"Bonus Rooms": More Than You Bargained For?
By Richard Stone, Extension Educator in Housing Technology
As “bonus rooms” become an increasingly popular feature of new homes they are following the trends set by new home construction in general. They are becoming larger, more complex, and more technically challenging. From modest beginnings as store rooms, craft rooms, and spare bedrooms, this extra space has now progressed all the way to becoming master bedroom suites with whirlpool tubs, showers, and fireplaces. Some bonus spaces are large enough to require additional heating, cooling, and ventilation systems. Planning for the complexities of a bonus space must be fully integrated into the design for the entire home. Even the simplest of bonus rooms requires extra attention.
Because truss manufacturers have adapted roof truss systems to provide the primary framing for these spaces, the building shell takes shape quickly. But, meeting the structural engineering requirements is only one of the steps toward successfully converting attic space into primary living space. After the truss system or primary framing for the bonus room has been completed, additional steps required to prepare the space for interior finishing include:
- Secondary framing, blocking, and sheathing
- Installation of supply and return ducting for the HVAC system
- Plumbing system rough-in including tub and shower fixtures
- Electrical system rough-in including low voltage components
- Insulation and air-sealing
Secondary framing in bonus rooms is more involved because many common elements of framing are not initially provided by the truss system. The "floor trusses" are present as part of each bonus room truss. The "bottom plate" for each bonus room wall needs to be installed as blocking at the correct position between trusses. Gaps must be sealed where the subfloor meets the bottom plate blocking. Since most trusses have 2x4 vertical chords which become the bonus room wall studs, using insulated sheathing on the attic side of the wall can increase thermal performance more easily and effectively than adding framing to thicken the wall cavities.
At the top of each bonus room wall, blocking must be installed at the "top plate" location. Space should be allowed for vent chutes and the sloped ceiling assembly should be sized to install sufficient insulation to meet R-value requirements.
A "band joist" assembly must be completed below every bonus room wall to provide a separation between the insulated bonus room floor over the garage and the adjacent attic spaces over the garage. The blocking and sheathing should be solid enough to attach and seal the air barrier after the floor has been insulated and to attach the drywall ceiling after air sealing has been completed. Blocking should also separate the bonus room floor from the floor system in the completely conditioned part of the house. The blocking and all penetrations made for mechanical systems should be air-sealed.
- Properly detailed framing in these locations provides the foundation for the effective insulation and air sealing that comes after the mechanical system rough-ins have been completed and sealed
- Blocking air movement through the bonus room floor contributes to health, safety, and durability as well as providing for increased comfort and energy efficiency
- The sealed floor system provides the barrier between the living space and potentially dangerous pollutants from the garage
- Preventing moist air from entering the floor assembly provides for durability of the structure
- Keeping heated or cooled air inside the living space and preventing drafts from outside saves energy
Insulation, air-sealing and managing the impacts of mechanical system rough-ins on the durability and energy performance of the building envelope will be addressed in future issues.
While it is helpful to study specific details for these critical assemblies, developing a strong understanding of the underlying principles of building science will position designers and builders to make better choices during every phase of the residential construction process. Building new homes for increased energy performance and durability must be held as the new standard of craftsmanship for the industry.