Building Science-Air Changes

My last posting I talked about air changes at 50 Pascals (ACH50) and air changes natural (ACHnat).  This week I’m going to discuss how much fresh air a home needs in a northern climate and a couple of the codes Minnesota has in force that work well.  One of these codes, the requirement for balanced ventilation, helps add durability to our structures.If a home has a low natural air change rate, moisture and pollutants can become trapped within the home, especially during the winter months when windows can remain closed for several months.  Elevated moisture levels can lead to mold concerns, can damage windows, and can affect building durability.  Indoor air quality can also be affected, VOC’s, formaldehyde, and other indoor pollutants can cause breathing difficulties and other health concerns.  It’s important that a home gets some fresh air, but how much is enough?

Suggested ventilation requirements at the national level are set by ASHRAE (American Society of Heating, Refrigeration, and Air Conditioning Engineers), specifically ASHRAE 62.2.  The formula used to calculate the fresh air needed when using a whole house strategy is:

total ventilation = 0.003 x the total square footage of the finished areas + 7.5 x (number of bedrooms plus one).

As an example, a 3 bedroom, 2,000 square foot home will require 90 cubic feet per minute of fresh air.  2,000 square feet x 0.003 = 60 CFM + (3 bedrooms + 1) = 4 bedrooms x 7.5 CFM = 30 CFM.  60 CFM + 30 CFM = 90 CFM total.

A heat recovery ventilator (HRV).

Minnesota’s requirements vary slightly from ASHRAE 62.2.  Minnesota’s requirement is:

Total ventilation = 0.02 x the total square footage of the conditioned space + 15 x (number of bedrooms plus one).

Using the previous example of a 3 bedroom, 2000 square foot home, the ventilation rate increases to 104 CFM.  If the ventilation is continuous (and balanced), the rate can be reduced by 50%.  Minnesota requires balanced ventilation, usually a heat recovery ventilator (HRV) is used.  The example house would be required to have 52 CFM of continuous ventilation.  I’ve written a blog detailing HRV’s, read that posting here.

In some areas outside of Minnesota, ventilation can be accomplished by an exhaust only system, usually a bath or kitchen fan is used to move air out of the home.  I’m not a fan of this type of ventilation in a cold climate, here’s why.  If you use an exhaust fan to remove air from a home, the home’s pressure becomes negative.  The pressure inside the home wants to be the same as the pressure outside, to equalize this pressure difference, outside air will be drawn into the home where ever it is able.  Ceiling, walls, doors, windows or a designed make-up air vent can become leak points.  If the outside air is cold, it’s going to increase the heating needs of the building.  Comfort is often times effected.  A balanced HRV unit will exchange some of the heat from the inside air leaving the building with outside air entering, reducing the heating need.

What if you have an older home, do you need mechanical ventilation?  The answer to this question is going to be based on the home.  Most older homes, such as the 1952 Cape I recently purchased have plenty of air exchanges, lots of leak points.  Some homes have undergone renovations at some time during their existence which may have tightened the envelope of the building.  If you have moisture on your windows (another blog I wrote which you can read here) during moderate winter temperatures, you may need mechanical ventilation.  The only way to really tell is by a blower door test.  I use a software program called Tectite that both controls the blower door and produces a finished report.  The report will estimate the infiltration rate.  A code complying 3 air changes per hour at 50 Pa will need additional ventilation.  Depending on the size of the home, 4 air changes may not.

Humidity condensing on a window.

Do you need mechanical ventilation?  The best way to tell is by a blower door test.  The moisture on window is also an indicator.  If you are building a new home in Minnesota, to be code compliant, it will have balanced mechanical ventilation.  If you are building a new home outside of Minnesota, build it tight and ventilate it right!

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