In New Zealand, much of our housing stock hasn’t been designed for long-term health and durability.
Homes often require significant repairs within 20-30 years of construction (or even sooner). Often the building budget prioritises visible features, while essential elements like moisture management, insulation, airtightness and ventilation are often overlooked.
A well-functioning home is like a balanced ecosystem — all parts need to work together to ensure the home is healthy, comfortable, efficient, and durable.
For over 20 years, we’ve been building and renovating homes, and with the renovated homes we are able to study why so many suffer from issues like mould, rotting framing, and rusted out structural steel elements. We have spent the time to understand where the construction methods and products went wrong and how they can be improved.
Without being airtight, insulation is compromised and acts as a filter – as air is dragged through it but it is never cleaned. Batts turn black over time and that dirty filter, along with all the
by-products of vermin, contaminate all the air that is being drawn back into the house. Window seals and corner junctions consistently fail in the same place and claddings with a high thermal conductivity like fibre cement and plaster can cause dew points to form on the inside of wall build ups where mould grows, timber rots and steel rusts out. Moisture kills homes.
Better technology is available and constantly being developed, and we are always working on and learning better ways of constructing homes. Our experience has taught us that with an open mind, a holistic approach to a project collaborating through the design and construction phase is critical to getting the best results, creating homes that today are much healthier and long-lasting.
To keep a home comfortable year-round, it's essential to treat the structure as a system that takes into consideration all weather conditions and extremes.
For the best results, planning from the start needs to involve a team working closely together, all with important roles to play, with architects, engineers and the builder all communicating from the outset and working towards the same goal. The team can then design the building with careful consideration around moisture and vapor management, solar design and how the thermal envelope works with insulation, airtightness, and ventilation as a whole system.
Houses can be modelled with PHPP and WUFI software which verifies a passive house design, thermal bridging and energy demand. This takes the guesswork out of the equation.
The changes in New Zealand’s H1 regulations that were implemented last year are a great start but still don’t take into consideration how things work together. The Passive House standard is something worth working towards and even if that doesn’t entirely fit for a specific project, high performance technologies from it can be adopted. For most people, a home is their main asset, and it makes sense to invest in a better standard that can be measured and quantified as a return on investment over time.
Think of insulation as sealed pockets of air that help maintain a stable indoor temperature.
Insulation is typically measured as an R-value which is the thermal resistance to heat flow per m². The higher the R-value, the more resistance to heat flow passing through it.
A well-insulated home is both warmer in winter and cooler in summer, which is often misunderstood. Overheating is not due to too much insulation and is often caused by poor design (i.e. a lack of effective shading or control of excess solar gain) and a misunderstanding of how these elements work together with solar design.
For insulation to work effectively however, it needs to be continuous around the entire building envelope and complete. Thermal bridging (or parts of the envelope that allow heat to pass through more easily) can drastically compromise a thermal envelope as well as a poor insulation install. A small gap of 5mm can reduce its effectiveness by 50%, as air leaks allow heat to escape like a fridge with the door open.
Properly installed insulation should also be protected from wind exposure to avoid “wind washing,” which also compromises its effectiveness which can be up to 80% — like wearing a woollen jumper in stormy conditions.
Cost wise, maximising the levels of insulation in a home is generally a small component to the value gained. If the installation is meticulously considered and correctly installed, and when it is in the environment it needs, it is a key component for cutting down on energy costs and creating a comfortable indoor environment.
Ventilation is equally important as we build more airtight homes but should be implemented in all homes no matter how air leaky they are.
An efficient, balanced heat recovery ventilator (MHRV) draws in fresh outdoor air through filters which remove dust, pollen, and pollutants while exhausting the same amount of stale indoor air and indoor pollutants. The heat recovery part of the ventilator uses the heat from the air being exhausted to bring the freshly filtered outdoor air to within 70-96% of the indoor temperature (system dependent) through a heat exchanging system, ensuring a continuous supply of fresh air. A MHRV can filter out moisture, mould spores, VOCs (volatile organic compounds), viruses and other pollutants, making your home healthier and more comfortable. It is important that the replacement air is drawn in from the outside of the building and not the roof space. The cost of a typical ducted MHRV system for a standard NZ family home generally costs around $8,000-$20,000 and some can be incorporated with heat pump systems.
