What Is Pile Weatherstrip? Understanding the A–G Specification System

Published 2026-10-11

Pile weatherstrip is a brush-like strip of polypropylene (PP) yarn that seals the gap between a window or door sash and its frame. It mounts in a kerf or against a stop, and its flexible fibers take up the tolerances that appear once a window or door is assembled, installed, and used. Flexible pile accommodates some gap variation, but its height and compression must suit the assembled window or door. That is why XINHAI publishes its strip as an A–G specification. This article explains the A–G specification system used for pile weatherstrip and how to read it when you are writing a purchasing specification.

Why Specifying by Dimension Beats Specifying by Name

Two buyers can order "a brush seal" and receive two completely different products. The base may be 4 mm wide on one and 25 mm on the other. The pile may stand 5 mm tall on one and 20 mm on the other. The same description, two incompatible parts.

Pile weatherstrip is not a commodity with a single size. It is a family of profiles with a set of independent dimensions that must match the application:

• The base width must fit the kerf or groove geometry.
• The pile height must bridge the gap between the sash and the frame after assembly tolerance is accounted for.
• The base thickness must be compatible with the groove depth and the retention method.
• The pile width affects how much material contacts the opposing surface.
• The fin, if present, must suit the sealing gap and deflect without excessive operating resistance.
• The strip length drives the cutting plan and the packaging.

For a purchasing engineer, a dimensional specification is also the fastest way to compare quotations. If three suppliers quote against the same A–G line, the comparison is real — but confirm density and construction options too, which manufacturers may publish separately ([example option matrix](https://www.ultrafab.com/product/ultra-fin-weatherseals/)).

The A–G System: A One-Line Language for Pile Weatherstrip

The A–G system assigns a letter to each controlled dimension so that a complete strip can be described in one line. This is the A–G system from XINHAI's official Standard & Specification, and the ranges below reflect what we produce on our lines.

A: Base Width — 4 to 25 mm

A is the width of the flat base that sits in the groove. Our range runs from 4 mm to 25 mm. Select the base width to suit the groove dimensions, retaining lips, tolerances, and clearance needed for installation.

A base that is too narrow will twist or pull out. A base that is too wide will not seat, and the pile line will sit high or crooked. If your groove is a nominal 5 mm, you are not looking for an "about 5 mm" strip — you are looking for the specific base width that fits your extrusion after the extrusion's own tolerance is considered.

B: Pile Height — 5 to 20 mm

B is the height of the pile above the base. Our range is 5 mm to 20 mm. This is the dimension that does the actual sealing, and it is the one most often specified incorrectly.

The pile should be slightly compressed in the closed position. It must not be so short that it fails to touch the opposing surface, and it must not be so tall that it prevents the sash from closing or forces the pile to fold over permanently. Gap tolerance across a production run, plus installation deflection of the frame, has to be included in the calculation — not just the nominal gap on the drawing.

C: Base Thickness — 0.8 mm ± 0.15 mm

C is the thickness of the base material. Across all our specifications, this is 0.8 mm ± 0.15 mm. It is a single value rather than a range because the base thickness interacts with the groove depth and any retaining lip. If your groove was designed around a different base thickness, confirm before ordering.

D: Pile Width and Configuration

D describes the pile itself — how wide the pile band is. This is where most of the variation in the catalogue sits.

Our D tiers are:

• 6.9(4P): 2.9 mm ± 0.3 mm
• 4.8(4P): 1.9 mm ± 1.5 mm
• 2P2L: 1.45 mm
• 3P1L: 1.45 mm
• 3P2L: 2.1 mm
• 3P3L: 2.68 mm
• 4P2L: 2.3–2.5 mm
• 4P3L: 3.4 mm
• 4P4L: 4.2 mm
• 5P5L: 4.7 mm
• 5P4L: 4.4 mm

A 4P2L (2.3–2.5 mm) tier and a 3P3L (2.68 mm) tier are different tiers with different widths. Choose the D configuration to suit your profile; if two configurations appear suitable, compare samples in your sash assembly.

E: Fin Height and Fin Material

Some strips carry a fin along the pile line. E covers two things: how tall the fin is relative to the pile, and what the fin is made of.

