Posted June 29, 2026
Yes, you can. Retrofit timber double glazing upgrades your existing wooden sashes to accommodate modern glass, allowing you to keep the timeless appeal of your timber windows while enjoying draft-free comfort.
By upgrading rather than replacing, you retain your home's architectural integrity and dramatically improve its thermal performance. Because retrofitting existing windows doesn't typically require building consent or trigger the H1 energy efficiency requirements of the New Zealand Building Code, it's one of the simplest upgrades a Canterbury heritage homeowner can make. While H1 compliance isn't required, we can increase the estimated R value of a timber sash from R0.19 to R0.70 or higher with the correct glass combinations, roughly 40% better than the minimum requirements for a new build.
You don't have to choose between historic charm and modern comfort. Canterbury's turn-of-the-century villas and 1920s Californian bungalows define the region's historic suburbs; Cashmere, St Albans, and Merivale among them. Many feature beautiful native timber joinery recognised under district plan character provisions and Heritage New Zealand frameworks.
The biggest challenge for owners of these homes is the harsh Canterbury winter: freezing frosts, bitter southerlies, and heavy condensation. Single glazed windows simply cannot cope. This is where Sharpglass can help, by retrofitting double glazing into your original timber windows and doors.
Retrofitting involves removing your existing timber sashes, precision-routering the rebate to accommodate a new double glazed pane, and refitting the sashes.
The whole process is designed around one principle: your original timber stays, and your windows simply work better.
Ready to see what's possible with your windows? Book a free heritage window assessment with Sharpglass.
Retrofit double glazing typically costs $XXX-$X,XXX per window, depending on sash size, glass specification, and the condition of the existing joinery. As an indicative guide for a Canterbury villa:
| Scope | Indicative Cost (NZD) |
|---|---|
| Single sash window frame (top and bottom sash, standard IGU) | $1,000 - $1,250 |
| Single sash window (top and bottom sash, Low E + argon) | $1,150 - $1,400 |
| Typical 3-bedroom villa (whole home, 25 frames) | $25,000 - $35,000 |
Note: These numbers are a ballpark and will depend on the job.
Factors that influence the final price include window size and style, the glass specification you choose, hardware restoration, and whether any localised rot repair is needed. Because the work is done sash by sash, you can stage the project room by room. Many homeowners start with bedrooms and living areas, then complete the rest over time.
Retrofitting is generally more cost-effective than full window replacement, which involves removing the entire frame and often triggers consenting and making-good costs that retrofitting avoids.
Yes, if retaining the look and fabric of your home is a priority, retrofitting is the superior choice. It upgrades your existing wooden frames to hold double glazing, preserving the exterior appearance while delivering modern warmth and dryness. Replacement, by contrast, means tearing out the entire window or door frame and sash to install modern aluminium or uPVC equivalents.
Early-1900s Canterbury villas were built using old-growth native timbers like Heart Rimu, Matai, Totara, and Kauri. These timbers are exceptionally dense and durable, and timber is a naturally effective insulator. Research from New Zealand institutes including Scion and BRANZ consistently highlights timber's strong thermal performance relative to standard aluminium. Tearing these frames out means replacing premium old-growth wood with fast-grown modern radiata pine or standard aluminium, a significant downgrade.
Wooden windows with double glazing are the happy medium: original joinery preserved, modern comfort gained.
The glass specification you choose directly impacts your home's comfort and heating costs. Building.govt.nz guidance shows that double glazing can roughly halve heat loss through glazing compared to single glass, and EECA notes that windows can account for a large share of a home's heating energy loss. The figures below compare each option against existing single glazing, with indicative savings for a typical Canterbury home (actual savings depend on house size, heating type, and usage):
| Glazing Type | Reduction in Heat Loss Through Glazing | Indicative Heating Savings | Indicative Annual Savings (NZD) |
|---|---|---|---|
| Standard IGU (air gap) | 30-40% | 10-15% | $200 - $350 |
| Low E IGU (air-filled) | 50-60% | 20-25% | $400 - $600 |
| Low E + Argon-filled IGU | 60-70% | 25-30% | $500 - $750 |
Argon gas is roughly 30% less thermally conductive than air. Paired with Low E glass, which reflects radiant heat back into the home, it delivers the strongest thermal performance available in a standard double glazed unit. The specification of your double glazed windows matters.
Unlike replacements that alter a building's proportions, retrofitting is seamless. By upgrading the existing sashes, original sightlines, shadow lines, and architectural details remain exactly as designed.
Retrofitting is generally more cost-effective than replacement, and you can stage the process room by room. It's also a sustainable choice that keeps historic timber out of landfills. And because the original building fabric is retained, retrofitting in most cases avoids building consent and heritage consent requirements entirely. The Window and Glass Association of NZ confirms that retro glazing existing windows generally requires neither building consent nor H1 compliance. It's a compliance-friendly route to a warmer home.
No. Retrofitting is a specialised, precision joinery process designed to carefully preserve your original timber while enhancing its insulation performance.
Yes. The same retrofit process works for 1920s Californian bungalows, art deco homes, and other character properties with timber joinery, casement and double-hung windows alike.
No. Skilled joiners compensate for the weight of modern IGUs by upgrading the traditional counterweights or installing concealed, heavy-duty spring balances.
You don't need a full replacement. Localised rot can be cut away and matching native timber precision-spliced into the damaged sections before the glass is fitted.
No. The main frames and existing joinery remain intact, and while sashes are at the workshop, openings are sealed with secure, weatherproof inserts.
Yes. Modern Low E coatings are virtually invisible, so you maximise heat retention while your villa looks exactly as it always has.
