Passive House Project in Asheville, N.C., Poses Many Questions and Lessons
Over the next few months, I will be following an interesting infill project here in Asheville. It’s an 842-square-foot house, following the Passive House Institute standards.
I checked out the project in West Asheville for the first time last week. The homeowner is a single fella who is contracting the home himself, with a fairly flexible time line. That will give us an ongoing opportunity to dissect different aspects and stages of the project very closely, and hopefully engage the readership to weigh in with philosophy, experience, and opinions. I’ll be shooting some video of the project as well, interviewing the architects and owner, so stay tuned. I live close by, so I will follow the project through to completion and habitation, chronicle a comparison of modeled to actual performance, and share the results of the HERS testing as well.
The home belongs to Chris Otahal, a structural engineer at Kloesel Engineering here in Asheville. He is working with Aaron & Calder Wilson at Wilson Architects, a husband-and-wife architecture team. I have had the pleasure of working with Aaron & Calder on a number of projects, including my own home. They are a very talented couple and bring a deep understanding of green building to this project, though this is their first Passive House.
Before beginning, Chris sought information from the US Passive House Institute in Urbana, Ill., which provided the team with energy modeling, insulation strategies, and HVAC sizing. (I’ll share some specifics on the services in later blogs.) With the Institute’s info and some further modeling, the team concluded that this particular design would be cooling driven—there will be more traditional energy used for cooling the home than for heating it. To keep the heating load as low as possible, the team implemented such strategies as airtight construction, superinsulation, passive solar design, and the use of custom glazing on the south side of the home (I’ll get into these in more detail in the future as well). Here’s a list of green features that the team has incorporated or will incorporate on the remainder of the project:
- The home has a small footprint, which minimizes land disruption and material use (the lot is .10 acres!).
- The structure holds a trim 842 square feet of living space.
- The marginal infill lot was unattractive to most, but Chris saw a diamond in the rough. The site has a stream on the south end and a sewer easement on the north end, limiting the potential building area. But Chris’s needs and the creativity of the team took the opportunity to put an interesting home on an interesting lot. The home makes use of the entire buildable area of the lot.
- Chris opted for a combination of triple-glazed windows, some with a low solar heat gain coefficient (SGHC), some with a relatively high SGHC.
- Exterior walls are 2×8 construction, 24 in. o.c. The cavities will be filled with blown cellulose, with 3 inches of rigid insulation on the exterior installed perpendicular to the studs.
- A rainscreen siding system was utilized, and the exterior finish will be corten steel panels attached to 1×4 furring strips.
- They used a high-performance weather barrier and drainage plane, VaproShield.
- A mini-split will be used to condition the home.
- They will be using a high-efficiency tanked water heater.
- Passive solar design utilizes large south-facing windows and limited north, east, and west glazing. South-side overhangs were properly sized.
- The 16-in. roof and floor TJI joists will be filled with insulation.
- An energy recovery ventilator will provide fresh air, pressure balancing, and preconditioning.
The project is at rough stage, so it’s a great time to investigate while the walls are still open. Interior insulation will start up soon, which brings us to our first opportunity to make this an interactive blog and construction process. Chris is still deciding on the best possible insulation product or products to use for the roof. It’s a 16-in. cavity that you can think of as a clean canvas. Free your inner green building Michelangelo to share your opinions and help the team find answers to the following conundrums they’re facing:
- Should they install a foam skin on the underside of the sheathing?
- 1. If so, should it be open cell or closed cell? (We are in a mixed humid climate, in the Blue Ridge Mountains. It’s a fairly unique climate: humid, some snow, some heat.)
- 2. Should another type of insulation be used, either by itself or in conjunction with foam?
- 3. What would the most effective depth be, taking energy efficiency and budget into consideration?
- 4. Should the roof be vented or unvented?
Here are my responses. Let me know if you agree or disagree and why:
- 1. For air-sealing purposes, I would opt for 3 to 5 inches of foam.
- 2. I would choose open-cell foam for its breathability.
- 3. I would fill the remainder of the rafter cavity with a blown-in product, possibly dense-pack, no-VOC fiberglass.
- 4. I would fill up the cavity. The mix of the two insulation types would be less expensive than all foam.
- 5. Unvented. It’s not necessary to vent with the use of foam insulation, which ensures that the conditioned space is pushed all the way to the exterior sheathing. The heat stays out of the envelope, so there is no need to vent hot air from the assembly.
What would you like to know about this project? I plan on investigating the choice and installation of windows, exterior cladding, and mechanical systems. Do you readers have burning questions that you would love to have addressed here more than anything else in the whole, wide world? If so, take a moment to post your question. I’ll kick it around to the team and respond in subsequent blogs. In the meantime, check out the Passive House Discussion at GBA, and another at JLC.
Check out pictures of the project at my Flickr page. Also, I will make a few architectural details available periodically. Here are a couple: