Moisture management in homes

Jul 15, 2025

Moisture is a hot topic in winter: it affects new and old housing stock when not managed correctly, resulting in higher energy bills, mould and bacteria growth.

Mould can degrade a structure rapidly and can have severe health effects once it establishes itself and contaminates the air you breathe. It needs a water source to grow and that generally comes through external leaks or internal humidity levels becoming saturated and condensing as the air becomes cooler. With indoor temperatures generally sitting around 20 degrees it’s a perfect environment for Mould and bacteria to establish.

External leaks in the cladding system and moisture barrier can create the water source needed for mould/fungi growth as the water makes its way into a building structure and cannot dry out.

Internal moisture generally comes from vapour build up as it is constantly being added to the internal environment through living conditions.

Human bodies release vapour through breathing and sweating, add that with boiling the jug, cooking, showering, washing and drying clothes etc and we are talking litres, it will at some point reach a saturation point if not managed. Cool air cannot hold the same amount of vapour (measured in Relative Humidity RH) as warm air so as the temperature cools it will condensate on cold surfaces like glazing or in a wall assembly on products with high thermal conductivity.

With the upgrade to the H1 (NZ insulation compliance requirements) in 2023 new homes subsequently became more airtight and the more airtight a home is the better, but the physics of the internal vapour change again and require additional management. This is where the H1 regulations have fallen short, like they did in the 1978 H1 upgrade, which altered how moisture was transported and could dry out with the methodology at the time, and in some cases contributed to leaky homes. Increasing insulation values and using better wrap systems are a great step forward in building better and more efficient homes that will last longer, but have created the need for mechanical ventilation to manage the internal vapour. Building and energy costs will continue to rise and a home that performs better is healthier, more comfortable, more efficient and lasts longer. Looking at the 20+ year cost of a home its a much more sustainable option to invest in building better.

You may have heard from people in the 50+ age group or be in that age group that grew up in an old cold home – that was fine. In older homes up to 1978, insulation wasn’t required and that old housing stock had building envelopes with materials like timber weatherboards that breathed quite freely and any leaks or internal vapour had enough airflow to be managed. Although very drafty, cold and inefficient with energy costs they were not creating perfect conditions for fungal/ mould and bacteria growth. As insulation was introduced with upgraded building code requirements the physics of the vapour transport changed. Accompanied with building materials like fibre cement and plaster (with high thermal conductivity values that bought the cold/ dew points inside the building envelope) became popular. Substandard detailing and poor installation added to the fire and problems appeared. Both leaks in external envelopes and dew points in wall assemblies or ceiling spaces that couldn’t dry out became common and then mould growth set in.

But, there are lots of easy and helpful things you can do to improve the vapour/moisture management in your existing home.

Have extraction fans in all wet areas and keep the doors closed when showering or doing the laundry
Make sure the kitchen extraction fan is on even when boiling the jug if possible
Don’t use portable un-flued gas heaters as they constantly release vapour
Wipe down cold surfaces frequently like windows or any surface that is prone to moisture build up.
Cover up fish tanks, toilet lids or any water source that can evaporate.
Dry clothes outdoors
Clean surface mould with a clean cloth and detergent then vacuum the surface and dispose of the cloth after wiping.
Choose building products with low thermal conductivity values
Open windows when practical.

In summer with a breeze opening windows is a great way to ventilate and cool a home naturally...

but it’s not always the ideal thing to do. The ability to control temperature and air quality is reduced. Depending on location external air might carry pollutants, allergens, or insects. And if there’s no wind you don’t always get the air exchanges you need depending on the occupants and size of the home to keep the relative humidity at healthy levels. In winter all the energy used in heating can be lost and it becomes an expensive exercise. Wet area extraction fans also help but as they extract air out of the building, air is drawn in through walls, windows, ceilings etc to take its place. Especially in older homes that air can be contaminating as it is drawn through over dirty insulation, vermin, potential mould/rot etc.

The best way to consistently control internal vapour is with a good ventilation system

and Mechanical Heat Recovery Ventilation systems (MHRV) make the most (financial and practical) sense. There’s a lot of different options but a balanced system will give the best results and is a fundamental principle of the passive house standard for a reason. They extract the same amount of air over a heat recovery system that heats the filtered fresh incoming supply air to within 70-98% of the temperature of the extracted air. A well designed system will deal with all that internal vapour, supply fresh filtered air and keep the energy required for heating and cooling to a minimum as it’s not being completely lost. A high performance home will take care of the wet area extraction as well. With an older home if you have moisture issues it won’t fix what is already happening in a structure if it has started but you won’t be feeding the water source needed for mould to grow rampant internally and the air quality will improve as it is constantly being exchanged. In summer if there are a lot of bugs around or no breeze there are summer bypass modes which turn the heat recovery part off and bring the night air in to help with cooling.

