The Hidden Moisture Problem in Modern Homes

Host: Where is this moisture coming from if it’s not coming from the outside?

Robert: Cooking, having a shower, having a bath—anywhere you’ve got steam. A lot of people don’t realize how much moisture we breathe out. Two adults in a room overnight breathe out one and a half pints of water. It’s a lot of moisture wandering around in your house.

In the winter, a lot of people get up in the morning and they’ll see condensation on the windows, so they can physically see it at that point. But the question I always ask is: where else is that happening?

Host: Right, so it’s not just turning up on the windows.

Robert: No. The reason it’s turning up on the windows is because you’ve got a cold surface area. As the warm, moist air strikes the cold surface, it condenses because as the temperature cools, the vapour becomes water. It goes from a gas—people understand humidity in a room—but what a lot of people fail to understand is what happens within your wall.

If it’s 0°C on the outside and 20°C on the inside, and you slice through the wall, you’ve got a sliding scale of how the temperature drops as it goes through the wall. Heat always moves toward cold. If it’s warm in the house, you’ve got high pressure, and on the outside, you have low pressure. One of the things I say to people is that if you could control the weather, would you?

Host: I’m sure a lot of people would. That would be nice.

Robert: You can’t control the weather on the outside, but you can control the weather on the inside without a lot of cost.

Host: Conceptually, thinking that you’ve got weather on the inside of the house seems like quite a big thing. You’re talking about moisture as a gas, which is just like clouds inside the house. I don’t think people are thinking about that enough. What is that gas going to do to your house?

Robert: In the winter, you have heat transferring slowly through the wall. Humidity is a gas; it’s an invisible vapour. As the heat goes through the wall, it takes the vapour with it. Then, as it cools, it condenses inside the wall.

To give you some idea of how much moisture goes through a wall, a study was done in Canada over the heating season. Over a six-month period, one square metre of drywall with two coats of enamel paint—so it isn’t raw, the enamel paint almost acts like an impermeable plastic surface—absorbed and allowed a third of a litre of water to pass through. If a room wall is five metres wide and two and a half metres tall, every single square metre has a third of a litre of water going into it.

Host: This isn’t sounding good.

Robert: It isn’t good, because as the moisture goes through the wall, your insulation starts getting wet. As your insulation gets wet, the moisture condenses further, and the insulation starts to collapse.

Host: So you’ve got the internal lining, the external cladding, and the insulation in the middle. Talk to me about how hot gas turns into water inside the insulation. How is that happening?

Robert: It is the same mechanism as a window. When the temperature scales between 20°C and 0°C, the gas starts to turn into moisture at around 13.9°C. It condenses around that temperature.

To look at outdoor weather as an analogy, clouds in the sky are fluffy, white, and beautiful. But as they rise over hills, they get colder and it rains—they condense. That is exactly what is happening inside the wall. When the inside of your window condenses and gets wet, it can feel depressing, but it’s fairly easy to fix by wiping it up. However, you’re talking about water condensing inside insulation within an inaccessible wall. Wet insulation cannot effectively stop heat transfer, so it stops working. Over time, you start getting mould in the wall because it doesn’t dry out. You create a breeding ground.

On a standard New Zealand house build, the framing stud timber is 90 mm thick. The highest insulation R-value you can achieve in a 90 mm wall is R2.8. “R” stands for resistance value; the higher the value, the better it is at resisting heat transfer. A high R-value means it takes longer for heat to leave a warm room, so you spend less on heating.

The problem is the timber itself. Timber only has an R-value of R0.7. PlaceMakers did a study on roughly 452 residential and commercial projects, and found that the average amount of timber in an external wall was 38%.

Host: Hang on a minute. You have a wall with vertical studs spaced 600 mm apart, horizontal dwangs running in between them, and these pieces of wood go right through from the outdoor cladding to the indoor lining across that 90 mm space. And you’re saying 38% of that physical wall is wood?

Robert: Yes, the average they found was 38%. So when someone tells you that you have a great wall with R2.8 insulation, that is only true for about 60% of the wall. The rest of it is less than R1.0, so you have to average it out.