Fin height is specified in one of three ways:

• Same as pile height
• Higher than pile height
• Lower than pile height

Fin material is plastic or fabric.

A plain (no-fin) strip is also an available option. Do not add a fin by default; add it because the specification calls for it.

F: Strip Length

F is the cut length of the finished strip. Strip length is customized to your specification. For window and door production, this usually means lengths matched to your cutting plan, your corner-jointing method, and your packaging.

Bring your required length, your tolerance, and your packaging preference to the quotation stage. Agree on supplied lengths and tolerances before ordering so they match your cutting plan.

G: Base Margin Width

G is the margin of base material that sits outside the pile line on either side. It varies with base width and pile width — there is no single value, because a 25 mm base carrying a narrow pile band has a very different margin from a 4 mm base carrying a wide one. G varies according to base width and pile width; confirm it on the drawing.

Material and Option Choices

Beyond A–G, four choices affect how the strip performs and how it is assembled.

• Material: PP yarn. This is the pile material we produce. It is the baseline for the yarn itself.
• Silicone or non-silicone. Silicone treatment is an option on the pile. Specify it when the specification calls for it; do not assume it is required for every window.
• Adhesive backing or non-adhesive. Adhesive backing suits assembly methods where the strip is bonded to a surface rather than inserted into a kerf. Non-adhesive is used where the strip is mechanically retained.
• Fin options: plastic fin, fabric fin, or no fin. As covered under E, a fin is an option, not a default.

The manufacturer-stated functions of these strips are anti-wind, anti-noise, and anti-dust. Those are the three jobs pile weatherstrip is asked to do in a door or window: reduce air infiltration, reduce noise transmission through the gap, and block dust.

How to Write a Spec Line

A complete pile weatherstrip specification in the A–G format looks like a short string of values, not a paragraph. The structure is:

A (base width) – B (pile height) – C (base thickness) – D (pile width/config) – E (fin) – F (length) – G (margin)

When you issue an RFQ, include:

• The full A–G line, with tolerances where they matter to you.
• The material and options: PP yarn, silicone or non-silicone, adhesive or non-adhesive, fin options (plastic fin, fabric fin, or no fin).
• The application: sliding or casement, kerf-mounted or surface-mounted.
• The color, if it is fixed by the profile.
• The packaging format and the annual or monthly volume.
• A drawing or a sample of the current strip, if you have one.

A sample is worth more than a description. If you can send a short offcut of the strip you are replacing, along with the groove it sits in, the dimensional match can be confirmed before anything is produced.

Common Selection Mistakes

Common specification errors include:

• Specifying pile height from the nominal gap only. Assembly tolerance and frame deflection are not in the nominal gap. A strip that seals on the drawing can fail in the field.
• Ignoring G. Base margin is invisible until the strip will not staple, bond, or seat.
• Treating 4P2L and 3P3L as interchangeable. They are different tiers with different widths. Match the tier, not the general appearance.
• Adding a fin by default. Fin height and fin material are separate decisions; specify both or specify plain.
• Forgetting the base thickness tolerance. C is 0.8 mm ± 0.15 mm across our specifications. If your groove was designed around a different base thickness, say so early.
• Ordering by color and photo. Colour tells you nothing about A, B, C, D, E, F, or G.
• Leaving length to the supplier. F is customized. Agree on supplied lengths and tolerances before ordering so they match your cutting plan.

Getting the Right Strip

Pile weatherstrip selection starts with dimensions but does not end there. Dimensions establish fit; also specify material, pile density, treatment, and performance requirements, then validate samples in the assembly. The work is in measuring the groove, understanding the gap tolerance in the closed position, and deciding whether a fin and silicone treatment are actually needed for the application.

XINHAI — Jiaxing Xinhai Doors And Windows Accessaries Co., Ltd. — manufactures pile and brush weatherstrip in Jiaxing, Zhejiang, China. The company was founded in 2018, operates more than 20 production lines across a 2,000 sqm facility, and states a daily capacity of 300,000 meters. ISO 9001 was announced in the company news in April 2019.

If you are specifying a strip, send us your A–G dimensions, your material and option requirements, and a sample or drawing if you have one. We will confirm the dimensional match and quote against your specification rather than against a description. Contact: [email protected], +86-573-86779825 (Mr. Adam Huang).