Building Science-An Interview with Gary Nelson

I think most of us know of this man, one of the founders of The Energy Conservatory and designers of the Minneapolis Blower Door.  I recently had the opportunity to chat with Gary.  My intent with the interview is a little different from other interviews that have featured Mr. Nelson.   I wanted to ask questions from a practitioner working in the field point of view.

I had some questions that have been bouncing around in my head for a while, but I was also curious about his experiences using the equipment he helped develop.  Gary and I chatted for over an hour and a half; his responses didn’t disappoint.

Randy-I live in a very cold climate, climate zone 7 of Northern Minnesota and often have to blower door test in very cold temperatures.  I recently conducted a test when the outdoor temp was  -20°F, (the interior space was heated to 68°F).  I use a laptop and software to control the blower door which asks for both the interior and exterior temperatures.  What effects do temperature, either very cold or very hot have on the test results?

Gary-“The inside and outside temperatures you put into the software, in theory, as long as the leaks don’t change size, should result in accurate corrections for temperatures.  It turns out the actual flow or CFM of air does change a little bit; it might be 10% or so if you go from 70°F to 0°F.  The correction that is done in the software corrects it so you should get the same result no matter what the inside and outside temperatures are.  But…leaks do change size, especially with humidity.  For a joint between a bottom plate and subfloor or any joint involving wood, in the summer the wood swells and those cracks probably get smaller.  There hasn’t been a lot of research on that, especially not recently.  Our calibration chamber is made out of ¾ inch OSB and is 8 x 8 x 16 feet; an OSB box.  In the winter, it has a little less than 1 CFM @ 50 Pa of leakage, and in the summer it’s about half that, and it goes up and down every year.  We measure the leakage rate at least every couple of months and have a graph that shows the leakage goes up and down regularly every year.  That tells me leaks definitely do change, and we think in the case of our calibration chamber, it’s got to be humidity because the temperature is pretty much the same year-round, but the humidity is way higher in the summer than it is in the winter.”

Randy-I’ve been testing some very tight houses recently, several below 1 ACH50 and I worry about these houses experiencing extreme negative pressures when exhausting equipment is in operation.  (I had one home end up with a -15 Pa while the dryer was operating.)  How should we be addressing make-up air in these houses?  I’m not talking combustions air, but make-up air for dryers, normal sized range exhaust hoods, (not the 1200 CFM monsters some people install), bath exhaust fans, etc.  Should we be installing an open or controlled vent to the exterior to address any negative pressures that these exhausting devices might create?  Is there a better way to address this issue?

Gary-“This is a big question that I keep hearing and asking.  I was out with CEE [Minnesota Center for Energy and the Environment] two winters ago when they were using the Aerobarrier system at a new apartment building. When they were done with one of the apartments it was less than 100 CFM50 of total leakage and Dave Bohac [Director of Research for MNCEE] and I had a discussion about how their kitchen fan was going to work.  What about their dryer?  What happens when they turn them both on at the same time? We didn’t have answers.

In houses that have no combustion appliances, I don’t have any problem with, when you turn the dryer on, the house goes negative 25 or even 50 (Pa).  It’s an intermittent thing.  When it’s below zero outside and you’re in a three-story house, the pressure at the bottom of that house could be -15 (Pa) because of the stack effect for weeks in a row.  At the top, it could be +15 (Pa).  Windows at the top of a house during real cold weather sometimes freeze shut.  There will be things like that.  If you have a big negative pressure for a long period of time, and you have some fairly large leaks where air is coming in through a wall and it’s hitting the back side of the sheetrock, making the sheetrock cold, you might have frost if you have big leaks.  But if the building is very tight, which it probably is because that’s why you’ve got the big pressures, and there aren’t any larger leaks, I’m really not too concerned about intermittent pressures up to about 50 Pa.  The reason for a 50 Pa limit is that at 50 Pa, it’s going to take about 10 lbs. of force to open an exterior door and there are safety standards that limit the force to open a door to 10 lbs. 