The current building code says the required R-value for Canterbury is R2.0. However, when you average your R2.8 insulation with the 38% of timber at R0.7, the wall does not actually achieve R2.0. People might think studs are only placed at 600 mm centres, but you also have double studs around sockets or doorways for structural support, as well as large lintels above windows. A lintel is a thick piece of wood that spreads the weight of the roof across the open span of a window. That lintel also has an R-value of 0.7. Either side of a window or doorway, you might have two or three studs. There is a lot more wood in there than just a stud every 600 mm.

We refer to these weakest points where heat escapes to the cold as thermal bridges, and the timber framing is the thermal bridge. This buildup of timber is why people see mould forming in the corners of older houses first. Last July, I used a high-quality thermal imaging camera on a standard heated house. While the surface temperature in the middle of the room was 20°C, the temperature in the corners was only 11.8°C.

Host: This is inside the room?

Robert: Inside the room. You’re sitting in this warm room feeling like life is good, but the corner is at 11.8°C, which is below the dew point, allowing it to condense right there in the corner.

Ideally, relative humidity should sit between 35% and 55% or 60%. If it drops below 35%, the air becomes too dry, which can cause nasal sores. Once humidity exceeds 65%, the room becomes too moist, creating a perfect breeding ground for mould and dust mites.

The dust mite is the single biggest cause of asthma. They feed on your dead skin flakes, meaning your bed, pillows, and furniture are full of them. Most people are actually allergic to the dust mite droppings. You breathe in their faeces, which triggers asthma. No house is completely without them, but a moisture problem allows them to breed rapidly. If the environment is dry and airy, they do not survive as well.

Host: Let’s go back to this moisture condensing in the walls. Even if a room maintains a healthy relative humidity of around 50%, you can still have moisture condensing inside the walls, ruining the insulation’s performance because it’s wet. Are we saying that the R-value drops even lower when the insulation gets wet?

Robert: Absolutely. Your house performs far worse than you think it does. Any cook will tell you not to pick up a hot dish out of the oven with a damp towel, because heat transfers through moisture very quickly. It is the exact same principle. Heat transfers through water much easier than through dry materials.

Host: With this concept of House 2.0, there are obviously ways to avoid this horror. What can someone do in the design, construction, and science of a house to prevent this problem from day one?

Robert: We follow international best practice, which is how Europeans build timber frames. We install a vapour membrane on the inside of the wall.

Host: Normal houses don’t have this?

Robert: Houses built to the current building code do not have it; it is not currently part of our building code. I would say 95% of houses don’t have it, but they should. Hopefully, future changes to the building code will make it mandatory, because the way we currently build is not fit for purpose for the inhabitants.

The system we use utilizes two membranes. On the outside, we use a weather barrier wrap that acts like Gore-Tex—it allows moisture out but won’t allow rainwater or outside moisture in. This is unlike standard building paper, which is not a true moisture barrier and can let water pass if it gets past the cladding. With our external moisture barrier, you could theoretically just leave the fabric on the house without cladding and it would remain waterproof.

Then, on the inside face of the timber framing, we install a vapour barrier membrane to stop interior moisture from entering the wall assembly.

Host: To visualize this compared to a standard New Zealand wall—which is just studs, insulation in between, cladding on the outside, and lining on the inside—you are adding a moisture barrier on the outside, a vapour barrier on the inside, and then additional horizontal bits of wood called battens before the internal lining. It’s almost like a double wall.

Robert: The battens run horizontally, attaching through the vapour barrier to the studs. This creates a secondary 45 mm services cavity where your electrical and plumbing lines run, and we fill this cavity with an additional layer of insulation. This drastically reduces the R0.7 thermal bridging of the studs because we are putting an insulation layer rated at R1.3 directly over them.

Instead of a standard 2.4-metre-tall stud acting as a continuous thermal bridge from top to bottom, the horizontal battens cross the vertical studs only at specific points. Your thermal bridge is reduced from a solid 2.4-metre line to just a few tiny square centimetres where the timber pieces intersect. Even the big lintels above the windows get completely covered by this insulation layer, dramatically reducing heat loss.

