Tales from a Pittsburgh Passive House retrofit

INTRODUCTION

I’ve grown to love Pittsburgh. While my wife and business partner grew up here—her family goes back generations in Pittsburgh—I’m a more recent transplant and have been really getting to know the city over the past few years. I love the city for its resiliency, its creativity, and its fiery civic pride. Remember Mayor Bill Peduto’s response to Trump’s “Pittsburgh, not Paris” justification for pulling out of the Paris Climate Accord?: “As the mayor of Pittsburgh, I can assure you that we will follow the guidelines of the Paris agreement for our people, our economy and future.”

Most of all, though, I’m drawn to the energy of renaissance that seems to be everywhere here. I’m not the only one that feels this energy. Lots of people see the Pittsburgh that I see and have been bringing investment dollars to the region. While that’s exciting, it has raised worries about gentrification and the potential loss of the buildings and the places that define Pittsburgh for Pittsburghers.

Given this civic backdrop and our belief at NK that, in the words of friend and colleague Timothy McDonald, the “new gravity” in architecture must be the climate impact of our buildings, we were excited to work on the Morningside Crossing project for local affordable housing developer A.M. Rodriguez, in partnership with Laura Nettleton of Thoughtful Balance.

A.M. Rodriguez’s remarkable vision for the project was to take a beloved yet abandoned 1897 school, Morningside Elementary, retrofit it and its 1929 addition to the Passive House standard, add a new Passive House wing, and offer it all to the community as affordable housing for seniors.

Honestly, it’s tough to imagine a design opportunity that checks off more things-that-I-think-matter boxes than this one:

  • Preserve embodied carbon of an existing masonry building? Check.
  • Slash operating carbon emissions through Passive House design? Check.
  • Rescue and revitalize a revered neighborhood building–on that many Morningside old-timers attended as schoolkids? Check.
  • Create affordable, healthy, and safe housing for Pittsburgh seniors? Check.
  • Create a community center for the neighborhood, hosting classes, community meetings, and civic live? Check.
  • Show that none of this is pie-in-the-sky, but that it all can be done at a cost that is comparable to conventional construction? Check.

And you know what? We did it, thanks in no small part to our collaboration with Sota Construction. This past October the team delivered the building (pre-certified PHIUS+) on budget and accepted its first residents. A leaky, abandoned masonry school became a model of high-performance building for seniors.

The project was not without its adventures, of course.

A PASSIVE HOUSE BUILDING IN THREE PARTS

Key to the project’s Passive House success was the teams’ approach to the project’s three elements: 1897 school, 1929 addition, and new 2018 wing.

We knew that the 1897 and 1929 portions would be a challenge to air seal to the Passive House standard, particularly since we were certifying with PHIUS, which currently does not offer a retrofit standard like PHI’s EnerPHit’s standard, that relaxes airtightness goals a bit for existing buildings.

So as a general principle on the project, we strove to overperform on the new wing in order to compensate for the challenges of the existing building. If you model the 1897 or 1929 retrofits as if they were standalone buildings, they don’t quite meet the standard. But when modeled as a whole that includes the 2018 new wing, the project hits the Passive House mark. In the end, the project missed the Passive House airtightness target, but got close enough to still perform as an ultra-efficient building. No small feat given the existing building we were working with.

WALL ASSEMBLIES

In detailing the wall assemblies for each of the three portions of the building, we aimed to keep it simple.

For the 1897 portion of the building, with its old masonry walls likely full of surprises, we applied a layer of spray foam on the interior face of the exterior walls, preserving the historic façade while also air sealing and super-insulating the assembly. A metal stud wall placed immediately inboard of the spray foam layer finishes the interior wall (gypsum) and provides a service cavity. Typically, we would avoid using spray foam as an air barrier, and its integrity will need to be monitored over time, but the complexity of the existing wall coupled with budget constraints dictated the solution for us. 

For the new front façade of the 1929 portion of the building, we settled on a simple exterior-insulated wall assembly, using 6 inches of mineral wool placed on Securock ExoAir430 sheathing (with liquid applied seams) for the assembly’s air and weather barrier. As with the 1897 wall retrofit, a metal stud wall finishes the interior wall and provides a service cavity.