Not all MHRVs are created equal though and there is a lot to consider to get the best results. A professional design will calculate supply and extract air flow rates and determine the size of a system. Good systems are calculated on 1ACH (Air Change an hour) and others work on .5ACH or less. Ducting should be in the building envelope and/or insulated so the energy isn’t lost in a roof space and exposed to outdoor temperatures causing condensation. There are also different levels of filters. With older homes Decentralised MHRV systems can be a cost effective option to consider – they are generally through wall units that work in pairs (as one unit extracts the other is supplied for a period of time and then they switch over). They will not give the same results as a centralised system but can still improve the indoor air quality significantly for a lower cost.

External moisture from wind-driven rain/leaks can be addressed with a high-performance vapour permeable weather & airtight building wrap as a secondary line of defence. Not a system with substandard tapes that peel off with temperature change and staple penetrations all through them that water can enter through. Added with a good cross-ventilated cavity system, and smart envelope detailing. It’s a small upfront cost to upgrade to a premium weather tight barrier system that is fully taped and sealed but the long-term payoff is huge. As cladding naturally degrades over time, water will eventually find its way in – so having that reliable secondary line of defence is important.

All homes have a lifespan and the best thing to do will depend on many specific individual factors like what value the building materials still have, how the home was built, the current health of the home and your budget. Building new you get the chance to get it right from the start but renovating the right way can also make huge improvements and it might make more sense economically as the most sustainable house is the house that’s already built. Picking the right team from the architect to the builder and subcontractors that understand the science behind how things work and implementing the correct installation is crucial. By energy modeling a house in the design phase through PHPP or WUFI software, it is possible to calculate the moisture transport and energy consumption so there’s no guesswork. Product selection payback periods can be calculated and cold spots that create moisture issues/ dew points can be eliminated creating healthy homes that will last.

Whether you’re looking to renovate your existing home or build new, one of the most important things to think about is building a team that will work collaboratively on a science-based approach to get you, the client, the best outcome possible.

Client Perspective: Creating a healthy, family home on challenging land

Mar 28, 2025

Building a custom home is a significant undertaking, but constructing on a tight site adds layers of complexity that require exceptional planning, expertise, and collaboration.

When our clients approached us about building on a flood zoned property, they were looking for a builder who could turn potential obstacles into opportunities, and ultimately create the healthy family dream-home they were looking for. This interview with the owner of project “Terrace Ave” (who also happens to be a tradesperson), offers an inside look at how technical expertise, transparent communication, and a collaborative approach resulted in a successful project that came in on budget while exceeding expectations. Their experience highlights the value of engaging a builder focused on creating homes that are not only beautiful but perform well in the Bay of Plenty’s unique environmental conditions.

Building on a Challenging Site

What did you see as the major challenges with this site and what did you want to achieve?

“The difficulties with this site in particular was that it is on a flood zoned area, so it was really important for us to have that chat with the builder from the get go, because we needed to know what we were in for, and how that was going to impact not only the budget, but the build itself, and what we could design on the site.”

Collaborative Design: Working with Builder and Designer from Day One

Did you work with the builder and the designer throughout the process?

“I called Toby up and said ‘Hey, we’re thinking about purchasing this piece of land, what do you think? Do you think you could come and have a look?’ And he said he would pop around from the office. From that point onwards, it was very easy – I knew it was just a good fit. When things are right it’s easy, and it just felt easy.”

What ultimately convinced you to proceed with Altbuild as the building partner for your family home?

“We decided to engage Altbuild from the get go. So yes, pricing was all part of that, but Toby was involved from the design point all the way through. So we worked with Lloyd Bassett Smith, our designer, Toby as project manager and us as the clients. We all designed the house sort of together, and I thought that was invaluable. Having the builders input into the design from a very early stage – it meant that we could change things if we needed to as we went. So we kind of talked about the budget all the way through.”

Finding a Trusted Partner in the Building Industry

As a tradesperson yourself, what were you looking for in a builder?

“As someone in the industry, one of the biggest things was getting a builder that we trusted. I have a little bit of an advantage, because I was working with Toby on other projects prior to this [as a sub-trade] so I knew how he treated his subcontractors. And I think that’s a big telling point you know, how you treat other people, not just the client who’s paying the bills. So I had a bit of an insight into that beforehand. And for me, it was important to have someone that was obviously skilled and competent in what they were going to do but also was kind and thoughtful and pragmatic. And with Toby and Mark there’s no ego, and you can tell that straight away.”

Problem-Solving Throughout the Building Process

What unexpected challenges popped up during your build? How did Toby and Mark work to solve those for you and your wife?

“I guess with every project there’s always something that doesn’t go super smoothly, but there was nothing here that was massive. But maybe that proves that, like they (Toby & Mark) were the buffer between stuff happening and us as the clients. So if there was anything that happened it was kind of like: ‘this thing is potentially an issue, here are the solutions, which one do you want to pick?’ There was no big, big issue that was unforeseen and then dumped on us and then, oh, by the way, it’s going to be this much extra money, and we didn’t know about it. There was nothing like that. But maybe that was part of going through the design phase altogether as well.”

Technical Excellence in Construction

There’s quite a lot of technical details to this house, how do you think the attention to detail and that technical expertise compares to others?