Host: How does that extra layer of insulation on the inside of the vapour barrier stay dry?

Robert Whitehouse: Because within that first 45 mm cavity on the warm side of the vapour barrier, the temperature doesn’t drop enough to hit the 13.9°C dew point. It will only hit the dew point further out in the wall assembly, safely past the vapour barrier. We generally work to a safe thickness ratio of two-to-one: a 45 mm batten cavity to a 90 mm stud frame.

Some people upgrade to a 140 mm framing stud with a 45 mm batten cavity to drastically increase their total wall insulation R-value due to the extra depth. However, a common misconception is that simply building a thicker 140 mm wall solves everything. If you don’t address the internal vapour problem with a membrane, a thicker wall just means you have more insulation to trap and hold that moisture, resulting in an even bigger wetness problem.

Going back to how internal moisture is created, people often say, “I have an extract fan in my bathroom”. It makes a lot of noise, which makes them think it’s working. But you can only extract air out of a sealed room if you are replacing it with incoming air. It’s like a beer keg: if you insert a tap at the bottom and open it, nothing flows out until you punch a vent hole in the top to let air enter. A standard bathroom extractor fan only pulls air effectively if a window is open to supply replacement air from outside.

BRANZ did a study that concluded extract fans do not work properly because people keep their windows closed. And understandably so—when you wake up and it’s 0°C outside with frost on the car, you aren’t going to open your bathroom window. Instead, people turn on a wall blower fan heater alongside the extractor. That clears the visible steam temporarily because warm air has a much higher capacity to hold moisture than cold air. But once they turn the heater off and leave, where does that trapped airborne water go? It drives straight into the un-membraneed walls and ceilings, which is why a massive percentage of bathrooms have chronic mould problems.

The same thing happens during cooking. A standard kitchen range hood extractor only captures about 70% of the moisture and heat; the remaining 30% dumps right back into the room because the fan lacks the volumetric capacity to handle all the steam. In England, where rooms are much smaller, building regulations require kitchen extractors to be powerful enough to exchange the entire room’s air volume three times every hour to clear moisture. In New Zealand, homes favor large, open-plan kitchen, dining, and living areas. You would need an industrial jet engine of an extractor fan to exchange that massive air volume, so most cooking moisture just stays trapped in the home.

This leaves us with a home filled with moisture-laden air, and it is significantly harder and more expensive to heat damp air than dry air because your heating system has to expend energy warming the water content within the air. It’s a compounding problem.

Host: So if you install this interior vapour barrier, the radiant heat passes through it but the vapour gas is completely blocked and stays inside the room. If the moisture can’t escape through the walls, your indoor humidity will rise and the air becomes unpleasant and harder to heat. What do smart companies do to solve that part of the equation?

Robert: That is exactly where a balanced mechanical heat recovery ventilation system comes in. The system is ducted to continuously extract stale, moist air from all the wet areas of the house—the kitchen, bathrooms, en suite, and laundry. At the same time, it continuously supplies fresh, filtered outdoor air into the habitable areas like bedrooms and living spaces.

People often mistakenly think these systems are designed to transfer heat from room to room around the house. They aren’t. Their primary purpose is to eliminate moisture by ensuring that for every single litre of stale air extracted, a literal litre of fresh outdoor air is brought in.

The outgoing warm, damp air and the incoming freezing outdoor air pass through a centralized heat exchanger unit. They flow through separate, sealed channels past each other so the air streams never physically mix, but the heat from the outgoing air transfers over to warm up the incoming fresh air. The moisture gets drained out of the system, and this exchange happens continuously, 24/7, all year round.

The system we use has an exceptionally high heat recovery efficiency rate of 94%. If it’s a freezing 0°C outside and your indoor air is 20°C, the incoming fresh air enters the rooms at a comfortable 19°C because 94% of that heat energy is recovered and retained inside the house envelope. If you think of heating energy as dollar notes, when you take a hot shower, you are spending money to heat that water. In a standard home, if you open the window to clear the steam, all those dollar notes fly straight out the window. With a heat recovery system, the heat exchanger acts like a banker that catches those dollar notes from the exhaust stream and injects them right back into the incoming air, reusing that energy over and over again.