The New Zealand Standard 4303:1990 recommends that indoor CO₂ levels be kept below 1,000 parts per million (ppm) as high CO₂ levels can cause tiredness, impaired judgement and fatigue. But in typical, unventilated bedrooms it’s common to find levels exceeding 3,000-5,000 ppm, especially on still nights, so no wonder we sometimes wake up feeling more tired than when we went to bed! This can be easily tested with an air quality monitor you can buy online for $50-100. A typical home without mechanical ventilation that is not airtight draws in air with contaminants through gaps and cracks in the envelope. The average person expels around half a pint of water vapour every night through breathing, and all the moisture from towels and bathmats drying out long after the fan has been switched off generally stays in the indoor environment until it can find a way out or through the building envelope. Pollutants from cooking, fires, viruses etc. all need to be managed to create a healthy living environment, and a good ventilation system can do that.
Weathertightness and airtightness are crucial for several reasons.
They prevent the house from structural decay from external water ingress, stop drafts and wind washing allowing insulation to perform effectively, and prevents vapour entering the structure through diffusion from internal living conditions.
For instance, during a typical winter night you will find an indoor temperature of around 20°C with a relative humidity of 50% and outdoor temperatures of 3 or 4 degrees Celsius. If the warm air is drawn through the construction build up as it cools, it loses its ability to hold the moisture. It reaches 80% relative humidity at 12.6 degrees (ideal conditions for mould growth) and then a dew point of 9.3 degrees, condensing and forming water droplets. Too often this is found inside the structure. The idea is to stop the moisture in the air from reaching the structure from either side.
The commonly used building wraps and detailing in our opinion should be upgraded. Pro Clima are doing some great work in education around weathertightness and airtightness and their products are not the only solution but are becoming more popular due to their ease of use and thorough detailing and installation instructions. On timber framed houses we like to use rigid air barriers in conjunction with other moisture managing products on the outside that are taped and sealed, and prefer to stick to timber based products as fibre cement has a higher thermal conductivity value and has a risk of shifting the dew point back into the thermal envelope.
An airtight envelope minimises the likelihood of moisture permeating through the building fabric, allowing for mechanical ventilation to be more effective at managing moisture and contaminants from indoor living. Various methods exist for designing and constructing airtight homes, with some systems, like Structurally Insulated Panels (SIPs) or Insulated Concrete Forms (ICF), making it easier to achieve superior airtightness. Each system has its pros and cons depending on site and environmental conditions. Timber frame construction is commonly used in Passive House designs, but requires careful attention to the airtightness layer. For instance, a 1mm gap in framing over a square metre can transfer 800ml of moisture through it in a 24-hour period.
Given that framing is often exposed to weather, it is common to see 1-2 mm gaps during expansion and contraction before being closed in and reaching a stable moisture content.
Air gets through gaps in skirting, sarking, trims, windows and doors, power points and light switches etc but there are products and systems to tackle this (such as Pro Clima’s INTELLO Intelligent air barrier) and with a little bit more thought around the detailing the whole envelope can be greatly improved from the current standard way of building.
Air Tightness testing with a blower door verifies whether a home is adequately sealed or is very air leaky. In New Zealand, standard homes would typically sit around 10-15 ACH (air changes per hour at a test pressure of 50Pa); this means the energy used to heat or cool a home is lost as quickly as it can be replaced through gaps and cracks in the floors, walls and ceilings.
In contrast, homes built to the Passive House standard must achieve no more than 0.6 ACH during a blower door test, significantly improving heat retention and reducing energy consumption. An airtight home can use 75-90% less energy to maintain a comfortable indoor temperature, depending on the climate and location.
Can you open windows in an airtight home and manually manage the ventilation?
In summer, open windows are great for letting in cool air in the evenings (if there is a breeze, windows on both sides of the house are open, and there is a temperature difference between inside and outside) but in winter all the energy used to heat the home would be lost. You can also leave a rangehood and bathroom extraction fan running 24/7 to expel indoor stale air but the air coming in would not be filtered or balanced and with some people the noise could be an issue. Most MHRVs have a summer bypass mode feature which diverts outgoing air around the heat recovery cell, so that incoming air is not warmed by the outgoing air. This allows the MHRV to draw fresh filtered air from outside into the home without heat transfer, helping with cooling.
Investing in a good thermal envelope with insulation, weathertightness, airtightness, and a decent balanced mechanical ventilation system is more than just a cost-based decision—it’s an investment in a healthier, more comfortable home and while there are plenty of other factors to consider when building the envelope should be the priority. While air conditioning units, fans, and purifiers can help, they do not address the entire ecosystem of your home. By designing these elements to work together from the start, you’re building a home that will last, be energy efficient, and provide a safer, more comfortable living environment for decades to come.
