The Goal
The primary objective of this deep energy retrofit was to create a new home for 475 High Performance Building Supply’s New York operation that aligns with their brand’s principles and meets specific environmental goals. These targets include improving energy efficiency, using sustainable building materials that are low carbon, and promoting a healthy indoor environment to support occupant wellbeing. The project adheres to passive house standards and aims to demonstrate a practical approach to sustainable design.
The design brief emphasized retaining as much of the existing structure as possible - this included the foundation, original walls, and concrete floor - to preserve the embodied carbon of those elements. Additionally, the strategy included the decision to not use any fossil fuel-based foam insulation in the enclosure. This approach was important to align with 475 High Performance Building Supply’s credo: Build like the future depends on it.
Preserving the existing exterior walls meant that any incremental insulation had to be added from the interior. Additionally, local building codes require exterior walls in NYC to be noncombustible and both PHI (Passive House Institute) and Phius require them to be very airtight and highly insulated to receive certification.
The Challenge
The garage lacked any form of insulation – so the deep energy retrofit required upgrading the enclosure on all six sides, including the floor, walls, and roof. The project team was mindful that changes to the structure enclosures, such as insulation and air barriers, could impact the assembly's ability to dry and would have to be carefully considered.
The exterior mass masonry walls posed a particular challenge. Trapped moisture or condensation can dramatically increase the risk of freeze-thaw in a retrofit, regardless of how long it had previously been standing. Masonry walls can be especially vulnerable to damage and deterioration due to their porous nature.
And because sections of the original masonry were to remain in place, along with the addition of new concrete masonry units (CMUs), the design team needed to find a solution that would allow installation of an assembly that met the passive house thermal requirements. The solution also needed to include air and vapor control from the interior.
The retrofit included a new high-performance low slope roof assembly, as well as a solution for the existing slab-on-grade foundation. The plan also included erecting new interior walls, to support the performance targets for acoustics, occupant wellbeing and comfort, and safety.
The Solution
Stone wool insulation was used in multiple applications to support noncombustible, vapor-open, and durable assemblies throughout the building to safely meet the high insulation values required for PHI and Phius certification.
A smarter approach to interior continuous insulation
An innovative solution – ROCKWOOL Smartrock – was used to insulate the exterior walls from the inside. Interior continuous insulation is not a new strategy for this type of retrofit; it involves installing insulation boards in continuous contact with the substrate from the interior. Continuous insulation with stone wool reduces energy loss through thermal bridging, does not impede the drying of the assembly due to its high vapor permeability, and stone wool is noncombustible, providing a solution for zero lot line construction. A traditional assembly would require the installation of a separate smart vapor control layer to control the movement of vapor through the exterior wall assembly.
Smartrock is an insulation board that features an integrated smart vapor retarder to provide thermal, vapor, and air control as part of the fully detailed assembly. This integrated smart vapor retarder utilizes INTELLO PLUS from Pro Clima, a membrane distributed exclusively in North America by 475 High Performance Building Supply.
The masonry was stabilized by being repointed and injection-grouted where needed. New CMUs were built on top, after which the VISCONN liquid-applied air barrier was added on the interior side of the exterior walls. The team then installed a 4-inch layer of Smartrock over the now structurally sound walls.
“The timing of our own renovation project and the availability of the ROCKWOOL Smartrock prototype was a happy coincidence,” says Floris Keverling Buisman, CEO of 475 High Performance Building Supply. “We were eager to install it in our own space to show how it contributes to a high-performing, air-tight, and vapor-smart assembly. And the contractor crew appreciated the efficiency they gained with Smartrock in not having to install a separate vapor retarding membrane.”
The smart vapor control technology, or vapor variability, means the assembly can adapt its permeance level (i.e. open or close) to changing seasons and varying moisture and humidity conditions, minimizing risk of damage from condensation and moisture build up in highly insulated walls. This in turn reduces freeze-thaw risk in masonry walls and the potential for entrapment of moisture within the building enclosure.
