“Ask an Expert” with Kris Knutson

Simple things to help improve your indoor air quality: Part 2

Indoor air quality is an important topic all year long – not just during winter when we’re spending more time indoors. Today we continue our series on simple things you can do to improve your indoor air quality, which we started back in March.

Read part 1 of this post.

In part 1, we told you that indoor air quality should be approached with four principles in mind: Elimination, reduction, ventilation and filtration. Part 1 focused on elimination and reduction. Today we are going to address ventilation, and more specifically, spot ventilation.

Spot ventilation, or point-source ventilation, exhausts air at the origin of the pollution.  This typically means using a simple fan to exhaust water vapor and cooking byproducts, which include smoke and combustion gases from a gas cook-top or range.  Water vapor also comes from bathrooms, pets, our own respiration and perspiration, and plants.  And yes – water is considered an indoor pollutant, especially when vapor accumulates. It can condense and collect on drywall, wood and other materials that are susceptible to moisture damage.  And if these materials remain damp, it creates the conditions for mold to thrive.  The good news is that controlling moisture in kitchens and bathrooms is easy and inexpensive to do.


Bathrooms are among the greatest indoor sources of water vapor. One very inexpensive way to help control bathroom moisture is to use a bath squeegee. When you’re done bathing, squeegee off all shower or tub surfaces, including the tub or shower pan area. This substantially eliminates the water that would otherwise evaporate into the room. Eliminating water at the source is always easier and more efficient than removing it with ventilation.

Keeping your bathroom dry using ventilation is also fairly simple and inexpensive.  Although newly constructed homes automatically have bathroom fans installed, older homes may not, or, the fan may not work properly, or is very loud making you not want to use it.  Follow these three key aspects to make sure your bathroom ventilation is effective:

1) Install a quality fan

Energy Star bathroom fans are a slam dunk in terms of efficiency and quiet operation.  To qualify for the Energy Star label, the fans must be quiet (2-3 sones or less), and electrically efficient.  For quietest operation, look for a fan with a very low sone rating – this is a measure of the fan noise projected into the room – a rating of 1. 5 sones or less is excellent.  Matching existing bathroom décor is easy, as a wide variety of colors, styles and sizes of fans are available.  Picking the right “sized” fan, or how many cubic feet per minute (CFM) you need is also easy.  California Building Code requires a minimum of 50cfm for a fan that is run intermittently and 25cfm for a fan that is run continuously (some fans can run at a very low level around the clock).  For larger bathrooms with jetted tubs or multiple fixtures, see Home Ventilation Institute’s “Home Ventilation & Indoor Air Quality Guide.”

2) Make sure your fan is installed, supported and vented properly
Once you’ve selected a properly sized Energy Star fan, make sure it’s installed using best practices.  Fitting the fan snugly into the drywall is important to make sure that moisture laden air does not sneak between any gaps between the fan and the ceiling.  This edge can be filled in on the attic side or bathroom side with regular painter’s caulk.

Use sheet metal duct, sealed at all edges and joints with mastic (not tapes) and fasten with three metal screws in each joint, and avoid sharp angles or elbows near the fan.  Join this duct to an appropriate roof jack, wall vent or other durable hood which vents directly outdoors.  Again, be sure the duct vents all the way outside and not into the attic. While tempting, this could quickly convert your attic to a mold farm.  Somewhere in this duct run should be a backdraft damper – it could be near the fan or the exhaust hood – but it should not be at both.  Consider serviceability when choosing where to locate the damper.

Finish things off with a vinyl or mylar backed insulation around the duct and seal all the edges with a matching tape.  This makes the duct quieter and prevents condensation in cold weather.  Finally, adequately support the sealed, insulated sheet metal duct with strong wire or other highly durable strapping material to the rafters.  This reduces stress on the fittings, fan and exhaust hood and is more durable if other attic work is ever undertaken.

3) Install controls that ensure your fan will run

With the fan and duct installed, you can use a standard wall switch to control the fan or you can use a fan control device.  This could be as simple as the classic “crank” or “twist” timer that provides up to 60 minutes of run time.  Or you could opt for a motion sensing/timer switch that runs the fan for a prescribed amount of time, but can be manually overridden when users leave the bathroom.  These will automatically run the fan if a guest is unaware of how to use it, and they allow you to manually set the fan for 30 minutes for example, after completing your shower and leaving the bathroom.

GreenPoint Rated recommends running a bathroom fan a minimum of 15 minutes after bathing.  Humidity sensing switches and fans are also available, but the durability and reliability of the sensors are still in question. Current California Building Code requires newly constructed homes to use an Energy Star bathroom fan with a humidistat control.


Like bathrooms, kitchens are a source of indoor pollution that is easily controlled with spot ventilation.  And because smoke, particulate matter, water vapor and combustion byproducts from gas appliances may be a danger to your health and the durability of your home, proper ventilation in this room is particularly important.

1) Install a range hood

Most homes have some form of ventilation in the kitchen- either a range hood vented to the outdoors or a recirculating range hood which circulates air within the kitchen. The best practice is to have a range hood vented to the outside. This prevents cooking vapor and particulate matter from entering the rest of your home.  In some cases, recirculation range hoods can be modified to vent outdoors. Consult the hood manufacturer for specific instructions on how to best install a duct to vent to outdoors. If you are renovating your kitchen or otherwise installing an all new hood and fan for your range, you have the opportunity to address the issue more comprehensively.

2) Choose a correctly sized fan
A too small fan will not be able to properly exhaust when many burners are in use, while a too large fan risks depressurizing the whole house.  When your house is depressurized, air from unhealthy spaces like an attached garage, a crawlspace, or an attic may be drawn inside.  Depressurization can also introduce carbon monoxide – an odorless, tasteless gas that can make you sick – into your home by back drafting a water heater or furnace flue, or a fireplace or wood stove. To minimize the risk of depressurization do not use a downdraft exhaust system.

California Building Code requires a range hood to have a minimum of 100cfm for a range hood used on demand. For hoods against a wall, HVI recommends a ventilation rate per linear foot of range of 100 CFM with a minimum of 40 CFM.  For an island hood, install 150 CFM per linear foot of range with a minimum of 50 CFM. The Home Ventilation Institute offers further guidance on fan selection.

I asked Judy Roberson, one of California’s leading experts on mechanical ventilation in homes, about high volume, professional grade or commercial style range hoods in a residential application.  She cautions that “professional or commercial range hoods are intended to be used with an engineered system, and installing them in homes should be avoided.”  Instead, she recommends “selecting an Energy Star labeled hood, which ensures efficiency, and it is quiet enough so people are more likely to actually use them when cooking.”  For optimal performance, she recommends “ducting fans directly to outside with smooth metal ducting and installing a backdraft damper for reasons of fire safety.”  She advises “carefully following range and hood manufacturer instructions for proper positioning and installation, and if the home has a heat recovery ventilation system, kitchen range hoods should not be ducted into them.”

Finally, an oversized range fan uses more electricity and speeds up air exchange in the home.  This means that in the summer when running air conditioning, or in the winter while running your furnace, you will be exhausting much of your conditioned air outdoors, which you would then have to reheat or re-cool upon its entry.

Check back in a couple of weeks when I will conclude this series with the topic of filtration.

Kris Knutson is a former Home Performance Contractor and consultant, and current Program Associate with Build It Green.  He manages the Ask an Expert Hotline, which provides California residents customized responses to green building questions for building professionals and the general public.  Submit a question!

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