Historic Phius Retrofit: putting together beauty, function, building science, and efficiency for a net-zero house
We’re excited to be in construction on our first historic residential Phius retrofit. Typical of these “firsts” (and most our works, thankfully), this project started with an inspired owner, whose vision was to update the old (1902) house in line with modern ideas about space, energy, technology, and health. Since it needed updating to the extent of a gut remodel, we had the opportunity to set the home up for the next century.
This kind of thinking is a key strategy for the best, fastest path to net zero emissions by 2050: radically reducing energy use of old buildings with only a modest upfront embodied energy input is needed in communities like Oak Park, where the greatest share of greenhouse gas emissions comes from heating our old, leaky buildings. Tearing down and building new is not such a viable strategy, as it requires huge amounts of upfront embodied energy in materials, while throwing away the old material.
Unlike cars, whose shape and construction materials have modernized significantly from the early 20th c., most homes are still constructed like this one: concrete foundation, wood framing, brick and stucco cladding. The overall mass, window area, and orientation are passive in nature since the home was built before large mechanical systems were a viable option. There are some significant changes, though—here’s what’s different:
- A welcoming back entry: homes of the early 1900’s typically treated the back of the house as a “service” area, with minimal openings and little storage. Outdoor space was focused on the front porch, which was most likely the main entrance. With more frequent car use, more stuff needing a place (mudroom), gardening, and outdoor grilling and playing, the back entrance needs a whole different expression–including some architectural love like the rest of the house. Here, as in a number of our other projects, we made the back entry on-grade with a sheltered patio outside; the ceiling height matches the main first level, so there are great sight lines from the kitchen out and down to the patio and back yard. Meanwhile, the big roof shades the east-facing windows while making a strong outdoor space.
- Insulation: the old house didn’t have any! The insulation retrofit includes all the surfaces of the thermal envelope, as continuously as possible. This required redoing the basement slab, which let us get a capillary break/vapor barrier, passive radon system, and insulation at the bottom of the house. These steps are needed to ensure a healthy interior. We used just enough HFO-blown closed cell foam to protect against condensation risk, and filled the rest with cellulose. To get the optimal insulation level, we framed a new wall a few inches to the interior, which let us create level, square, and smooth interior surfaces.
- Airtight vs. Leaky: given that the original house was built before the membranes, tapes, sealants, etc. that we have now, it was no surprise that the house leaked like a sieve. But you can’t have a healthy or energy efficient interior with a leaky house. Our insulation strategy needed to align with an airtightness strategy that accounts for all penetrations (windows, exhaust vents, etc.) and connects between surfaces (slab, foundation, framed walls, roof). Following window installation and slab and insulation work, we used a product called AeroBarrier, a non-toxic aerosolized compound that fills all the gaps as the house is pressurized with a blower door during installation. This typically isn’t needed on new construction, but can be a godsend on old buildings with complex geometry.
- Gas vs electric: this is a decarbonization project, and like all our retrofits: we’re switching off of gas, using heat pumps for heating, cooling, water heating, and the clothes dryer, and induction for the cooktop. We’ve planned enough solar PV on the garage and addition roofs to generate enough energy to offset the annual electricity consumption. The energy bills will be near zero, with no gas bill or service charge. This is a way of making the home healthier: cooking and line leaks inside the home have been shown to be significant pollutants, often making homes more toxic than the outdoors. And the big-picture energy future is that electrification plus renewables equals zero emissions—it’s where we need to go. Backup power is supplied by batteries, and soon there will be options for using electric cars as backup power sources for homes.
- HVAC: older homes used a lot of coal or gas to heat their uninsulated, leaky shells; our model is reverse of that, insulating and tightening enough that only a small amount of energy is needed to maintain comfort. Options for heating and cooling include air-source heat pumps plus heat pump water heaters, or ground-source “(geothermal”) heat pumps that can heat, cool, and provide hot water. Ground-source is more efficient (although air-source is very efficient, a good option), but tends to require a single ducted system, versus smaller distributed systems common with air-source. And all tight houses need good ventilation—we always use balanced energy recovery ventilation (ERV) systems: they extract pollutants from kitchen, baths, laundry, workshops, cat litter boxes, etc., and provide equal fresh air to bedrooms and living spaces. About 80% of the energy of the outgoing air is transferred to the incoming air, so it’s an efficient way to maintain healthy, filtered air. MERV 13 and higher filters can even remove significant amounts of pollutants like PM2.5, the culprit in our wildfire-tainted skies this summer. I think of the ERV as the “lungs of the house,” maintaining fresh air continuously.
Putting all this together with energy modeling and third-party quality control during construction allows us to pursue the Phius certification. By meeting the stringent metrics and QC, the owners are ensured optimal performance for the life of the building.