Windows: repair or replace, and how to read the sticker

The pitch is that new windows will pay for themselves in energy savings. They very rarely do; the utility savings from replacing decent double-pane windows are small next to the cost. That does not make new windows a bad idea. It makes them a comfort, appearance and maintenance decision, and those are fine reasons. Here is how to tell which decision you are making.

When repair wins

• The frames are sound wood and the problem is a broken sash cord, a stuck sash, a cracked pane or failed glazing putty. A window repair Pro fixes all of it for a fraction of replacement.
• The windows are old single-pane in a house you care about. A good storm window over a restored original insulates about as well as a new double-pane unit, and the original will outlast the replacement.
• The draft is around the frame, not through the window. Caulk outside, weatherstripping inside, and foam in the gap behind the trim fix that.
• One or two units have fogged. That is a failed seal in the insulated glass, and the glass can be replaced in the existing frame.

When replacement wins

• Rot in the frame or the sill that has gone past the surface.
• Vinyl or aluminum windows from the cheap end of the market that have warped, whose seals have failed across the house, or whose hardware is broken and unavailable.
• Windows that will not stay open, will not lock, or cannot be used as an exit from a bedroom, which is a safety issue.
• You are residing the house. Doing the windows at the same time is far cheaper than doing them later.

Reading the sticker

Every new window carries a label from the National Fenestration Rating Council. Two numbers matter.

• U-factor: how much heat passes through. Lower is better. Around 0.30 or less is a good window in a cold climate.
• Solar heat gain coefficient: how much of the sun's heat comes through. Lower is better in the South and on west-facing walls; a little higher is fine, even useful, on the south side in the North.

Everything else on the sticker, visible transmittance and air leakage, matters less. The Energy Star mark means the numbers meet the standard for your region, and it is what the tax credit is keyed to.

Full frame or insert

An insert, or pocket, replacement fits a new window inside the old frame. It is faster and cheaper and leaves the old frame, sill and any rot in place, and it loses some glass area. A full-frame replacement takes it all out to the rough opening, lets the installer flash and insulate properly, and is the right answer whenever there is any doubt about the old frame. The estimate should say which one you are getting.

The installation is the product

The same window installed by two crews performs differently. What matters is the flashing at the sill and the sides, the foam or backer rod in the gap, and the caulk outside. Ask how the sill will be flashed and whether the crew is the company's own. And get the permit; in most places windows require one, and the inspection checks the egress and the safety glazing.