By Greg Stopher
Part 1: Basic Residential Ventilation and The Bathroom Fan
With all of the energy updates, efficiency upgrades, and weatherization programs today, I have to wonder if the indoor air quality of homes is beginning to suffer. I have personally noticed a rise in moisture problems in homes with increased energy efficiency where ventilation was not properly considered. Is saving on your energy bills really worth sacrificing you health or can we have the best of both worlds – an efficient and healthy home?
In older homes, like the home I lived in as a young child, we didn’t experience moisture problems and there was always plenty of fresh air. For one, we pumped all kinds of energy through the un-insulated walls when we burned cords and cords of wood. Most of the doors and windows were so drafty in that old house that it was probably like having the back and front door open. We were basically heating the outdoors, like my dad would say when we actually did leave the door open.
My parents decided to build a new home on the property next to our old house. It was a simple home with an unfinished basement. The focus was on energy and it was well insulated, then wrapped with more insulation. The windows were top of the line for the time with good u-value and low air leakage. The windows were wrapped and insulated using spray foam and the home turned out to be pretty tight and efficient. Soon after we moved in, my dad noticed condensation on the windows and higher humidity in the house. We had bathroom fans but with three boys with the second level to ourselves, we didn’t use them much. In the end, my dad was convinced that the home was too tight so he removed the foam insulation from around the windows. The moisture problems mostly went away along with the efficiency. Could my parents have continued to enjoy a home that was energy efficient while also solving the moisture problem? Yes, they could have!
In today’s homes there is an increased focus on the R-value and tightness of the home. We don’t want the heat that we paid $3 to $4 a gallon for to exit through leaks in the home. I support energy efficiency, adding insulation, and getting a house as tight as you can. But the one thing that is often overlooked or not the top priority on the list of energy updates is ventilation. Ventilation should always be considered first when upgrading the energy efficiency of a home.
The entire home should work as a system and the ventilation has to be part of that system. Ventilation can be achieved in a variety of ways. Bathroom fans, kitchen exhausts or other ventilation where moisture is exhausted at the source is part of the system. Other types include passive ventilation as well as whole house mechanical ventilation systems. Whatever the system is, it must work effectively with the super tight, extra efficient homes of today.
The most common ventilation a homeowner will see is the bathroom fan. Code requires there to be 50 CFM intermittent or 20 CFM continuous ventilation in a bathroom. CFM is cubic feet per minute, which means that a single bathroom fan should exhaust a bathroom at a rate of 50 cubic feet of air per minute because it’s intermittent. Is this rate of exhaust adequate to rid all that moisture from the shower to the outdoors? There are a lot of factors that will have an impact on the adequacy of the fan. Is the fan sized to accommodate the length of ducting or resistance the ducting puts on the fan? An 80 or 100 CFM fan might not be able to overcome the resistance of 100 feet of ducting so it might only be providing about 30 to 40 CFM of exhaust. To put this in simple terms, a bathroom fan will have a CFM rating at a certain resistance (usually grains of water, w.g.). A bathroom fan that has more resistance than it was tested at will have a lowered CFM. Ducting is added up in linear feet along with the linear feet of the fittings in the ducting, using a formula, to come up with the amount of resistance. Larger diameter duct equals less resistance, more fittings, bends, and turns equals more resistance. The fan must overcome this resistance to provide the necessary ventilation. Most do not. A lack of exhaust can create very humid air in the bathroom, resulting in a mold or mildew.
Take a large bathroom, an eight by ten foot space with nine-foot walls, which is 720 cubic feet. A 50 CFM bathroom fan will take a little less than 15 minutes to clear a bathroom that size. Do we run the fan for 15 minutes after our shower? Some of us might, but do your kids or other family members? One remedy for not running the exhaust long enough is to put a timer on your bathroom fan. It is a good idea to run it during and 20 to 30 minutes after you’re done showering, so it should run for a good 30 to 45 minutes to exhaust all that moisture from the shower.
Why not just install a larger exhaust than required? I definitely agree that exhausts are sometimes undersized, especially when there is added resistance in the ducting, but a bathroom fan can be oversized as well. A bathroom fan that is pulling 175 cubic feet of air from a 320 cubic foot bathroom may be overkill, especially if it’s left on all day. An oversized fan will exhaust a lot of warm heated air unnecessarily and result in heat loss. A bathroom fan already takes a lot of heated air out of the home. Another disadvantage and potentially dangerous situation is that too much exhausted air results in a high negative pressure in your home and can lead to back drafts and carbon monoxide infiltration from combustion appliances or the garage, if the garage is attached. I do believe that a slightly negative pressure is better than a positive pressure in your home. It is important to make sure that the attached garage is slightly negative of the home and everything should be okay. A positive pressure can potentially push unwanted moisture into your wall cavities where it can condense and create even more problems.
In conclusion, a bathroom fan should be correctly sized for the bathroom size and the number of fixtures in the room. The bathroom fan should also be used and used long enough to bring the moisture in the air back to the normal levels. Remember, you can have ah home that is energy efficient while successfully solving moisture problems.
Please Note: Properly ventilated fans should always vent to the outside, never into a crawlspace or attic.
Part 2 of the ventilation series will cover kitchen exhaust and whole home ventilation systems, as well as, other air quality hazards that are lurking in the home.
Greg Stopher has over 16 years of experience in the construction field and earned a degree in Construction Technologies from the University of Alaska – Southeast. His company, Stopher Construction, LLC, is a general contracting company specializing in remodeling, custom finishes, additions and new home construction projects. He can be reached at 907-321-2350.