Find the source without a flame
With the window shut, observe movement of a light tissue near the frame on a windy day. This is only a rough indication, not an airtightness test. Cold air moving down a cold pane can feel like a draught even with intact seals. Check visible damage and whether the sash closes correctly; do not force the hardware.
Keep designed ventilation open
Do not block trickle vents, combustion-air openings or mechanical ventilation. Airtightness and planned ventilation are different tasks. A loose or damaged seal may need the correct replacement profile, not another thick adhesive layer. The wrong strip can prevent closing or damage fittings.
Agree the repair first
In a rental, document the gap or damage and check local rules and the lease before altering frames or fittings. This is general technical guidance, not German rental law applied abroad. Do not attempt external work at height. Ask the responsible repair service to assess misaligned windows.
Use curtains with care
A curtain may improve perceived comfort but does not repair a leak. Leave radiator valves and heat flow unobstructed; watch for condensation hidden behind fabric. Measure humidity and inspect cold surfaces after changes. If the frame is sound, compare comfort and ventilation before buying more materials.
Common questions
Does cold glass mean the seal leaks?
No. Cold glass can create downward air movement without a gap in the seal.
Can I seal every opening?
No. Intended ventilation and combustion-air openings must remain functional.
Sources and limits
Technical sources below are in German, checked for the original guide on 4 October 2026. These adaptations do not establish local rental law, installation rules or tariffs. Follow local requirements and the instructions for your actual system.