Jonathan Sinclair’s home, a 225-year-old timber frame in New Hampshire, is getting a facelift. Sinclair is residing the building with fiber-cement lap siding, and he plans to install the siding over a vented rainscreen—a thoroughly modern detail.
“My question comes from building an adequate soffit vent that will flow air,” Sinclair writes in this recent Q&A post.
As he explains, the original builder included 2×6 soffits with a 2 1/2-inch-wide gap between the building and the edge of the soffit. That gap was then covered with molding. Sinclair plans to incorporate this gap into the top of his rainscreen so that air will have a continuous path from the bottom of the house to the attic. Ridgevent 20 (manufactured by Air Vent Inc.) at the top and bottom of the rainscreen will keep out the bugs.
“Does anyone have an opinion if this will allow adequate air flow to act as a soffit vent or will I need to drill holes up through the soffit beyond the crown molding through the 2×6 to give adequate soffit venting,” Sinclair asks. The house has no ridge vents and but it does have gable-end vents at the peak.
GBA editor Brian Pontolilo replies that it’s generally fine to vent a rainscreen into an attic, although Sinclair’s building inspector may have other ideas. If the inspector is okay with it, there is still the question of whether it provides an adequate amount of intake area for effective roof venting.
The International Residential Code (IRC) does include some guidance on how to calculate what’s called the “net-free ventilating area” for a roof, which you can read more about here. Pontolilo also suggests an earlier GBA article on the topic (see the first entry in the “Related Content” sidebar below as well as this article…