But once you get a house that’s down under 100 CFM50 and you have exhaust appliances larger than 100 CFM, now you need make-up air. There are make-up air units designed to be interlocked with exhaust appliances and that have heaters to heat the incoming air.  Electro Industries is a Minnesota company that makes one that I’ve heard good things about because all the required dampers, heater controls, and interlocks are included in a fully designed system.  I think there are possible solutions using an ERV or HRV but am not aware of anything on the market yet.  A solution for dryers is to use a heat pump dryer that doesn’t exhaust air from the house.  I almost put one in my house during a recent remodel but decided against it when I heard they were noisy.  I have recently heard that noise isn’t a big problem.

Randy-I have been seeing various issues with attached garages, many of these garages are conditioned.  The current ANSI/RESNET/ICC 380 standard prohibits attached garages, whether conditioned or not, from being included in either the volume calculation or the blower door test of a home.  In your opinion, is this exclusion a mistake?  If so, why?

Gary-“I am a member of the 380 committee and was partly responsible for the standard.  The ICC and IRC codes require that there be an air barrier between a house and a garage.  It does not require an air barrier between the garage and the outside, and as far as I could find, I did some research on this about six months ago, I couldn’t find much at all about any energy efficiency requirements for garages.  If you decide to fully condition a garage, I don’t believe there are any requirements for a vapor retarder, or an air barrier in the exterior walls.  But the thermal envelope of the house must contain a continuous air barrier according to codes.  It has to be part of the thermal envelope, but they don’t really define “air barrier” very well.  When you read the RESNET 380 standard for doing a blower door test, it’s not perfectly clear what the intent of the test is as far as what it’s measuring.  I’m lobbying for making this more clear.  I think that what it should say is “the purpose of the test is to test the integrity of the designed, continuous, primary air barrier.”  I think it’s important that it’s primary because in the case of a garage, the garage to the outside surface may have an air barrier.  The same with a three-season porch.  I have a sun porch that has R-40 walls, triple glazed windows, and is very air-tight.  But it’s also insulated and air-tight on the inside and because of the way RESNET works and the way the international codes work, that’s not considered conditioned space because it doesn’t have a heating system out there.  I think it should be clearer that the door from my porch to the outside should be open and the door between the home and the porch should be closed [when conducting a blower door test] because the primary air barrier is the air barrier in the thermal envelope separating warm from cold.  Same thing with a garage, it’s the wall between the house and garage that needs to have the air barrier.  One change in the 380 standard that we recently made was we now require that you open a door or window between the garage and outside during the blower door test so that you’re testing the air barrier between the house and garage. (Update: Gary lost this fight and the standard was changed back so that you should close the garage doors and windows during the blower door test.  This may have to go out for public review so is possibly not the final word on this.)

Randy-I recently had a home test at .33ACH50.  The home is code built, nothing special in the way of air sealing or insulation, a complete surprise it tested that tight.  I re-measured the home twice and conducted 3 more tests because I didn’t trust the data, all with the same results.  Have you had any blower door tests or testing experiences that caught you by surprise?

Gary-“I have not personally been surprised by any tests, but I’ve never tested a house that wasn’t designed to be airtight and found that it was .33 ACH50.  I’ve certainly heard of some poured concrete apartment buildings where each unit was less than .5 ACH50. 

Early on, back in the early 1980’s, one of my mentors was Dave Robinson who was a physicist that designed the first super insulated house in Minnesota back in the late ‘70’s. His brother built a super insulated house where he used just two large pieces of poly for the air and vapor barriers.  He owned a rubber and plastics company and had sources where he could get like 8 mil sheets of poly that were 30 feet wide and a hundred feet long.  He figured out how to cut and fold them in a way that there was one joint between the wall and the ceiling and one joint between the wall and the floor.  It was something like .2 ACH50. I had never seen anything like it.  I had an early blower door that I made that used flow measuring nozzles designed by Harold Orr from Saskatoon.  I had to use the smallest nozzle that was probably about an inch and a half in diameter and I was puzzled by how long it took to get the house up to pressure.  When I turned the blower door off, I could hear the air whistle for a long time as it went back through the blower door fan.  This was because the air in the house was compressed and had nowhere to get out except back through the small nozzle of the blower door.  I think it was at least 20 years before I heard of another similar experience.  I think that recently, especially with Zip sheeting catching on, it’s gotten a lot easier to make tight houses.  I’m sure there are a lot of them getting down to 1 ACH50 or less without the builder realizing it.

