When my friend Laura Murphy mentioned that her neighbors in Ripton, Vermont, Chris and Zoe Pike, stayed warm last winter by burning just half a cord of firewood, I was intrigued. So I tracked down the Pikes to learn a few more details about their house.
It turned out — surprise, surprise — that the Pikes’ house was designed by Chris Corson of Belfast, Maine. In fact, the Pikes’ house is a virtual replica of the well-publicized Passivhaus that Corson built in Knox, Maine. (GBA has published two stories about the Knox house: Striving for Passivhaus Affordability and Cold-Climate Passivhaus Construction Costs. Chris Corson’s JLC article about the Knox house was titled .)
Chris Corson, the founder of , is one of two New England builders — the other is Carter Scott in Massachusetts — who have been justly praised for building high-performance cold-climate homes that don’t break the bank.
A tight thermal envelope with high R-values
The Pikes hired Alex Carver of to build their house. Construction was completed last fall.
The house sits on a slab-on-grade raft slab foundation that includes 12 inches (about R-50) of expanded polystyrene (EPS) foam. “Branch River Plastics delivered the EPS foam right to my shop, which is two miles away from the building site,” Carver told me. “We precut the four outside corners. I had to miter the corners, using a combination of hand saw cuts and a hot wire. The house is a rectangle, so that was easy.”
The 14-inch-thick walls are a variation of the Klingenberg wall, with 2×4 bearing walls sheathed with OSB and vertical TJIs attached to the exterior side of the OSB sheathing. The TJI bays were insulated with dense-packed cellulose, and the 2×4 bearing wall was insulated with 3.5 inches of Roxul mineral wool, for a total wall R-value of about R-52. The attic is insulated with about 36 inches of cellulose (R-100 plus).
The walls of the Pike house do not have any fiberboard sheathing (the type of sheathing used at the Knox house) on the exterior side of the vertical TJIs. Instead, the walls use a European membrane (Solitex Mento) on the exterior to retain the cellulose insulation.
“The most tedious work was the vigilant air sealing and taping,” Carver told me. “It was a challenge.” The work paid off. The Pike house tested at 0.40 ach50, easily beating the Passivhaus airtightness target.
The house has triple-glazed Intus windows with vinyl frames. The roofing is steel. For more information on the construction process, see the blog written by Zoe Pike, . (Zoe’s blog does an excellent job of .)
Not much firewood was needed
Like the house that Corson built in Knox, Maine, the Pikes’ house is designed to be heated with a single 12,000 BTU/h Mitsubishi ductless minisplit unit. “We have a minisplit, but we didn’t turn it on all winter,” Chris Pike told me. Instead, they heated their house with a Morso 3112 wood stove. The stove has a very small firebox that takes 12-inch firewood.
I spoke to Alex Carver, the builder, about the stove. “Chris Corson did not want the Pikes to have the wood stove,” Carver told me. “But Chris Pike was adamant that he lived in a wood lot and wanted to burn wood.”
Chris Pike said, “Chris Corson predicted we were going to burn 1/2 cord of firewood. I told myself, ‘I’m going to put up a cord and a third.’ But we used less than 1/2 cord of firewood for the entire winter.”
I was curious to know how carefully the Pikes measured their firewood consumption. “We stack our firewood on pallets, and each pallet holds a 1/3 cord,” Chris Pike told me. (See Image #4, below.) “At the beginning of winter, we had four of those pallets. We burned one and a half pallets of firewood, along with about half a pallet of construction scraps.”
After Chris Pike e-mailed me some photos of his firewood stacks, I concluded that he measures the firewood carefully.
A cord of seasoned hardwood contains about 24 million BTU, so a 1/2 cord contains about 12 million BTU. If the Pikes’ wood stove has an efficiency of 65%, then the house required about 7 million BTU of heat last winter. (To get the same amount of heat from an oil heating system with an efficiency of 85%, you would need about 59 gallons of fuel oil.)
Learning how to run a wood stove in an airtight house
It can be tricky to operate a wood stove in a very tight house, so I asked Chris Pike if their Morso stove had any problems with draft. “We installed a dedicated air inlet,” he told me. “Morso calls it the ‘mobile home kit,’ because it’s often used when the stove is installed in a mobile home. The air intake goes right to the back of the stove. We installed a short length of metal duct, but the fresh air duct transitions to PVC in the wall.” The 2-inch diameter duct is controlled by a ball valve purchased from a plumbing supply house. [Author’s postscript: GBA readers should note that the use of PVC duct in this location may violate code requirements and may constitute a fire hazard. For further information on this topic, see Comments #3, #15, and #17, below.]
