Reducing Condensation in New Zealand Homes

When we’re talking with clients who are planning to build a passive home or a high-performing home, one of the key reasons they are looking to build this way is because of the benefits when it comes to condensation. Here in New Zealand, many homes, both old and new, that are built to the standard New Zealand Building Code, will experience ‘crying windows’ or high levels of condensation on their home’s windows.

So why does this happen?

When thinking about condensation, an important element to understand is the dew point. The dew point is the temperature below which water vapour in the air will start condensing to liquid. This is important in buildings, because anywhere there is a dew point, there is a risk of condensation and mould growth. Condensation will form on any object when the temperature of the object is at or below the dew point temperature of the air surrounding the object.

When the indoor temperature cools down the air cannot hold as much water vapour as when it was warm. This results in the water vapour condensing to become a liquid. It then becomes visible on cold, hard surfaces such as glass and windows. Moisture can be harder to see on other surfaces, but still penetrates carpets, fabrics and any other absorbent surface, often making them feel cold and damp. Green Architects also outlines that much of New Zealand housing stock has mould growing in the walls, this is because the dew point occurs in the fabric of the construction. If not dealt with, the condensation combined with mould spores which are present in (amongst other things) dust, leads to a build-up of mould, which, in a lot of cases, is hidden in walls, degrading the fabric of the construction and insulation.

Mould is a leading health concern in New Zealand’s homes. In fact, in 2020, a report by Stats NZ and the Building Research Association says about 28,000 kiwi homes were reported to be always damp and have visible mould. Now we know that visible and hidden mould in the walls and ceilings are both concerns, we imagine this number would be much higher. This report also showed that the presence of mould was associated with asthma, more frequent colds and flu, and an increased number of sick days and care days. People who lived in a house that was always damp were more likely to have had colds or flu in the last 12 months, and to have asthma. Similarly, living in a dwelling that was cold enough to see breath in winter was also associated with higher rates of sick and care days, asthma, mould, colds, and flu.

So how do we reduce condensation?

Managing condensation is a three-pronged approach. Firstly, we want to reduce the moisture in the air in the home, then secondly we can improve the quality of the windows we use, and thirdly, we want to heat homes to reduce the chance of reaching the dew point.

Everyday activities within the home, such as cooking, showering, washing clothes and breathing contribute to the moisture in the air. According to BRANZ:

• Breathing & perspiration creates 3 litre of moisture per day

• Cooking creates 3 litres of water per day

• Bathing creates 1.5 litres of water vapour per person

• One load of clothes in the washing machine creates half a litre

• Unvented clothes drying creates 5 litres of moisture per load

One of the key ways to reduce moisture in the home is through adequate ventilation. Managing moisture by using mechanical extracts and ventilation (opening windows and doors) is essential for ensuring a healthy indoor environment. Without proper ventilation, this can severely reduce indoor environmental quality, increase the risk of damp and mould, and affect the health of occupants.

In passive homes, one of the foundational pillars is using a mechanical ventilation and heat recovery (MVHR) system for this exact reason. Using a MVHR system brings fresh cold fresh air into the house, using the heat from the warm stale air exiting the house to heat up this fresh air. It is crucial to understand that the fresh air and stale air do not mix; so the air coming into the house is warm and fresh. You can read our full blog post on MVHR systems here. Using an HRV system for ventilation is another option, which are installed in your home’s roof cavity and work to draw in fresh air that is forced into of your home through a series of ducts. As this new air is pushed in, the stale air is forced out and reduces the dampness that causes mould, mildew and condensation build ups.

Now we have reduced the moisture in the home, let’s talk about windows.

The type of window you instal in your home does make a difference when it comes to condensation. To put it simply, the warmer the glass surface, the less chance there is of condensation forming. Let’s dive into the details:

• Single-glazed window panes of glass will often be the same temperature as outside, so when we reach -5 degrees overnight (or even colder!) here in Wanaka, the window pane will lower too. The colder window pane leads to condensation because when indoor temperature cools down the air cannot hold as much water vapour as when it was warm. This results in the water vapour condensing to become a liquid. It then becomes visible on cold, hard surfaces such as glass and windows. Single-glazed windows have twice the heat loss of double glazing.

• Double-glazed windows reduce the likelihood of condensation as the pocket of air between the two panes is intended to help maintain the indoor temperature (provided the house is adequately heated) and in doing so, keep the internal pane closer to the internal temperature, which means that condensation is less likely to occur. When rooms are poorly heated, double or triple glazing will not be able to reduce heat lost via conduction through a window. In this scenario, condensation may occur on the inside of windows regardless of the energy efficiency of the glazing (due to the low relative temperature of the glass surface).

• Triple-glazed windows consists of three panes of glass separated by air gaps to provide comparatively high levels of heat retention and noise reduction. With three panes of glass, there are two controlled spaces that can both be filled with argon gas. These two controlled spaces help keep the internal window pane closer to the internal temperature. Triple-glazed windows typically allow for higher levels of humidity and lower condensation than double-glazed windows.

• Low Emissivity (Low-E) coatings on double or triple-glazed windows can increase their effectiveness even further. This coating lets light and heat in, but helps prevent heat from escaping by reflecting the heat back into the room.

• The frame material used on your windows plays a significant role in their performance too. We have an entire blog post on glazing specifically which you can read here, but let’s recap the main points. Aluminium is light, strong and durable but is a poor insulator. We recommend using thermally broken aluminium windows which have insulation between the interior and exterior parts of the frame. Another option is uPVC window frames made from unplasticised polyvinyl chloride, which is a good insulator. It is light, low maintenance and steel-reinforced for strength. Using timber for framing is the traditional approach and is an excellent insulator (performs similarly to uPVC), however does have higher maintenance involved to maintain performance.

Below you can see the comparison table from Smarter Homes which shows the R-value of window and frame combinations. The higher the R-value the higher the performance – less heat will transfer from one side to another.

Now we have reduced the moisture in homes by ensuring effective ventilation and looked at the quality of our windows, now we need to ensure our home is heated properly.

Here in the Queenstown Lakes District, we experience very cold winters with temperatures regularly dipping well into the minuses overnight. Keeping homes warm during winter in this region often sees homes using heatpumps 24/7 or burning through trailer-loads of wood.

One of the driving forces behind our company is changing the way we build, so in turn, we can change the way we live. We are passionate about building well beyond the current minimum standards in the NZ Building Code so people can experience an easier and healthier way to live.

As Team Green Architects, one of our preferred high-performance architects in the region, puts it, “by building a better thermal envelope, you will have a much lower energy requirement to heat your home. Passive House buildings being 10x more efficient than Building Code minimum construction, so your heating is a tenth of the cost of a traditional kiwi build.”

It can seem daunting when faced with the possibility of building better. Some of our clients have chosen to take the path towards a complete passive home, while others have added higher performing elements to an existing design. The choice remains yours.