Randy-Given your education and experience in building enclosures, how did your personal homes (current or past) perform during a blower door test?  Did you design any unique or outside the norm details that improved performance?  Did you set a “tightness” goal (if you built new or did a major renovation)?

Gary-“My house was built in 1907, we moved in in ’78, and in ’80 I started working for the state energy office as an engineer designing an energy audit and got interested in making houses more efficient.  We had terrible ice dams.  It’s a one and a half story house, with a roof that was half rotten because of the ice dams and the condensation in the attic.  So, we took the roof off and made it a full two-story house.  We added 3 or 4 inches of foam board on top of the original wood sheathing.  We used nail-based sheathing that was made from chip board laminated to 3 inches of high-density polyurethane foam on about half of the house and the other half we found some cheap 4-inch-thick high density EPS foam that was designed for use under roadbeds.  The plan was to make the foam airtight.  We held all of the sheets of 4’ by 8’ foam about ¼ of an inch apart and we squirted one part polyurethane foam in that joint so it would expand and completely fill the joint.  We put in new triple pane windows. Unfortunately, this was before low-e or argon or good edge spacers and they all ended up failing.  I knew about air sealing attics, so we did a good job with that.  We got the house, which was about 2400 square feet, including the basement, down to around 1000 CFM50 or 3 ACH50. I don’t know what it was to begin with because I didn’t have a blower door then, but by the time I finished, I had a blower door.    We couldn’t get to some areas to seal including a front porch, 2 bay windows, a sun porch and a garage.  There were places we really couldn’t get at the foundation well.  There were places we couldn’t get at the rim joist very well.

In 2017, all of those problem areas were fixed as part of the new renovation that also added about 900 square feet of floor area.  We ended up deciding to take all of the siding off because we found that the joints between the sheets of foam had all cracked.  There were a few places where cedar shingles had turned black or were cupping and when we took the cedar shingles off, we found the sheathing underneath was black.  The places that were black were located where house air was slowly leaking out through the failed joints in the foam.    These failures didn’t cause the measured airtightness to change much; it was still about 1000 CFM50 when we started the renovation in 2017. But they did allow for enough air to exfiltrate and condense on the cold surface, which was the chip board, to cause deterioration.

Now that we are done [with the second renovation], the house airtightness measures about 500 CFM50 or about 1 ACH50. If I were going to build a new house, I would plan for it to be less than .5ACH50.  It’s really not hard to do.

Something I want to bring up that you didn’t ask about but that was an issue in our renovation is radon fans.  I imagine all of the houses you build have at least some protection for radon.  Because you’re into air sealing, you also try to make the slab as tight as possible.  Before our recent renovation our house had a couple long term radon tests that measured about 4 pCi/L so I wanted to make sure that a sub slab depressurization system, including the installation of a fan, were done.   The architect, general contractor and subs were all experienced with radon protection and I was traveling a lot so left them alone other than suggesting the smallest RadonAway fan be used since the slab addition was only about 450 square feet. The day before the slab was going to be poured, I did a quick look and thought everything looked great except for the poly under the 4 inches of XPS.  I thought we had discussed leaving the poly out but didn’t think it made sense to remove it although I was aware that, if installed, it really should go on top of the XPS.  I didn’t think about how the slab or poly was going to be sealed to the EPS concrete forms or how the plumbing penetrations for the new full bath would be sealed.  By the time I had a chance to think about this and do testing the basement was largely finished, including drywall and finished trim hiding all these details. When the house was completed and I did some testing I measured that when the radon fan was turned on it depressurized the house by about 3 Pascals and was sucking about 70 CFM of air out of the house.  This means that I really didn’t need the top-of-the-line ERV that was installed for ventilation.  I also measured the pressure in the sump that is located far from where the radon fan is drawing from and found it to be -11 Pa with reference to the house when what is needed is probably -1 or -2.  So, I sealed the sump cover and all the joints in the new slab that I could get to and then restricted the outlet of the radon fan to get the pressure in the sump to be about -2 Pa and have estimated that it’s still drawing about 25 CFM of air from the house.  The fan seems to have gotten noisier since I restricted it so I’m now looking forward to replacing it with a variable speed fan that won’t need to be restricted and will be both quieter and more efficient.  I gave a talk about my experience at a radon training symposium and learned from several of the instructors and attendees that my experience was typical, at least for Minnesota, in how slabs are sealed in new construction.  I now recommend that house depressurization due to radon fans should always be measured and that the amount of air that is drawn from the house by the radon system should count as a whole house exhaust ventilation system.  I’m guessing this is a much bigger problem than people are aware of.”

