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Figure 5. Maintaining good ventilation is a particular challenge when insulating a vaulted or cathedral ceiling.
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By Brad Oberg
The sloped portion of the cathedral ceiling found in most bonus rooms presents another challenge to proper insulation. These areas are subjected to the same solar exposure as conventional attics but do not have the same available space as attics to provide for a larger volume of insulation, although the structural depth can be altered to provide additional space for insulation.
When insulating these surfaces, be sure to protect the ventilation strategy of the building. A typical ventilation strategy is a perforated soffit and continuous ridge vent. If this strategy is used, it’s important not to obstruct the flow of air to the ridge vent. When applying the sheathing to the back of the insulation at the knee wall, leave 1" minimum clearance at the top of the backer, insulation, and framing between the top of the knee wall and the roof sheathing. Batt insulation is available in higher density products, designed for restricted spaces such as these ceiling areas, which will maintain higher R-value without blocking the ventilation path.
When filling the cavity with insulation, be sure to use a rafter vent between each ceiling joist to protect the ventilation path. This will allow air to find its way over the knee wall and out the ridge vent. While this will slightly compromise the R-value of the assembly because it does not completely fill each ceiling joist cavity, it is a necessary trade-off to ensure proper attic ventilation.
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.
Related: Building Airtight Insulated Cathedral Ceilings
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