Yes — fitting draught-excluding brush strips to a sash window is a job most handy homeowners can do themselves. It requires no licensed trade work, just some basic tools, careful measuring, and a little patience. Done well, it can make a noticeable difference to warmth and comfort, and it supports the goals of Energywise's home insulation advice for reducing heat loss in New Zealand homes.
Why Draught-Proofing Your Sash Windows Matters
Double-hung sash windows are common in older New Zealand villas and bungalows, particularly in Auckland, Wellington, and Dunedin. Their sliding design means there are multiple gaps where cold air can sneak in — along the sides of the sashes, between the two meeting rails, and at the top and bottom. In a cold, damp NZ winter, these draughts can significantly increase heating costs and reduce indoor comfort. Draught-proofing is also relevant to the Healthy Homes Standards for rental properties, which require adequate draught stopping.
What Are Brush Strips?
Brush strips (sometimes called pile weatherstripping) consist of a rigid carrier — usually aluminium or plastic — with a dense row of fine polypropylene bristles. The bristles compress gently against the window frame or sash as it slides, sealing the gap without preventing movement. They are ideal for sash windows because, unlike foam tape or rubber seals, they allow the sash to slide freely while still blocking draughts.
Tools and Materials You'll Need
- Brush strip weatherseal (sold by the metre or in kit form — available at Bunnings or Mitre 10)
- Tape measure and pencil
- Junior hacksaw or tin snips (to cut aluminium carrier)
- Flat-head screwdriver or small pry bar
- Cordless drill and small drill bits
- Screws (usually supplied with the strip)
- Fine sandpaper
- Cleaning cloth and methylated spirits
Step-by-Step: Fitting Brush Strips to a Sash Window
1. Assess the Window and Identify the Gaps
Open both sashes fully and inspect where the draughts are coming from. The main areas to seal are:
- The vertical channels (the grooves the sashes slide in, on each side)
- The meeting rail (where the top and bottom sashes overlap in the middle)
- The top of the upper sash and the bottom of the lower sash
2. Measure and Cut the Strips
Measure each run carefully. Cut the brush strip to length with a junior hacksaw or tin snips. For the vertical channels, you'll need two strips per channel — one for each sash — so they don't overlap when the window is closed.
3. Clean the Surfaces
Wipe down the channels and rails with a cloth dampened with methylated spirits to remove dust, old paint, and grease. Good adhesion or a flush screw fit depends on a clean surface.
4. Fit the Strips into the Channels
For the side channels, slide or press the brush strip into the groove so the bristles face inward against the sash edge. Many strips are designed to be tapped or screwed into a routed groove. If your channels are too narrow, you may need to lightly sand or plane them — go carefully to avoid damaging the frame.
For the meeting rail, screw or press a strip along the face of one rail so the bristles compress against the other rail when the window is closed.
For the top and bottom, fit strips to the head of the frame (top) and the sill rebate (bottom) so the sash closes against them.
5. Test the Movement
Slide both sashes up and down. They should move smoothly with gentle resistance. If a sash sticks or is very hard to move, the strip may be too thick for the gap — try a narrower pile height.
6. Check for Remaining Gaps
On a windy day, run your hand around the closed window to feel for any remaining draughts. Small gaps at corners can be filled with a flexible, paintable acrylic sealant.
Choosing the Right Brush Strip
Brush strips come in different pile heights (typically 3 mm, 5 mm, or 9 mm) and carrier widths. Measure your gap before buying — a strip that's too tall will jam the sash, while one that's too short won't seal properly. Check the product packaging for the recommended gap size it suits.
| Location | Typical Gap Size | Recommended Pile Height |
|---|---|---|
| Side channels | 2–4 mm | 3–5 mm |
| Meeting rail | 3–6 mm | 5 mm |
| Top/bottom of sash | 3–8 mm | 5–9 mm |
Gap sizes vary between windows — always measure your own.
Does This Work Require a Building Consent?
Fitting brush strips is maintenance work and does not require a building consent. It does not alter the structure or weathertightness envelope of the building in a way that triggers consent under the NZ Building Code. However, if you are considering replacing the sash window entirely or making structural changes to the opening, check with your local council, as that may be a different matter.
When to Call a Professional
This is not restricted trade work — no licence is required to fit brush strips. However, consider calling a qualified joiner or window specialist (you can find local tradespeople on tradedirectory.co.nz) if:
- The sash cords are broken or the sashes are jammed and need repair before sealing
- The timber frames are rotten or swollen and need planing, patching, or replacement
- The window is a heritage or character feature and you want to preserve it correctly
- You're unsure about the condition of the window or the right product to use
A joiner can also advise whether full draught-proofing, secondary glazing, or window replacement would give you better value for money in the long run. BRANZ has useful research on the performance of different window retrofit options in the NZ climate.
