The Airport House: Measuring Results

Wrapping up the series with a look at how well the house has performed

Editor’s note: This is the last in a series of guest blogs by Reid Baldwin about the construction of his house in Linden, Michigan. For a list of previous blog posts on GBA by Reid Baldwin, see the “Related Articles” sidebar below. You can read his entire blog here.

We have now been living in the house for about nine months. I would like to wrap up this series of articles by discussing the measured performance. I’ll start with the blower door test.

One of our goals during construction of the house was to make it airtight. Of course, no house is ever completely airtight. Some houses leak a little and other houses leak a lot. The standard way of measuring airtightness of houses is a blower door test.

A fan is installed into the front door so it blows air out. An equal amount of air comes in through whatever leakage paths are in the house. The technician adjusts the fan speed while measuring the pressure difference between the inside of the house and the outside of the house. That determines the rate of airflow required to establish a 50 pascal pressure difference. The air flow rate is compared to the volume of the house to calculate the number of air changes per hour, called ach50.

A low ach50 number is good. We had a blower door test performed on our previous house, which was built in the early 1990s. The ach50 was about 10. That house had many of the symptoms of poor air sealing: rooms that were uncomfortably cold whenever it was windy in the winter, for example.

New construction houses now usually have an ach50 of between 3 and 4. To be certified under the very rigorous Passive House standard, a house needs an ach50 of less than 0.6. Builders of Passive Houses go to great lengths to get to that level. For our house, the heating and cooling load calculations assumed an ach50 of 1.5, although I hoped for better.

Floor plan complicated air-sealing

The floor plan of our house is not ideal for air sealing. The fact that only some of the house has a second story and the ceiling in the hangar is midway up the second story meant that we had more wall/ceiling intersections to seal. We did a number of things to improve the airtightness.

The worst areas in most houses are the ceilings and the rim joist. To reduce air leakage through the ceilings, we:

• Located the attic accesses either in the garage or in the gable ends to avoid leakage around attic access doors.
• Avoided recessed can lights.
• Used spray foam around the intersection of the walls and ceilings.

To reduce air leakage around the rim joists, we:

• Applied a peel-and-stick membrane on the exterior extending from the foundation concrete to the wall sheathing.
• Used spray foam on the interior

To reduce air leakage through the walls, we:

• Taped the joints of the OSB sheathing.
• Caulked around the framing on the interior side
• Selected high-quality casement windows.

So, did these things work? Yes, they did. Our blower door test result of 0.82 ach50 proves it.

As discussed in a previous article on heating and cooling, the Manual J design heat load for my house is 34,000 Btu/hour at a design temperature of 7°F. To check this, I used a method of calculating the heat load based on fuel use as described in this GBA blog by Dana Dorsett.

The heating demand is satisfied by a combination of internal gains and furnace output. The contribution of the furnace is calculated based on historical fuel use data. To calculate the contribution of internal gains, this method relies on a guess at the home’s balance point, which is basically the outdoor temperature at which internal gains would keep the house at the design indoor temperature. A better insulated home will have a lower balance point than a poorly insulated home. Homes with a lot of solar gain or other internal gains would have lower balance points. Dana recommends guessing 60°F for a house with 2×6 walls and 65°F for a house with 2×4 walls.

Instead of simply guessing what the balance point was, I attempted to determine the balance point by applying Dana’s method to four different months of fuel use data. For each of the four months, I tried four different levels for balance point, 50°F, 55°F, 60°F, and 65°F. The estimated heat load gets lower as the presumed balance point gets higher. However, the slope is steeper for warmer months. (That is probably why Dana recommends using mid to late winter bills.)

The negative slope results from assuming that a lower proportion of the heat comes from internal gains as the balance point increases. (This does not imply that modifying the house to have a higher balance point would reduce the heat load.) I graphed the results (see illustration #3 below). In theory, the lines for the different months should intersect at the actual balance point and design heat load. In practice, noise factors like varying occupant behavior cause some divergence from the theory. Based on the graph, I believe the balance point is in the ballpark of 56°F and the heat load is in the ballpark of 28,000 Btu/h.

The lines for October to December seem to cross as expected. However, the line for January appears to have shifted. This could be due to noise factors or could represent an actual change in the heat load. I would expect some drop in the heat load because we improved the insulation in our attic around that time, as mentioned in a post I wrote in GBA’s Q&A forum.

