Work has been moving quickly on the brick row house in Brooklyn, New York, where remodeler Jose Maldonado is carrying out a gut-rehab renovation and deep energy retrofit to super high-performance Passive House standards, guided by architect James Wagman and Passive House consultant Cramer Silkworth. JLC caught up with Cramer Silkworth this week to talk about progress on the job and discuss the energy efficiency part of the project.
"They're moving right along," says Silkworth. "All the insulation is in, they got the mechanicals roughed in, and plumbing is going in. But because of the weather, the addition off the back is still open. They haven't been able to get up to the roof to work on that, either. So they're going as far as they can inside, but pretty soon they may hit a point where they have to get the rest of the exterior work done before they can get more of the inside done. But with the weather improving somewhat, I think Joe is going to try to shift his focus and get some of the outdoor work done this next week too."
The building can't be blower-door tested for the first time until the rear wall is closed in and the new addition is sealed up. That's a concern, says Silkworth, because the blower door is a quality control tool for testing air-tight details. "I want to do it very soon," he says, "because they're moving quickly on other parts of the inside. We're rapidly approaching the point of having stuff covered up in some rooms. But having worked with Joe once already, he knows what the tricky spots are for air-sealing, and we've been doing visual inspections all along. I don't think anything that has been covered up is going to be a problem."
Windows present another timing issue, Silkworth notes. "We're trying to get our order finalized, because they're coming from Europe and they will take two and a half months to get here. But we needed to triple-verify the rough openings in all the masonry in the front, because once you put in the order, that's it. You can't call up later and tell them, 'We want something a little different, can you send it next week.'"
The window installation involves finicky details. "At our meeting last Tuesday we were talking about exactly how the windows are going to be installed," says Silkworth, "what sort of spacing we wanted to allow for insulation to go around the window frame, instead of meeting the rough masonry, and also just how to take advantage of some of the rough opening space to make them look from the front as if they are a regular window instead of some crazy Passive House window with a beefy frame." Each of the openings in the existing brick facade is slightly different, notes Silkworth, "so we are going to go to each one and see how much space we have to fill with insulation after we get the unit blocked and shimmed and everything, and then how we fill the rest of the space with as much insulation as we can, and then bring the interior air barrier membrane and wrap that into the window opening and get it taped right to the window frame, so that we have thermal and air barrier continuity for every window."
The home's mechanical systems are typical for a Passive House project: a mini-split air-source heat pump for heating and cooling, and a high-performance energy recovery ventilator (ERV) for fresh air and humidity control (see slideshow, "Brooklyn Passive House Mechanical Systems"). Silkworth has specified a 30,000-Btu/hr (2.5 ton) ducted Mitsubishi mini-split. "That's the size of the condenser outside," he says. "Inside, we have two 9,000-Btu air handlers." The design heating load for the building is actually well below the 18,000 Btu/hr the two indoor units can supply, he explains—closer to 10,000 Btu/hr—but the system's real-world output during the coldest days of winter will be lower than its "nameplate rating" determined under standard laboratory conditions.
The ducted mini-split is a step up from Mitsubishi's basic wall-mounted system. Says Silkworth: "In these nice brownstones, nobody wants to see a wall-hung mini-split. But also, given the long, narrow shape of the building and the room layout, I like to run ductwork to the front and rear rooms from an air handler that is centrally positioned. So if a floor has a bedroom in the front and a bedroom in the back, and a bathroom or two in the middle, I will locate the air handler in the ceiling of the bathroom in the middle, and have a short return directly into the unit from the corridor or the stairwell, as the central pathway for the return air. Then I run supply ducts to the rooms in the front and back. Sometimes I can do two floors from one unit, particularly if the stairwell for the two floors is permanently open, with no doorways or anything."
In this case, Silkworth says, it's more practical to give each floor in the home its own air handler. Both are linked to the same outdoor compressor and coils, which operate in either condensing or evaporating mode, depending on whether the interior side of the system is calling for heating or cooling. "Each air handler inside responds to its zone as it needs to," explains Silkworth. "So it's communicating to the outdoor unit and saying, 'I need heating now, so kick on.' They have to sort of cooperate, though — one can't be heating while the other is cooling, because the outdoor unit has to be in one mode or the other, at least with these simple residential-scale systems."
The ducted system gives Silkworth the flexibility to address the different needs of different rooms while optimizing the efficiency of the full system. "I like the ducted units because they can mix the air around," he says. "You can put it in fan-only mode, and it will just mix a floor or two worth of air together. So if there is sun coming in in the southern bedroom or the living room, you put the unit in fan mode and it will mix it up and help move that heat and spread it around."
For a reliable fresh air supply in the very tight house, Silkworth has specced a Zehnder ComfoAir 350. "That's the same unit I used in the last house," he says. "It's a very popular size for homes in the 2500-to 3500-square-foot range, depending on the number of bathrooms. Zehnder makes a few different sizes, but they are pretty much the only name in town for Passive House type stuff. It's pricey, but it's superefficient, and it's very quiet, which is important with a Passive House—it's so well-insulated and air-tight that there's very little noise coming in from outdoors, so you don't want to put in noisy equipment."
Commissioning the system before the house is occupied is important, says Silkworth. "That's going to be true of any mechanical system, really," he says. "You should have somebody commission it and make sure everything is in the right place and operating properly. With the Zehnder, or any heat recovery system, what you want to ensure is that the airflow in is exactly equal to the airflow out, so the heat recovery process is balanced and operating most efficiently. That's the biggest part of the Zehnder commissioning—just balancing the airflows so your potential recovery efficiency is met."