If an older sash window rattles or admits a draught, the fact that two replacement strips are both marketed for sash windows does not make them the same product. Mighton lists FWS (Flexible Weatherstrip) for the top of the top sash, with a flexible hinge, while SWS (Standard Weatherstrip) has a barbed foot and flexible fin and is described for positions including the staff bead and sash head. Their published installation-groove dimensions are also different. The correct first task is to identify location and existing attachment, not to order from a color photo.
What the manufacturer actually differentiates
| Product label | Issuer's stated use/form | Published preparation dimension — not a recommendation to cut |
|---|---|---|
| FWS | Flexible-hinge-type weatherstrip described for the top of the top sash | The product page specifies a 3 mm wide × 6 mm deep groove in timber |
| SWS | UV-stable PVC strip with a barbed foot and flexible fin, described for a staff bead, sash head or sash position | The page specifies a 2.5 mm groove for the product |
Those numbers describe manufacturer-stated new-fit preparations on the undated product pages. They do not prove that a groove in an old or protected sash is the same size, that it was made for Mighton, or that a current FWS or SWS strip fits it. No instruction here authorizes routing, machining or deepening original timber.
Find the original location before naming the seal
Identify the upper sash, lower sash, staff bead, head and meeting rail by their positions in the complete window. Then record where the worn material is seen: at the top of the upper moving sash, pressed into a bead, in a prepared groove in timber, or fixed to a separate carrier. A flexible lip visible from one direction may conceal the mounting base. Do not prize out a sound strip for the sake of a profile photograph.
If the window no longer opens smoothly, note whether paint bridging, timber swelling, damaged sash cords or a distorted bead could also be involved. A draught symptom by itself cannot identify the particular product. If an upper sash is unstable or hardware seems unreliable, do not operate it further just to inspect the gasket.
Photographs and measurements worth collecting
- Take one whole-window image from indoors, with the top and bottom sash clearly identifiable. Mark where you feel air rather than assuming that location must contain the bad seal.
- Photograph the head/top-sash junction and an accessible staff-bead section separately, with natural light revealing how the material attaches.
- If an already detached fragment exists, lay it on a ruler and photograph its end profile from two directions. Record approximate back shape, fin or flexible hinge, overall section and whether any barbs are visible. Do not treat a soft, crushed used sample as the manufacturer's uncompressed dimension.
- If an existing groove can be seen without disturbing the sash, note its location and take a non-contact close-up. Do not scrape paint, cut timber or drive a probe into it. If its dimensions are unclear, write groove unverified rather than guessing 2.5 mm or 3 mm.
- Note sash material, known approximate age, prior weatherproofing work, paint or conservation status, and any seller/installing contractor documentation.
Send the evidence with the exact FWS/SWS product names and ask the supplier or qualified sash specialist whether the existing interface and window condition can accept that product. For a listed/historic building or significant original joinery, obtain conservation advice before any irreversible work.
A sash-location comparison an owner can complete without tools
Describe these locations separately on a sketch or annotated photo: (A) top edge of the upper sash, (B) staff bead contact, (C) meeting rail, (D) bottom/threshold contact. Then add a second column: “existing visible strip / no strip / cannot see without removal.” FWS is a more relevant maker document for the stated top-of-top-sash function; SWS is a more relevant maker document when the barbed-foot fin is associated with a staff-bead or sash-head position. Neither assignment proves installation in a historic or altered sash.
If an already detached, freely accessible fragment exists, photograph the end view next to a ruler, note whether the mounting foot looks barbed or hinged, and identify any deformation; do not cut a sample off a working window. Record an existing groove width only if it can be observed safely without removing hardware. The cited 3 mm × 6 mm (FWS) and 2.5 mm (SWS) figures are product-page preparation dimensions, not instructions to rout a conserved or unknown timber sash. The SWS source quoted here does not supply an equivalent verified groove depth: leave that field blank, never derive it from FWS.
Sample question for a sash specialist: “The suspected strip is at location [A/B/C/D], appears to attach by [observed interface], and the window is [identified/unknown vintage]. Which issuer specification and conservation constraints apply before any removal or machining?”
Two wrong assumptions to avoid
“FWS and SWS are both white PVC, so either will work.” Color and material are weak identifiers. One product has a flexible hinge and a described top-sash location; the other uses a barbed anchoring foot and multiple positions. The manufacturer does not state universal interchangeability.
“I can cut whichever groove is on the product page.” These dimensions are not a general sash-restoration specification. A new cut can irreversibly change original timber, meeting geometry, water shedding and sash movement; protected fabric needs an appropriate preservation decision. Photograph and preserve the original interface instead.
“The air leak is at the meeting rail, so it must be a Mighton SWS problem.” A window has several interfaces and a symptom may result from fit, alignment or other materials. Identify the precise observed seal and locate the correct original source documentation before selecting a replacement.
What is verified, and the last gap before a purchase
Mighton's undated manufacturer FWS product description explicitly states use at the top of the top sash, flexible hinge construction and a 3 mm × 6 mm timber groove. Its undated SWS product page identifies a barbed-foot flexible-fin strip, the stated staff-bead/head/sash uses, and a 2.5 mm installation groove. Not verified: a particular property's vintage, an existing groove's origin, suitability for historic fabric, wear diagnosis, exact part match or current seller stock. The website headings “Description / Specification” are the locators; no edition-stamped fitting drawing was used for a match claim.
Original manufacturer sources
- Mighton Products, Flexible Weatherstrip FWS, “Description / Product Code FWS” (undated current product HTML): https://www.mightonproducts.com/shop/sash-window-hardware/draughtproofing/standard-weatherstrip/flexible-weatherstrip-1-8m-length-white/
- Mighton Products, Standard Weatherstrip SWS, “Description / Product Code SWS / Download Documents” (undated current product HTML): https://www.mightonproducts.com/shop/sash-window-hardware/draughtproofing/standard-weatherstrip/standard-weatherstrip-2-4m-length-white-sws/
Bottom line: work from the sash position and attachment profile toward manufacturer identification, then decide any intervention with the right specialist. Do not create a new groove to see whether a catalog strip might fit.