The Ins and Outs of Ventilation

When designing a new home, most people give a lot of thought to a home with an efficient layout, warm pleasing interior, hansome elevations and plenty of light and views outside. One characteristic that is often overlooked, however, can make the home harder to keep clean, may reduce the life of the structure, increase the operating costs and can even make the occupants sick. It’s Indoor Air Quality, or IAQ, and it’s controlled by well designed ventilation.

We often associate pollution with transportation, manufacturing, and heavy transportation like trucks, buses and trains because it’s pretty easy to see in the air. Research has found, however, that often the air inside our homes is often more polluted than the worst smoggy day in a major city. Common activities in our home can reduce the air quality; cooking, bathing, cleaning, and even breathing all contribute to the pollutants in our homes. The materials we build and furnish our home with can also contribute to this indoor pollution, because most modern materials will “off-gas” when they are new – think of the smell of a freshly painted room or a newly laid carpet for instance.

How do we make sure that the indoor air is as clean as or cleaner than the outside air? Ventilation, of course! There are three types of ventilation; natural, mechanical, and infiltration. Natural ventilation is the means of using open windows to draw air through by use of the stack effect or wind. In mechanical ventilation a whole house fan or other power driven vent fan is used. Infiltration ventilation is the air exchange forced through leaks in the building envelope. Though the least desirable, infiltration is the most common way our homes are ventilated.

Natural and Infiltration Ventilation –

In the days before Air Conditioning and central heating, deliberate ventilation was accomplished with the use of windows and doors. Many traditional home features that are desired today, such as high ceilings, transom windows and wide porches, were put there to allow the owner control over the movement of the air! Moving the air through and around the house was the only way the occupants could stay comfortable. A home was often “operated” by opening and closing windows throughout the day, and if it was a very hot day, the occupants moved out to a sleeping porch to take advantage of the cooler night air! Older homes also had a lot of “accidental” ventilation – the leaks around radiator pipes, chimney flues, wood sash windows, and wide wood trim often kept the house aired out even when the occupants were trying to keep the house closed up.

These days, very few people have the time to “operate” their homes – they expect them to stay comfortable on their own and not require sleeping outside when the weather is hot. Most homes are also better insulated and tighter than they used to be. While we may be more comfortable without much work there are some drawbacks. Most heating and cooling in North America is done with a forced air system, typically in the form of a furnace with an air-conditioning coil. Warmed or cooled air is moved around the home by a powerful blower and ductwork. This equipment, however, can often set up pressure differences between different parts of the home and even between the interior and exterior which can make even a relatively tight home leak (infiltrate) air. This infiltration can cause a lot of problems and change the IAQ, and is in some ways worse than the “old” leaky homes.

When a house from the early 1900s leaked air, it was often through large gaps between building materials and the cavities in the walls. Moisture that was in the house would move very easily out of the house through these air currents, as well as through the open windows. When insulation came along it was simply tucked into the cavities, and buildings eventually started experiencing moisture problems because water would become trapped in the walls. Mechanical ventilation in the form of kitchen and bathroom fans was introduced and helped with moisture, but now created a negative pressure inside the house which would pull air with dust, mold, and pollen out of the gaps in the walls and ceilings! To make matters worse, once we started sealing up those gaps in the 1970s and 80s we found that the moisture was again being trapped in the walls because the air was no longer flowing even with the vent fans running, and mold and indoor pollutants began getting worse due to higher moisture levels in the homes. It seemed like “tight homes” were causing the problem, but it was the lack of proper ventilation!

Mechanical Ventilation –

Most people are familiar with the acronym HVAC. They know it has to do with heating and cooling a building – but the “V” is there for a reason – it’s Ventilation! Heating, Ventilating and Air-Conditioning systems always have a connection to the exterior on commercial systems to bring in fresh air to replace ‘stale’ air inside the building. Most residential systems, however, do not incorporate this “make-up” air in their design because it has been assumed that the house will be leaky enough to introduce enough air to keep the inside fresh. Once the homes are sealed up, that is no longer the case.