He continued, “We also have a Zehnder ComfoAir 200 HRV. When the Zehnder unit goes into defrost mode, it shuts down the outdoor air intake, which can depressurize the house. So we learned to shut off the Zehnder unit — to power it right down — before lighting a fire in the wood stove. I later figured out that the Zehnder has an ‘open fire mode’ that can be engaged instead of turning the unit off. That way the wood stove drafted much better during the initial start. We also learned that our electric clothes dryer will put the fire right out. When the dryer is on, the smoke will back-puff from the stove and the fire will go out. So we never use the clothes dryer when the stove is lit. It’s just a management issue.”
Alex Carver provided another example of the Pikes’ “active management” approach. “The Pikes insisted on having a gas range, and the range hood isn’t vented,” Carver told me. (Their range hood has a recirculating charcoal filter; the kitchen also has an exhaust grille in the ceiling connected to the Zehnder HRV.) “The CO2 levels are being monitored. I was over at the Pikes’ house for dinner a few weeks ago — we were a party of six — and the CO2 monitor was on the counter. Chris noticed that the CO2 levels were rising, so he turned up the Zehnder a notch, and the CO2 levels came down.”
Were the indoor temperatures low?
I wondered whether the Pikes’ low rate of fuel use could be attributed to low indoor temperatures. The answer was no. “When we woke up in the morning, the indoor temperature was about 65 degrees,” Chris Pike told me. “But if the day was sunny, it was 74 in the afternoon.”
Chris Pike was interviewed by journalist Ted Cushman in January; the interview led to a blog post on the JLC website titled Chris Pike told Ted Cushman that “bedrooms on the north side of the house run slightly cooler than the sunnier south-facing rooms … but they’re still comfortable.”
Pike also told Cushman, “In the evening, my wife might start a small fire in the wood stove after the kids are in bed, just for us to hang out by. … And by ten or eleven o’clock the fire goes out and we go to bed.”
The colder the outdoor temperature, the earlier the Pikes would light their evening fire. Pike told me, “When the outdoor temperature was 10 degrees or less, we would light a fire in our wood stove in the evening at about 6 o’clock. The next morning, I would look at the weather forecast. If it was going to be sunny, I wouldn’t touch the wood stove. Otherwise we would light a fire in the morning.”
The HRV has a heating element
I asked Chris Pike about his electric bills. “They’ve been running between $75 and $90 a month,” he said. “The Zehnder HRV has an electric heater that draws 700 or 800 watts, so that’s a fairly significant draw when it’s on. It just comes on when it’s cold outside. We could have put in a glycol ground loop instead of using the electric heater, but that would have cost $3,000 to put in.”
I asked whether the Pikes were pursuing Passivhaus certification. “I’m not,” Chris Pike told me. “I have a house that does what I want it to do.” As it turns out, however, the builder, Alex Carver, is planning to pursue Passivhaus certification at his own expense — even if the process costs him thousands of dollars. “We consider it an advertising expense,” Carver told me.
I asked Chris Pike, “If you could build the house again, would you do anything differently?” He responded, “I can’t think of anything. The house has been working great. It’s performed beyond our expectations.”
Construction costs were reasonable, Pike told me. “I like Chris Corson’s approach. As he says, you don’t have to be a millionaire to have a high-performance house. Our house is under 1800 square feet, and we built it for around $165 a square foot, including the well and the septic system, but not the land.”
No radiant heat required
At the end of the construction process, Carver was proud — but he still had a few worries.
“I was very surprised to have that much glazing in a house with no radiant heat in the concrete slab,” Carver told me. “That scared me a bit. When the house was completed in the fall, it took a couple of weeks before the slab finally started warming up. It took a long time to get it up to temperature.” (In a followup e-mail, Carver emphasized that this warming-up process was a one-time event, not something that will recur every autumn.)
After monitoring the performance of the house over its first winter, Carver’s worries have evaporated. “I’m very impressed about how the slab is performing,” he said. “For me, what is so awesome is that so little energy was required to heat the house last winter.”
Martin Holladay’s previous blog: “New Lakesideca Products — May 2014.”