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ArticleTechnical Article

Weatherization and Ventilation

With the recent weatherization funding increase by the American Recovery and Reinvestment Act (ARRA), there is an excellent opportunity to expand weatherization efforts to more houses and apartments. The ARRA provides over $5-billion for low-income household weatherization through existing community programs. It also funds weatherization through the Energy Efficiency and Conservation Block Grant program for cities and counties across the United States. Part III, Section 1121 of the act allows up to $1,500 tax credit for homeowners to improve energy efficiency, including weatherization. Weatherization of older structures with the addition of ventilation will allow for increased energy efficiency and improved indoor air quality (IAQ) compared to weatherized structures without ventilation.


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Homes have become much more thermally efficient, but they've also retained more air contaminants inside, while water vapor from daily activities such as breathing, cooking, and bathing causes condensation on surfaces inside the walls, enabling mold to compromise the structure. However, with mechanical ventilation, thermally efficient houses can preserve their structural integrity and create a healthier environment for the inhabitants.

Over the past 30 years, builders of new homes have taken this ventilation lesson to heart. They've been incorporating a variety of fresh-air intake devices, energy recovery devices, and ventilating fans into new construction. Health-savvy home buyers and building codes are now demanding effective home ventilation. But many older single-family and multifamily buildings have never been improved in this way. Many of them are still behind even in basic ventilation, such as kitchen and bathroom fans.

For the next couple of years, weatherization improvements will be more attractive to homeowners because of the stimulus money that's coming to the states from the U.S. Department of Energy (DOE). This provides an opportunity for weatherization and renovation professionals to drive business while maintaining structural integrity in homes, improving energy efficiency, and improving the health of residents of older dwellings.
Reasons to Ventilate
Historically, houses and apartment buildings were constructed to allow an abundance of outside air flow. They had relatively few IAQ problems, since air was flowing in and out regularly. But they were also hard to heat or cool. As energy became more expensive, builders started tightening up the gaps, adding insulation and replacing older windows and doors. As a result, energy bills became lower, but there were unintended consequences.

In tight dwellings, air is trapped and becomes stale, humidity and contaminant levels rise, and IAQ goes down. Although weatherization construction practices have improved through better manufacturing and legislation, some building materials can introduce contaminants into the inside air: for example, drywall that off gasses and corrodes metal pipes. Chemicals can also be brought in by dry cleaning clothing and curtains. Even plants, pets, and other sources of moisture can hurt a house and eventually the people in it, if that moisture isn't reduced by ventilation or dehumidification. It is a well-established fact that mold exacerbates asthma and allergies in humans.

The above is only a partial list of items that can impact IAQ. So when you decide to weatherize a house, you should seriously consider adding mechanical ventilation to ensure a healthy indoor environment. DOE and state guidelines agree; they typically recommend the addition of ventilation if the house or apartment will have or has had extensive air tightening work done.

Historically, the practice was to avoid adding mechanical ventilation by tightening up the house to the point where outdoor air requirements could be met through leaks and operable windows. The idea was to seal up the house only to the point where you'd still get about 0.35 ACH (air changes per hour) leaking in through the cracks in the building, based on a blower door test.

A measurement of 0.35 ACH is pretty good natural ventilation, but the blower door test results give an average annual value for the leakiness of the house, not an absolute value. When there are no driving forces, no wind or temperature difference between indoors and outdoors, the house is ventilated less than the amount indicated by the test. So the house is generally much tighter than the blower door test indicates. If ventilation decisions are based on the blower door average, it's like wearing the same clothes all year long based on the average annual temperature outside.

The blower door test is a great tool for finding leaks and measuring how much weatherization has been done. But it has little bearing on the need for ventilation. Mechanical ventilation ensures that no matter what's going on outside, you can control the air change in whatever way is best to save energy and create a healthy indoor environment. The rule "Make it tight and ventilate right" applies to both new and existing houses.

