Blower Door Test Completed! 0.45 ACH50!

Energy use in a building scales with air infiltration.  Decreasing air infiltration can have a huge impact on heating costs.  The plot in this article (http://www.bluegreengroup.ca/blog/page/19/) illustrates things nicely.  Heating energy decreases linearly with decreasing infiltration while conduction losses through the insulation change little with air infiltration.   A tighter air barrier has several implications.  It leads to lower energy bills and it also leads to a more resilient structure that is less susceptible to moisture.   In addition, balanced mechanical ventilation provides fresh air more effectively than random infiltration and is a controlled route for energy capture/release through an HRV/ERV.

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,

147 CFM50/5913.8 sq. ft = 0.025 CFM50/sq.ft.  

The standard requires 0.05 CFM50/sq.ft.!  Our number is good!  In terms of air changes,

(147 CFM50 x 60 min/hr) / ( 19082.5 cubic feet) = 0.45 ACH50!!

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).

ACHnat = ACH50 / 14.8
=  0.03 air changes per hour.

the heat lost, Q, due to a single infiltration event would be

Q = m Cp DT

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

Q = Dair x ACHnat x Vb x Cp x Gair

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.:

Q = 1.225 kg/cu. m. x 0.03/h x 540.36 cu. m. x 1.006 kJ/kg/C x 69283 C.h = 1383170 kJ

which can be converted to 384 kWh.  At 0.93 ACH50 the heat lost would be 804 kWh which leads to an energy savings of about 420 kWh.  This is a highly simplified method but gives some indication of the order of magnitude for savings.  At our current 5 year projected electricity rate we would save about $97 per year if heating with electricity.  Since I will be heating with wood, it will save about 50-60 junks annually.  This begs the question:  Is reaching 0.45 ACH50 worth it?  In terms of the money saved on energy compared to the effort involved I would say probably not.  If we had reached 0.93 ACH50 I would have been pleased but at 0.45 ACH50 it gives us some buffer for issues as construction moves along.

All of the air sealing tasks required were well documented in the plans but it did require us to come up with a well thought out, methodical approach.  We couldn't have done this without discussing details in depth with my passive house designer, Mike Anderson (Passive Design Solutions).  In the past three weeks we completed an amazing amount of work:

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!




Photo 1-1 Brad Dunn setting up the blower


Photo 1-2  DG-700 pressure and flow gauge.  The knob below is used to modulate the speed of the blower.  The speed of the blower is changed until the pressure reads -50 Pa.  The flow rate in CFM is then displayed on the screen.


Photo 1-3.  While acquiring CFM, the message on the screen was LO.   This was a good sign! Trying to capture screen shots was almost impossible.

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Comments

  1. Just wondering why you went with 2x8s for the the exterior framing instead of double stud or staggered? I mean, it's clearly working out for you! But wanted to know your thinking.

    Thanks for blogging, it's an excellent resource.

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    Replies
    1. Hey, good question. This particular wall system was developed by Passive Design Solutions in nova scotia. They have several wall systems but this particular one was easy to implement with locally available materials. Most double stud wall systems require some sort of blown in insulation. Blown in insulation is can be more expensive than batts which is the case in NL. batt insulation is a good cheap option which works fine if installed correctly. the 2x8 wall system is just part of the wall. A 2x4 service wall on the interior will be insulated also. So this is still a kind of double stud wall, just not the kind people are used to seeing. later, Hope you continue to enjoy the blog!

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    2. Ah, I didn't realize you were going to have a service wall as well, right on.

      Just for interest, the closest I've seen to this on the avalon is the net-zero house that K&P construction built in flatrock a few years ago. I believe it was 3 staggered 2x4 walls filled with roxul. No exterior foamboard, sheathing on outside, poly on inside of second wall. Not sure what the ACH@50 was though! Best of luck with the rest of the build.

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    3. Yes, K&P does a double stud wall system. I believe, the double stud wall is an exterior 2x4 wall, a 3.5" insulated space, vapor barrier, then an interior insulated service wall. I know that the house you are talking about is certified to be NetZero ready. The NetZero standard now requires 1.5 ACH50 I believe which is the same as R2000. So their house had to meet at least that standard. I believe they did much better than 1.5 ACH50. Although everybody uses ACH50, its not necessarily the best standard. The problem is that surface to volume of a house increases as you make homes smaller. The main component of leaking air comes from fenestrations and holes in the air barrier. This makes it much harder to seal up smaller homes leading to a situation where its hard to meet smaller ACH50 so you get penalized for building a small house right from the beginning using the ACH50 metric. Personally, I think that CFM50/sq. ft. is a better metric. Thanks for the good words, more articles to come!

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    4. Great blog; I've been along since almost the beginning.

      You are correct on the K&P wall design; I've spoken a number of times with the owners of K&P as I was designing plans for my own double stud wall house. Search youtube for roxul, net zero ready, newfoundland and you'll see them take you through the relevant points of the wall, insulation, and air sealing (as much as a short video can do). You can also search for a powerpoint presentation which has some further details. I believe they achieved 0.75 ACH50.

      Their new design does not use batts exclusively in the walls; they have blow in cellulose from the vapour barrier out to the sheathing, and then batts inside of the VB. They had a mock up of that at the 2017 home show.

      When I priced up blow in cellulose vs batts vs open cell spray foam for my plans (3.5" outside stud, 3.5" space, 3.5" service wall - total 10.5" wall), I seem to recall the raw material for fibreglass batts was about $2600, roxul was about $7500, and I calculated about $3000 for the cellulose at my (mostly) retail prices.

      I never priced the cost of batts installed, as I figured I would do those myself. Open cell spray foam installed was about $12k, blown cellulose was about $10k. After watching many videos on dense pack cellulose installation for walls, I fail to see how that roughly $7k labour cost is justified, but I could certainly be wrong. Hopefully I build the house next year, not sure yet what insulation I will use.

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    5. Installing batts yourself is a good way to save a few dollars if youre on a tight budget. Dense fiberglass is another option and is about the same cost as cellulose installed. It does not provide the hygric buffering properties of cellulose. IMO cellulose is better...depending on application. Bill Gifford (home shield) is certified in dense pack cellulose. At the time I designed this house, there was nobody here doing it and I changed the wall construction to match materials methods available here. If I had my time back, and there were installers, a dense pack wall would have been preferable.

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  2. I believe Bill may have gotten certified after I sent him my plans and indicated the St. John's required the installer of dense pack cellulose to be certified. Does Flatrock inspect and require them to be certified?

    I had read that cellulose was better in that regard than fibreglass.

    If you did go with dense pack cellulose, would you have still used the exterior foam? If I understand it correctly, you have 4" exterior foam, 2x8 wall with fibreglass batts, and then a 2x4 service wall, also with batts?

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    Replies
    1. you are correct. The wall system for dense pack was a completely different design.

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