If your house has an attached garage, you may have a bonus room — that is, a bedroom above the garage. For new home builders, bonus rooms make sense: including one is relatively inexpensive for the builder, and it’s seen as a desirable feature by home buyers.
From a thermal or comfort perspective, however, most bonus rooms are disasters. They are often cold in winter and hot in summer, even when equipped with a supply register delivering conditioned air from a furnace or air conditioner. There are several reasons why this is so:
- A bonus room has more surfaces exposed to outdoor conditions than a typical room.
- Because of the need to address tricky air-sealing details at kneewalls, most bonus rooms have very leaky thermal envelopes.
- In homes with forced-air heating and cooling systems, the duct serving a bonus room is often undersized, poorly insulated, leaky, and unusually long.
Make sure you have an air barrier
In a new home, most builders use “room in attic” roof trusses to create a bonus room.
While these trusses simplify the framing details for construction workers, they have an Achilles’ heel: They lack a bottom plate under the kneewalls. The lack of a bottom plate complicates air sealing details and greatly increases the chance that the bonus room will have a leaky air barrier.
In most cases, the insulation contractors quickly stuff batts in the kneewalls, and then stuff batts in the joists separating the garage from the bonus room.
This photo shows the ceiling framing above a garage. The fiberglass batts installed in the bonus room kneewalls are hanging down into the joist bays. Needless to say, neither the framers nor the insulation crew thought to install an air barrier near the…