Determine How Much Ventilation Is Enough

The most widely accepted, and probably the best ventilation standard to use is ANSI/ASHRAE Standard 62.2-2007 "Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings." ANSI is the American National Standards Institute, a private, nonprofit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international standards so that American products can be used worldwide. ASHRAE is the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, the professional organization that represents engineers, contractors, and manufacturers of HVAC products that are used in all kinds of buildings.

ASHRAE 62.2 has been adopted by California and Maine as part of the building code for new construction and is now being considered by several other states and jurisdictions. It is also cited as the ventilation requirement for most green building programs. LEED for Homes from the U.S. Green Building Council, the Indoor Air Plus package from Energy Star, and the National Green Building Standard from the National Association of Home Builders all use 62.2 as the ventilation standard. 'I'he standard requires continuous ventilation at a low rate that's based on the number of occupants and the size of the house or apartment. It calls for 7.5 CFM (cubic feet per minute) of air change for each person, plus 1 CFM for every 100 square feet of floor area. It assumes that two people will be in the master bedroom (see "Calculating CFM").




There is an easy way to calculate the ventilation that is recommended by the ASHRAE 62.2 standard. Take the number of bedrooms plus 1 times 7.5 plus the floor area in square feet divided by 100 ([BR+1] 7.5 + SF/100).


The standard also provides a table to make sizing even easier (see Table). This table takes into account the approximate floor area in square feet and the number of bedrooms to determine the air change rate. For example: to calculate the ventilation for a 2,500 ft2 house with three bedrooms, follow the 1,501-3,001 ft2 line over to the 2-3 bedroom column to find that we need 60 CFM of continuous ventilation.


Table. Ventilation Standards [per number of] Bedrooms


Floor Area (ft2)
 <1,500 30 45 60 75 90
 45 60 75 90 105
 3,001-4,500 60 75 90105
 4,501-6,000 75 90 105 120 135
 6,001-7,500 90 105 120 135 150
 >7,500 105 120135


This is referred to in 62.2 as whole-building ventilation and is commonly referred to as general ventilation. ASHRAE 62.2 was developed primarily for new construction, where it's relatively simple to get things right the first time. Existing houses and apartments more than a few years old are probably not built to the requirements of current codes. The ASHRAE 62.2 Committee recently wrote an appendix for existing houses as guidance on how to address the standards requirements in existing structures. To make it easier to meet the standard, the appendix allows the whole-house fan flow to be increased to account for deficiencies in local ventilation, such as in kitchens and bathrooms.

What Type of Ventilation Is Right for the Job?
Various green building programs, utility incentives, and ASHRAE 62.2 let you provide exhaust ventilation, supply ventilation, or balanced ventilation. Here are the pros and cons of each of these approaches.

Exhaust ventilation is the simplest way to ventilate and is used throughout the United States. This kind of ventilation can be created with an upgraded bath fan that meets the sound specification for general ventilation or with an additional fan placed elsewhere in the house. It might be a single fan or an inline remote fan that's ducted from more than one air intake. However, the incoming air is not filtered.

Supply ventilation uses a dedicated fan or air handler that's part of the central heating or cooling system to push air into the house. This allows filtering or tempering of outdoor air before it's introduced inside. A downside of using an air handler is that it is driven by a high-power motor, so it's expensive to run. Also, a duct has to be installed from the outside to the return side of the air handler and sized properly to bring in the required amount of air.

You can lower your operating costs by running the air handler one-third of the time, as long as you bring in 3 times the air flow that a continuous system would provide. The air handler must operate once every three hours under this scenario to provide a comparable level of IAQ. But other problems arise when you use supply ventilation, including extreme draftiness when the system is running at high speed for ventilation only. In humid climates, a dehumidifying supply ventilator may be the best solution, since it can dehumidify the hot, humid outdoor air before it's introduced into the house.

Balanced systems provide nominally balanced amounts of both supply and exhaust ventilation. What does this mean? If 60 CFM of ventilation is needed, this type of system is designed to exhaust 60 CFM of stale air and supply 60 CFM of fresh air. This can be done with a single unit capable of recovering heat from the exhaust air. Or it can be done from two separate fans without heat recovery. It can be done with a centrally ducted unit, or it can be done with a single unit mounted in the ceiling.

