Roof Ventilation Installation and Attic Insulation in Boston, MA

Intake and exhaust have to balance

Ventilation is measured in net free area: the open square inches a vent delivers after screen and louvers. The residential code sets 1 square foot per 150 square feet of attic floor, and 1 per 300 when 40 to 50 percent sits high on the roof and the rest at the eave. Confirm your number with the building official.

• Intake should meet or beat exhaust, never the reverse
• Exhaust that outruns intake pulls heated air from your ceiling
• A vent's stamped size is not its net free area
• Every rafter bay needs a baffle above the insulation

The mistakes we find in Boston attics

Exhaust with no intake

Somebody adds a ridge vent, the attic still runs hot, so they add a powered fan. Now the roof takes makeup air from the house through every ceiling gap. A strong fan can backdraft a water heater.

Two exhaust types on one attic

Gable louvers plus a ridge vent short-circuits the system. The ridge draws from the nearest opening, the gable, and the soffits stop moving air. Pick one path and close the other.

A ridge cap over an uncut ridge

We pull ridge vent off roofs every year and find solid sheathing under it. The slot was cut too narrow, stopped short, or never cut. It vents nothing.

Boston building stock vents differently

Story-and-a-half capes

The biggest ice dam generator we see. Bedrooms sit under the deck, insulation packs the rafter bay solid, and the eave cannot breathe. Post-war capes and ranches across West Roxbury, Saugus and Lynnfield are full of them. The fix is a vent chute in every bay, or closed-cell foam instead.

Three-deckers and flat center decks

The form matured around 1890 in Dorchester, and survivors are sheathed in boards, not plywood. Hipped slopes leave almost no ridge and the center deck none. Those get edge and deck vents over a sealed ceiling, as our guide to preventing ice dams on a Boston triple-decker explains.

Mansards and First Period antiques

A Back Bay or South End mansard has no soffit and no ridge. Venting happens at the seam between the slate face and the low-slope top, or not at all. Ipswich holds 59 houses with pre-1725 elements: hand-hewn rafters, gapped boards, no ridge path.

Insulation, and what comes out first

Insulation only works over a sealed ceiling. Blow cellulose across open top plates and you have built an expensive air filter. Seal first, then insulate full depth over the exterior wall without crushing the vent channel.

• Duct every bath and kitchen fan outside, never into the attic
• Weatherstrip and insulate the attic hatch like a door
• Box and seal recessed lights before burying them
• Bag out wet or moldy insulation instead of adding on top