By Brad Oberg
Once the sloped portions of the bonus-room envelope have been properly insulated, it's time to deal with the more traditional flat ceiling. Roofs in general are constructions that allow larger volumes for insulation, so they can be effective locations for increased insulation levels.
While the size of this flat portion of the ceiling may vary depending on the size and design of the bonus room, it should be insulated in the same manner as a traditional attic space. But again, take care not to interfere with the ventilation strategy. The insulation baffles installed to preserve the ventilation at the cathedralized section of the bonus room should be extended up the ceiling joist cavities, so they also prevent the insulation in the flat ceiling from interfering with the attic ventilation.
This detail is the same as what you would do in a traditional attic assembly at the eaves of the roof. With bonus rooms, though, extend the baffles at the transition from cathedralized insulation to standard attic insulation.
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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