Updated: Mar 8
As I sit here, all huddled up in a blanket on a cold night at home, I made a quick basic heat loss calculation for my house. If I make it as airtight as a passive house, leaving insulation as it is (only in the ceiling currently), how much money on heating could I save? The answer is above 10% savings on heating cost, at an outdoor temperature of minus 3 degrees Celsius. What a shockingly high number! That would look nice in my bank account.
So, what is airtightness and why does it make such a significant difference to our heating bill? In this article, we will explore and explain questions you might have regarding airtightness.
What is air tightness, and how is it measured?
What are the benefits and drawbacks of airtight buildings?
How airtight should I build?
How can I make my project more airtight?
What are the available air barrier options?
What should you look out for in your air barrier?
Is it costly to construct airtight buildings?
1, What is airtightness, and how is it measured?
Airtightness is a fancy term for measuring the cracks and gaps in our buildings that let warm air escape to the outside. These gaps and cracks can be around windows, doors or joints of different building materials, or air travelling through the building materials themselves.
Generally, cold outdoor air enters our buildings due to:
Air pressure. Wind creates pressure on a building. If the outdoor wind pressure is higher than indoor air pressure, it could push the outdoor cold air through the building gaps. Once inside, the wind pressure would further drive the warm indoor air out through the opposite walls of where it enters. Air pressure doesn’t need wind; as long as the air pressure on the outside of the house is stronger than the inside, air infiltration can occur.
Temperature differences. Nature prefers balance. The warm indoor air tends to escape and draw in cooler outdoor air. Drought is a typical result of this air movement. The higher the temperature difference between the indoor and outdoor, the more droughts created.
Building materials. Many of our building materials let air travel through them usually. We might think of our plasterboards stopping air from moving into walls, but this is not the case—air flows right through them.
Airtightness is measured by the number of air changes per hour using a “blower door test”. The higher the number of air changes in an hour, the lower the building’s airtightness.
In a “blower door test”, all windows and doors except the building’s front door are shut. A blower door which is a fabric door with a fan, a manometer and calibrated plates, is attached to the front door. The house will be pressurised and depressurised with around 50 Pascal. Once the pressure is stabilised, we could understand how much airflow is moving through the fan per minute using the manometer. We could then divide the airflow number with the house’s volume to determine how many air changes per hour.
According to BRANZ, we lose somewhere around 6-11% of all heat created through air leakage; all those little gaps and cracks. There is, however, a big misunderstanding within many in the industry: The fact, that many new buildings have six air changes per hour does not mean that the air is replaced six times per hour. The latter is only accurate when the building is under immense pressure (usually at 50 Pa pressure). A study in Finland on the air pressure difference between indoors and outdoors in buildings without a ventilation system found a pressure difference of only minus 5Pa (1). In actuality, the air inside our homes is replaced about 0.3 times every hour- roughly once every 3 and a bit hours. Still a lot if you think about it.
The World Health Organization recommends, for residential properties the air changes per hour should be between 4 and 5. This number is recommended for homes that do not use a ventilation system. According to official resources, in NZ on average, standard new buildings have 6.5 air changes per hour; structures built between 1960 and 1980 have 8.5 air changes per hour.
We should aim to build more airtight to save on energy costs as it is comparatively cheap to make a building envelope more airtight.
2, What are the benefits and drawbacks of an airtight building?
The benefits of an airtight building are:
It is dramatically cheaper to run and will save on heating costs. It is healthier and more comfortable to live in as fewer droughts occur.
It lowers CO2 emissions over the long term as heating energy creates CO2 emissions.
It cuts down on outside noise and creates a quieter indoor environment.
Less moisture enters the building structure. It is quite crucial as most insulation materials lose efficiency if moisture gets into them, in some cases above 50%. Therefore there is a lower potential of structural damage or mould formation due to moisture.
The drawbacks of an airtight building are:
Airtight buildings should have a ventilation system or need to be well ventilated to ensure enough healthy air supply.
Different applications and situations require alternative air barriers (vapour tight, vapour open, air barrier material). The wrong air barrier or location of the air barrier can cause damage to the building structure. From an environmental perspective: right now, the high-performance air barrier wraps are all made out of petrochemicals—plastics. In the future, we need them to be made out of recycled plastics or natural materials.
3, How airtight should I build?
There are different schools of thoughts regarding airtightness in a building—the passive house school and the natural building school. However, there is a lot of overlap and people with slightly different philosophies.
In a passive house school, the goal is to build a house with 0 air changes per hour. A maximum of 0.6 air changes per hour is accepted to be certified for a passive house.
In a natural building school, there is no clearly defined airtightness goal. In NZ, the goal appears to be around 2.5-3 air changes per hour for a residential house. Once people move into the home, the air changes will increase, as a cat door, internal access door to the garage or open bathroom window will bring the building to about four air changes per hour.
Why is there a difference?
In a passive house, the key focus is on reducing energy demand to heat the building. High-performance air barrier wraps—plastics, are in NZ often used to achieve the target. A mechanical ventilation system also has to be used to supply the building with fresh air and remove moisture and toxins.
A natural building focuses on utilising natural products, such as wool insulation and natural ventilation. The designer may opt for slightly higher air changes but chooses building materials that could help deal with the moisture level in the air and reduce toxicity. Using natural materials in the right combination helps to lower the actual need for ventilation as the materials take parts of the humidity, CO2 and other toxins out of the home. A mechanical ventilator is often not used in this case except for the kitchen and bathroom extraction vents.
4, How can I make my new building or renovation more airtight?
