Where an Old House Loses Heat

Ask most people why an old house is drafty and expensive to heat, and they’ll point at the windows. The windows matter, but the bigger losses usually hide where you rarely look: up in the attic and down where the house sits on its foundation.

A house is a chimney

In cold weather, the warm air inside rises, just like smoke up a chimney. It leaks out wherever it can at the top of the house, into the attic, and as it leaves it pulls cold outside air in at the bottom: through gaps at the basement rim joist, around basement windows, and through every crack in between. This is the stack effect ^{1 2}.

Old houses are especially good chimneys. In a balloon-framed house the stud bays can run from the basement to the attic, there are chimney chases, old light fixtures, and plumbing runs through the ceilings, and nothing was air-sealed when it was built. ENERGY STAR points out that while drafts around windows are easy to feel, the leaks that really raise the heating bill are the hidden ones in the attic and basement ².

How big is it? Air leakage can account for 5 to 40 percent of a building’s heating and cooling costs, according to the National Park Service, and the leaking air also carries moisture into walls and attics where it can do damage ¹.

Seal before you insulate

Insulation slows heat moving through a material. It does very little to stop air moving through gaps. A fluffy layer of attic insulation over an open stud bay is like a sweater in the wind: the warm air just flows through it. That’s why the order matters: seal the air leaks first, then insulate ^{1 2}.

ENERGY STAR’s tell-tale for an attic leak is dirty insulation: dust filtered out of moving air darkens the insulation over a gap ².

What to do first

For historic houses, the National Park Service sorts energy improvements by how much they disturb the building ¹:

Little alteration, do these first: - Reduce air leakage. - Add attic insulation. - Install storm windows. - Insulate the basement or crawlspace. - Seal and insulate ducts and pipes. - Weatherstrip doors and add storm doors.

More alteration, think hard: - Replace windows. - Insulate wood-frame walls. - Insulate masonry walls.

The first group gives the most savings for the least money and harm. On windows in particular, the Park Service is blunt: the idea that replacing windows alone brings major energy savings, often used to sell replacements, “is simply not true” ¹. A single-glazed storm window over an old single-pane window roughly doubles its insulating value, to about R2, and protects the old sash too ¹. (The Residential Construction course has a lesson on restoring old windows.)

Get an audit

Before spending serious money, the Park Service recommends an energy audit by an independent auditor with no stake in selling you anything. A good audit reviews a year or two of utility bills, estimates the insulation in each part of the house, and uses a blower door, a big fan that depressurizes the house, often with an infrared camera, to show exactly where air leaks in ¹. Retest after the work to see what it achieved ¹. Some utilities offer basic audits for free ¹.

A safety note

Tightening a house changes how it breathes. Once the big leaks are sealed, have a heating technician check that gas or oil furnaces, water heaters, and dryers still vent properly, since a tighter house gives carbon monoxide more chance to build up ². A very tight house may need mechanical ventilation for fresh air ¹.

Try it

On a cold, windy day, light an incense stick and hold it near the attic hatch, the basement rim joist, an outlet on an outside wall, and the window trim. Watch where the smoke wavers or gets sucked in. Then go up in the attic and look for dark, dirty patches in the insulation. Map what you find; it’s your sealing list.