A properly designed HVAC system will follow a set of design criteria laid out in the ACCA Manual J. This series of calculations shows how much heating and cooling each room in a home needs, what it’s air supply should be, and even how much fresh air should be introduced into the home, room by room. It’s this part of the Manual J calculation that’s often omitted which can make homes unhealthy. Most older homes leaked at a rate of 1.75 to 2.5 ACH – or Air Changes per Hour.
Newer, “tight” homes leak at .25 to .35 ACH, or in other words, when under pressure, the air inside will be replaced every 3 to 4 hours. That might not be quick enough to vent out moisture or cooking odors or the fumes from that new carpet or paint. .25 ACH might even make a room feel “stuffy” because carbon dioxide from the occupant’s breath is concentrated.

A quality HVAC system will bring in outside air to replace the “stale” air and keep the pressure on the house as close to neutral as possible. A house under no positive or negative pressure will not leak air and therefore won’t be pulling pollutants like mold, dust or pollen in through the cracks. When the fresh air enters the house through the HVAC system, it can be filtered and brought up to the desired temperature before it enters a room, which means there will be no drafts.

How can we do better than a make-up air intake on our HVAC system? An Energy Recovery Ventilator, or ERV is a good choice. With this system, air is exchanged constantly through a set of smaller, dedicated ducts in the home that bring in fresh air and exhaust stale air through a heat exchanger. The more advanced heat exchangers – the kind we need in central Texas, also exchange humidity, so they reduce the humidity of the incoming air and send it back out with the “used” air. Advanced ERV’s can also adjust the pressure on the house as conditions change, such as doors opening and closing and wind currents outside, and they filter the air to a very high standard often referred to simply as HEPA, which effectively eliminates pollens, dust and molds that exist naturally in outdoor air.

Natural Ventilation – again!

OK, so we now know that we should seal our houses up tight and ventilate them correctly to reduce moisture, reduce pollutants, and to stay healthy – but it’s still possible to ventilate our houses naturally even with these improvements. Some days are just too nice to stay locked inside our Heated, Ventilated and Air conditioned boxes! We know that if we open our windows on a day with low humidity that the house will “air out” and we will save electricity and money, but how do we take advantage of the nice weather?

Well placed operable windows and doors with insect screens are the answer. There are several ways to take advantage of natural breezes in any house, such as opening more windows on the leeward, or “down-wind” side of the home, or opening all the windows upstairs and some of the windows downstairs to create a chimney effect, but your results may not be optimized if your house is not designed to take advantage of the breezes.

A home designed for good natural ventilation through the windows and doors will have good orientation of the windows to the wind, porches or walls positioned to direct extra air to the openings, and will even have operable windows in a clerestory, cupola, monitor or Tower to create a breeze where there is none by the chimney effect due to warm air rising and pulling fresh air in through the lower openings. If you are designing a new home – ask about these features. An added benefit of properly placed windows is day-lighting which will also save energy.

Ventilation has been one of the most neglected characteristics of new homes but as we’ve seen here – it shouldn’t be! Make sure that when you are designing or remodeling to remember how important the quality of the air you breathe can be to your well being as well as your wallet and make sure to ask about it during the design process.

Todays information on Ventilation is part of an ongoing series called Building Performance with Intenion. Refer to our April and July newsletters if you are a new subscriber to our newsletter. Thank you for reading!

Green Living Tips!

I welcome inspirations from your home to put into future newsletters. Email them to me; Cammi Klier. Thanks!!

If you read our July newsletter last month you may be interested in learning more about energy audits. You now have an opportunity to see one in action! Next Tuesday, September 7th at 2pm, a full scale energy audit will be performed at a residence in Southwest Austin. A blower door test, a duct leakage test, and a thermal imaging scan will be performed. The homeowner, Ron Van Sickle, is hosting an open house for people intererested in observing and learning more about the process. Please RSVP to Ron at 512-745-1394.

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