The new 2018 wing features a hybrid wall assembly, with two inches of polyiso insulation on the exterior, taped ZIP sheathing for the air and weather barrier, and fiberglass batt insulation in a wood 2x6 stud wall for interior insulation.

EXISTING MASONRY BEARING WALLS

Despite our carefully laid-out plans for the building’s exterior walls, we still had a tricky thermal bridge issue to address in the 1897 portion of the building: the existing, big masonry bearing walls, integral to the building’s structure, jutting from the exterior wall into the building’s interior. You'll remember that our Passive House retrofit wall assembly for the 1897 building is spray foam on the interior face of the existing exterior wall. So every bearing wall that intersects that wall creates a massive thermal bridge. What to do? Simply doing away with these bearing walls through demolition was neither practical from a cost perspective nor advisable from a structural one.

Through thermal analysis of the problem we devised a straightforward solution. By continuing the inboard assembly of spray foam, service cavity, and gypsum wall along the face of the bearing walls, we created insulated “sleeves” for those walls. Cold from the exterior wall does run into the interior bearing walls, but after a few feet it dissipates safely inside these “sleeves.” Even though the R-value of the masonry wall is very low, if run along a long enough distance, that R-value accumulates into a significant number. That’s what the insulating sleeves accomplish: forcing cold transfer to travel along the length of the masonry bearing walls instead of escaping from the faces of those walls. The key design question then becomes how long must the sleeves run along the bearing walls to successfully transform those bearing walls into thermal breaks, not bridges. The answer in this case? Five feet.

WINDOW BUCK ADVENTURE

Despite having prior access to the existing building and doing plenty of forensic work to understand existing conditions, it was impossible to fully know what to expect when it came to the 1897 building’s window openings.

Our goal, of course, was to create a constructible, cost-effective, and replicable solution for window installation that would work for most existing conditions. We eventually got there, but it was definitely a design journey.

Our initial concept was to create a two-by window buck to create a solid, level surface, raised above the exiting stone sill. Spray foam would fill in and insulate from the interior, and the window would be placed at the center of the existing masonry wall.

Value engineering eliminated that two-by buck, however, replacing it with a ¾ inch plywood buck. Meanwhile, we discovered a real mess at many of the existing window openings: disintegrating masonry, rotting wood, metal embedded in the stone sill, gaps in the masonry left by the scar of the old window.


So we proposed to combine the plywood window buck with a wood infill element to fill in much of the scar, plus brake metal trim to cover the rest of the stone sill and jamb (and the any remaining old window scar). Sota was skeptical about using the brake metal, and preferred that the new windows sit in the opening to cover the old window scar, eliminating the need to mask the existing stone sill and jamb in the first place.

This approach had obvious advantages in terms of cost savings and ease of installation, but we needed to tackle its hygrothermal implications. We were concerned about how cold the surface of the interior sill might get immediately behind the window, and the potential condensation risk that posed. We explored adding a 1/2” by 4” length of XPS insulation immediately inside the window frame to address the issue. We ran the thermal analysis and it worked.

But was this additional element of the detail, and extra installation step, really necessary? Upon closer examination and thermal analysis, we determined that the detail without the XPS insulation still kept the surfaces inside the window warm enough to avoid condensation risk.

We had our cost-effective and replicable solution!


CONCLUSION

We encountered other adventures during the design and construction of the project, but by problem solving with a topflight contractor, committed subs, and an engaged owner, the team succeeded at navigating those while also hitting our budget and schedule. The result is super satisfying as an architect. The seniors that live in this restored community landmark breathe fresh, clean air and enjoy consistent, comfortable temperatures 24/7, 365 days a year. If the power ever goes out, the building’s “passive survivability” (thanks to Passive House design’s envelope-first approach) will make the building a refuge of safe temperatures, whether it is freezing or sweltering outside. What was once an abandoned building, a liability for the neighborhood, is now a community asset again, providing affordable housing for the older generation while offering a model for ultra-energy efficient living that safeguards the future of the younger gener