Why New Zealand Homes Still Feel Cold Even With Double Glazing.
Double glazing has become one of those terms people throw around in housing conversations almost automatically now. It sits somewhere alongside stone benchtops and walk-in wardrobes on the list of things that are apparently meant to make a house “high-end”. For many people building or renovating, the conversation starts and ends with one question: “Does it have double glazing?” But windows are a lot more complicated than simply counting the number of panes of glass.
The reality is that many homes with double glazing still perform poorly because the rest of the window system, and often the overall design itself, has not been properly considered. A window is not just glass. It is an entire assembly made up of framing, spacers, seals, orientation, shading, installation method, and thermal detailing. If one part performs badly, it can compromise the performance of everything around it. That’s why some homes with expensive glazing still overheat in summer, lose heat in winter, suffer from condensation, or feel uncomfortable despite ticking all the right marketing boxes.
A lot of the performance issues actually begin with the frame itself. Standard aluminium frames are highly conductive, meaning they transfer heat and cold very efficiently. In winter, they can effectively behave like little thermal bridges around the perimeter of the glass. That’s why thermally broken frames, or frames made from non-conductive materials such as uPVC or timber composites, make such a significant difference. Even then, the frame alone is not enough. Where the window sits within the wall also matters enormously. Ideally, windows should be recessed into the warmer part of the wall assembly rather than sitting exposed toward the colder exterior line. Poor placement can increase thermal bridging and reduce the effectiveness of the insulation layer around the opening.
Then there is the glass itself. Not all double glazing is equal, despite how it is sometimes marketed. High-performance insulated glazing units generally include Low E coatings, argon gas filling, and properly designed thermal spacers between panes. These details might sound technical and slightly boring at first glance, but they have a huge impact on how the home actually performs day to day. Low E glass helps reduce unwanted heat transfer while still allowing natural light inside. Argon gas improves insulation performance between the panes. Warm-edge thermal spacers reduce condensation risk around the edges of the glazing. Remove these elements, and “double glazing” can quickly become a fairly average-performing system carrying a premium price tag.
The airtightness of the joinery system matters too. This is one reason why European-style tilt and turn windows often outperform many standard sliding window systems commonly used in New Zealand. They typically create a much tighter compression seal when closed, reducing unwanted air leakage and improving thermal performance. Anyone who has stood beside a rattling sliding window during a Canterbury southerly already understands this concept emotionally. But before diving into technical specifications, there is something even more important: good design. A high-performing home should start with orientation, window size, proportions, and shading strategy before anyone starts selecting glazing options. If these fundamentals are wrong, no amount of expensive glass will completely fix the problem afterward.
Large north-facing windows can be fantastic for winter solar gain, but they need correctly designed shading to stop overheating during summer. West-facing glazing is even more challenging because late afternoon sun is lower, harsher, and much harder to control. Without protection, homes can become uncomfortably hot very quickly. This is where deep eaves, louvres, external blinds, pergolas, or other shading systems become critical. And importantly, the protection needs to happen outside the glass. That’s a detail many people misunderstand. Once sunlight passes through the glazing, the horse has bolted, and the heat is already inside the home. Pulling curtains or internal blinds afterward is a bit like trying to stop a barbecue from getting hot by putting oven mitts on after you’ve touched it. Internal coverings help with glare and privacy, but they are far less effective at preventing solar heat gain compared with external shading.
At the same time, good window design is not purely about thermal performance. Windows shape how a home feels emotionally as well. They influence views, natural light, privacy, ventilation, and connection to outdoor spaces. A carefully positioned window can completely transform the atmosphere of a room. That balance is what makes window design surprisingly complex. Too much glazing can create overheating, glare, and heat loss. Too little glazing can make spaces feel dark and disconnected. Large feature windows may look dramatic in a magazine, but if they are poorly oriented or unshaded, they can also turn the living room into a conservatory by 5pm.
This is why experienced designers matter so much. Good design is not about chasing trends or copying what looks impressive on Instagram. It is about balancing comfort, performance, durability, aesthetics, and practicality together into one cohesive solution.
So before getting too excited by the phrase “double glazed”, it is worth asking a few deeper questions. Are the windows recessed properly into the wall assembly? Are the frames thermally broken or non-conductive? Is the glazing specification actually high performance? Do the windows have adequate external shading? Are the seals airtight? Has the design balanced comfort, views, daylight, ventilation, and privacy properly?
The next time you see a spectacular architectural home covered in floor-to-ceiling glazing on a hillside or across a magazine spread, it is worth pausing for a second and asking: will that house overheat, and does it actually have any meaningful external protection at all? A beautiful wall of glass might look incredible, but if it turns the lounge into a sauna every afternoon, the house is probably showing off more than performing.
