By Brad Oberg
Proper design and insulation of bonus rooms will help to maintain good energy efficiency, but they can only work as well as other important elements: mechanicals and ductwork.
When sizing mechanical equipment, be sure to consider the bonus room over a garage. It is essential that the HVAC contractor not only includes the space in the load calculations but also he or she calculates it correctly.
Correctly supplying the bonus room with heating and cooling is equal in importance to creating a high performance thermal enclosure. Calculations must be done correctly and not assume heat loss or gain based on average conditions; compared with other rooms in the house, bonus rooms have a much greater surface area in proportion to the floor area. As a result, simple allocation of conditioning based on the additional floor area is not enough. The specific details of the room and their corresponding insulation values need to be kept in mind and modeled correctly during the load calculations.
The next challenge is delivering the conditioned air. Bonus rooms must have both supply and return ducts, and they must be balanced with the rest of the home. Keeping ductwork in conditioned space is always optimal, but with bonus rooms it may not always be feasible. A thermal envelope of insulation --the most crucial component to efficiency--should surround a bonus room over a garage. Unfortunately, that leaves almost no place to effectively place ductwork without displacing insulation.
If there are no internal framed walls in the space (like a closet or other space with interior partition walls) the next best place to put the ductwork is in the attic. When floor diffusers are needed, routing ductwork through the garage ceiling may be necessary; this would displace insulation, but low supply air at the floor of a bonus room in a heating climate is important enough to warrant this. It may be complicated to route ductwork to the bonus room while keeping it out of exterior walls, but it is not impossible. Be sure to insulate and carefully air-seal all ductwork located in unconditioned spaces.
Climate plays a significant role in determining the location of the supply diffusers and return grilles. For example, in a heating climate, the supply diffusers should be located in the floor or low sidewall on the bonus room, while the return should be in the ceiling or high sidewall. Supplying warm air low in the room will help to attenuate the cold floor; the high return will promote good mixing within the space.
Conversely, in a cooling climate, the air is best supplied and returned at the ceiling or high sidewall. In this scenario, the cold air naturally drops as it flows across the room. The hot air naturally accumulates at the ceiling of the room where it is returned to the unit, thus promoting good mixing.
Brad Oberg is co-founder and chief technology officer of IBACOS,a building-science company based in Pittsburgh. He has directed extensive research into the integration of ventilation strategies, residential ductwork design and performance, improved residential airtightness approaches, and durable construction approaches.
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