Locate the gap before replacing a seal
Cold air may enter at the moving panel, meeting stile or frame joint. Check whether the panel closes fully and whether visible weatherstripping is torn, flattened or missing. A replacement seal cannot correct a panel that sits out of alignment.
Choose the right seal profile
Pile weatherstripping, flexible gaskets and frame seals have different roles. The channel size and original profile determine what fits. Ask whether the proposed work addresses the actual air path and whether closing and locking will be checked afterward.
Moisture needs a separate explanation
Condensation on the room-side glass is different from fog trapped between panes. Water in the sill can also involve drainage or installation conditions. Keep drainage openings clear and avoid covering them with sealant while trying to stop a draft.