Seal Tight & Ventilate Right
As the home performance contracting industry moves to improve the energy efficiency, comfort, safety, and health of existing homes by applying building science principles, one of the most ubiquitous problems building analysts are finding is that, even recently, most homes are not built using airtight construction methods.
Seal Tight: Measuring a Home’s Air Tightness
Air leakage in homes is measured in Air Changes per Hour (ACH), which compares the amount of air that a home exchanges every hour with the amount of air that can fit in the home. Ideally, a home should change over about 35% of the air inside of it every hour (0.35 ACH). This is enough air exchange to remove moisture, stale air, and contaminants, and provide sufficient fresh air for occupants to breathe. In addition to being delivered in the proper quantity, fresh air should also be of high quality. Unfortunately, almost all existing homes suffer from either excessive quantity or poor quality of ventilation due to unintentional air leakage, and many suffer from both. The goal of air draft sealing is to reduce the quantity of air leakage that takes place in the home while improving the quality of air that enters the home. Often, a ventilation system is required to deliver high quality fresh air in the appropriate quantities.
In older homes, a blower door test typically reveals air leakage rates of 0.6 to 1.0 ACH, or about twice to three times the amount of desired air exchanges. This large amount of air exchange comes from a variety of sources. Although we frequently associate drafts with leaky windows or doors, the easiest places to make the largest gains in reducing air leakage while spending the least money are the attic, crawl space, basement, and garage:
Common Attic Air Leaks
- Penetrations (holes) in the ceiling and wall frames where light fixtures, wires, plumbing, heating ducts, and other mechanical components of the home protrude into the attic
- Wall top plates – the joint where the drywall connects to the wall frames at the ceilings are usually 1/4 inch wide or larger on both sides, and there can be more than 200 lineal feet of walls. This adds up to about a 10 inch square hole!
- Dropped ceiling cavities and chases, often over cabinets or behind showers, can expose large areas of interior walls to the attic. A 1 foot wide, 8 foot long chase could expose over 100 square feet of interior surfaces to attic temperatures. This condition is called a thermal bypass because a relatively small hole allows air from one place to move into cavities where they can affect the surface temperatures of areas much larger than the hole itself.
- Open wall cavities, or open floor cavities in multi-level homes with knee wall attics. Open cavities are considered to be thermal bypasses, basically exposing the entire ceiling of the rooms below as well as the floors of the rooms above to attic conditions.
Common Crawl Space, Basement, or Garage Air Leaks
- Penetrations as described above
- Large thermal bypasses around cavities, especially under bathtubs, showers, and staircases
Warm air tends to rise out of holes in the attic during winter, and be replaced by cold air that is pulled in from the basement. This driving force is called the stack effect, and it is the principle on which chimneys, furnace flue pipes, and smokestacks work. In order to reduce the stack effect in a home, workers start air sealing in the attic.
Insulation Works Better in a Tight House
Not only do these holes allow air that we paid to heat leak directly outdoors, penetrations and thermal bypasses greatly reduce the effectiveness of insulation. Fiberglass and loose blown cellulose, the two most common insulation products, don’t work as they are designed to when air is moving through them, just as a wool sweater doesn’t keep a person very warm when the wind picks up. Finally, thermal bypasses allow attic or crawl space air to leak right past insulation, eliminating its effectiveness across large portions of the home.
Better Indoor Air Quality in a Tight House
In addition to all this excessive air leakage wasting energy and harming comfort, the attic and crawl space aren’t exactly places I would think of if I wanted to step out for some fresh air. Here are just a few of the particles residing in most attics and crawl spaces that might be in our “fresh air” stream:
- Rat feces
- Rat poison
- Mold and mildew
But a Tight House Doesn’t Always Mean Better Indoor Air Quality
Once, I did an energy audit at a small home built in the late 1970s, one story on a concrete slab, with new dual pane vinyl windows and new doors installed. A blower door test revealed that the home’s air leakage was right around 0.35 ACH. However, there were still lots of air leaks and thermal bypasses in her attic, and her duct system, which ran through the garage and attic, was pretty leaky too. This customer had the appropriate quantity of air exchange in her home, but since almost all of the air exchange came from the attic, her air quality was poor. Obviously, this is still not a great situation. The windows had cost close to $10,000, and while they had clearly made her home less drafty, they had also harmed her air quality. For half the money, she could have sealed up all the air leaks and duct leaks in her home (about $3,000) and installed new attic insulation (about $2,000), ending up with much more significant energy savings at half the cost. She also would have had cleaner air, as the drafts through the old windows were certainly healthier than drafts from the fiberglass-insulated attic and the garage where she stored the car, lawnmower and gasoline.
