How Tight is Too Tight?
Builders should build tight and ventilate right to avoid indoor air quality problems.
By Steve Easley
I often get questions from builders who are concerned about the impact that tightening up houses to both reduce energy use and meet new energy standards has on the building. The most common question? Will tightening a home to reduce air leaks make the home too tight and lead to indoor air quality problems? The short answer is "no," but it is also important to ventilate properly.
There is a misconception that a loose home with lots of air leaks is adequately ventilated. The second misconception is that a leaky home is a healthy home.
The idea that a tight home is unhealthy stems from the recognition that a home needs to be ventilated. This is true but how reliable the ventilation strategy is is what really matters. Simply having lots of "avenues" in the building envelope for air to infiltrate does not ensure adequate ventilation.
For a home to get fresh air, you need both a hole to the outside and air movement. Most ventilation air flow only occurs is there is a temperature difference between inside and outside as well, but only during the coldest weather.
I will address how unreliable air gaps can be as a ventilation strategy later. But tightness, of the ventilation rate, is only one factor. Even leaky homes can have bad indoor air quality for a variety of reasons that have nothing to do with the air exchange rate. Off-gassing of building materials, excessive moisture due to over humidification, backdrafting of fireplaces and combustion appliances, and normal household activities that generate fumses, such as from cleaning products, all contribute to bad indoor air.
For example, consider that many homes today have humidifiers and homeowners who are clueless about how to operate them properly. And dust mite populations grow exponentially in relative humidities over 55%, and the byproducts of dust mites can be allergens, which cause allergic reactions and asthma.
Factors that Affect Air Quality
Infiltration through cracks and gaps in a building enclosure is neither a reliable source of fresh air or an effective means of exhausting bad air. This type of ventilation is accidental at best.
When considering healthy indoor air quality, you need to consider all the factors that impact a healthy home.
*Pollutants and their source strength *How many hours a day people are exposed to a pollutant source *The ventilation or dilution rate and how consisent the ventilation rate is *A person's sensitivity to the air quality. (Some people are more sensitive to some pollutants than others, i.e. pollen, dust mites, chemicals, and so on.)
Instead, the prevailing wisdom is to "build tight and ventilate right." That is, stop the air leaks in a house and install a controlled ventilation system that will deliver fresh air to occupants, dilute indoor air pollutants, and exhaust air from the locations in a home where the most moisture and pollutants are generated.
Unreliable Air Exchange
If you add up all the cracks and gaps in a typical home, that will equal about a 150-sq.-in. hole that is open all year long. A typical house will have more than 2,000 linear feet of cracks and gaps. These breaches through the building enclosure are not necessarily located where the occupants need fresh air. Moreoever, the rate of air flow depends on the outside weather, which is constantly changing and therefore unreliable.
In the winter months in cold climates, you have greater driving forces from higher winds and greater temperature differences. In the spring, summer, and fall, these driving forces are much less or even non-existent. This causes home to be over-ventilated in times of cold, windy weather, driving up energy costs, and under-ventilated the rest of the year. In the winter months, heated air inside the home wants to exfiltrate to colder outdoor conditions. As this heat transfers from inside to outside, it carries moisture-laden air with it. In the summer months, if the home is under negative pressure, just the opposite can occur, with the house drawing hot, humid outdoor air inside through air leaks.
The Solution: It's a Control IssueI consider most homes today to have "accidental ventilation" rather than controlled ventilation. A controlled ventilation system brings in a specified amount of air on a regular basis, avoiding the fluctuation of natural infiltration and exfiltration. In addition to ensuring a clean, consistent supply of fresh air to improve indoor air quality, a controlled system will not over-ventilate in the winter months and unnecessarily increase energy bills. A well-designed ventilation system will introduce an adequate amount of fresh air into the home, and will exhaust moisture and indoor air polutants, even at times when natural driving forces to not exist.
The difference between natural and mechanical is readily apparent in the chart below, which shows a wide fluctuation in natural ventilation rates that vary with climatic conditions, but a steady rate of mechanical ventilation, which is what is desired.
Many manufacturers offer solutions for controlled ventilation. These products include energy recovery ventilators, air to air heat exchangers, and systems to bring in fresh air and exhause stale air without creating unwanted pressure imbalances that lead to building envelope moisture problems.
One very clever and economical system is made by Honeywell. This Honeywell Y8150 unit costs less than $300 and it provides a fresh air ventilation system. It is basically a controlled damper and a duct that brings in fresh air outdoor air via a duct to the return air side of a standard forced air heating and air-conditioning system. It is designed to bring in fresh air when the HVAC system runs. When it operates, it also turns on a bathroom exhaust fan so that there are no pressure imbalances in the home. It has a controller, a humidity sensor, damper, and outdoor sensor. The unit operation is based on the ASHRAE 62.2 residential indoor air quality ventilation standard. What is really smart about this system is that you simply program in the number of bedrooms, the square footage of the house, and the ventilation rate and the controller figures out the rest.
the bottom line is that my field of experiences have taught me that leaky building shells cause problems and increase chances for mold growth. I see far less moisture and durability problems in energy-efficient, well-contructed homes.
People want homes that are affordable to heat and cool, as well as healthy to live in. Today we have the ability to do both cost-effectively. Pay close attention to properly sealing the cracks, gaps, and bypasses on the inside of a house. On the outside, make sure windows and doors are flashed and a breathable weather-resistive barrier is properly installed and sealed.
If you think that building a drafty building increases a homes's indoor air quality, think again. The best practise is to follow the adage: "Build tight, ventilate right."
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