Trenching, Closing up, and a Blower Door Test

After Virginia's last blog entry, I realize just how technical this blog has been. For those of you that enjoyed Virginia's post (and based on all the comments and emails, there are a lot of you), I must warn you that this post returns us to some of the more technical aspects of the house. But what can I say, I actually find the technical stuff fascinating! Oh well, hopefully Virginia will chime in now and again to provide a broader perspective on this whole project. Anyway, there's a lot new to report and I digress...

Although most of the wiring in the house is complete, until recently there was no connection between the house and the utilities.  Terry had to dig a trench from the house out to the curb so that the utility companies could run the appropriate cables to connect the house.  But after the conduit was buried, the telephone company broke the string that was to be used to pull their cable through.  Rather than digging up the pipe, Robin tried using a vacuum cleaner to suck through a new string.  This worked like a charm and within a few minutes, we used the string to pull through the telephone cable.  So at this point, we have electricity running to the house; only two outlets are actually wired for power, but at least we can plug things in instead of using a generator. Although I don't look forward to getting a new electric bill every month, it's kind of neat to see a brand new meter reading 00000 at the very beginning of its life.

Brand new meter (Click image for more photos.)

One of the benefits of constructing a SIPs house is that in addition to being very well insulated, there are very few places for air to infiltrate the house; the house is very tight, so cold air stays out in the winter.  The downside of this is that you need some kind of mechanical air-exchange unit such as a Heat Recovery Ventilator (HRV).  The upside is that much of the heat loss due to air infiltration (which is most of the heat loss in a typical house) is eliminated.  To see how tight the house is and to find and eliminate as many air leaks as possible, a blower door test is often performed.  Such a test uses a fan to maintain a specific pressure difference between the inside and outside of the house while the rate of air flow (leakage) out of the house is measured.  Of course, such a test can only be performed once all the exterior doors and windows are properly installed (i.e. when the house is properly "closed up").

The builder wanted to do a blower door test before the drywall is installed.  This would provide an opportunity to fix some of the leaks that are found.  Since the drywall is scheduled to be installed in just a few days, it was critical to install the exterior doors so that the house would be properly sealed for the test.  There were five exterior doors to install, three sliding glass doors, one door to the garage, and the front door.  The sliding glass doors were large and heavy, so putting them in was not as simple as you might imagine. Fortunately for Robin, my daughter Kyra had the day off school and was able to help out.

Kyra helps out (Click image for more photos.)

The front door, by comparison, was much easier to install. Having these doors in place makes the house look and feel quite a bit more "finished" and gives the home a certain "welcome home" feel.

Front Door (Click image for more photos.)

After the house was closed up and ready to go, Stan Kuhn from Energy Auditors showed up for the preliminary blower door test.  Energy auditors is a company that specializes in providing an unbiased energy analysis and recommendations for improving energy efficiency in homes.  Stan will be doing an energy star rating of our home when it is complete and this preliminary blower door test is part of the process.  Performing the blower door test is qualitatively simple -- the house is closed up with the exception of one door, and a large frame with a flexible panel and a fan unit with attached computer is sealed into this doorway. When the unit is turned on, the fan pulls air out of the house and blows as hard as necessary to maintain a pressure difference of 50 Pascals (Pa). Measuring how much air is being moved by the fan tells you how much air is leaking into the house.

Blower door test unit (Click image for more photos.)

When the blower door test is in progress, you can look around for leaks by producing some smoke near a potential leak (near an outlet, door, or window). The smoke allows you to see where air is infiltrating the house, allowing us to find some of the major leaks and get them plugged before the drywall goes in. All in all, a pretty interesting process.

So how did the house perform? Well, to answer that question requires a little background and some technical information. Blower door test measurements are typically stated in one of two ways: cfm50 (cubic feet per minute at a 50 Pascal pressure difference), and ach50 (air changes per hour at a 50 Pascal pressure difference). The first one (cfm50) gives the raw leakage rate for the house but does not take into account how large the house is. Roughly speaking, a house that is twice as large will generally have twice as much air leakage even if it constructed to the same tightness standard. The second unit (ach50) takes house size into account and tells you the number of complete air changes (when all of the air in the house has been replaced by outdoor air) per hour.  If you find this topic interesting, Martin Holladay, an advisor for GreenBuildingAdvisor.com, has a nice blog entry on Blower Door Basics that's worth reading.  I'll summarize the relevant information here.

David Keefe, the manager of training services at the Vermont Energy Investment Corporation, writes in an article on Blower Door Testing that "tight houses tend to measure less than 1,200 cfm50", while Dan Chiras, author of The Homeowners Guide to Renewable Energy, says "a really good measurement is around 500 to 1,500 cfm50".  According to energy experts John Krigger and Chris Dorsi, authors of Residential Energy: Cost Savings and Comfort for Existing Buildings, existing "homes with 1,500 cfm50 are difficult to improve".  This is why it's so important to improve the tightness of the house before it is finished.  Our preliminary blower door test results gave 470 cfm50, so the house looks to be very tight.

Blower door test raw result (Click image for more photos.)

As mentioned, a more meaningful measurement would be the ach50 value.  While the cfm50 values are given at the time of the blower door test, the ach50 values must be calculated after taking into account the volume of the house.  My own estimate gives a preliminary value of approximately 0.9 ach50 for our house.  To give this some context, Holladay states that new home builders in Minnesota, where tightness is very important, routinely achieve 2.5 ach50 while the Canadian R-2000 program has an airtightness standard of 1.5 ach50.  While our results look quite good, I do note that the Passivhaus standard is an incredible 0.6 ach50, and while we hope our number will come down as the house is completed, it is unlikely that we will reach such an impressive value. But we'll keep our fingers crossed just the same.