Why Are New Builds Overheating and Costing More?
Warmer. Drier. Healthier. That’s the promise.
Across New Zealand, the reality is falling short of expectations. Homes that should outperform old ones are instead overheating in summer, leaking warmth in winter, and generating higher energy bills than expected. The frustration for homeowners is that these issues are not the result of “too much insulation”, as oversimplified headlines often suggest. They are the outcome of multiple overlapping variables, such as design flaws, material choices, and a Building Code that rewards minimum compliance instead of long-term performance.
In this article, we analyse the technical causes of an overheating building and demonstrate how design decisions compound into comfort issues. We outline what homeowners can do to prevent or correct these failures if they find themselves in this situation.
The Core Problem: Design and Performance Gaps
Orientation:
Orientation is one of the most overlooked drivers of overheating. A house with large panes of west-facing glass absorbs the full blast of the afternoon sun. Without eaves or shading, these living areas act like solar ovens at precisely the hours when solar gain is at its strongest. What’s intended as a feature to maximise natural light instead becomes a liability, creating persistent discomfort and higher cooling demands in summer.
Shading is not optional; it’s critical. West-facing windows in particular demand careful design attention. We explored this in depth during the Superhome Shading Webinar. The full recording is available here, for those who’d like to dive deeper.
Aluminium Joinery:
Windows are responsible for at least 30% of a home’s heat loss during winter. High-performance windows, such as those with deep thermal breaks and insulated frames, can provide double the insulation compared to standard thermally broken aluminium windows. Upgrading can save homeowners 15% in energy costs.
The majority of these standard frames are non-thermally broken, meaning there’s no barrier between the inside and out. Because aluminium is highly conductive, the frame itself acts as a heat bridge. In summer, it channels solar heat indoors and radiates it into the room. In winter, the process reverses, drawing interior warmth outside and undermining comfort.
This is not a small margin of inefficiency; it is a design flaw that forces the homeowner into a cycle of higher energy use year-round.
At this point, the burden often shifts onto the occupant. They are advised to “manage” overheating by closing curtains during the day. Behaviour becomes the patch for poor design. While closing curtains may delay heat gain, it is not a solution; it comes at the cost of natural light and livability. Occupants are forced to choose between comfort and daylight because the joinery itself is not doing its job.
The Super Windows Webinar Slides include clear visual examples of recessed installation and diagrams that show exactly how thermal bridging works in practice. They make it easy, even for those with no background in windows, to see and understand why detailing and installation matter so much. You can view the full slide deck here.
Glazing:
Glazing choices deepen the divide between performance and failure. To the untrained eye, double-glazed windows can look identical, but their performance can be worlds apart. The distinction lies in the engineering. High-performance glazing is not just two panes of glass separated by a cavity; it is a carefully designed system that combines thermal insulation, argon gas and high-quality Low-E coatings on both panes of glass.
Low-E coatings allow visible light to enter, while reflecting infrared radiation outward in summer and bouncing heat back into the home during winter. By contrast, a low-quality “double-glazed” window with only a single internal Low-E coating traps solar heat in summer and prevents warmth from escaping at night. The result is a space that overheats by day and refuses to cool by evening, turning what should be a comfort feature into a persistent problem.
Urban Setting:
Urban context can make overheating problems significantly worse. Many new builds are located in high-density subdivisions paved by asphalt, concrete and other heat-retaining surfaces. These materials absorb solar energy during the day and release it slowly at night. This phenomenon, the Urban Heat Island effect, means cities stay hotter for longer.
In dense developments, where airflow is blocked by neighbouring structures, the effect compounds. Limited ventilation prevents heat from dissipating naturally, trapping it around homes and amplifying internal temperatures. Even well-designed houses with high-performance insulation and glazing can struggle to maintain comfort under these conditions if external shading, ventilation, and site planning aren’t considered.
Mitigating the UHI effect requires a combination of strategies: planting trees and greenery to provide natural cooling, using reflective or lighter-coloured surfaces, and incorporating external shading and ventilation into building design. In high-density urban areas, these measures can be the difference between a home that stays comfortable and one that becomes a solar oven in summer.
Building Code:
Overheating issues are often compounded by New Zealand’s Building Code. H1 compliance is commonly treated as the standard to aim for, when in reality it is only a minimum requirement. By international standards, the airtightness, insulation, and glazing requirements are low. A property can fully comply with the Code yet still underperform in real-world conditions. For instance, windows that aren’t installed recessed will fail to achieve their advertised thermal performance, reducing comfort and efficiency.
Energy Modelling:
One of the most overlooked tools in preventing overheating is energy modelling, yet many New Zealand homes are built without it at the design stage. Without energy modelling, designers can’t reliably predict how a home will behave under real conditions. Overheating risks will remain invisible until keys are handed over and the occupants move in. At that point, fundamental design decisions can’t be easily changed.
Once It’s Built, It’s Too Late to Fix Easily
The harsh reality is that overheating problems discovered after completion cannot be solved at their root. Orientation cannot be rotated. Rooflines cannot be reshaped. Glazing ratios cannot be reduced without major reconstruction.
Retrofitting becomes the only option. Ventilation systems can be installed, external roller screens or shading structures can be bolted on, and glazing units can sometimes be swapped for higher-performing alternatives. But these are not equivalent solutions; they are compensations. They disrupt lives, carry additional costs, and often deliver only partial relief.
Ultimately, homeowners end up paying twice: once for the build itself, and again for the corrections that could have been avoided with performance-based planning.
What Can Be Done:
Before Building or Buying:
Build for performance, not minimum compliance.
Homes built to the current Building Code may meet H1 requirements, but these are minimum standards that do not anticipate future climate or performance expectations. Homes built to today’s building code will underperform tomorrow, when summers are hotter and energy costs are higher. To avoid chronic overheating, focus on orientation, glazing, insulation, and shading from the start.
Passive House and Superhome Approach:
Adopting Passive House (international standard) or Superhome standards ensures homes maintain comfortable temperatures year-round. Key strategies include:
- Exceptional airtightness and insulation
- Elimination of thermal bridges
- Mechanical ventilation with heat recovery
- Optimised window placement and shading
The outcome is consistent comfort year-round and as much as 75-90% lower heating and cooling demand compared to code minimum construction.
In New Zealand, the Superhome Movement promotes similar principles and adapts them to local conditions. It provides open-source design resources, peer networks, and real-world case studies. While homes built to these standards may have slightly higher upfront costs, they deliver lower running expenses, healthier indoor environments, and increased long-term value.
Energy Modelling:
The most powerful tool for closing the performance gap is also the most underutilised: energy modelling. Simulations show how a home will behave before the first foundation is poured.
Designers can:
- Identify overheating risks
- Test shading strategies
- Optimise window placement and glazing ratios
Energy modelling is cost-effective, typically $2,000 – $3,000, less than 1% of the total project cost and is often included as part of an architect’s service. A small change during the planning stage, such as altering window placement, adding shading and using high-performance windows, could prevent years of discomfort and high energy bills.
After Overheating Is Discovered:
Retrofitting Options:
Once a home is built, correcting overheating is more difficult and expensive. Structural elements like orientation, rooflines, or glazing ratios cannot be changed without major reconstruction. Retrofitting options include:
- Installing ventilation systems
- Adding external roller screens or shading structures
- Upgrading to high-performance glazing
These measures are compensations; they cost more, disrupt lives, and usually only partially solve the problem.
External shades:
Where options like deep eaves aren’t feasible, external shades are one of the most effective solutions, typically costing between $2,000 – $4,000 per window.
Overheating Recap:
The path forward is clear: Insulation is not the enemy. The real drivers of overheating are poor orientation, inadequate glazing, aluminium thermal bridges, and a Building Code that confuses legality with real-world performance. Addressing these issues requires smarter, performance-focused design from the very beginning.
By following Passive House and Superhome principles, optimising orientation, specifying high-performance windows, eliminating thermal bridges, and incorporating shading and ventilation. Homeowners and designers can create homes that remain comfortable year-round while dramatically reducing energy use.
For existing homes, targeted retrofits such as external roller screens, high-performance glazing, or improved ventilation can help reduce discomfort or energy costs.
Ultimately, prevention beats cure. Investing in thoughtful design, quality materials, and energy modelling today ensures homes that are not only livable and comfortable now, but resilient to the hotter, more unpredictable climate of the future.
Need expert advice?
At Koffman, we help you select the best option based on your project’s design, climate zone, and performance goals.
Whether you’re planning a new build or assessing your current windows, our team will walk you through what matters, so you can make a confident, informed decision backed by clear, technical insight.