Unvented attic: is converting to a sealed roofline right for your house?

Most Bay Area attics are vented: outside air moves through the eaves and gables, and the insulation lies on the attic floor, keeping the house below at a comfortable distance from whatever the attic is doing. An unvented attic flips that. The vents are closed, insulation goes on the underside of the roof deck, and the attic becomes part of the house.

That's a design decision, not a product choice. This page is about when the design makes sense, what California's Residential Code requires, and what it changes for the rest of the house's life. For the material itself, open-cell versus closed-cell and where foam earns its place in general, see spray foam insulation.

The question to start with: what's in your attic?

An unvented conversion costs more than insulating the attic floor, so it has to buy something the floor can't. Usually that something is the ductwork.

In a vented attic, ducts and air handlers sit in air that runs close to outdoor temperature, and on a summer afternoon a vented attic is often hotter than outdoors. The Energy Commission's compliance manual says as much: on hot days a typical vented attic runs hotter than outside, and a poorly ventilated one can run much hotter. Every bit of leakage and every surface of duct is exchanging heat with that air. Move the insulation to the roofline and those ducts are now inside the insulated house.

So the picture looks like this:

• Ducts, air handler or furnace in the attic, inland climate. San Jose and the Santa Clara Valley (climate zone 4), Concord, Walnut Creek, Dublin, Pleasanton and Livermore (zone 12). Long, hot summers, equipment in the attic: this is where a conversion makes its strongest case.
• Ducts in the attic, fog belt. San Francisco, Oakland, Berkeley and the bayside cities (zone 3). The case is weaker. Summer attic heat is milder, and sealing and insulating the ducts where they are, plus a well-insulated floor, often gets most of the way for less.
• No ducts or equipment in the attic. Some houses heat with wall or floor furnaces, or run their ducts under the floor instead of overhead. Then the attic floor is the right place for insulation, and a vented attic is the simpler, cheaper design.

If you're weighing materials for a normal attic floor job, the insulation types comparison is the better starting point.

What California's code requires

The rule is Section R806.5 of the California Residential Code, "Unvented attic and unvented enclosed rafter assemblies." Read with care, it's a short list of conditions, and an unvented attic is allowed only if all of them are met. In the 2025 edition:

1. Inside the envelope. "The unvented attic space is completely within the building thermal envelope."
2. No Class I vapor retarder on the attic floor. Interior Class I vapor retarders aren't installed on the ceiling side, so the attic can still dry toward the house.
3. Wood shingles or shakes need a vented airspace of at least 1/4 inch between the roofing and the underlayment above the sheathing.
4. Vapor control in cold zones. Climate zones 5 to 8 need air-impermeable insulation to be a Class II vapor retarder. Those are the national code's climate zones, not the Energy Commission's 16, and they don't apply to the Bay Area.
5. Insulation placement for condensation control. If only air-impermeable insulation (typically spray foam) is used, it goes in direct contact with the underside of the roof sheathing. If air-permeable insulation is used directly under the sheathing, rigid board or sheet insulation goes above the deck, sized to Table R806.5. Combinations put the air-impermeable layer against the deck and the air-permeable layer beneath it.

Behind those conditions is one problem: keeping the underside of the roof deck from getting cold and damp enough to collect condensation. That's why the code cares so much about what touches the sheathing.

The Energy Code adds its own layer. When a roof or ceiling is opened so the space between the framing becomes accessible, the compliance manual treats it as altered, and rafter roofs carry a mandatory minimum of R-19. How the rest of your zone's requirements are met, prescriptively or by a performance calculation, is something to settle with the building department before the foam truck arrives. The Title 24 page explains what counts as an altered ceiling.

Gas appliances and combustion air

This is the part that most often turns a simple-sounding job into a project.

A gas furnace or water heater in a vented attic may be drawing its combustion air from that attic. Seal the vents and that supply is gone. The Department of Energy's Building America guide for converting existing roofs to below-deck spray foam is direct: combustion appliances installed in the attic "should be direct-vent sealed-combustion appliances," and it lists combustion safety and controlled mechanical ventilation upgrades among the requirements for success.

In practice that means the HVAC contractor is in the plan from the beginning. If the equipment in the attic is due for replacement anyway, the conversion is a natural time to switch to sealed-combustion or electric equipment. If it isn't, the conversion may need to wait for it.

What else the conversion involves

The DOE's retrofit guide lists the scope, and none of it is optional:

• Inspect the roof first. It says to inspect the existing roofing for any deficiencies. Foam on the underside of a leaking roof is a bad combination.
• Block the soffit vents, and the rest of the attic vents, on purpose.
• Fire protection. Install a thermal barrier or ignition barrier over the foam where required. The guide notes that not all spray foams require one; the product's listing decides.
• Manage the attic air. It calls for conditioning, dehumidification or controlled ventilation to the attic. An unvented attic isn't a sealed box left to its own devices.

And, as the attic ventilation page notes, the old vent openings stop being part of the ventilation math entirely, because there's nothing to ventilate.

What it means later

A conversion lasts as long as the roof deck it's attached to, so think about the next twenty years:

• Roof leaks are harder to trace. With foam covering the underside of the deck, you can't see a stain spreading on the sheathing from inside the attic. Roof maintenance matters more.
• Reroofing is different. Roofers work over a foamed deck, and replacing sheathing means replacing the foam on it. Tell any future roofer before they start.
• Rodent work comes first, not later. Do any rodent exclusion and cleanup before the foam goes on. Entry points at the eaves are far easier to find and close while they're visible.
• Removal is a bigger job. Loose-fill insulation on an attic floor can be vacuumed out. Foam stuck to a roof deck comes off by hand, if at all.
• Ember exposure goes down. With no attic vents, there's no vent opening for wind-blown embers to enter. Fire Safe Marin notes that roofs without vents are inherently more resistant to wind-blown embers, and in the Bay Area's hillside neighborhoods it's a real, if secondary, benefit.

Is it right for your house?

A conversion is worth a serious look if the ducts or air handler are in the attic, you're in a hot inland zone, and either the roof is in good shape or a reroof is coming anyway. It's a harder sell in the fog belt, in houses with no ducts upstairs, and anywhere a gas appliance in the attic isn't ready to be replaced.

If you're trying to decide, book a free inspection, or send us photos of the attic, the ducts and any equipment up there. We'll look at what's in the attic and the roof's condition before suggesting a design, and if a well-sealed vented attic with sealed ducts gets you most of the way, we'll say so.