Insulating with Glass
At some point in almost every renovation or new build conversation, windows become the sticking point. The architect wants more glass. The engineer is worried about heat loss. The client is looking at the price difference between double and triple glazing and wondering if it's actually worth it.
These are legitimate tensions, and the answer is genuinely "it depends" — but not in the hand-wavy, non-committal way that phrase usually gets used. It depends on specific, knowable things: your climate zone, your building orientation, how much glazing you have, what your heating system looks like, and what you're actually trying to achieve. This article is about helping you think through those questions clearly, so you can make a decision you'll still feel good about in ten years.
Glass isn't the enemy — bad glass specification is
The building industry spent decades treating glass as the thermal weak link — the thing to minimise in the interest of energy efficiency. Add more insulation, reduce the window area, keep the glazing small. That advice made sense when the only option was single glazing with a U-value around 5–6 W/(m²K). It makes much less sense now.
Modern high-performance glazing — triple-glazed units with Low-E coatings and noble gas fills — can achieve U-values below 0.5 W/(m²K). That's approaching the performance of a well-insulated wall. The glass that was once a liability is now a genuine design asset, capable of capturing solar heat in winter, providing daylight without cold draughts, and contributing to passive solar performance that reduces heating demand.
The problem isn't glass. It's glass that's been specified without thinking through the full picture.
The three questions that actually determine your glazing spec
Before anyone starts talking about specific products, these are the questions worth answering first.
What climate are you building in? Queenstown and Christchurch have cold winters with significant heating demand — the performance gap between double and triple glazing is meaningful and the payback is real. Auckland's mild winters make the case for triple glazing less clear-cut, particularly on a budget-constrained project. This doesn't mean Auckland homeowners should accept poor glazing, but it does mean the cost-benefit calculation is different.
How much glazing do you have, and where? A modest amount of north-facing glazing with good overhangs is a solar asset in winter and manageable in summer. A wall of west-facing glass in a poorly ventilated room is a very expensive problem. The orientation, size, and shading of glazing matters as much as the U-value of the glass itself.
What are you comparing against? Upgrading from single to double glazing with a Low-E coating and argon fill is almost always worth doing — the performance improvement is large and the cost increment is modest. Upgrading from already-good double glazing to triple glazing is a finer calculation that depends on your specific situation.
What actually moves the performance needle
If you're comparing glazing products and trying to work out where to spend your money, here's the rough priority order:
| Low-E coating Highest impact | Reflects heat back into the room rather than letting it escape. Should be standard on any window specification — the cost premium is small relative to the performance gain. |
| Gas fill (argon minimum) High impact | Reduces heat transfer through the gap between panes. Argon is the cost-effective standard. Krypton and xenon offer further improvement at higher cost — worthwhile for triple glazing. |
| Warm-edge spacers High impact, often overlooked | Eliminates the thermal bridge at the glass edge. Should be non-negotiable but often isn't specified explicitly. Confirm with your supplier. |
| Third pane (triple glazing) Climate-dependent | Meaningful improvement in cold climates and for large glazed areas. Less critical in mild climates or where budget is constrained — get the double glazing spec right first. |
| Frame material Don't ignore it | The frame can account for 20–30% of a window's total heat loss. A well-specified glass unit in a thermally poor frame still underperforms. Timber and uPVC frames consistently outperform standard aluminium. |
The renovation calculation is different from new build
In a new build, glazing decisions happen early and everything else gets designed around them. In a renovation, you're often working with existing openings, existing framing, and a budget that has to stretch across many competing priorities. The calculus changes.
You have a 1980s home with single glazing throughout. The insulation is also inadequate. Budget is limited.
Where to start: Ceiling and underfloor insulation first — highest impact per dollar. Then glazing replacement, prioritising the rooms you use most and the elevations with most heat loss (south-facing in NZ).
What to specify: Double glazing with Low-E coating and argon fill as the baseline. Warm-edge spacers. Thermally broken aluminium or timber frames. Triple glazing where budget allows, particularly for large openings or south-facing windows.
What not to do: Replace windows without also addressing insulation. The two work together — a well-glazed house that's still losing heat through a poorly insulated ceiling isn't performing as well as it could.
The conversation your window supplier may not be having with you
Window suppliers are generally good at talking about their products. They're less consistently good at helping you understand how those products fit into the broader performance of your building. A salesperson who quotes you a centre-of-glass U-value without mentioning the whole-window figure, or who doesn't raise the question of spacer type, or who doesn't ask about your orientation and climate — that's a gap worth filling yourself.
The questions to ask:
What is the whole-window U-value, including frame? The centre-of-glass figure is always better — make sure you're comparing the same thing across products.
What spacer type is used? Warm-edge as standard, please.
What gas fill? Argon minimum; krypton for triple glazing.
Where is the Low-E coating positioned? The answer affects both heat retention and solar gain, and should be chosen based on your climate and orientation.
Want to understand how solar heat gain (G-value) interacts with your glazing spec? That's covered in our next article on G-value — the other number on your window datasheet that's just as important as U-value.
