Short answer. A Q-LON seal that appears flattened after years in a closed sash is not identified by its compressed silhouette. The manufacturer’s published recovery result is a performance description for named products, not a certificate that a particular old, abraded, painted or misaligned seal will spring back—or that a thicker replacement is safe. Schlegel lists polyurethane foam and a polyethylene facing as principal Q-LON layers [F01]. Its pages for QL 3097 and QL 55 each refer to an EN 12365 Part 4 Class 6 result, up to 90% recovery [F03][F04]. Neither figure measures the seal in your window.
Inspect four conditions separately
| Observation | What it can suggest | What it does not establish |
|---|---|---|
| Body looks thinner at the regularly compressed closing edge | The field measurement may be of the used profile | The original uncompressed maker dimension or product code |
| External film is worn, torn, loose or split | There may be damage to the visible outer protective layer | Exact foam chemistry or a replacement with matching backing |
| Window or door is hard to close | Closing resistance should be investigated | That adding or removing seal thickness will fix hinge, latch or alignment problems |
| A draft is felt at a particular location | That joint merits location-specific investigation | That the weatherseal rather than sash adjustment, glazing or a frame joint is responsible |
Schlegel’s performance classification treats working range, linear compression force, deflection recovery and recovery after ageing as separate topics [F05]. Therefore a tall uncompressed cross-section, a class number and a closing-force complaint must not be merged into one supposed “seal size.” A separate accepted article is already assigned to decoding EN 12365 six-position classifications; this guide instead addresses observable ageing and a safe evidence record.
A non-destructive identification sequence
- Photograph the closed assembly from inside, marking the exact joint where a draft, light or uneven contact is observed. Repeat with the door/window open, without pulling a damaged seal apart.
- Capture a square-on view of the unmodified attachment area: frame material, receiver shape, and where the active sealing face makes contact. If there is an intact spare offcut already loose, lay it beside a scale; do not pry bonded or fragile seals out to make one.
- Make two distinct records: used installed height (state that it was held compressed) and candidate manufacturer profile drawing. Do not write the former as the latter. Include millimetres or original units, instrument resolution and uncertainty. If repeat readings are possible, note variation across three points rather than cherry-picking the tallest.
- Record damaged film, swelling, missing corners, paint coating, and contact witness marks as observations, not material diagnoses. Compare adjacent edges with the same lighting. Avoid applying heat, solvent, lubricant, glue or a trial thick strip to “restore” shape.
- Find the window/door system label, manufacturing year or installation papers, and any original seal code. Ask the manufacturer/fabricator whether its current service part is approved for that particular model, generation, groove and compression gap.
What to ask when the old seal does not bounce back
Send the fabricator: full opening photo, 90-degree profile/mount view, location of symptoms, existing label/code, approximate vintage and any drawing that supports it. Ask (a) which original seal was specified for that exact sash/frame, (b) whether the drawing shows nominal uncompressed profile geometry, (c) whether a current compatible replacement is documented, and (d) whether alignment or hardware adjustment needs professional inspection before selecting weatherstrip.
If the maker cannot match the part, the honest result is “no confirmed exact replacement from these records yet.” Do not transform it into “discontinued,” “will never fit,” or an informal Amazon size recommendation. Close the sash gently only if it operates normally; a binding exit door, rated door or damaged frame belongs with a qualified tradesperson.
Example: why a percentage is not a shopping dimension
“Up to 90% recovery” [F03] does not mean you can multiply the height of a flattened installed QL 3097 by 1.1 to reconstruct its original height. The original code, shape, test conditions and compression history matter. Likewise a profile without the same attachment backing is not a fit candidate solely because its foam dome visually matches.
Original evidence and scope
- [F01] Schlegel, Q-LON seals product range, undated current web edition,
Materials & Colours > Film/Foam/Insert; https://www.schlegel.com/products/qlon-seals/ (accessed 2026-10-12). - [F03] Schlegel, QL 3097, undated current web page,
SKU QL 3097 > compression-recovery product feature; https://www.schlegel.com/products/seal-technical-data-sheets/q-lon-3097/ (accessed 2026-10-12). - [F04] Schlegel, QL 55, undated current web page,
SKU QL 55 > compression-recovery product feature; https://www.schlegel.com/products/seal-technical-data-sheets/q-lon-55/ (accessed 2026-10-12). - [F05] Schlegel, Products, undated current web page,
Why specify Schlegel weatherseals? > EN 12365 fields; https://www.schlegel.com/products/ (accessed 2026-10-12).
Limits: No field test, no original seller part authentication, no EN test reports or exact installed-fit evidence. This is non-destructive condition recording, not a promise of restored airtightness or a guide to altering a certified assembly. Related owned material: /measure/bulb-and-compression-weatherstrip/ (dated 89) and NW0151 EN-classification draft (69 read-only); those bodies must not be silently replaced.