Drafty sash windows and doors: find the leaks, then fix them cheapest-first

On a windy January night in an Edwardian flat, the curtains at the bay window move with every sash shut. That is warm air leaving and cold air coming in through gaps, and in an old San Francisco house most of them are in the windows and doors: tall wood sashes that rattle, a front door with daylight under it, a fireplace whose damper never got closed. Before anyone prices new windows, find out exactly where the air gets in.

Most of those leaks are cheap to stop once you know which seam is open. SF Planning makes the same case in its window standards: a properly weatherstripped single-glazed sash can greatly reduce or eliminate air infiltration, which it calls the place where most energy is lost, and repairing old wood windows is the first option it tells owners to explore.

#Where an old house leaks

Most residential buildings put up in San Francisco before the Second World War got wood windows, SF Planning notes, often two sashes in one frame. In the classic double-hung, each sash hangs on cords that run over pulleys at the top of the frame to weights hidden in pockets inside the wall. Every part can leak: the top of the upper sash, the check rail where the sashes meet, the side channels, the bottom rail at the sill, and the pulley holes, which open straight into the weight pockets.

Bays multiply all of it. The bay-window Italianates of the 1870s were built with two-story bays, and the Stick-style houses that followed often had box bays (SF Planning, Bulletin 18), so one front room can hold three sashes and three times the seams. Doors add the gap at the bottom and the strip along the latch side, and a fireplace adds an open chimney.

#Find the leaks first: a 20-minute test

Pick a windy day, shut every window and outside door, and turn the heat off so the furnace doesn't stir the air. Start on the south and west sides of the house: SF Planning's window guide notes that windows facing south and west wear out sooner than ones facing north and east. You need incense sticks, a saucer, a few sheets of paper, a flashlight and, for the last test, a helper.

1. Look before you test

   Walk each room in daylight. ENERGY STAR's signs of a leaky window or door are worth knowing: a draft with it closed, visible gaps around the trim, dust or insects getting in, outside smells, peeling paint around the frame (ENERGY STAR). Old sashes add their own: rattling in the wind, cracked putty around the panes, a sash that sits crooked.

2. The incense test

   Hold a lit incense stick close to each seam: the check rail, both side channels, the top and bottom of each sash, the trim joints and the pulley holes. Where the smoke bends sharply or streams away, that seam leaks. This is ENERGY STAR's method; do it over a saucer, keep the stick clear of curtains and blinds, and drown it in water when you finish.

3. The paper test

   Close the window or door on a sheet of paper. If it slides out easily, that edge needs a tighter seal, usually thicker weatherstripping (ENERGY STAR). Try the top, middle and bottom of each side; old sashes rarely sit evenly.

4. The flashlight test, after dark

   With a helper outside, sweep a flashlight slowly along each frame from inside. Wherever they see light, air gets through too, by ENERGY STAR's rule of thumb. It is the quickest way to size the gap under a door and along its latch side.

5. Make a leak map

   Sketch each wall and mark every leak with a letter: S for sash, T for trim, D for door, P for pulley, F for fireplace. That map is your shopping list, and if you hire the work out, it is the most useful thing you can hand a pro.

#The fix ladder: start at the bottom rung

Climb only as far as the draft makes you. Each rung below gives the effort and the money in words, not prices, and says whether a renter can usually do it without asking. The bottom rungs are the ones you can undo, which is why they come first.

1. Free · two minutes

   Latch every sash

   An unlatched double-hung leaves the two sashes loose where they meet. Push the upper sash all the way up, close the lower one and turn the lock; on many old windows that alone closes up the check rail. If the lock no longer reaches its keeper, the sash needs adjusting, not replacing. Renters: yes.

2. Costs nothing · one minute

   Close the fireplace damper

   A cold fireplace with its damper open is a hole in the roof. The Chimney Safety Institute describes the damper as the plate that seals the fireplace when it isn't in use; open it fully before any fire. If yours no longer seals, a tight top-sealing damper is a job for a chimney sweep, and the fireplace guide covers the rest. Renters: yes.

3. Next to nothing · ten minutes

   Block the bottom of the door

   A draft snake or a rolled towel along an outside door stops the worst of it tonight. It is the renter's door sweep, and it moves out with you.

4. Cheap · an evening

   Rope caulk on windows you won't open

   A soft, putty-like cord pressed into the joints of a sash that stays shut until spring, then pulled off. It handles the check rail and side channels of a window locked for the winter. Try a corner first on old or soft paint. Renters: usually yes.

