This post first appeared on the Green Building Advisor Website.
Work on the “barndominium” project is nearly complete after 18 months of construction. You can read parts 1-5 here on GBA (linked at the bottom of this post) and on this blog, links to the right labeled Barndominium Project. This final post will discuss mechanical systems, blower door numbers, along with the challenges and benefits of this type of building method.
Heat loss calculation
Because this project was constructed in a heating dominated climate (climate zone 7), the design of the heating system was crucial, and it started with a heat loss calculation. The calculation was performed by the heat pump manufacturer using Wrightsoft HVAC design and sizing software. The manufacturer was given the design, construction type, insulation levels along with the projected air tightness for the project. They used a simplified method for calculating the air infiltration of the building, this input uses a poor, good, or best range of air tightness instead of using actual air infiltration rates. Our goal for air tightness of the entire building was 1 ACH50, I don’t believe they thought that level of air tightness could be attained using a post and frame construction method, which resulted in a much higher air infiltration estimate on the heat loss calculation. More on the actual blower door test results later.
The heating load for the 3,480 square feet of living quarters was calculated at 46,767 Btuh at the heating design temperature of -17°F. As you can see by the heating and cooling calculation, infiltration rates are projected to be nearly 44% of the heating load with walls and window heat loss accounting for the majority of the remaining losses. I don’t have the inputs used to calculate the load, but I believe they are oversized by around 10,000 Btuh. Cooling loads for this project are minimal, but part of the system includes a forced air, ground source heat pump that is supplying the upper level with heat as well as the entire living quarters with cooling.
As for the 7,200 square feet of heated storage space, (maintained at 64°F) the heat loss was calculated at 93,574 Btuh, again with infiltration being the largest heat loss component at 38.4%. The walls and garage doors accounted for an additional 32.5% of the heating load. The storage space does not contain any cooling equipment.
The calculated heat loss for the entire structure was estimated at 140,341 Btuh, or nearly 12 tons of total capacity.
The Heating System
The homeowner was adamant about using a ground source heat pump as the primary heating and cooling system. We ended up installing 11 tons (132,000 Btu) of total heat pump capacity, a closed loop system with 11 vertical wells drilled between 180 and 200 feet. A total of 4600 feet of tubing in the loop field was used for heat transfer, roughly 400 feet (supply and return) down each well plus another 200 feet of supply and return installed as the manifold, buried 8 feet deep, between the home and the well locations. The system itself consists of a two-ton forced air system (water to air), and a two- and seven-ton hydronic (water to water) heat pumps, the two-ton hydronic system supplies in-floor heat to the living space with the remaining seven tons used to heat to the storage area floor. The system was first started in February of this year with well water temperatures in the mid 40°F range. We noticed that when all 11-tons were in operation, we were able to drop the loop temps (incoming water temperatures) down to mid 20°F. We are using a glycol antifreeze in the system in both the loop field and in the in-floor radiant tubing (it took three 55-gallon drums to achieve the required water to anti-freeze ratio for the system). There is 11,000 feet of 5/8” pex tubing placed on roughly 8” centers supplying the heat to the concrete floors.
We also installed a back-up system, a Lochinvar propane gas boiler/on-demand water heater. The main purpose of the Lochinvar is domestic water heating, but it also has the capability to add heat to the in-floor system should the heat pumps be unable to maintain comfort or require maintenance. We were worried about the GSHP not keeping up during a polar vortex when we can see outside air temps drop below -40°F for a time, or during a late season cold snap when the loop field temperatures may be lower than desired. Having a back-up should one of the heat pumps require maintenance is also a plus.
Balanced Mechanical Ventilation and Dehumidification
Minnesota has required balanced mechanical ventilation in all new residential construction since the early 2000’s, typically an HRV is installed in every new home. Because of the air sealing methods and triple pane windows selected, I suggested that we instead use an ERV. I’ve been leery about making a change to an ERV because of the slight increase in indoor humidity an ERV can create. Windows are typically the weak spot in my climate, they become dehumidifiers when temps are below 0°F and humidity levels are above 30%. Most homes I test during the winter run in the high teens to mid-20% humidity levels with minimal water or frost accumulation on a window. At 30% or higher, double pane windows will be damp, at 40%, the window is very wet or frosty. Wood windows will show signs of water damage after just a couple years. This becomes much less of a concern when a good triple pane window is used.
We installed a Broan AI series self-balancing ERV. The system is set up so that exhaust is drawn out of each bathroom, (no other bath fans were installed, mainly to limit the number of penetrations through the air control layer) laundry room and kitchen area and fresh air is supplied into the return side of the forced air duct system. The system is set to maintain a 35% RH in the living quarters.