In relying on the Manual J calculations, I specified a two-stage furnace with an output rating of 39,000 Btu/h on high stage, against the HVAC contractor’s recommendation. On mornings with single-digit temperatures this winter, however, the furnace ran continuously without satisfying the thermostat setpoint. I have not yet figured out why that happened. Is the heating load actually substantially higher than suggested by the Manual J calculation or the calculation based on fuel usage? Is the furnace not actually putting out its rated output capacity? (I did verify that the fan speed changes as expected when it is supposed to switch from low stage to high stage.)

One theory is that the small fan that goes with the small furnace is not able to distribute the heat throughout the relatively large house. I didn’t worry too much because the house never got uncomfortable and the temperature came up to the setpoint rapidly once the sun came out.

Another decision that the HVAC contractor pushed back on was my decision not to install a humidifier. On that front, the data say that I was right. The indoor relative humidity did not go below 30% over the course of the winter.

The Manual J calculated cooling load for the house was 18,000 Btu/h at 88°F. The smallest central air conditioner that was readily available was a 2-ton (24,000 Btu/h) unit. I haven’t attempted to systematically measure the cooling load. Anecdotally, I notice that the air conditioner runs about half of the time on the few occasions that the outdoor temperature has reached the mid 80s this summer. I wish I would have spent the extra money for a two-stage air conditioner. When only one zone is calling for cooling, it is sometimes uncomfortably cold close to a supply register. If we had a two-stage unit and a zone controller smart enough to use the low stage when only one zone is calling, that problem would be mitigated.

Out of curiosity, I installed a remote temperature sensor in the attic while dealing with the insulation issue. On a sunny day, it is not uncommon for the attic air temperature to be 20°F to 25°F warmer than the ambient outdoor air temperature. So, for cooling, the delta T through the ceiling is commonly three times larger than the delta T through the walls.

The relative humidity on the first and second floors seems to average about 50% in the summer without supplemental dehumidification. In the basement, the temperature stays in the low 70s without ever calling for air conditioning. The relative humidity in the basement often pushes 60% if I don’t run a dehumidifier.

Overall impressions

During the process of designing, building, and living in this house, I have learned a lot about building science, HVAC, and the housing construction industry. Fortunately, I learned most of it in time to incorporate that knowledge into my house. There are a few things that I learned about too late in the process to apply. I got a much better result in terms of energy use and comfort than I would have gotten if I simply hired the usual cast of professionals to do what they usually do.

The cast of professionals that I hired also learned some things. It seems that only a tiny fraction of the industry is actively applying a lot of what experts know about how to build houses. Hopefully, my project made that fraction a little bit less tiny.

1. | | #1

I must have missed it in one
I must have missed it in one of the blog entries, but what did you end up doing with the roof insulation?

2. | | #2

Response to Greg
When you say roof insulation, I suspect that you are asking about attic insulation. We have a vented attic with blown cellulose on the attic floor. There is no insulation at the roof.

The insulation issue that is mentioned in the article deals with one small area that did not get blown cellulose when the rest of the attic did. The insulation contractor made a hole in the closet ceiling in order to access the attic. (The attic access was in a gable end, but they didn't notice that.) Obviously, they couldn't blow cellulose over that spot until the hole was repaired. After the hole was repaired, they came back and installed fiberglass batts in that spot. Once winter came, I noticed that the ceiling in that spot was much colder than the rest of the ceiling. We made them come back and do blown cellulose there.

3. Expert Member
| | #3

reid

4. | | #4

Tight air seals
There was a good reason why old houses had some air leaks. The inside of a house needs a way for the moisture from bathrooms, kitchens, and basements to escape. The incidence of allergies to mold and other contaminated is documented to have increased over the recent decades, and some research connects the tight seals of new houses to these medical problems. 30-50% moisture content in a home is a nice breeding ground for these allergens. Builders and homeowners should be required to install dehumidifiers as standards.

5. GBA Editor
| | #5

Response to Catherine Brooks
Catherine,
Unfortunately, your post is a mixture of half-truths and falsehoods.

