Most windows need replacing after 20 to 30 years. The clearest signs are fog between the panes, drafts, soft or rotting frames, and sashes that stick or won’t stay open.

The signs

1. Fog between the panes. The seal failed, and the insulating gas is gone.
2. Drafts. You can feel air moving near closed windows.
3. Rot or soft wood. Press the sill with a screwdriver. If it sinks in, water got in.
4. Sticking or dropping sashes. Worn balances and warped frames.
5. Condensation or frost on the inside glass in winter.
6. Rising energy bills without another explanation.
7. Single-pane or aluminum windows, which are well below modern efficiency.

Repair or replace?

One foggy pane in a good frame can often be fixed with glass replacement. Rot, multiple failures, or windows over 20 years old usually point to full replacement.