If you’ve been reading my blogs for a while, you know I see a lot of problems with heated, attached garages in my market. Cold and wet vehicles entering a confined space with minimal natural air exchange is a recipe for disaster. I usually suggest a negative pressure or balanced air change strategy to reduce humidity levels in these spaces. Because of the sheer volume of this garage/storage space, controlling humidity by air exchange with outside would have resulted in a very large air exchange system, probably some sort of commercial unit. We chose to install a large dehumidifier instead. We went with the Quest HI-E Dry 195, a 195 pint per day dehumidifier. We are still playing with the humidity setting, trying to determine what the best RH for the garage is. At one point this past winter, we had five ice- and snow-covered construction vehicles inside the garage while we were working on the interior space. Between the floor drains and dehumidifier, humidity was never an issue.
The Electrical System
With all 11 tons of the ground source heat pumps operating at the same time, the electrical draw is 36 amps at 240 volts. About 8.6 kWh. In comparison, when all the LED lights are on in the home and storage area, the electrical draw is 50 amps at 120 volts, around 6 kWh. We installed a Leviton electrical panel, if we install the smart breakers for the GSHP system, we could track the energy use of each heat pump separately, I’d also like to see the dehumidifier usage tracked. Something I will suggest to the homeowner.
Nearly the entire building has lighting controls so that the homeowner is able to turn lights on and off using his phone, set lighting scenes, and schedule light routines to fit their schedule. An integrator (What Is a Smart-Home Integrator? – Fine Homebuilding) was brought on to the team to assure all the controls were communicating correctly with the controlling software, phones and tablets, and voice command devices.
The Final Blower Door Test
The blower door test of this large structure was a pleasant surprise. Our goal was less than 1 ACH50 for the living quarters and around 1 ACH50 for the garage/storage space. The total conditioned area is 10,680 square feet including the second story living space and a mezzanine space, which is also part of the living space. The total volume is 161,940 cubic feet with 32,340 cubic feet the living space portion. A total surface area of 27,136 square feet of which 6,040 is the surface area of the living space.
I conducted a total of three blower door tests, all RESNET multipoint negative pressure tests.
The living space results were:
536 CFM50, .99 ACH50, .089 CFM/square foot of surface area.
The storage/garage space results were:
906 CFM50, .42 ACH50, .043 CFM/square foot of surface area.
The total structure results were:
997 CFM50, .38 ACH50, .038 CFM/square foot of surface area.
Ideally, I would have liked to see the living space CFM leakage rate plus the storage/garage CFM leakage rate equal the total structure leakage rate. There is a difference of 445 CFM. This is the leakage rate between the two spaces. The question is, what is the actual leakage rate to outside of the living space and what is the actual leakage rate to outside of the storage/garage space? There are a couple ways to test this, the topic of an upcoming blog post. In the end, both the storage/garage area and living quarters will have lower final blower door numbers.
I’ve been involved in building dozens of post and frame structures throughout my career, including a couple “barndominiums”, but this was the first one I’ve tried to make higher performance.
The things I didn’t like. Most of the construction took place over the coldest months in climate zone 7, meaning we were building, insulating and air sealing in temperatures around 0°F, something I don’t recommend. The 18-foot sidewall height made the insulation and air sealing a challenge, especially air sealing the ceiling. The trusses on 6-foot centers also complicated the task. I wasn’t a fan of how the water management for the windows turned out. I had a specific detail in mind, the post and frame contractor refused to change their framing methods, resulting in a weird detail. I think it will work fine, it was just too complicated and took too long to perform. I also do not like the fact that this roof does not have solid sheathing, we have already run into a problem with this design, which only uses purlins on two-foot centers for the roof panel attachment. We had a small ice dam form in a valley this year, (the ice dam was the result of freeze/thaw cycles we experienced this past winter, not typically common). The ice right at the bottom of the valley backed water up where it found a path under the four-foot-wide valley W-flashing and into the attic. Solid sheathing, or at least some sort of membrane over the purlins would have eliminated the water issue.
The things I liked. Working with the MEP contractor (who also happens to be my brother). He and his team understood what we were trying to accomplish in both air sealing and overall performance and took the time to think through how they were going to address penetrations in the control layers, designing the mechanical room to maximize space, and think ahead so that future changes won’t drastically change the planned performance. I also liked the substantially reduced thermal bridge in the framing members. I recently visited a couple of RR Buildings high performance post and frame projects, Kyle, the owner of RR Buildings, mentioned that someone had calculated the thermal bridge for his project at 8%, pretty impressive when most homes are closer to the 15-20% range.
I had a conversation with the homeowner on the electrical costs to operate the building since the GSHPs were started. The end of this winter saw more cold weather than the beginning, a good test of the system. The owner indicated the electric bill has been around $400 to both heat and operate the building. By comparison, another, much smaller structure on the property costs $400 per month just to heat.
Would I do another? I think so, with the right customer and budget. This was a fun project to work on. Given the size and scope, this one was not a “sustainable” build, but the project would have been built regardless. At least we were able to reduce the operational costs somewhat and hopefully give the customer the generational property he was looking for. With a little luck and some dedication from his descendants, the structure should still be standing more than 100 years from now.