Indoor air quality (and control of interior moisture levels) are obviously important issues. Good ventilation depends not on random air leaks through the walls and ceiling (as you advocate) but on making the envelope as airtight as possible and providing a mechanical ventilation system.

In general, homes that comply with green building programs, or built according to building science principles, are more likely to have high indoor air quality (and less likely to have high indoor humidity or mold problems) than homes that have a high rate of air leakage.

6. | | #6

Very impressed with what you have accomplished - especially since you seemed to be bucking the norm of builders/contractors!
You probably will not do this since your house is now "buttoned up", you are already experiencing short-cycling of your AC, and you are in a predominantly heating climate zone but your comments on attic VS ambient air temps bring me to comment. Was just installing 8 more plastic air ventilation chutes in my attic joist space today (actually replacing some much narrower foam chutes that allow much more radiation) and took a thermometer up while I was working. Outside air temp (mostly cloudless day) was 82; attic temp - 95. I have previously measured over 100 degrees with similar outside temps but before I started installing ventilation chutes. This is in a 1930's brick house, Climate Zone 5, roof running almost North-South with no soffit vents (I have 6 double-hung windows I leave open during the Summer and close in the Winter) and with a very high roof peak. My hypothesis is that the roof deck cools more quickly and radiates much less heat into the attic if the entire underside is somewhat encapsulated with venting chutes (I also believe this is why the reflective foil products lower attic temps). In my case, I only install chutes about 10' down from the (vented) peak to allow interior attic air to enter the bottoms of the chutes when the chute interior space gets heated and moves to the ridge vent through faster thermosyphon action. To date I have covered roughly 1/3 of my roof underside and plan to install about another 48' of venting chutes - not even the entirety of my roof deck, just the part with the most solar gain. I'm hoping to reduce Summer delta-T between the outside ambient and attic air temps to around 10 degrees or less.

7. | | #7

Response to David
When I see that the attic temperature is substantially higher than the outdoor temperature, it is very tempting to try to reduce my cooling load by improving the air exchange in the attic. Based on the various threads on GBA regarding attic fans, I have not pursued that approach. With the attention to air ceiling at the ceiling in my house, it may be in the narrow class of houses where an attic fan would be effective, but relying on the attic floor insulation seems better. We know that works all year whereas I would be guessing that an attic fan would be helpful just for the cooling season.

Your approach seems like an interesting experiment. If I understand correctly, you are concentrating the solar gain in narrow spaces between the rafters in hope of creating a chimney effect. Do you have a measurement plan so that you will be able to report on the effectiveness?

8. Expert Member
| | #8

In heating dominated climates the attic heat is beneficial.
Roofing materials with a low solar reflective index (SRI), including most asphalt shingles turns the roof into an unglazed solar collector. Even though convective cooling occurs on sloped roofs (primarily on the exterior, even with vented roofs) the additional heat gain to the attic lowers the amount of heating energy use for the house, and increases cooling energy use. In heating dominated climates (zone 4 & up) the heating season energy savings almost always outpaces the additional cooling energy use, for a net reduction in annual energy use. The higher the R-value of the attic, the smaller the effect, but it's still possible to measure it At IRC 2015 code-minimum R-values the effect is negligible.

Additionally, the higher attic temperatures lower the relative humidity in the attic, and drives moisture out of the wood. This is true in both heating & cooling dominated climates. While "cool roof" shingles can lower energy use in cooling dominated climates, the lowered attic temperature does increase the average moisture content of the roof deck, sometimes to consequential levels.

Attic fans usually use more power for the fan than it offsets in cooling energy use by the AC, and can often increase the energy used by the AC unless the attic floor is completely air tight. The exceptions would be small solar powered attic fans that produce their own power, and do not excessively depressurize or pressurize the attic. In one Florida study in lowered the cooling energy use by a large single-digit percentage, but usually air sealing and bringing the attic insulation up to code minimums or higher is a better investment.

• Solar and Hot Water at the Airport House

The roof of even a small aircraft hangar provides plenty of room for a sizable PV system

• Airport House: Experiments in Home Automation

The pleasure of playing with technology makes up for the effort and expense of installing it

• Airport House: Choosing and Installing an ERV

No, tight houses do not need to breathe, but they do need a source of fresh air

• Airport House: Heating and Cooling

Insulation levels play a role in selecting the most appropriate system

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