The dual density of Smartrock also gives it a degree of flexibility to conform to the uneven substrate while the high-density surface is more rigid for fastening and taping. This was particularly helpful for the rear wall of the space that had both CMU and uneven brickwork on which the boards were able to be secured, which helped level out this section. Interior wood framing was installed inboard of the Smartrock and its cavities filled with ROCKWOOL Comfortbatt® R15 for a combined R-value of 31 on the exterior wall assembly. (Each assembly design is unique to meet the needs of particular projects; consult with the ROCKWOOL Building Science team for guidance on the use of interior wall cavity insulation with Smartrock.)
A functional – and beautiful – roof
The deteriorating 1st floor roof of the original building was in a state of complete disrepair; it was demolished and after a second floor was built, the new space was covered with a flat, nail-laminated timber (NLT) framed roof structure.
NLT framing has been part of New York City’s building code since 1938; the decision to use this traditional approach needed to be combined with noncombustible building enclosure that would protect neighboring buildings in the event of a fire. It also supported 475 High Performance Building Supply’s goals for carbon sequestering, was a more affordable option than cross-laminated timber (CLT) framing for this size project, and created an opportunity to leverage the spacing in the NLT as a beautiful wood design element.
The roof assembly included an average of 10 inches of ROCKWOOL Toprock® DD equating to an R-value of 38. 475 High Performance Building Supply specified one of their own products – SOLITEX ADHERO VISTO – as the air and vapor control layer and temporary protection membrane between the insulation and the exposed NLT framing, before the insulation and roof membrane were added on top.
Toprock DD supports a high-performing and durable roof assembly with its thermal, vapor permeable, and noncombustible properties.
Indoor comfort and occupant well-being are hallmarks of passive house design
The interior walls were filled with ROCKWOOL AFB® stone wool batts to control sound and enhance occupant safety with industry-leading fire resistance.
Repurposing the original garage for additional space
The original concrete slab of the garage was preserved to reduce waste from the renovation; it was covered with a vapor barrier and insulated with a layer of 2.5” thick ROCKWOOL Comfortboard® 110 prior to being finished with two layers of plywood and New York-sourced maple flooring.
The outcome
475 High Performance Building Supply achieved their goal of creating a comfortable, energy-efficient, and sustainable workspace for their team that will be PHI and Phius certified. Early blower door tests gave the project a 0.3 on airtightness. The new high-performance building envelope, including multiple applications of stone wool, contributed significantly to this effort.
The project was made possible through funding from Empire State Development.
The exterior walls feature a new innovation in high-performance building enclosure design: ROCKWOOL Smartrock. Interior continuous insulation is an effective strategy for retrofits of mass concrete and masonry buildings. Using stone wool insulation in these assemblies is now easier with the introduction of Smartrock as the boards include an integrated smart vapor retarder.
Conclusion
Retrofitting historic buildings like the 369 Warren St. townhouse in Brooklyn to passive house standards presents significant challenges, particularly in managing moisture and ensuring the durability of the mass wall structure. The original masonry walls, which had plenty of life left in them, posed a risk of freeze-thaw damage due to their porous nature. The design team had to find a solution that would meet thermal, air, and vapor control targets without compromising the building's integrity or encroaching on lot lines. This required a smart insulation strategy that could be implemented from the interior, preserving the exterior facade while enhancing the building's durability and energy efficiency.
The introduction of ROCKWOOL Smartrock provided an innovative solution to these challenges. Smartrock's integrated smart vapor retarder allows the assembly to adapt to changing moisture and humidity conditions, minimizing the risk of damage from freeze-thaw cycles. This not only improved the building's thermal performance but also ensured its long-term durability, making it a model for future retrofits of historic masonry buildings.
Architects and builders now have more options to achieve a high-performance enclosure design while preserving aesthetics and historical integrity.
