Venting a Tricky Old Roof
Retrofitting undershingle intake and ridge vents helped to cool and dry this 120-year-old attic.
Synopsis: While old houses can have enormous appeal, they also can have big problems, including air-circulation issues. In the course of various renovations to an old house in Rhode Island, FHB editorial adviser Mike Guertin upgraded the ventilation to bring the attic into the house’s building envelope. Because Guertin couldn’t install air-intake vents at the eaves, he had to use rooftop air-intake vents. He then installed ridge vents to balance the system and allow it to circulate properly. This article includes information about the way to calculate the proper amount of venting for any roof.
I’ve been working on this old house for a few years now. Among other projects, I retrofitted the house with central air conditioning, which has duct runs in the attic, and updated the bathroom, swapping the original claw-foot tub for a walkin shower. But I’m not the only remodelling contractor who has worked on this house since it was built in the 1880s. The roof has likely been replaced a few times, most recently with asphalt shingles and roofing underlayment. And at some point in the 1970s, the attic was insulated with loose fill.
As soon as I started working on the house, I knew that it might have attic-ventilation issues. After all, when the house was built, it wasn’t insulated and couldn’t have been as tight as it is today. Rather than strain their budget, however, the homeowners agreed to keep a close eye on the attic. After a couple of years, it became clear that the two small gable-end vents weren’t providing enough airflow to keep the attic cool and dry. In the summer, the temperature skyrocketed during the day and didn’t cool down in the evening. In the winter, moisture condensed on cold surfaces.
One approach to fixing these problems is to insulate the underside of the roof with spray foam, which makes the attic semi-conditioned space and brings it into the building envelope. But there are several challenges to this approach, including the high cost of installing spray foam. I decided to use a more cost-effective method and installed a balanced attic-vent system, which uses intake vents (typically installed in the soffits) and exhaust vents (typically installed at the ridge). the system creates steady airflow that helps to keep the attic cooler; carries away excess moisture vapor, reducing the chance for condensation and mold growth; and reduces the likelihood of ice damming.
Shingle-over ridge vents were a no-brainer for the exhaust vents, but choosing the style of intake vents was a bit trickier. The eaves on this house project only 8 in. from the sidewall, and the soffit boards are applied to the underside of the sloping rafter tails, which meant there was not enough room to install intake vents in the soffit. Venting drip edge would have been my next choice. But the eaves are filled with loose-fill insulation. In fact, the insulation blocks the first 2 ft. of the rafter bays. In the end, I opted to use shingle-over intake vents. These specialty vents look like a one-sided ridge vent and can be installed anywhere within the first few feet of the eave. I installed them about 2 1⁄2 ft. up from the eave edge, just above the insulation level.
This roof is roughly 36 ft. long. It took one person one day to retrofit the intake and exhaust vents. The attic is now noticeably cooler in the summer, and it stays dry in the winter.
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