Mechanicals-Adding Mechanical Ventilation to an Old Home

Something I suggest before any renovation to an older home that includes improvements or changes to the water, air, vapor, or thermal control layer is to have a blower door test conducted.  The improvements may include but not limited to window replacement, new siding that includes the addition of a water resistive barrier (WRB), improvements in insulation, or an addition to the home.  A pre and post blower door test will indicate any changes to the natural air exchange rate.  Less natural air exchanges are good for controlling energy costs, and increase comfort, but may not be so good for indoor air quality or keeping indoor air dry during the winter months.  Controlling humidity is the main reason Minnesota requires balanced mechanical ventilation in every new home.

What can happen?  I told this story once before on this blog, but I’ll tell my experience with window replacement having an effect on the “tightness” of a home again.  Several years ago, I was asked to replace four windows on the upper story of a 100-year-old farmhouse.  The space was previously 2 bedrooms.  Now the homeowner only used the space for storage, and partially sealed the area during the winter. There was no heat source on the upper level, so the temperature probably dropped into the 30’s and 40’s during the winter

months. The windows that where replaced were old and very leaky.  The air that leaked through the windows helped keep the humidity level low during the winter.  The first spring, after the windows were replaced, I was called to the home to investigate water damage in the ceiling.  (The homeowner thought there was a roof leak, I had replaced his roof a couple years earlier.)  One look in the attic and I could see where the water was coming from. The entire roof deck was covered in frost.  There had never been any issue with frost or water before the windows were replaced, so what was the cause?  The window replacement tightened the space just enough to raise the temperature and humidity level.  Science says warm air can hold more moisture.  There were several areas of attic knee wall storage where I could easily reach through the insulation and touch the roof deck.  The slightly warmer, moist air was leaking into the attic because of the “stack effect”, the warmer air holding more moisture was meeting the cold roof deck, achieving the dew point and turning to frost.  All because 4 windows were changed raising the temperature of the space a few degrees, who would of thought?  Had I conducted a blower door test on this home, would I have realized I tightened up the structure enough to warrant mechanical ventilation?  Probably not, but that may not be the case for all homes having changes made during remodeling.

At what point do we need to worry about balanced mechanical ventilation?  That’s kind of a tricky question.  It depends!  The average house that has a blower door test of over 5 air changes per hour at 50 Pascals (5ACH50) probably has enough natural air leaks in the home to control moisture levels.  (Some homes in my area add central humidifiers to the forced air heating system to try to increase humidity during the winter, never a good idea!)  The location of these natural air leaks may be affecting indoor air quality and possibly the durability of the structure.  They are definitely paying more to heat and cool the home.  If this same house has a high moisture load, such as an unsealed dirt crawlspace, leaking foundation or basement, or a large number of family members that cook, bathe, and breathe in the home, may need additional air changes to control moisture levels.  I always recommend to my customers to know the humidity level in the home, keep it around 30% during the winter in a cold or very cold climate.

A heat recovery ventilator. The two upper insulated ducts are fresh air from the outside and stale air to the outside. The lower left duct is fresh air to the home and the lower right duct is stale air from the home.

Above 3 ACH50 but less than 5 ACH50, I suggest an air exchanger, but it may be able to be smaller or operate less than one installed in a new, tight home.  A high moisture load or a family that smokes indoors, burns lots of candles, fries lots of food without using a vented range hood (or uses a ductless range hood), or has several teenagers taking long showers should operate an air exchanger, usually a heat recovery ventilator (HRV) in cold climates.  A home that has a decent indoor air quality, no smoking or candles and operates a vented range hood while cooking may be able to limit the operation depending on the humidity within the home.

A home tighter than 3 ACH50 (read about ACH50 and blower door testing here), needs mechanical ventilation, at least during the winter months.  If the home is very tight, less than 1.5 ACH@50PA and the HVAC system controls the temperatures during both the heating and cooling seasons (the occupants do not open windows often), the air changer should operate year round.  Windows that are frequently open during the summer could reduce or eliminate the need for mechanical ventilation during warm weather.

Will updating and remodeling an old home have an effect on the natural air changes of the home?  Probably.  Can the changes have an adverse effect on the home and the health of it’s occupants?  Possibly, the only way to know is to test.  I currently charge $200 for a basic blower door test or a code required test on a new home.  (Within 75 miles of my home.)  I offer a second test for free if needed.  This test does not include looking for air leaks, just the air change rate with the blower door operating.  That blower door number will tell you if you require mechanical ventilation.

How much ventilation will be needed?  If you live in Minnesota, the formula for figuring ventilation needs is:

Total ventilation (cfm) = (.02 x square feet of conditioned space) + (15 x (number of bedrooms + 1)) 

This calculated cfm rate can be reduced by 50% if the ventilation equipment operates continuously.  If you live outside Minnesota, you will be following either a local or state code or ASHRE 62.2, which uses a similar formula for cfm calculations.  To my knowledge, there is no cfm reduction for a continuously operated HRV or ERV allowed in the International Residential Code.

Want more information about mechanical ventilation and air exchangers, here’s a link to my blog post Mechanicals-Heat Recovery Ventilators here.

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