A window has several different barriers to air and water. Weatherstripping around an operating sash is not the same as the insulated-glass seal or the flashing behind the frame. The visible symptom needs to be matched to the correct system.
In this guide
Tools and preparation
- Flashlight.
- Camera.
- Window model information.
- Safe ground-level access.
Time is a planning estimate, not a timed field test. Material prices vary; check the correct parts and local prices before buying.
Steps and safe checks
1. Inspect the moving seal
Open the window only as directed and look for missing, torn or compressed weatherstripping. Check whether the sash closes and locks evenly. Do not compensate for misalignment by adding thick adhesive foam.
2. Observe fixed joints
From safe access, look for visibly split or detached sealant at suitable stationary joints. Record where the defect is rather than scraping it out immediately. The joint may require a specific sealant and backing.
3. Identify drainage before sealing
Consult the manufacturer’s diagram for weep openings and drainage paths. Leave these clear. Water that enters a designed track needs an exit; indiscriminate caulking can make a leak worse.
4. Match the repair to the evidence
Use the correct replacement seal for an operating sash. Refer uncertain perimeter leakage or between-pane moisture to the window maker or installer. Visible caulk alone does not establish whether concealed flashing is sound.
Check the result
A useful inspection distinguishes a damaged weatherstrip, a suspect fixed joint and a drainage opening. Do not treat them as one caulking job.
Common mistakes
Common errors include blocking drains, mixing incompatible sealants and assuming new perimeter caulk will fix every rain leak.
When to call a professional
Use a qualified installer for high access, concealed flashing, frame rot, repeated leakage or uncertain compatibility with a warranty.
What can happen if you postpone it?
A failed weather seal and a designed drainage gap are not the same. Applying sealant indiscriminately can trap water behind a frame and make diagnosis harder.
Prevent a repeat
Inspect after severe weather and during seasonal maintenance. Keep dated photos of changes and save the window model and installation documents.
Sources and scope
- Andersen: Care and Maintenance Guides
- Andersen: Product, Care and Maintenance Information
- South Carolina Energy Saver: Caulking
Source check: November 4, 2026. AI-assisted educational writing checked against the linked documentation; no firsthand testing or licensed professional review is claimed. Manufacturer instructions and applicable local requirements take priority.