5. Cheap · an afternoon

   Shrink film over the window

   Clear film on double-sided tape, tightened with a hair dryer. ENERGY STAR rates it an easy project that reduces drafts and condensation on the glass, and taped to the trim it covers the sash and trim joints at once (ENERGY STAR). Condensation matters in a fog-side bedroom; the mold and condensation guide explains why. Renters: yes, after testing the tape on a hidden spot.

6. Low cost · an afternoon

   Pulley seals

   Small plastic caps over the pulley holes, so the sash still runs but the weight pocket stops breathing into the room. ENERGY STAR suggests them for older double-hung windows whose pulleys still work. Renters: ask first.

7. Low cost · about an hour a door

   A real sweep and threshold

   A sweep on the inside face of the door, or a door shoe or bulb threshold at the sill, replaces the towel for good; DOE's guide lists door sweeps and bulb seals among the standard weatherstripping types. Measure the daylight under the door before you buy. Renters: ask first.

8. Moderate · a weekend

   Weatherstrip the sashes

   Spring-metal or vinyl V-strip in the side channels, a foam or bulb seal at the top of the upper sash and the bottom of the lower one, and a seal at the check rail. DOE's guide names tension seals, foam, felt and interlocking metal channels for joints like these. Read the lead box below before you scrape or sand anything.

9. Moderate · a day

   Caulk the joints that never move

   Weatherstripping is for parts that move; caulk is for parts that don't. Seal where the trim meets the wall, inside and out. ENERGY STAR uses caulk for gaps of a quarter inch or less, and non-expanding foam or backer rod for wider gaps behind the trim, a job it rates moderate to difficult.

10. Significant · a pro's job

    Have the sashes restored

    Broken cords, sashes painted shut, putty falling out: a window repair pro can take the sashes out, rehang them, reglaze and weatherstrip. SF Planning notes that many windows from the late 1800s and early 1900s were made of dense old-growth heartwood and still perform very well.

11. Significant · one-time

    Interior storm windows

    A second pane fitted inside the old window, sealing the whole opening while the original sash stays. SF Planning suggests them for energy and noise, and notes that an interior storm window does not need a building permit. Renters: with the landlord's okay.

12. Highest · permit required

    Replace the window

    Last, not first. In San Francisco a replacement window needs a building permit, and Planning checks what goes in; the windows trade page and the permit guide cover glass, Planning review and lead-safe rules.

#Renting? What's yours to do, and what's the landlord's

California's Civil Code 1941.1 lists effective weather protection of the roof and outside walls, including unbroken windows and doors, among the basics a rental must have. It says nothing specific about weatherstripping, so start with a polite written request and keep a copy. This is general information, not legal advice.

#When the cold isn't coming from the windows

If every window passes the incense test and the room is still cold, look up and down. DOE's air-sealing guide notes that leaks hide where warm air rises into an attic and where walls meet foundations, and that a do-it-yourself pass won't find the small, hidden ones; for those it recommends a home energy assessor with a blower door and an infrared camera.

• The attic hatch

  A loose hatch is a chimney for warm air. Weatherstrip its edges; the insulation above it is a job for an insulation contractor.

• The floor over the garage

  In a Sunset or Richmond tract house the living floor sits over the garage (SF Planning). A cold floor there is an insulation question more than a weatherstripping one.

• Outlets on outside walls

  Air can move through the electrical boxes in an outside wall. Anything behind a cover plate is for a licensed electrician or an insulation pro.

• Flues and vents

  Around a furnace or water heater flue, DOE's guide says to leave the sealing to a professional contractor. Gaps around plain water pipes are fair game for caulk.

#Things people get wrong about drafty old houses

• Heard this?New double-pane windows are the only real fix.Tap for the factsWhat's trueSF Planning says a properly weatherstripped single-glazed sash can greatly reduce or eliminate air infiltration, and that many wood windows from around 1900 still perform very well. Fix the leaks, then decide.
• Heard this?Spray foam will seal a window for good.Tap for the factsWhat's trueNot around a sash that moves. ENERGY STAR's trim-sealing project calls for non-expanding foam made for windows and doors, and warns against forcing filler into the gap so hard that the frame bends.
• Heard this?An old house needs its drafts to breathe.Tap for the factsWhat's trueA house needs ventilation it can control, not leaks it can't. DOE pairs air sealing with planned ventilation, especially where gas appliances burn indoor air.
• Heard this?Window film will pull the paint off.Tap for the factsWhat's trueENERGY STAR says the tape comes off the trim easily in spring without harming the finish. On old, soft or flaking paint, test a hidden spot first, and in a pre-1978 house don't let it lift paint at all.

Find the leak before you buy the fix. The cheapest draft-stopper is knowing exactly which seam is open.