the same level fairly close to the peak of the roof and in fairly close proximity. There are no vents in the dormer at all. While the entire attic is a continuous space, the vent configuration did little to facilitate true circulation.

Complicating matters, the design of our roof exterior prevents us from using the most common solution--soffit vents. These would provide continuous intake venting under the eve of the roof and are common on retrofits in our neighborhood. However, our home has beautiful rafter tails that we'd have to remove if we used that approach.

Our solution? Eve vents! We found 3" circular vents that we installed between each rafter tail--here's a close-up on one installed on the side of the dormer. The great thing is you can't even see them from the street unless you really look.

Half a dozen of these along the sides of the dormer will provide a new current for cool air to enter and will greatly increase circulation. We'd also like a ridge vent, but will have to wait until we re-roof in a few years. Our roof is very complicated. So we will be relying on the mushroom vents for outake right now.

Before finishing we needed to make sure a continuous path for air flow wasn't blocked when we install insulation. Typically, this calls for rafter trays that are attached to the under side of the roof and preserve a 2" gap between insulation and the roof. However, we've also heard these have a disadvantage--condensation can form on the under surface and run down the edge to create moisture spots. So, we decided against them. The way that the insulation is set into the channel, we already have a gap between the insulation and the roof.

However, we did find one good use for the sample tray we bought. I cut 1' square channels and stapled them to the framing over each circular eve vent to ensure a gap for air to flow by preventing the insulation from bunching up and clogging the vent over time.

So, the additional intake locations and a functioning fan should put us in a better place and insure our roof has a longer lifespan. Soon I'll be continuing the insulation work by finishing the walls and then insulating the ceiling.

CAVEAT: There are a million ways that a roof can be vented and/or insulated. Everyone has strong opinions. We read and listened to experts and experimented and did calculations and Jeannie fretted a lot over this. This is our documentation of how we did ours and your roof/attic will be completely different. To each his own.

Posted by aaron on November 7, 2004 at 3:08 PM (7)

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