The system acts like an octopus, running supply and extract ducts throughout the whole house to manage the entire home as a singular, balanced ecosystem.

When designing for high energy efficiency, the ratio of a home’s internal air volume to its external surface area is critical, because surface area is where you lose heat. A highly articulated, narrow, or H-shaped house has a massive external surface area relative to a small internal air volume, making it incredibly expensive to render energy efficient. The structurally ideal shape for efficiency is a sphere, so the closer a house layout is to a compact footprint with maximized internal volume, the better the building will perform. People often ask if our energy-efficient homes feature solar panels. Solar panels are great for generating power, but they do not make the fabric of the house itself energy efficient. True efficiency is about how well the house retains its energy after the sun goes down, because minimizing heat loss directly minimizes the amount of energy you must pay to put back in.

Most people only focus on the performance of their house when their monthly power bill arrives and they see a shocking $500 charge. My family lives in a comfortable 220-square-metre home, and our total combined energy bill for the entire year—covering all cooking, hot water, and heating—was just over $2,200. And that cost heats the entire house uniformly. Every single room sits at the exact same comfortable temperature. In standard or older uninsulated housing, people have to close off unused rooms and avoid heating them to save money, and stepping into those unheated zones feels like freezing.

According to health guidelines, the ideal indoor sleeping temperature for healthy adults is 18°C, and for young children or the elderly, it is 20°C. Sleeping in environments below 12°C can cause serious, long-term damage to your respiratory system. Yet so many people go to bed in winter and can literally see their breath in the bedroom air, which indicates the room has dropped to a freezing 6°C. They pile on heavy blankets, hot water bottles, and beanies because the bedroom feels profoundly cold and damp from high indoor humidity. Even if they manage to stay warm under a pile of blankets, they spend eight hours breathing in freezing, damp, mould- and mite-laden air that damages their lungs, leading to persistent respiratory infections that they can’t seem to shake off for weeks.

It is alarming that so much of the housing stock in this country forces people to live in environments where they cannot thrive. We have an outdated building code that merely lets you survive. MBIE is currently working on an overhaul of the building code, with the next major performance changes slated for 2028. Under these upcoming plans, every single house will be legally mandated to receive an official energy efficiency performance rating certificate, graded from A down to F, much like buying a modern kitchen refrigerator.

When you look to sell a house in the future, buyers will be highly educated. If a house lacks these airtight membranes and ventilation systems, it will grade out as a poor D or F rating, and it will become incredibly difficult to sell. Critics argue that updating building mandates will drive up initial house construction costs. While upfront costs may rise slightly, the drastically reduced monthly running costs will save homeowners far more money in a very short timeframe than that initial capital expense. It is cheaper in the long run. Furthermore, making our nationwide housing stock healthy and energy efficient would save our public health system billions of dollars.

When a nation’s housing requires drastically less power, the cumulative drop in household electrical demand alleviates immense pressure on our national grid. As our wider transport sector transitions rapidly over to electric vehicles, the power grid can comfortably balance the surging demand for EV charging because our high-performance housing sector is pulling significantly less power. Everyone wins.

When you set out to build your next home, don’t just look at a short-sighted metric of how many square metres of space you can squeeze out of a contractor per dollar today. Ask yourself: why are you building this house, and what product are you creating for the long-term health, comfort, and financial future of your family? A home should provide unconditional health and comfort, but a poorly designed house fails its occupants, leaving them freezing in the winter and sweating in the summer. Investing in building science is a better investment for your wallet, a better environment for your family’s health, and a better outcome for our planet.

Host: Thank you, Robert. This has been an absolute eye-opening discussion. Thank you very much.

Robert: Cheers, thank you.

The show is brought to you by Energy Efficient Homes, based here in Christchurch, New Zealand. Their integrated design and build approach brings architecture, building science, and construction together to create high-performance homes that are healthier, more comfortable, and built for the future.