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:
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 batts. In the absence of an air barrier, outdoor air will easily filter through the batts and enter the bonus room at the unsealed crack between the subfloor and the bottom of the drywall on the kneewall. [Photo credit: Matt Bowers]Once the interior side of the kneewall is covered by drywall, it isn’t obvious that there is a large crack under the kneewall that allows outdoor air to entry the bonus room. If…