Choosing Window and Door Joinery: Aluminium, Timber and Thermally Broken

Because the frame is a large part of a window’s heat loss, not a neutral surround. A window is really two decisions, the glass and the frame, and a high-performance pane set in a cold frame is let down at its edges. BRANZ treats the frame and the glazing together as one whole-window result for exactly this reason.

It is easy to focus on the glass and forget the frame carries it, seals it and, in the wrong material, conducts heat straight out of the room. The pane, the double or triple glazing, is covered in our guide on double versus triple glazing. This article is about the other half of the decision, the joinery, and how the two are specified together so the window works as one system rather than a good pane undone by the frame around it.

There are four you will meet in a New Zealand home: standard aluminium, thermally broken aluminium, timber, and composite or uPVC. They run from cheapest and coldest to warmer and dearer, and each carries the glazing you specify, so the choice is about how much heat the frame itself lets go.

Standard aluminium is the default in most volume housing because it is strong, slim and inexpensive, but bare aluminium is an excellent conductor, so the frame runs cold. Thermally broken aluminium keeps the same look and strength but adds an insulating barrier inside the frame. Timber insulates naturally and performs well without any break, at a higher cost and with maintenance on the exposed exterior. Composite frames pair a timber or insulated inner with a hard-wearing outer skin, and uPVC frames, common in Europe, insulate well but are less established here. The table sets the practical differences side by side.

Which one suits a given home depends on the performance you are after and where each window sits, which is why the frame is never chosen in isolation from the glazing and the orientation.

Thermally broken joinery is aluminium framing with a low-conductivity barrier set between the inside and outside faces of the frame, so the two are not in direct metal-to-metal contact. Heat cannot short-circuit through the metal, so the inner surface stays warmer and the frame loses far less heat.

The break itself is usually a strip of polyamide or similar material bonded into the frame during manufacture. Picture a standard aluminium frame as a single piece of metal running from the warm room to the cold outside air: whatever the glass does, the frame hands heat straight out and draws cold straight in. The thermal break interrupts that bridge. The result is a frame whose inner face sits closer to room temperature, which cuts heat loss and, just as importantly, keeps the surface above the point where moisture forms. It is the standard way to bring aluminium joinery, the material most New Zealand homes already use, up to a level that suits a warm, dry home, and it pairs naturally with the airtight, well-insulated approach we describe in our guide to airtightness and why it matters.

A cold frame chills the air beside it and its inner surface drops below the dew point, so moisture condenses on the frame and the glass edges. A warmer frame, thermally broken or timber, stays closer to room temperature, so it feels more comfortable to sit near and it stays dry.

Condensation is not a mystery or a fault in the house so much as physics: when a surface is colder than the dew point of the room air, water forms on it, the same way a cold drink beads in summer. BRANZ points to cold internal surfaces as a leading cause of the winter condensation and the damp, mouldy reveals many New Zealand homeowners know well, and standard aluminium frames are a common offender because they run so cold. The comfort side matters too. A cold frame and glass edge create a draught of chilled air that falls into the room, so you feel cold near a window even when the air temperature reads fine. Warmer joinery removes both problems at once, which is why the frame is a health and comfort decision, not only an energy one. Good ventilation still has its part to play alongside the joinery.

The frame, the glazing and the orientation are specified together as one whole-window result, matched to each elevation rather than chosen off a single price. PHINZ modelling and BRANZ guidance both treat the window as a system, so the joinery is settled alongside the glass, not after it.

In practice that means a north elevation gathering winter sun is a different decision from a cold, shaded south face or an exposed coastal wall, and the frame and glass are matched to each. This is the kind of question that is cheap to solve on paper and expensive to fix once the joinery is ordered, so at Ecotectural it is settled early through the approach we call Around One Table, with the architect, an energy-literate engineer and, where it is useful, an energy consultant deciding it together. Orientation drives much of it, which is why we plan the glazing around the sun, as our guide to natural light and space in home design explains. The joinery specification then follows the light, the exposure and the performance target for the home, rather than being a late substitution to hit a number. You can see how that whole-of-home thinking runs through our design and build service.

The frame matters as much as the glass, because it is a large part of a window’s heat loss. Standard aluminium is the coldest and most condensation-prone; thermally broken aluminium adds an insulating barrier inside the frame; timber and composite insulate naturally at a higher cost. Whichever you choose, the frame is specified alongside the glazing and the orientation so the whole window performs as one.

Getting this right is the difference between a window that feels warm and stays dry and one that streams with condensation on a cold Nelson morning. Ecotectural has designed and built architectural homes across Nelson Tasman since 2006, settling the joinery, the glazing and the orientation together at the concept stage rather than leaving them to a late choice. If you are planning a high-performance home, the best first step is to arrange a consultation before the window schedule is locked in.