Sealing gaps around window frames is one of the most cost-effective ways to make your home warmer and quieter. In New Zealand's variable climate — from Auckland's damp winters to Otago's bitter frosts — even small gaps can let in significant cold air and moisture. The good news is that most draught-proofing work is well within a confident DIYer's ability, requires only basic tools, and can make a noticeable difference to your comfort and energy bills.
Why Gaps Appear Around Window Frames
Over time, the timber or aluminium framing around windows moves slightly as your house settles and as materials expand and contract with temperature changes. This is especially common in older NZ homes built on timber piles. The result is small gaps between the window frame and the surrounding wall lining or exterior cladding — prime entry points for cold draughts, moisture, and even insects.
Addressing these gaps also supports the intent of the NZ Building Code around weathertightness (Clause E2) and can contribute to meeting Healthy Homes Standards if you're a landlord.
Tools and Materials You'll Need
Before you start, gather the following:
- Flexible sealant / caulk — a paintable acrylic sealant for interior gaps, or a polyurethane or silicone sealant for exterior use
- Foam backer rod — for filling wider gaps before sealing
- Draught-excluding tape / self-adhesive foam strip — for gaps between the opening sash and the frame
- Caulking gun
- Utility knife and scraper — to remove old sealant
- Methylated spirits or sugar soap — for cleaning surfaces
- Masking tape — for neat edges
- Safety glasses and gloves
You'll find most of these at Bunnings or Mitre 10. For timber-specific products, PlaceMakers is another solid option.
Step-by-Step: Sealing Interior Gaps
These are the gaps between the window frame/architrave and the interior wall lining (usually GIB plasterboard).
- Inspect and clean — Run your hand around the frame on a cold day to feel for draughts. Use a scraper to remove any old, cracked sealant. Clean the surface with sugar soap and let it dry fully.
- Apply backer rod if needed — For gaps wider than about 6 mm, press foam backer rod into the gap first. This gives the sealant something to bond to and prevents it sinking in.
- Mask up — Apply masking tape either side of the gap for a clean finish.
- Apply sealant — Load a paintable acrylic sealant into your caulking gun. Apply a smooth, continuous bead into the gap, holding the gun at roughly 45 degrees.
- Tool off — Wet your finger or a caulking tool and smooth the bead in one pass. Remove the masking tape immediately before the sealant skins over.
- Paint when cured — Once fully cured (check the manufacturer's timeframe), you can paint over acrylic sealant to match your wall colour. Resene and Dulux both offer good interior paint ranges suited to trim work.
Step-by-Step: Sealing Exterior Gaps
Exterior gaps — between the window frame and the cladding — are more critical from a weathertightness perspective.
- Use a UV-stable polyurethane or silicone sealant rated for exterior use.
- Ensure the surface is bone dry before applying — never seal over damp timber or cladding.
- Check that existing flashings and sill trays are intact. If they look damaged or incorrectly installed, stop and get a carpenter to assess before you seal anything, as trapping moisture behind cladding can cause serious rot.
- Apply the sealant in the same way as interior work, tooling it off neatly.
Important: If your home has a cavity-based cladding system (common in homes built after the leaky building era), do not block weep holes or drainage channels at the base of windows. These are essential for moisture management.
Draught-Proofing the Opening Sash
If the draught is coming through the gap between a moving sash and its frame (rather than between the frame and the wall), sealant isn't the right solution — you need draught-excluding tape.
- Self-adhesive foam strip is the simplest option and suits most timber and aluminium windows. Clean the rebate surface, peel and stick.
- Brush-pile or rubber-fin seals last longer and suit sliding sash windows well.
- Replace worn or compressed existing seals before adding new ones on top.
Choosing the Right Sealant — Quick Guide
| Location | Recommended Type | Paintable? |
|---|---|---|
| Interior frame to GIB | Acrylic / latex caulk | Yes |
| Exterior frame to cladding | Polyurethane or silicone | Check label |
| Wet areas (e.g. laundry) | Sanitary silicone | Usually no |
| Wide gaps (>10 mm) | Expanding foam + sealant over top | Varies |
Sika offers a well-regarded range of sealants suited to NZ conditions and is widely available at trade and hardware stores.
When to Call a Professional
While draught-proofing is generally DIY-friendly, call a qualified carpenter if you notice:
- Soft, spongy or discoloured timber around the frame — this may indicate rot caused by long-term moisture ingress, which needs proper repair before sealing
- Gaps wider than 15–20 mm or frames that are visibly out of square
- Damaged or missing flashings — incorrect flashing is a leading cause of weathertightness failures in NZ homes
- Mould growth on the interior wall near the window — this may point to a condensation or moisture issue needing professional assessment
For significant window repairs or replacements, check whether a building consent is required with your local council. Many like-for-like repairs fall under Schedule 1 exemptions, but it's always worth confirming.
You can find qualified local carpenters and Licensed Building Practitioners on tradedirectory.co.nz.