Randy-In your opinion, should there be changes to the current codes and standards?  Are the current codes of 3 or 5 ACH50 sufficient or should they be changed?  If so, at what point are we “tight enough”?

Gary-“I think code should probably be 1 or 1.5 ACH50, not 3.  Three was a good start to get you used to it and into it.  People were worried that if we make the code too tight, we are going to have all these problems with indoor air quality and so on.  I almost think you’re more likely to have issues at 3 because at 3 you can have a ventilation system that’s not working at all and a lot of the time you don’t have a noticeable problem.  When our house was at 3 ACH50 we could get by with only running our exhaust only ventilation system some of the time.  Now though the house is much tighter and my experience with even tighter houses is that if the ventilation system stops working, you figure it out pretty quickly.  It gets stale, you get condensation on your windows, people notice the odors.  You don’t notice that on houses that are a little leakier.  I think it’s safer to have a really tight house because when something isn’t working right, you’ll know it.

 Randy-In a related question, should there be any changes made in the way we test?  Do the current testing standards of using RESNET/ICC380, ASTM E779 or ASTM E1827 provide us with the best blower door test results?

Gary-“I think the current test standards are okay.  None have a really good definition of what the air barrier is and the intent of the test and how to prepare the building.  For single family homes I think the RESNET 380 standard is probably the best and I think it’s going to get better.”

Randy-There has been a lot of discussion about moving away from the ACH50 number to CFM/Sq Ft of surface area.  In your opinion, should the industry make the change?

Gary-“I definitely think the codes should get away from ACH50 and move to CFM50/Sq Ft.  I think if you did that, the number should probably be about .065.  There is no exact equivalent between ACH50 and CFM50 per square foot; it depends on the surface to volume ratio of the house. The surface area should include the whole envelope surface, including below ground; “all six sides of the box”.  The RESNET standard says to use the exterior dimensions of the walls.  I have always taught to use the inside dimensions, thinking it’s the usable floor space that matters.  It seems crazy to me that if you add six inches of insulation on the outside of your walls that you get credit for increasing the floor area.  It turns out the reason that’s in the standard is because that’s what is in an ANSI standard that was developed by real estate agents.  Real estate agents don’t want to go inside the building to measure the floor area and they want it to be as big as possible, so they measure on the outside.  For some reason we just adopted that standard for measuring floor area.”

Randy-Is there a new technology or a change to the way we currently test that has you excited?

Gary-“I don’t think so. There is a pulse technique from the UK where you use a tank of compressed air to make a quick pressure pulse that is analyzed to calculate the airtightness. I think it’s probably ok and may have some special applications, but it doesn’t help you find the leaks.  To me, the main reason to use the blower door is not just to get a number, it’s to help you figure out where the leaks are.”

RandyWhat lessons did you learn in your career that changed the way you design or manufacture your equipment and/or software?  In other words, did you have a moment of discovery that had an influence on the company you founded?

Gary-“I did. I was driving from my hometown of Two Harbors [Minnesota] to Minneapolis one day, thinking about how we were going to seal our blower door into a doorway, and I came up with this idea of four “L” shaped pieces of plywood that were slotted, kind of like the current frame, but I used plywood instead of aluminum.  It was simple, light, cheap and worked really well.  Then we came up with the idea of the cam levers. 

In my work over the years, the most rewarding thing has been working on research projects measuring things to figure out how to make buildings more energy efficient.  The data logging program TECLOG was designed for one of these projects studying back drafting and the Duct Blaster came from collaborating with several researchers on studies about residential duct leakage and distribution system efficiency.  We developed techniques for both measuring and locating duct leaks that were used in the first studies of energy savings due to the repair of duct leaks.  We worked on a project with researchers from Ecotope, a consulting firm in Seattle, and Washington State University on a grant from DOE to design a device to measure flows though air handlers better.  This resulted in the TrueFlow that just now is being improved and will be available as a wireless digital device with no tubes or wires to deal with.