We didn’t walk away from this job because the house was “tight enough already”. Instead, we sealed up the attic, ducts, and garage, and made the house even tighter. Then we installed a timer-controlled ventilation system in her home to exchange the 0.35 Air Changes per Hour of ventilation. We also replaced her furnace and water heater. The results were amazing. Her house was as efficient as it could possibly be without gutting the walls – energy modeling showed that we reduced her energy consumption by close to half. It was very quiet and comfortable to be in. And, despite not feeling drafty, it didn’t feel stuffy or clammy either, thanks to the ventilation we installed. We were able to seal tight and ventilate right!
Often, we work with clients whose doors and windows are leaky enough that we can seal holes in the attic, basement, crawl, and garage and they still have a ventilation rate of 0.35 ACH or higher. Sealing up air leaks and dense-packing walls to reduce air flow always reduces or eliminates pathways for air quality contaminants to enter the home. Is it better than it was before? Absolutely! Cracks around doors and windows are certainly a cleaner source of air than moldy basements with fiberglass insulation and asbestos ducts. However, that doesn’t mean that these are ideal sources, either.
Ventilate Right: Types of Ventilation Systems
Ventilation systems are recommended or required by the Building Performance Institute (BPI) and Home Energy Rating System (HERS), the two leading national organizations that set standards for energy efficiency in homes and train contractors to properly improve the performance of homes.
Exhaust-only ventilation systems are the systems most homeowners are familiar with. They are generally located in bathrooms to exhaust moist air, but are also increasingly being used as inexpensive whole-house ventilation. A high quality exhaust fan that is rated for continuous, quiet operation, along with an automated controller that turns the fan on just long enough to provide the right amount of ventilation, can be installed for less than $1,000 in most cases. Exhaust-only ventilation is great because it is low cost and simple. Unfortunately, an exhaust-only system can only control the place where air is removed, and for every cubic foot of air that is removed from a space, another has to come in from somewhere to replace it. And even if your home is half as leaky as it was before, we can still stir up contaminants while bringing in a small amount of ventilation from I-don’t-know-where. Still healthier than it was, but not optimal.
Supply-only ventilation systems typically consist of an intake duct that runs from outside the house. This duct should be installed in a location far from the driveway, road, exhaust flue pipes, plumbing vents, attic or crawl space vents, or other places that might have poor air quality. The intake duct connects to the return side of the furnace and has a damper installed so that it isn’t leaking air in all the time. However, when the ventilation controller calls for air, the furnace fan turns on and the intake duct damper opens, delivering fresh supply air to the inside of the home. Supply-only ventilation is good because the installer can choose an optimum location to bring in the fresh air. Supply-only is usually around the same price as exhaust-only, sometimes more depending on conditions at the home. The downside of supply-only is that, if the ventilation turns on during the winter and the furnace isn’t also running, the inside of the home will get a periodic cold blast of air. This wastes energy and harms comfort. Also, since supply-only ventilation pressurizes the house, stale air inside the hose has to find a way out. This can occasionally lead to moisture being driven into wall cavities and other places that can cause moisture problems.
Balanced Heat Recovery Ventilator (HRV) and Energy Recovery Ventilator (ERV) systems exhaust stale, moist air at the same rate as they bring in fresh outside air.
Heat Recovery Ventilators have a built-in heat exchanger that transfers heat from the stale exhaust air to the incoming fresh air. In fact, most HRVs transfer heat with an efficiency of 70% or more. So, if it is 35 degrees outside and 7 degrees outside, an HRV can warm the incoming air to almost 60 degrees without using any energy to create that heat!
Energy Recovery Ventilators are capable of transferring moisture as well as heat, which is preferable for humid climates, where during the summer, much of the air conditioner’s energy is expended in removing moisture from the inside air. HRV and ERV systems typically have four ducts connected to the main unit:
- An intake duct running from a clean outside fresh air source to the unit
- A supply duct (or ducts!) delivering fresh air to the living area and/or bedrooms of the home
- An exhaust duct (or ducts) removing stale, moist air from the kitchen and/or bathrooms
- An exhaust duct depositing stale pulling exhaust air from the unit to an outside location
Better Air Quality and Energy Savings with Balanced Ventilation
HRVs and ERVs provide superior quality ventilation by allowing the source of the fresh air to be chosen, by tempering the incoming fresh air without consuming heating or cooling energy, and by removing moist, stale air from the locations most likely to produce it. They can even be fitted with filtration systems to remove pollen and other allergens from intake air. The biggest downside is cost. Professionally installed, these systems cost anywhere from $1,500 to $5,000, depending on the complexity and size of the job.
At first glance, the idea of sealing up holes in a home to keep air from exchanging, just to poke a hole in the home and install a machine that exchanges air, seems a bit absurd. However, particularly in homes that are made very airtight, sealing tight and ventilating right saves orders of magnitude more energy than is consumed by the ventilation equipment, while ultimately making the home’s occupants much healthier. If you are considering performing air draft sealing, I strongly urge you to also consider whether ventilation would improve your family’s health and quality of life.
Copyright September 2014 by Advanced Home Energy
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