Short answer. In Simonton's official double-hung replacement-parts illustrations, the flexible weatherseal bulb sits on the underside of the lower sash and extends across its bottom width. The distinct fin-seal weatherstripping—also called wool pile—is a fuzzy strip used on the sides, outside and top of either sash. Those named positions are more useful than a guess based only on a loose piece of black rubber. [S:SIMBULB] [S:SIMFIN] [S:SIMPARTS]
Make a position-based distinction
| Where the weatherstrip is missing | Manufacturer category to investigate | What the source establishes |
|---|---|---|
| Bottom edge of the lower sash | Weatherseal bulb | Soft bulb seals across the lower sash's full width |
| Side or upper sash surfaces with a fuzzy/finned strip | Fin-seal weatherstripping | Wool-pile/fin category, not automatically a bulb |
| Middle meeting rail | Another component or a model-dependent seal | No exact substitute established by the two parts pages |
| Sliding patio, casement or horizontal-slider sash | Different product family | Double-hung part map must not be generalized to these units |
The manufacturer describes that the bulb can deteriorate with age and should cover the lower sash width; it does not give one universal cross-section or part number for every Simonton double-hung window. [S:SIMBULB] The fin-seal page names position and material class, not the dimensional size needed for a particular model. [S:SIMFIN]
A practical identification card
Photograph the window front-on with both sashes and the latch visible. Point to the specific lower-sash bottom edge or vertical track. Include a close-up of the mounted material with its receiver or groove visible without pulling the component free. If a sample is already detached, photograph it beside a ruler, noting base width, exposed pile height or bulb profile as observed measurements, not manufacturer specifications. Keep the original label and approximate installation date as separate fields. Do not assume a forum participant's “Simonton” description establishes your product's series.
Why a retailer's nearest profile is not a fit approval
Online discussions may recommend an apparently similar T-base bubble seal when an owner posts dimensions. Those are unverified field reports, and a reseller's closest catalog item does not identify an OEM compatible part for an unknown Simonton window. In particular, a T-slot base width from a photo cannot establish whether a different seal's bulb shape, compression, operating clearance or track interface works in your unit. No claim about substitute fit is made here.
When contacting Simonton or your dealer, send the identification card and ask for product series/revision, the named lower-sash weatherseal or fin-seal item, current availability, exact mounting interface, and authorized replacement guidance. If the original model cannot be established, the useful answer is that this evidence does not confirm a compatible replacement; keep the opening functional and seek service advice rather than forcing a guessed part.
Evidence and limits
Simonton first-party Weatherseal Bulb page, Identify the Part, source; Fin Seal Weatherstripping, source; Replacement Parts for Double Hung Window map, items L and M, source. All accessed 2026-10-12, current undated HTML (no original PDF edition). Archived E01 claim IDs W300-C003/W300-C004 support only the bulb's location/full-width description. Distinct from generic /window/pile-weatherstrip-identification/ because this article shows one manufacturer's two component positions, but final editor must confirm this is not duplicated by an independent Simonton lane.