Heat transfer and glass: why modern windows aren't the weak spot everyone thinks they are
Glass has a reputation problem. Ask most people whether windows are good or bad for a building's thermal performance, and they'll tell you: bad. Glass is where the heat escapes. Glass is the weak spot. You want a well-insulated house, you want fewer windows.
That was a reasonable position in about 1985. Modern glazing technology has moved on considerably since then — and understanding what's actually happening in a contemporary window unit changes how you should be thinking about specification, budget, and where to invest in your building envelope.
First, the number that matters: U-value
U-value measures how readily heat passes through a material. The lower the number, the better the insulation. A single pane of glass has a U-value of around 4.8 W/(m²K) — meaning heat moves through it very easily. A solid timber-framed wall with decent insulation might sit around 0.2–0.3. Old single glazing wasn't just a weak spot; it was a gaping hole in the thermal envelope.
Here's how things have changed:
That's not a small improvement. Going from single glazing to a well-specified triple-glazed unit is roughly equivalent to adding 150mm of insulation to a wall. The glass that was once the problem is now — in high-performance form — barely distinguishable from the wall around it.
What's inside the unit that makes the difference
The gap between panes in a double or triple-glazed unit isn't just air — or at least, it shouldn't be. Air conducts heat reasonably well. Noble gases like argon, krypton, and xenon are denser and conduct heat less readily, which is why filling the gap with them drops the U-value further. Argon is the most common and cost-effective. Krypton performs better but costs more. Xenon is the high-performance option you'll typically only see in Passive House-certified units.
Low-E coatings — the part most people don't know about
Low-emissivity coatings are where a lot of the modern performance gain comes from, and they're genuinely clever. A thin metallic layer is applied to one of the glass surfaces inside the unit — invisible to the naked eye, but doing a lot of work.
Here's what it does: long-wave heat radiation (the warmth your body and your heating system emit) gets reflected back into the room rather than passing through the glass. Short-wave solar radiation (sunlight) is still allowed through. So you get the light and the passive solar gain, without the heat loss.
The part everyone overlooks: edge spacers
Here's the detail that separates a well-specified window from one that looks good on paper but underperforms in practice. The spacer bar sits around the perimeter of the glass unit, holding the panes apart and sealing the gas fill inside. Traditional spacer bars are aluminium — cheap, durable, and thermally disastrous.
Aluminium conducts heat very efficiently. A traditional aluminium spacer creates a thermal bridge right around the edge of every pane of glass, which is why you often see condensation forming in a strip around the perimeter of windows even when the centre of the glass is fine. The glass is performing; the spacer is undoing some of that work.
Modern warm-edge spacers made from stainless steel, fibreglass, or polymer composites address this. They're not exotic or expensive — they're standard in any window product worth specifying — but it's worth confirming they're included, particularly if you're comparing quotes where the price difference seems hard to explain.
| Single glazing Avoid | High heat loss, condensation on glass, cold surfaces. No place in new construction and worth replacing in renovation if budget allows. |
| Double glazing, air-filled Minimum | Better than single, but leaving performance on the table. Fine for low-budget projects, but specify Low-E at minimum. |
| Double glazing + Low-E + argon Good | Solid everyday specification for most NZ climates. Good value, widely available, meaningful improvement over standard double glazing. |
| Triple glazing + Low-E + krypton/xenon Best | Required for Passive House. Worthwhile in cold climates or for large glazed areas. Higher upfront cost offset by heating savings and comfort. |
| Warm-edge spacers Always specify | Non-negotiable in any high-performance window. Eliminates edge thermal bridging. Should be standard — confirm if not mentioned. |
What this means when you're actually choosing windows
Window specifications can be confusing, and suppliers don't always make it easy to compare like for like. When you're looking at quotes or product data, the number to focus on first is the whole-window U-value — not just the centre-of-glass figure, which is always better and often the one that gets quoted in marketing material. The whole-window value includes the frame and edge effects, which is what actually determines how the window performs once it's installed.
From there: confirm the gas fill (argon as a minimum), confirm the Low-E coating is present and which surface it's on, and confirm warm-edge spacers. If a supplier can't answer those questions clearly, that tells you something too.
Next up: G-value — the other glazing number that determines how much solar heat your windows let in, and why getting it wrong can make a well-insulated house uncomfortably hot.
