Think Wood: Hi Ruth, Bobby. To get us started can you explain a bit about Passive House, for those who might be less familiar?
Ruth Mandl: Passivhaus is a building standard that was developed in Germany in the early 1990’s. It focuses on high-performance construction through the use of extensive insulation, energy-efficient windows, and the use of an air-tight membrane that wraps the entire structure, reducing the impacts of hot and cold weather. Passive House design can offer huge energy savings and ultimately cut harmful greenhouse gas emissions. When done right, all of these factors can reduce the operational energy consumption of a building by 80 to 90%.
TW: How did you get started with Passive House design?
RM: Our own house was actually our first Passive House project and now serves as a great test case. Working with an existing building from 1889, it was important to us to keep and reuse as much of the original moldings and millwork as possible, while updating the building to the Passive House standard. We removed all of the wood finishings and stored them in a tent in our backyard. Once the high-performance thermal envelope was completed, we reinstalled most of these materials; what we couldn’t reuse, we donated for reuse. The project demonstrated to us that it is possible to achieve a high-performance Passive House renovation while retaining salvaged materials.
TW: How does CO Adaptive approach Passive House design in renovation and adaptive reuse projects?
Bobby Johnston: Depending on the context and condition of the building, our approach is to carefully consider what we can retain from the original construction while updating the thermal envelope to meet high efficiency standards. The goal is always to update the building for airtightness without demolishing the interiors. We are starting to focus much more on how we can approach adaptive reuse and renovations with minimal waste, opting to focus on deconstruction rather than demolition. It’s a trade-off between minimizing a building’s embodied carbon by retaining as much of the existing structure as possible and the operational benefits that you achieve through Passive House design.
TW: How do Passive House structures perform in different climates?
RM: Given the ever-hotter weather due to climate change, buildings in the U.S. need to mitigate heat as much as cold temperatures, particularly in southern climates and during summer months. Passive House was originally developed for Germany—a relatively cold climate—but it also works really well in a hot climate, as long as you’re preventing the sun in the summer from penetrating the interior. The key is in getting your building envelope to work harder than your active systems—in turn reducing your reliance on HVAC systems.
TW: What role can prefabricated timber systems play in Passive House design?
RM: Along with minimizing demolition in renovations, we are starting to look more rigorously at prefabrication and modularity in our adaptive reuse and renovation projects—and how we can make use of demountable timber panels, ultimately prefabricating portions of a Passive House envelope for retrofit applications. Thermally, wood is superior to steel and concrete and offers added thermal mass. All of our Passive House projects use wood as the primary material. Wood provides more versatility and given its natural renewability, it lends itself to our goals of regenerative, circular design. There’s more modularity and prefabrication in new builds but we haven’t seen a lot in the retrofit market, especially in an urban context. I think that’s really what is needed. So we’re working on a prototype here at the office, doing just that.
TW: Is Passive House part of a broader vision for your architectural practice?
RM: Very much so—we’re constantly evolving what sustainability means to our practice, but we know at the core we want to think as systemically and regeneratively as we possibly can. Cutting embodied and operational energy is definitely a part of that. In urban environments, these principles are best applied in adapting historic building stock, reducing demolition when it can be avoided, and respecting and optimizing the materials that we’re using and reusing. To do this, it makes sense to use low carbon building materials—like timber—and design for disassembly with adaptability in mind, so that things can be taken apart, reused, and become part of a circular design loop.
BJ: Our passion is for retrofitting historic and aging buildings but we found that there can be a significant cost to doing that. We’d like to find ways to curb those costs and make Passive House design more affordable and accessible. Prefabricated timber systems, modularity, and systemized assemblies are part of that vision. I think our cities—in particular those with larger, older building stock—need solutions that can be easily implemented. We want to retrofit existing buildings to higher performance while doing it in a way that is affordable and not completely disruptive. At present, there’s a gap in the market and our goal, as a practice, is to fill that gap.
Ruth and Bobby live in Brooklyn, New York, where they completed a self-commissioned renovation of their own brownstone, transforming a beautiful, century-old building into a highly resilient home to take their family into the future, while respecting and celebrating the past. Their firm’s name CO Adaptive takes inspiration from the same biological term used to describe the process by which a bee adapts to a flower, just as the flower adapts to the bee.
