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Click to enlarge image.
Figure 1: Bonus room detail: air-sealing at the ceiling line of the garage. Click to enlarge image.
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By Brad Oberg
The floor of a bonus room above an unconditioned garage space presents some unique insulation challenges. The first is to ensure the safety of the room by isolating it from the garage below. Isolation must include complete airsealing at the ceiling line of the garage (Figure 1). Unless this is done first, all other components may be compromised.
This is also a good time to re-route any utilities that have been incorrectly placed in the ceiling cavity. If the garage ceiling contains runs for any temperature-sensitive utilities, such as water pipes, drains, or ducts, the design should include an insulated, sealed cavity for their passage.
Insufficient insulation is a common problem in a floor over garage. An uninsulated floor is subject to the same extreme temperatures as exterior walls, so people who walk on the floor will notice if it is colder than the rest of the house. To ensure the homeowner’s comfort, the floor-over-garage assembly requires a specific insulation strategy with an increased amount of insulation and good air-sealing.
Floor assembly framing members are larger than those for the typical exterior wall. This allows for more insulation to be installed within the assembly. Code typically requires a higher R-value, and this is not a place to trade down. It is important that the insulation completely fill the floor-joist cavity to get the full effect of the R-value. If there is an oversized cavity because of deep joists or trusses, make sure insulation is in contact with the plywood subfloor and that the entire perimeter band surface is fully insulated and sealed.
Most bonus room floors have conventional framing, so the ceiling board of the garage and the sub-floor of the bonus room will "sandwich" the insulation, while the framing members will cap the sides. Install blocking in each joist cavity below the knee walls, then seal it to encapsulate the insulation.
The finished space between the garage ceiling and the attic floor provides the final component in this space, controlling horizontal movement of cold air from ventilated portions of the roof to under the floor. The result will be an energy-efficient and comfortable room all year round.
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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