I’ve been asked this question a few times. “At what point do I need to add mechanical ventilation to my home?” The answer, it depends. With this blog, I’m going to focus on rule of thumb recommendations along with a calculation for a more accurate recommendation. I’m also going to dig into the Minnesota Energy Code for the rules in new construction which may be a good recommendation for older construction.
The purpose of mechanical ventilation in a cold and very cold climate is to control interior humidity levels and bring in fresh air to improve indoor air quality. This blog is written for the control of indoor humidity, ventilation for indoor air quality requires a different approach, a topic for another posting. It’s preferable to control the air exchange rate rather than allow the home to exchange air through air leaks in the envelope. New construction in Minnesota requires balanced mechanical ventilation in all new residential construction along with a blower door test of 3 air changes per hour or less with a test performed at 50 Pascals of pressure. The required ventilation rate for Minnesota can be calculated using the formula:
Ventilation rate (CFM) = (.02 x square feet of conditioned space) + 15 x (number of bedrooms)
Straight forward calculation. A 1,500 square foot home with 3 bedrooms will required 90 CFM of fresh air. This rate can be reduced by 50% if the ventilation is continuous. We in Minnesota typically use heat recovery ventilators (HRV).
If you are outside Minnesota, check with your local authority having jurisdiction or building inspector. You may be using the ASHRAE 62.2 formula.
How about an older home, built before the requirement of balanced mechanical ventilation? How does a person know when they are too tight? First, I recommend a blower door test. The testing I do uses software where the natural air exchange rate will be estimated. A note about natural air exchange estimations, they are just that, an educated guess. Depending on the location of the air leaks, difference between inside and outside temperatures and the direction and how hard the wind is blowing, etc… this estimation may be off by a lot. For me, it is my best guess at the exchange rate and what I’m going to use for a recommendation.
What if you’re not using software and need a manual calculation for the natural air leakage rate? You’ll need a couple more formulas.
ACHnatural = (CFM50 x 60) / (n-factor x volume of house)
ACHnatural = ACH50 / n-factor
The n factor was developed by Lawrence Berkeley Laboratory in the mid 1980’s to more accurately calculated the natural air change rate. The table gives the n-factor used in the calculation and is dependent of location in the country, how shielded from wind the structure is, and the number of stories the building has.
Back to our example. This time we have actual blower door test results. The 1,500 square foot home has a volume of 16,000 cubic feet and moved 2000 cfm through the blower door fan. This gives us 7.5 air changes per hour at 50 Pascals (7.5 ACH50). The home is located in northern Minnesota, is in a heavily wooded rural area and is a two-story home. The n factor number from the above table is 14.9.
ACHnatural = 7.5/14.9
The natural air change rate for this home is estimated at .5 air changes per hour. (Think of that exchange rate for a minute. The entire volume of conditioned air inside the home is being exchanged with outside air every 2 hours. If the outside air happens to be -20°F…ouch!) We now need to change the air change rate to the actual CFM air leakage rate of the home to compare with the suggested fresh air needs of the home.
The volume of the home is 16,000 cubic feet x .5 natural air change rate = 8,000 cubic feet per hour. Next we need to change the cubic feet per hour calculation to cubic feet per minute. 8,000 cubic feet per hour / 60 minutes gives us a natural air change rate of 133 cubic feet per minute. Our recommended air change rate for the 1,500 square foot house from the previous calculation is 90 cfm intermittent or 45 cfm continuous. The natural air change rate is much higher than the recommended mechanical air change rate. No mechanical ventilation needed (and I would be recommending air sealing to the homeowner).
Let’s say the blower door number was lowered by 50%. Then will we need mechanical ventilation? 133 CFM x 50% = 67 CFM. Now we are getting to the point where we may need some mechanical air changes. This is where the “depends” comes in. Where do the homeowner’s like to keep their humidity level? Is there any moisture on the windows during the winter? Do they have a bunch of house plants? An aquarium? Is there another moisture source in the home such as a bare dirt floor? (The dirt floor should be addressed regardless of the air change rate.) Is there smoking in the home? Are the bath fans that are used during bathing? Depending on the answers to these and several more questions, I might recommend just cycling a bath fan for a few minutes every hour to increase the air change rate slightly. This might be accomplished by placing the bath fan on a programable timer. We are dealing with an older home built before the energy code requirements. I wouldn’t suggest installing an expensive HRV unit to achieve a few additional CFM’s per hour. Now, if the natural air change rate where a little lower, I would be suggesting an HRV unit.
Back to the question, at what point do I need to add balanced mechanical ventilation to a home? If you are meeting the current required air tightness level for new construction, you I will be suggesting mechanical ventilation. If your above 3 air changes per hour at 50 pascals, then it depends.