How to Replace a Worn Brush Seal on a Sliding Window

Replacing a worn brush seal on a sliding window is one of the easier DIY jobs around the home — no special licences required, and most homeowners can complete it in under an hour with basic tools. A fresh seal stops draughts, reduces outside noise, and helps your home meet the warmth and insulation expectations set out under the Healthy Homes Standards.

What Is a Brush Seal and Why Does It Wear Out?

A brush seal (sometimes called a pile weatherstrip or fin seal) is a strip of fine synthetic fibres — usually polypropylene — set into a rigid carrier, typically aluminium or plastic. It runs along the meeting stiles or the sill track of a sliding window, creating a flexible barrier against draughts, dust, and light rain infiltration.

Over time, the fibres flatten, become matted with grime, or simply break down from UV exposure and repeated friction. Signs it needs replacing include:

• Visible draught or cold air around the closed window
• Fibres that look crushed, sparse, or discoloured
• Increased outside noise compared to when the window was new
• Condensation forming more readily on the glass edge

Tools and Materials You'll Need

• Flathead screwdriver or a seal-removal tool
• Scissors or a sharp utility knife
• Tape measure and pencil
• Mild detergent and a cloth
• Replacement brush seal (matching pile height and carrier width — see below)

Replacement brush seals are available from glazing suppliers, hardware stores such as Bunnings or Mitre 10, and specialist window hardware suppliers. They are sold by the metre or in rolls, so measure your window carefully before buying.

Step 1 — Identify the Correct Seal Size

Brush seals come in different pile heights (typically 5 mm, 7 mm, 9 mm, or 12 mm) and carrier widths. Before removing the old seal, measure:

1. The pile height — the length of the bristles from the base of the carrier to the tip.
2. The carrier width — how wide the rigid backing strip is.
3. The total length needed — measure each run separately and add 10% for trimming.

If you can, take a short piece of the old seal to the store to match it exactly. Aluminium-carrier seals are common on aluminium joinery; plastic-carrier seals suit uPVC frames.

Step 2 — Remove the Window Sash

Most sliding windows can be lifted out of their track for easier access:

1. Slide the sash to the centre of the opening.
2. Lift it firmly upward into the top track, then swing the bottom outward and lower it free.
3. Lay the sash on a padded surface (an old towel works well) to avoid scratching the glass.

If the sash feels stuck or the track is damaged, don't force it — see the professional advice section below.

Step 3 — Remove the Old Brush Seal

The carrier usually slots into a groove machined into the frame or sash. Use a flathead screwdriver to gently prise one end free, then pull the strip along the groove. It should slide out with moderate resistance. If it's held by adhesive as well, work slowly to avoid gouging the frame.

Once removed, clean the groove thoroughly with a damp cloth and mild detergent, removing any old adhesive residue, grit, or mould. Allow it to dry completely.

Step 4 — Cut and Fit the New Seal

1. Cut the new brush seal to length with scissors or a utility knife — a clean, square cut gives the best result at corners.
2. Starting at one end of the groove, press the carrier firmly into the slot. It should click or slide in with hand pressure. Work along the length gradually, keeping the pile facing the correct direction (towards the mating surface it will contact).
3. At corners, cut the seal at 45° for a neat mitre, or butt-join two pieces tightly — either method works for draught control.
4. Check that the pile makes light, even contact with the opposing surface when the window is closed. If the pile is too tall and causes the window to drag heavily, you may need a shorter pile height.

Step 5 — Refit the Sash and Test

Re-hang the sash by reversing the removal steps: lift it into the top track, then lower the bottom into the lower track. Slide it back and forth several times to seat the seal evenly. Close the window and run your hand around the edges — you should feel no significant draught.

Maintenance Tips

• Clean the tracks every six months with a vacuum and damp cloth to reduce wear on the new seal.
• Avoid using oil-based lubricants on brush seals — a dry PTFE spray on the track is preferable.
• Inspect seals annually, especially after a harsh NZ winter or a prolonged dry summer.

When to Call a Professional

While brush seal replacement is DIY-friendly, call a qualified glazier if:

• The window frame or track is bent, corroded, or cracked — this may affect weathertightness compliance under NZ Building Code clause E2 (external moisture).
• The glass is cracked or the glazing compound is failing — glass work should be handled by a professional glazier for safety reasons.
• The sash won't lift out cleanly or the hardware (rollers, latches) is broken — a glazier can source the correct replacement parts for your joinery brand.
• You're a landlord and the window is in a rental property — Healthy Homes Standards require draught-stopping, and a professional can document the work.

You can find qualified local glaziers on tradedirectory.co.nz.

Sources & Further Reading

• Energywise — Draught-stopping your home
• BRANZ — Weathertightness and joinery guidance
• Tenancy Services — Healthy Homes Standards
• MBIE Building Performance — NZ Building Code
• Bunnings — Weatherstrip and seals range
• Mitre 10 — Door and window seals