Air-Tight Window Details for a 150-Year-Old Brick Home

As part of the deep energy retrofit of the Moothart Residence, we needed to figure out how to install new windows in the existing masonry openings that would be air-tight, weather-proof, and could account for the fact that none of the openings were exactly the same size. They also needed to work well with the 3 1/2” of rigid insulation we were adding to the outside of the brick wall. And, we wanted to avoid removing the interior window trim. (The project was not a gut retrofit, so maintaining the interior was part of the design brief.)

Our solution was to create plywood “buck boxes” that could be independently levelled and squared within each rough opening, and would overhang the new insulation layers. These boxes would also be part of the continuous air barrier.

Below we show how the process looked, both in theory and in practice.

Window Detail: Buck Boxes

Below is an installation process diagram we developed to explain the multi-step process of installing the window bucks, windows, and siding.

The installation process we developed was as follows:

1. Brick air barrier. The brick walls were painted with elastomeric paint, which provided a vapor-permeable air barrier. This was our primary air barrier layer, and we wanted to make sure the window buck tied tightly into this.
2. Window buck. A box was constructed out of 1” plywood. We provided at least 1/2" clearance to the rough opening on each side, to enable us to level and square the buck box within the rough opening. This also allowed us to standardize the size of the new windows, despite having slightly different masonry openings. The box overhung the brick by 3-1/2” so that it would be flush with the rigid insulation. The gap between the brick rough opening and the buck box was filled with low-expansion spray foam insulation.
3. Connect the air barriers. We did not want to rely on the spray foam for air-sealing, as it often fails to complete fill the cavity and can contract over time. So we used a vapor-permeable air barrier membrane (Henry Blueskin) to seal from the projecting sides of the buck box back to the elastomeric paint.
4. Framing. To support the new cladding, we specified 2x4 framing on the flat against the existing brick walls, attached with masonry screws. We also installed framing on both sides of the window, so that we would have something to attach the “buck flanges” to (in step 7).
5. Rigid insulation, first layer. A 1-1/2” layer of polyisocyanurate was installed between the 2x4 framing, and attached to the brick with adhesive and masonry screws.
6. Rigid insulation, second layer. Then, a 2” layer of polyisocyanurate insulation was installed over the framing, to provide continuous insulation and eliminate thermal bridging. The seams between the insulation were taped to provide a drainage plane.
7. Buck flanges. On top of the second layer of insulation, we installed 3/4" thick “buck flanges,” which were attached both to the buck box itself and to the framing below (from step 4). These would ultimately be flush with the 3/4" furring (step 11). The purpose was to provide a surface for attaching the flanged windows. This allowed us to have a relatively conventional window installation process, with conventional flashing details.
8. Window flashing. Standard window flashing was installed – except that we also added flashing tape from the buck flanges to the surface of the insulation to form a continuous drainage plane.
9. New windows. New high-performance windows were installed – again, using conventional flanged window installation methods. For air-tightness, we specified a backer rod and sealant between the window and the buck box. This tends to provide a better air seal than low-expansion foam.
10. Window flashing. The window was flashed, again using conventional methods.
11. Furring strips. 3/4" vertical furring strips were added, anchored through the second layer of rigid insulation back to the 2x framing. This provided a place to attach the cladding, as well as a ventilated cavity that improves the assembly’s water management and durability.
12. Engineered wood cladding. Finally, prefinished engineered wood cladding and trim was installed over the furring strips. The product was chosen because it is lightweight, easy to work with, inexpensive, low maintenance, and low embodied carbon.

Window Installation

And here’s how it looked in practice:

Key Take-Aways

Overall, the installation was a success. Here are some key takeaways and lessons for other projects:

• Excellent air-tightness. The retrofit achieved near Passive House performance, with air-tightness coming in at 2 ACH50 (compared to 6 ACH for a typical new home, and 10+ ACH pre-retrofit). The windows were extremely air-tight, solving a key area of leakage in the original home. We utilized a similar detail in another retrofit project that is on-target to achieve Passive House performance.
• Collaboration with GCs and trades. We worked closely with the installation crew on the details and sequencing. This collaborative approach was essential in developing an approach that was buildable in an efficient way, as well as achieving performance targets.
• Keep things conventional whenever possible. As far as possible, we used common materials and installation methods, including conventional window installation and flashing details, and standard siding and rainscreen details. This kept construction efficient and cost-effective.
• Find expression through construction. Throughout this project, we used the details and construction methods as an opportunity for architectural expression—for example, the exposed edge plywood trim that expresses the buck box, and creates a juxtaposition between historic and contemporary that is used throughout the project. Details like these that flow naturally from the construction methods also tend to be material-efficient and cost-effective.

This window detail was a key part of achieving a high-performance, durable, low-maintenance home. The project is a net energy producer, using less energy than it generates with its rooftop solar array, and has received a Cincinnati Design Award and USGBC Regional Leadership Award—proof positive that good design and exceptional performance can go hand-in-hand.