The Goal
Design and build a new custom midcentury-modern style home adopting high-building performance strategies to focus on comfort, indoor air quality for health, and even the “luxury” of a very quiet and clean indoor environment without the restrictions or cost of certification. Homes built to the “Pretty Good House” standard still feature a high degree of thermal control, helping to proactively meet changing codes for greater energy efficiency in many regions across North America.
The Challenge
Selling high performance can be a challenge in communities where energy is relatively affordable. Many environmentally and sustainability-conscious builders, like Catalyst, have therefore modified their discussions with homeowners by focusing on the other benefits of high performance, namely thermal comfort, indoor air quality, acoustics, and even fire safety. For these clients—who started the project with aggressive sustainability and environmental goals in mind—achieving all the benefits of high performance without significantly increasing the cost would require strategic planning on the part of the architect and builder. They would need to ensure the building envelope still met the requirements for airtightness, thermal control, vapor permeability, and moisture control from below grade to the roof and everything in between—the features which will support long-term durability of the structure while also minimizing its carbon footprint.
The Solution
The design-build team adopted several principles that contribute to high performance and which have become common practice for them, including:
- moving from 2x4” framing on 16” centers to 2x6” on 24” to reduce thermal bridging;
- swapping out commodity OSB sheathing and house wrap to an integrated sheathing with both air and water control features;
- replacing minimum code R-13 required cavity insulation with ROCKWOOL Comfortbatt® R-23 in those 2x6 cavities; and,
- pushing for continuous exterior insulation (2” of ROCKWOOL Comfortboard® 80) with rainscreen under cladding details whenever the budget allows.
The clients had already specified a smaller square footage in keeping with the first tenet of PGH, as well as fewer windows but in key locations and with excellent U-values. The architect’s initial design also included a reduction in concrete and unnecessary expense by proposing a crawlspace rather than the full basements typical in the Kansas City market.
The builders also recommended a few other important changes to the preliminary schematic plan that would make the home exceedingly high performance in comparison to code-built homes in the area. The first relates to the crawlspace where they opted to use ROCKWOOL Comfortboard® 80 to insulate the floor; they placed a vapor barrier over the 3” ROCKWOOL Comfortboard® 80 to protect it from abrasion and to keep the assembly code compliant as well as airtight.
To ensure the continuity of thermal control the builders also installed ROCKWOOL Comfortboard® 80 on the interior of the foundation walls of the crawlspace. This keeps all the mechanicals and the structure of the crawlspace within the conditioned space and reduces any thermal bridging or vapor condensation concern at the floor joists. Once that framing was installed, they treated the rim joist cavities as above-grade walls and used ROCKWOOL Comfortbatt® between the joists.
At the wall assembly they also advocated for a continuous layer of 2” ROCKWOOL Comfortboard® 80 outboard of the integrated WRB and sheathing. This exterior insulating layer reduces thermal bridging as well as offering vapor permeability, pest resistance, a reduction in sound transmission, not to mention improved fire resistance.
Stone wool insulation is also naturally moisture resistant and provides excellent drying ability; the builders wanted to further improve the exterior wall’s durability by providing a rainscreen. The combination of rainscreen plus vapor permeable stone wool insulation created an assembly that would be effective in managing moisture for the stucco-clad walls.
The architect had initially proposed an elegant and interesting vaulted roof truss with substantial overhangs cantilevered out over the exterior walls; the builder suggested shortening those trusses to end in plane with the wall framing to accomplish two important goals: the first being the simplification of the air barrier strategy from exterior wall sheathing to roof sheathing, and the second being the ability to run the mechanicals through a conditioned attic rather than insulating the ERV ducts and air sealing lighting penetrations.
The builders specified ROCKWOOL Comfortbatt® R-30 between the 24” OC roof trusses, which could friction fit and then be held secure with some nylon strapping. They also designed an over-roof structure to frame directly on top of the sheathed roof truss assembly, to be layered with an additional 5” of ROCKWOOL Comfortboard® 110 for further reduction of thermal bridging and a total roof R-value of 50+. The 2x6” over-roof frame could cantilever over the exterior walls to continue to offer the narrow fascia dimension and modern aesthetic the architect and client had worked so hard to achieve, all while providing superior thermal performance and sound reduction for the metal roof that was planned.
The interior of the house was also fully insulated with ROCKWOOL Safe’n’Sound® to improve acoustic comfort and fire safety. The non-direction fiber orientation and increased mass of stone wool insulation make it effective in reducing sound transmission between rooms. It is also naturally noncombustible; it can resist temperatures up to 1,177°F (2,150°C), will not generate smoke or harmful gases in the event of fire, or contribute to the spread of flames.
