Blower Door Test Completed! 0.45 ACH50!
Infiltration is typically measured with a blower door test. A fan blower is placed in a door to the exterior. The fan is then turned on and the differential pressure it creates causes the house to leak from exterior to interior (depressurization) or vice versa (pressurization). Typically, the air leakage is measured at 50 Pascals of depressurization. The air volume flow rate is determined from the fan parameters and then translated into cubic feet per hour. Dividing this by the volume of the house gives the ACH50 (Air Changes per Hour @ 50 Pa). Most new homes are about 3-5 ACH depending on how well the air barrier is installed but there is no way to tell unless a blower door test is used to verify air tightness. The PHIUS+ 2015 standard requires 0.05 CFM50 per square foot of gross envelope area. (The gross envelope area is specified during modelling and according to my WUFI Passive report, the area was 5913.8 sq. ft and the volume was 19082.5 cu. ft.). This corresponds to 295.69 cfm or 0.93 ACH50 i.e. 295.69 cfm x 60 mins/hour x 19082 cu. ft.
Brad Dunn (Amerispec NL.) was on site around 9:00 am on the morning of the test. I did the general tour showing him the various air sealing details. He was impressed and said that he would be very surprised if there were issues. The system that he was using can be set up with several discs that allow for progressively lower air flows. We used the smallest disc that he had available with his system. After turning on the fan, and adjusting the fan speed to get about 50 Pa, the meter read LO! I was happy! It took a few seconds to get the numbers. They were hard to capture with pictures of so I made note of them. At 50 Pa, the average measured value for CFM50 was about 147 cfm.
Now for the numbers.....Using the gross envelope area,
The standard requires 0.05 CFM50/sq.ft.! Our number is good! In terms of air changes,
The required air changes from the infiltration test had to meet 0.93 ACH50 so I am quite happy with this result.
The Energy Star home sealing specification indicates that we are in Zone 2. Since the house is a 2 storey dwelling, and in a moderatley exposed location, a factor of 14.8 can be used to convert ACH50 to ACHnat (natural air changes).
where m is the mass of air, Cp is the heat capacity, and DT is the temperature difference between the air inside and the temperature of the infiltrated air. The assumption here is that air naturally infiltrates constantly during the year according to ACHnat. This is a fairly simplified assumption but should, in principle, work over long time periods to give an approximate estimate. That being said, infiltration then needs to be summed over the whole heating season, while accounting for varying temperature outside. This is where the concept of heating degree days (or hours) comes in. So the equation for heating becomes
where Dair is the density of air, Vb is the volume of the building, and Gair is the heating degree hours for air infiltration. This expression has several variants and also appears as a term in equation 6 in this publication for WUFI Passive. The WUFI report for my house provides an estimate for Gair. In my report it was referred to as "degree hours ambient heating" and has a value of 124700 F.h/year (69283 C.h if converted to Celsius). After converting all units to S.I.:
1. Insulating the exterior walls.
2. Erecting the OSB air barrier.
3. Tape sealing and caulking joins in the OSB air barrier.
4. Tape Sealing the OSB air barrier to the window bucks, the rim joist space and the slab.
5. Spray foaming rim joists.
6. Spray foam, backer rod, caulking around all windows and doors.
My father was on site for 21 days working 9 hour days. I was on site during the weekdays working 4.5 hour days and working 9 hour days on the weekends for a total of 310 person-hours.
Our initial test result of 0.45 ACH50 is a testament to our methodical approach and diligent attention to details. 0.45 ACH50....an amazing result on my first passive house!