Heat recovery ventilators move heat from one airstream to the other to preheat the incoming air. Energy recovery ventilators move both heat and moisture from one airstream to the other to cool and dehumidify the incoming air. Whichever system you use, it is desirable, but not required by ASHRAE 62.2, to have the ability to increase the ventilation rate for periods of high occupancy or activity. This may be done by using any other intermittent bath fans that you have, or by using fans that have the ability to boost air flow speed temporarily.
Performance Standards
Being able to increase the ventilation rate semi-permanently is useful for the owners of multifamily buildings, to accommodate a higher-than-average number of occupants. Several state codes and ASHRAE 62.2 require the fans to be rated by the Home Ventilating Institute (HVI) to ensure proper performance. The criteria include air flow, sound and wattage, and efficiency for heat recovery ventilators and energy recovery ventilators. Sound is a critical issue because many times people turn off or disable noisy vent fans (see "Sound Ratings").



The HVI sound ratings are based on testing in a certified lab and are expressed in units called sones. A sone is an internationally recognized measurement of sound output. Sones translate decibel readings into numbers that correspond to the way people sense volume, and follow a linear scale, like inches. Sone readings offer quick sound comparisons for laymen and engineers. The Table shows how common sone levels affect how we feel.

ASHRAE 62.2 requires that a continuous whole-building or general ventilation fan be rated at 1 sone or less. Range hoods and other bath fans should be rated at 3 sones or less. The HVI Certified Product Directory includes performance data on about 3,000 fans and other ventilation devices. It is updated monthly at


Table. How Sounds Compare


 Environmental Sounds
 Sone Level
 How We Feel
 Traffic noise
 Conversation with added noise
 4.0 Normal conversation
 Calm office
 Night in the suburbs
 1.0 Comfortable zone
 Rustling shrubs
 Free of noise


 ASHRAE 62.2 requires continuous ventilation, but it makes an exception for intermittent systems that are operated at a much higher rate than the typical fan running continuously. In other words, operating the ventilator one-third of the time at 3 times the rate is just as good as continuous flow at the lower rate if the duty cycle is less than three hours. Read ASHRAE 62.2 for the details. The bottom line is that continuous air flow at a low rate is much simpler than using intermittent air flow.

Obviously, energy cost and energy use are key considerations when specifying and installing mechanical ventilation. A standard consumer fan may cost $60-$75 per year to operate continuously. However, an Energy Star-rated fan may cost only $10-$40 per year for electricity.

Final Thoughts
There are a number of other things to consider when you are installing mechanical ventilation. They include type, fittings, and configuration of ducts; fan pressure capability; and control strategy. Specifiers and installers need to be aware of these details to ensure that the ventilation system works as intended. Properly configured and installed mechanical ventilation is a must for an airtight, energy-efficient structure. And an airtight, energy-efficient structure is a must to maintain healthy IAQ. Remember: Make it tight and ventilate right.




Home Ventilating Institute (HVI)


HVI represents a wide range of residential ventilation products sold in North America. Its Certified Rating program, conducted by independent laboratories, tests and verifies ventilation performance. HVI maintains an online directory of about 3,000 certified products and performance data that is updated monthly.

Web site:
HVI Certified Product Directory:


Energy Star

Another indicator of performance is the Energy Star Web site, where you can find fans that meet the Energy Star criteria for sound and efficacy. To earn the Energy Star designation, products must meet the strict energy efficiency guidelines set by EPA and DOE.

Web site:

Energy Star Vent Fans:


Don Stevens is the National Research and Development Manager for Panasonic Home & Environment Company. The company's Weatherization Resource Web site contains guidelines on selecting proper ventilation for optimal energy efficiency. It also includes online videos and information about the importance of ventilation; weatherization codes and standards; ventilation strategies; and training on duct design and installation.

Reprinted by permission of Home Energy magazine. Copyright 2010.


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Weatherization and Ventilation:  Created on April 12th, 2010.  Last Modified on January 25th, 2012


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