Using Passive House Principles to Retrofit an 1895 House

The Moothart Residence is a deep energy retrofit and contemporary re-cladding of an 1895 house in Cincinnati, Ohio. In addition to the energy efficiency measures, the renovation also included a new kitchen, powder room, and laundry on the first floor; a new bathroom on the second floor; and finishing an unfinished attic.

The home was projected to achieve a 73% reduction in energy use from pre-retrofit conditions. The first year of actual data suggests that this prediction was low: the home achieved an 91% reduction from pre-retrofit conditions—and this is BEFORE we include the on-site solar array that is producing 15% more energy than the home is using. That’s enough to power the home, charge the owner’s electric vehicle, and have additional leftover, making the house a net energy producer.

How did we achieve this? By employing Passive House principles of super-insulation without thermal bridges, air-tight construction, and high-efficiency mechanical systems. These principles provide an excellent framework for high energy efficiency—even in historic buildings like this one. And, as we’ve discussed previously, energy efficiency is the crucial first step toward building decarbonization.

These strategies, however, can be challenging to implement in an older house. Here’s how we incorporated each one:

Continuous insulation. The first principle of passive buildings is to keep the heat in with excellent insulation. Here, we wrapped the house with 3.5” of polyisocyanurate rigid insulation, resulting in R-25 walls and an R-42 roof. Importantly, the insulation is continuous, eliminating “thermal bridges”—elements that break through insulation and can be superhighways for heat loss. This included removing the home’s eaves so the wood rafters would not poke through the insulation wrapper.

Thermal mass. One advantage of the existing home was its solid brick construction, including a central chimney spanning from the basement to the attic. Thermal mass, when kept within the home’s thermal envelope, can help regulate indoor temperatures and reduce heating and cooling loads, especially in the “shoulder seasons” of spring and summer, with warm days and cool nights.

Passive solar design. Most of the home’s existing windows were south-facing—ideal for harvesting sunlight to help heat the home. The glazing properties of the new windows were optimized to let in the right amount of solar gain. On the east and west façades, we added shading elements, since solar gain from these orientations was detrimental. While optimum orientation is not necessary to achieve Passive House levels of performance, it certainly helps.

Air-tightness. Air leakage is a significant source of heat loss—so passive buildings aim for an air-tight envelope. That can be especially difficult to achieve in old buildings, but we developed a series of details to address common problem areas. The brick was painted with elastomeric paint that served as a continuous air barrier. The existing eves were removed, and an air barrier membrane was wrapped from the roof deck to the walls to create an air-tight transition. Spray foam insulation was used to seal the rough stone foundation and joist bays in the basement. And the windows were carefully detailed with “window bucks” that provided excellent air-tightness. Together, these achieved air-tightness roughly 4x better than a conventional new home, and reduced air leakage by 80% from pre-retrofit conditions.

Fresh air with heat recovery. Once we have good airtightness, we need to bring in fresh air for occupants. Like most Passive Houses, this project includes an Energy Recovery Ventilator that delivers fresh, filtered air, and recovers heat from the exhausted air. Not only is this more energy efficient than a leaky home, it also results in better indoor air quality, since fresh air is delivered where it’s needed, and is filtered rather than leaking in unintentionally through dusty cracks and crevices.

High-efficiency appliances. With the envelope and ventilation systems optimized, the largest loads in Passive House projects tend to be appliances, lighting, and plug loads. To bring these loads down, we specified energy-efficient appliances and equipment, including an induction cooktop, a condensing dryer, and 100% LED lighting. This also helps to reduce cooling loads, since efficient appliances create less waste heat.

High-efficiency mechanical systems. Once we’ve minimized loads through energy efficiency, we meet the remaining needs with simple, efficient systems. In this case, an air-source heat pump delivers both heating and cooling, and a heat pump water heater provides hot water. The existing ductwork was air-sealed and re-used for the first two floors to save cost; and a ductless mini-split was installed for the newly-finished attic. All-electric systems minimize the home’s carbon footprint while also eliminating one of the largest sources of indoor air pollution: combustion appliances.

Solar PV. Only after energy efficiency has reduced our energy use by 70-80% do we look at renewable energy. At this point, a relatively small solar array (7.6kW) provides not only 100% of the home’s energy needs, but also charges the owner’s electric vehicle, with extra to spare. A battery can easily be added in the future to improve the home’s resilience against power outages and extreme weather.

These same passive building principles can, of course, be used in new construction as well—in single-family homes (see, for example, our Iowa City Passive House), multi-family developments (see, for example, Fairwood Commons), and increasingly in commercial projects (see, for example, Sol’s Cincinnati offices). They can be used in Heritage buildings that don’t allow the “exterior wrap” approach used here—see, for example, our Myers-Heckman Residence, which uses continuous rigid insulation on the building interior in order to preserve its historic façade.

The Moothart Residence shows that these strategies have vast potential to reduce energy use and achieve a truly Net Zero Energy, decarbonized built environment.