“From my experience, there are different tiers or levels of craftsmanship with every trade, you know. And so I would put Altbuild at the very top of that, of that tier, on par with, you know, the best around and attention to detail and forethought into the specific detail, so that you’re not having to chase yourself later to achieve the detail that you wanted at the start, you know? So there’s that thinking 3-4-5, steps ahead. That’s what differentiates the best from good, if you know what I mean.

There’s a lot of steel work in this house, it’s a very contained site, and a very contained building, as far as we have a lot of open space, so we don’t have a lot of walls, spare walls to run cabling and piping and all that sort of stuff. So all of the steel work, all of those holes and penetrations had to be spot on, and they were all thought of and done at engineering and design stage. It wasn’t a last minute thing on site ‘by the way, now we have to get this one replaced,’ or whatever it was. All just thought of beforehand.”

The Handover Experience

So, on handover day – you’ve got all the bells and whistles that you designed in with everybody along the way, what did that feel like?

“It was very surreal. It didn’t feel real at the start. You know, it felt like I was walking around someone’s grand design house just going for a tour. It took a while for it to sink in. Just, just amazing. What an incredible feeling, you know, just this thing that is especially for us. From seeing this land go from nothing, just with a couple of feijoa trees, and then going through the whole design and just living that – you live through it. It’s the biggest thing that’s happening at that point of your life, so it becomes all consuming. So then at the very end, it’s like this big release.”

Living in a Healthy Home

What can you tell us about building and living in a house with these high performance elements?

“From the start, we wanted a house that was not just beautiful aesthetically, it needed to perform for us as a family, and part of that was the feeling of what it feels like to live in a really healthy house. And so all of those design choices, along with a lot of Toby’s input along the way, were made with that end goal in mind. Ultimately, I don’t even think about it, right? I just live in the house. But when I think back and I reflect like we’ve just come through winter, all of our living is upstairs, and so it was just completely warm, dry. There were no issues. I don’t think we got sick this winter, you know, like to be honest, it’s not something that, I’m thinking about all the time. It’s just a lovely place to be.”

And going from your old place to this, how has living in a healthy home changed how you live?

“Our old house was quite damp. In the winter, it was quite dark, and it was hard to heat and then it was really hard to keep cool in the middle of summer, when it was really hot. So for us, and for my understanding of what a high performance house is, it’s the opposite of all those things. So it’s a dry, clean environment to have your family in. It’s easy to cool, easy to heat. And we have a solar system with batteries, so that means it’s also cheaper to run.”

Communication and Budget Precision

What was the communication process like during the build?

“We got weekly emails with photos of progress on the site, and then a detailed description, and then just any time of the day I could call or text with any questions. And Toby is still on my favorites list, we would talk almost every day.”

Because you understand the construction industry, what do you think is the most valuable thing that Altbuild brings?

“The most valuable skills that Altbuild brought to this project for us were:

Now that it’s all said and done, we stuck to budget within about half a percent, something crazy, and I know from experience and talking to other people that that’s quite rare.
Their pragmatic approach. What I mean by that, is that they’re very approachable, like I said, there’s no ego. So it’s not like you’re scared to say, Hey, can I get a bit of clarification on this? Or why is this happening? We never felt silly for asking potentially a silly question. Even though I come from the industry and a lot of that dialogue was very easy – my wife doesn’t. So we felt very comfortable.
We’re in good hands and just trust. Trust is ultimate, because the builder is spending your money, and at the end of the day they have to do what they say they’re going to do. And that’s what Altbuild did right the way through.”

Ongoing Support and Relationship

How was the after handover support?

“There’s always minor teething issues as the building settles. It was the same thing, it wasn’t an issue – it all got sorted out. And we’re still, like, good friends now, you know? And that’s awesome. There’s so many horror stories of people falling out with their builders at the very end, or people being not quite satisfied or whatever, but I just don’t have a bad word to say, honestly.”

What would you say to someone who is thinking of building a performance house?

“They need to find a builder that understands what they’re trying to achieve. I haven’t met any other builders locally that have such a focus on performance in the home. And I do chat to quite a lot of builders about that sort of thing, especially going through this process.”

Conclusion

Creating a healthy family home on a challenging site requires more than just standard architectural building expertise, it demands foresight, technical knowledge, ECI – early contractor involvement, and a collaborative approach. This client’s experience demonstrates the value of tackling complex projects with specific performance goals in mind, by working from the start as a team.

The success of this project highlights the value of engaging a builder at the earliest stages, from initial land assessment through collaborative design and into construction. With Altbuild’s focus on high-performance homes, the result wasn’t just a visually stunning house, but a healthy living environment that performs efficiently year-round.

For those considering building on difficult sites or prioritising health, comfort, and energy efficiency in their new home, this interview outlines valuable insights into what makes a successful building experience. The combination of technical excellence, transparent communication, and budget precision created not just a house, but a healthy, comfortable home perfectly suited to family life in the Bay of Plenty.

If you’re considering building your own home in the Bay of Plenty, we’d be happy to share our approach and discuss how we might help bring your vision to life.

The Importance of thermal envelope design and construction implementation

Nov 14, 2024

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.