We could not achieve airtightness only by purchasing a few airtight products, such as better-sealed windows or doors. To successfully obtain an airtight building, we also require a designer to help specify more efficient building details. Both of these are important.
Airtight products such as air barriers improve the airtightness of a building significantly. An air barrier is a system of materials, designed to form an air enclosure boundary, which separates the indoor air and the outdoor air. Designers often use an air barrier to cover the building materials; joints or corners of the building materials; and wall openings, such as windows, doors, electrical equipment or plumbing.
A good designer is also essential. He/she could help specify useful joint details and well-sealed products like windows and doors. He/she also often creates a service cavity to limit the number of holes put into an air barrier.
Making a new building more airtight is much simpler than in a renovation. For the latter, extensive work is required when installing the air barriers, such as removing windows, doors, appliances and wallboards/exterior cladding etc. It can be done in stages or partially.
5, What are the available air barrier options?
Wraps go around building materials to stop air movement through building materials and close small gaps. A special tape is used to seal the joints in between the layers of the wraps. A wrap can be applied behind the wallboard, behind the exterior cladding or both. The most commonly used wrap is the outer wrap.
An inner wrap is a wrap that is fixed right underneath the wallboards. It has only one purpose: to keep the air from escaping. In a climate such as the South Island of NZ, an air barrier should generally be installed behind the internal wallboard to keep moisture from entering the building structure. The reason is that the moisture/vapour pressure is mostly higher inside the building than outside.
An outer wrap is a wrap that is fixed underneath the battons of the exterior cladding. The outer wrap keeps the air from escaping the indoor and propels the outdoor air and the driving rain away from our building structure. In NZ, we often require a plastic building wrap for the most commonly used building products to keep the moisture out. Considering that, an external air barrier, which serves both purposes, often makes financial sense. As the additional cost for the outer wrap is a meagre (around $1000 for a 150sqm house), this is the most commonly used air barrier in NZ.
Air barrier included in wallboards
Some wallboard products have already an air barrier included in them. Utilising these products saves time and money in the installation of a wrap. The hard surfaces of the wallboard products also potentially simplify the sealing process. Additional tapes, sprays or wraps are required to seal doors, windows and other wall openings. Options of such wall boards include some Magnesium Oxide, specific plywood and OSB boards.
Different wallboards have certain environmental and health concerns, such as their ability to be recycled or biodegradable, level of off-gassing volatile organic compounds, and other harmful substances released. However, we will not discuss them further here.
Air barrier included in insulation
In our research, we did find some spray-in insulation products that advertise included air barriers. While we have no experience with this type of air barrier; the likelihood of air leakage is in our view higher than other products. Insulation might not be sprayed evenly in every little nook and cranny (unless applied outside of the structure and therefore without any gaps); furthermore, timber frames themselves let air travel through them. Spray-in insulation is less likely to eliminate this. Also, there are some health concerns, particularly around Spray Foam Insulation products (2).
Wall finish products
In some instances, the best option is to apply a wall finish, that will also act as an air barrier. A cement lime plaster is such a material. Lime cement plaster is one of the leading reasons that buildings in Germany are more airtight than in other places. Lime and cement plaster is a standard wall finish in houses there (my parents, grandparents, friends and other relatives all have a cement lime plaster finish on their walls). If no renovations are required for some time to come, this is an excellent method for retrofitting buildings.
6, What should you look out for in your air barrier?
While all air barriers have the same primary goal: to stop any air from escaping the building, there are other vital tasks they should fulfill. Some materials also prevent vapour/moisture from travelling through them. Depending on your values, such as the plastic used, site conditions, roofs and floors and budget, different options should be used.
It also helps if you could watch out for unsealed holes that are put into the air barrier. Ensure you inform all trades that you want to have an airtight building, and be aware to or remind tradespeople to seal the holes appropriately wherever and whenever the air barrier is penetrated.
Lastly, an air barrier is a product that should last a long time. Replacing it is not an easy task, and also rather expensive. Choose a quality product and quality tapes. Tapes that crease easily or don't fasten well will limit the longevity of the air barrier.
7, What is the rough cost of airtight buildings?
The cost of constructing an airtight building is not easy to assess, as the construction involves many products.
For air barriers, fitting a 100 square meter building with an interior air barrier wrap will average about $3,500 to $5,500 plus few days of labour. The cost for an external air barrier is only a couple of hundred dollars more than your general building wrap, so no significant differences in price. Wallboards with included air barriers tend to cost twice as much or more than regular gypsum wallboards (but are also often stronger than regular gypsum wallboards).
A mechanical ventilation system costs around $6,000-$15,000, depending on the number of rooms and brand it can also cost more.
High-performance windows and doors vary in pricing. The cost is anything from $1,000-$2,000 per square meter. It is also beneficial to look into the cost of sealing all the openings properly once these windows/doors are installed.
The airtightness level of residential housing in New Zealand is higher than the recommended standard of WHO. While higher is often a good thing, it is not when it comes to buildings leaking air.
An airtight building is healthier and more comfortable to live in and highly energy-efficient and cheaper to run. To construct it successfully, quality airtight products, suitable air barriers and good designers are all essential. Property owners should also carefully plan out ways of expelling the increased moisture level in the house.
It is easier to make a new building more airtight than in a renovation. Also it is crucial to check if the air barrier should be inside or outside the timber-framed wall. This ensures that there is no excessive moisture buildup in the construction frame.
For any questions on what type of air barrier or way to increase airtightness in your building might be of most use, get in touch with us!