Drafty house in winter? Why it feels so cold, and where the air gets in

Why is my house so cold? It's not only the windows

If you've been asking why your house is so cold when the heater is on, you're in good company. Bay Area winters are mild on paper, which is part of why so many houses here were built with little or no insulation and never got any. Then a January week of 40-degree mornings arrives, the heater runs and runs, and you still feel cold air around your ankles in a room with every window shut.

ENERGY STAR's list of common home problems puts "drafty rooms" under the attic, "cold floor in the winter" under the basement and crawl space, and "hot and cold rooms" under the attic, crawl space and ducts. Windows and doors are on the list too, but they're one item among several. In many older Bay Area houses, the big leaks are above the ceiling and below the floor, where nobody looks.

This page is sorted by what you feel: drafts, cold floors, one cold room, and the damp chill of the fog side.

Why a drafty house feels drafty: it's a chimney

Warm air rises. In winter, heated air collects at the top of the house and pushes out through every gap it can find: around recessed lights, at the tops of walls, around plumbing stacks, through an attic hatch with no weatherstripping. The house doesn't run out of air; it replaces what leaves by pulling cold outdoor air in at the bottom, through gaps at the floor, the crawl space, the foundation sill and the baseboards.

That's the stack effect, and it's why the draft you feel near the floor is often caused by a hole in the ceiling. The Department of Energy's guidance for existing attics notes that sealing holes in the attic ceiling reduces the house's "suction," and it says to air seal attic floor penetrations first, before insulating. Two-story houses feel it more, because the taller the column of warm air, the harder it pushes.

Our air sealing page lists where the leaks usually are in a Bay Area attic, so we won't repeat the whole list here. The short version: the ceiling is the top of the chimney, and plugging it slows the whole draft.

Victorians, Edwardians and walls that open into the attic

Some pre-war houses in San Francisco, Oakland, Berkeley and Alameda were framed differently from modern ones. In balloon framing, the wall studs run continuously from the foundation up to the roof, so a wall cavity can be an open shaft from the crawl space to the attic. Warm air from inside the walls, and cold air from under the house, travel straight up and out.

The Department of Energy has a job aid for exactly this: block the tops of balloon-framed wall cavities from the attic with rigid material such as lumber or gypsum board, sealed so air can't pass. In an old house, that blocking is often the single most useful thing done in the attic. It also matters if the walls are ever filled with insulation, because the blocking keeps the fill from pouring into the attic. Our wall insulation page covers that part.

Old houses also bring safety checks with them, like knob-and-tube wiring and flues that need clearance. Those get looked at before any sealing starts.

Cold floors: what's under the house

If the floor feels cold through your socks, look underneath it. A lot of older Bay Area houses sit on a raised perimeter foundation with a crawl space below, vented to the outside by design. That means the underside of your floor is, in effect, outdoors.

Three common finds under older floors:

• No insulation at all. Many older homes were built that way and never changed.
• Batts on the ground. Insulation that was installed decades ago without proper supports and has sagged or dropped onto the dirt, sometimes helped along by rodents.
• Holes in the floor. Gaps around plumbing, drain lines and heating ducts, where cold crawl-space air comes straight up into cabinets and closets.

Insulating the floor over the crawl space, held tight against the subfloor, and sealing those penetrations is the fix. A vapor barrier on the ground is often part of the same job. Our crawl space and underfloor insulation page covers the materials and the moisture checks that come first.

Houses on a concrete slab don't have a crawl space to fix, so if your floors are cold on a slab, this section isn't your answer, and we'll say so at the inspection.

One room that never warms up

When the whole house is fine except one room, look at what's around that room.

• A room over the garage. The garage ceiling is that room's floor, and the garage is effectively unheated. Our garage insulation page explains what goes in between the joists and the fire-separation rules that apply.
• A room under a low corner of the roof, or a bonus room with sloped ceilings and short kneewalls. The kneewalls are often open at the back, with the cold attic right behind the drywall.
• A room at the end of a long duct run, especially if the ducts run through the attic or crawl space with damaged insulation. The air arrives lukewarm.
• A room with a thin patch above it. Insulation gets pushed aside when someone installs a fan, runs cable or crawls to a far corner, and it rarely gets put back.

The fog side's damp chill

On the fog side (climate zone 3, from San Francisco down the Peninsula and along the East Bay shoreline), "cold" often isn't about the temperature. It's damp, gray air that never really warms up, and it can show up in July as easily as in January. Houses that leak air let that damp outdoor air straight in. A tighter attic floor and an insulated attic won't change the weather, but they mean the heat you do use stays inside instead of being replaced by sea air every hour.

Inland, in the Tri-Valley, Walnut Creek, Concord and the Santa Clara Valley, winter nights tend to run colder than by the Bay, and the same leaks simply cost more to heat through.

What we fix, and what we don't

To be clear about scope: the leaks we fix are in the attic, the crawl space, the garage ceiling and the walls: air sealing the attic floor, blocking open wall cavities from above, blown-in insulation in the attic, underfloor insulation, garage ceilings and existing-wall insulation.

Windows, exterior doors and their weatherstripping, fireplaces and dampers, and outlets and switches are other trades' work. If a door has daylight around it or a window rattles in the wind, that's a job for a window or door installer, and it may be worth doing too.

Where to start

1. The attic floor first. Sealed, then insulated. It's the top of the chimney, and it affects every room below it.
2. Then the floor, if you have a crawl space and the floors are cold.
3. Then the problem rooms: over the garage, under kneewalls, or at the end of a duct run.
4. Then windows and doors, if drafts remain.

If the heating bill is what sent you here, our page on the high heating bill takes the money side of the same problem.

If you book a free inspection, mention which floors feel coldest and whether the house has a crawl space; that tells us where to start.