Building Materials – Attic Venting and Internal Moisture Control

This section of the Building A Home In New Zealand addresses the topic of Attic Venting And Internal Moisture Control. This is designed to help make your home dry after the Thermal and Noise Wall Insulation section made it warmer. This section is intended to be read with the other Building Material Sections: Windows, Solar Heating and Electrical, and Heating before or in conjunction with the Choosing a Builder and Designing A House topics.


Attic Venting

This section is a somewhat confrontational topic.  This is where NZ and the rest of the world really differ. The rest of the world has published hundreds to thousands of papers on the topic in favor of venting the attic as both a form of temperature control in the summer months, as well as a form of moisture control in the winter months. The argument is that modern houses are more sealed and insulated and gradually build up moisture as a result of everyday living. Activities like cooking, showering, drying clothes, and breathing produce moisture inside your home. You then heat your home in the winter and have the windows closed so the warm moist air can’t exchange with the drier colder air outside.  The warm inside air rises and escapes through recessed lighting and collects in the attic.  During cold nights, the moisture condenses on the underside of the roof and runs down to the insulation or soffit and causes mold and decomposition. The research has shown that venting the attic space removes that captured moisture and prevents the condensation and resulting problems.

This was also the idea in NZ when in 1975, venting the attic was part of the building code. Since then, NZ changed its mind and stated in E3/AS1 1.1.4.b that the attic is now an insulated space and shall be enclosed with no ventilation.  Coincidentally, the 1980’s started the Leaky Home crisis in NZ, many of which didn’t have the problem monolithic cladding systems. Even BRANZ in 1988 stated that ~40% of moisture damage in buildings then from Otago resulted from severe wintertime roof space condensation where moist air migrated to the cold roof spaces and was unable to escape.  Now, attic ventilation is accepted again, but may not be a focus and can be overlooked by both builders and designers, which may continue to result in moisture buildup and condensation in attic spaces.

There are different types of attic ventilation available. We recommend a passive system that is designed to give you the minimum ventilation needed to remove moist air from attics. Though active (powered) systems may be more effective, they tend to break and require maintenance which never seems to happen.  Common passive venting options may include gable vents, can vents or mill vents, and ridge vents.  Ridge vents tend to be very effective but are also more costly.  One thing to keep balanced is the potential for air intake as well as exhaust. Hot moist attic air can only escape if there is incoming air to replace it, so intake vents are also vital.


Internal Moisture Considerations / Ventilation

First, internal moisture is not a bad thing.  As humans, we need some moisture in the air for comfort with the ideal humidity levels from 30-50% as recommended by the US EPA states and the Mayo Clinic.  However, most problems in NZ occur when moisture levels rise so high that they result in condensation.

Condensation is when water reverts from an airborne state back to a liquid state and is the opposite of evaporation. The amount of water that air can hold depends on the temperature, with higher temperature able to hold more water than colder air.  As moist air meets a cooler surface, the air temperature cools to the point where it cannot hold all of its water (dew point). The water then condenses on the colder surface. Two things are required for condensation to occur, high humidity and cooler surfaces.  If either of those is removed, then so is the possibility for condensation.

We have previously talked about ways to remove colder surfaces like increasing the insulation within your home. We have also talked about higher performance windows. Even the experts in NZ agree with this. An article from BRANZ concerning condensations states the following.  “Changing from single glazed windows to double glazing with standard aluminium frames may not get rid of condensation problems. Aluminium is a good conductor of heat. On very cold days, the inside of the frame can be almost as cold as the outside, and moist air inside the house then condenses on the frames and runs down.” Choosing the right windows with higher R-values in the entire window can help prevent condensation.

Alternatively, preventing moisture build up can also help prevent condensation problems.  The best way to prevent moisture buildup is to remove it using proper ventilation.  Using ventilation to remove warm moist air from where it is most often generated will transfer that moisture outside and keep inside humidity levels lower. Below are some areas where ventilation is required/recommended

  • Bathrooms
  • Kitchens
  • Clothes Drying
  • Gas Heating



This is a major source of humidity in most homes.  During a shower, huge amounts of water are transferred to the air. To remove the water, most building codes recommend a fan OR a window that can be opened in the bathroom.  However, we recommend a fan be installed even if there is a window. Houses have more inside moisture problems in the winter colder outside temperatures prevent you from opening your windows regardless of the need to vent moisture. Therefore, install an exhaust fan that vents outside and not into the attic (read the section on attic venting).  You will also want the fan to run for at least 15 minutes after you finish your shower to remove most of the moisture in the air. The best way to do this is to use a switch timer.  The most popular NZ switch timer is a 7-minute timer, which is not enough. Spend the extra $10 and purchase the variable timer which will allow you to choose how long you want the fan on. While you are installing them, they are also great for toilet only regions too to remove unwanted smells so you don’t have to open that window in cold weather either.



These are required to have ventilation as well. Your kitchen designer should include a range hood in the kitchen design. One point is to use it to remove water vapor from cooking food and not only to remove smoke when something is burning.  The process of heating food and water drives moisture into the air. Removing moisture when it is generated and venting it outside, and not into the attic, will prevent that water from increasing indoor humidity.

More simple advice is to cook with the lids on pots and pans. This does two things; it traps the heat and the water vapour in the pan where it can be used to cook your food faster. Since the heat is trapped in the pan, it will also require less energy to cook your food since the heat will not be escaping.  Since the water isn’t escaping, you won’t be increasing the indoor humidity as much.

We recommend getting a demonstration of your extractor fans in the store before purchasing one. Don’t settle for the least expensive if it won’t work for you. You will want one that extracts lots of volume, as well as one that is quiet.  You will choose not to use a fan that is too loud or doesn’t work well.


Clothes Drying

Most clothes drying in NZ is done on a laundry line outside. This saves a lot of energy as well as making sure that unwanted moisture remains outside. The problems come in the winter months with extended rainy periods and you aren’t able to use the outside lines. Most people frequently use indoor drying racks. However, indoor clothes drying contribute significantly to the indoor humidity.  If you are having moisture and condensation problems, you may want to shift drying clothes outside under a porch, into the garage, or wait until a dry day to do laundry.

If you have a mechanical clothes dryer, ensure that it is vented properly outside. It may be tempting to shortcut the installation process by venting indoors, but this has two major safety issues. First, the moisture from the clothes will vent inside and increase indoor humidity levels. Second, the lint from the dried clothes will remain inside and pose a fire hazard as it accumulates.


Gas Heating

We have seen a number of people that need extra heating in the winter and install a portable gas heater. These are high output and heats a room very quickly.  These heaters have some significant safety concerns though when not used / vented properly. When gas burns, it creates CO2 and water. If these exhaust products are not vented outside, they will build up inside of your home. Though CO2 is dangerous, the constant source of water will increase humidity inside your home. If the gas fire doesn’t burn efficienly, CO can be formed instead of CO2, which is odorless and considerably more toxic than CO2 and can result in death. Please vent any gas burning heaters or decorative fireplaces outside.


Next Steps:

This brings to a close our short discussion topic concerning Building Materials relating to Attic Venting and Internal Moisture Control.  The next section in the Building Materials discussion is Heating.