Those kinds of projects have been more rewarding for me than the cam lever design.  Some of my best friends are still people I got to know working on them.”

Randy-When I purchased my first blower door, it was a tool that energy auditors/raters needed to properly do their jobs.  Today, educated building contractors are purchasing blower doors to build the best houses they can.  Did you foresee this shift in trades purchasing your equipment? Many of these builders will be learning to use your equipment from videos on-line instead of in the “classroom setting” where I first learned to use the equipment.  Is there a piece of advice you could give to a builder just learning your equipment?

Gary-“When I first saw a blower door, in around 1980, I thought that within five years every builder would have a blower door.  Houses are going to be airtight, because already in 1978, the Swedish building codes required 3 ACH50 for single family houses and 1 ACH50 for multi-family buildings above three stories.  So, I thought, oh, the market is builders.  Well, builders have never been a big market for us.  But I never changed my mind and still think builders are eventually mostly going to have blower doors because they are the ones who need it. 

Is there a piece of advice I could give a builder? Yes, read the manual, watch the video.  I try to talk people into the idea this isn’t hard.  We are just measuring the air going out of the fan, which is the same as the airflow coming in through the leaks, and we are measuring how much pressure it creates in a building.  It’s a simple concept and it’s really pretty easy to use.  I would always teach people how to do a simple one-point test using the DG-1000.  We have really good customer support, including Paul Morin [TEC’s technical support leader who has done as many blower door tests as just about anyone], and we will talk for hours to help you figure stuff out.  We are always quick at responding to questions.”

Randy-Are you currently working on any projects?  (I consider “happily retired” a project.)

Gary-“I am not happily retired and have no intention of ever retiring.  Well, I probably will, but as long as I am enjoying my job and feeling useful, I really like what I do!  I tell people who ask if I’m going to retire that, you know, I retired in about 1990.  I haven’t worked since then; it’s just been fun working with the incredibly talented people at The Energy Conservatory.

There are several interesting things I’m working on now.  One is a fog puffer made from an e cigarette and a turkey baster that we just introduced as a kit.  I’m happy with the quality of the fog it makes but am anxious for feedback from people that will be using it in the field every day before possibly making some improvements.  There is another commercially available fog generator that I like that I’m working on modifying to be more useful for field use when more fog would be helpful. I’m also helping with our work to improve the accuracy of the blower door during windy weather.  I designed a simple T fitting for the end of the outside tubing that both reduces wind noise and protects the end of the tube from getting rainwater in it and that will soon be a standard part of every blower door that we make.   And we are working on our TEC Auto Test software to make it detect wind and make better decisions about when to make longer time averages of data points.  The most important thing to do to improve accuracy when it’s really windy is to take longer readings.    I continue to participate in several committee meetings to improve ventilation standards and building test procedures.  I’m really looking forward to getting back to attending conferences in person again.”

Randy-I asked a related question to Gary’s comments:  Would it be better to run a multi-point test or single point test when it’s really windy?

Gary-“Either one, but you should make multiple readings.  If what you care about is CFM50 or ACH50, in theory, making more measurements of the flow at 50 Pascals will get you better accuracy than taking the same amount of time measuring at different pressures.  A procedure I like is to first make a 30 second baseline reading and then a 30 second reading at about 50 Pa using the CFM@50 mode on you gauge.  Repeat this, including a new baseline, 5 times.  The range of values you get for the 5 tests is a pretty good estimate of the range of values that the true value falls between and the average of the 5 values is your best estimate.  If all 5 values are less than what is required to pass the test, you can be pretty confident that the house passes.  Unfortunately, we currently don’t have software that supports this procedure, but I hope we will add this or something similar in the near future as we work on improving accuracy in windy weather.  If you are doing a multipoint test with TEC Auto Test, turn Wind Assistant on and it will set the time averaging period to 30 seconds (the default for Wind Assistant).  After you finish the test look at the results to see if the resulting confidence interval (the +/- %) is good enough.  If you want to get it to be smaller, repeat the test but change the time averaging period in Wind Assistant (in Settings/Wind Assistant Suggest When to Use) to 60.   Keep in mind that this is still a work in progress so give us a call if you need help.”

A big thanks to Gary and The Energy Conservatory for their time.

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