Cathedral Ceiling Best Practices
Check out photos from HERS rating projects in Western Massachusetts, with detailed descriptions.
Properly insulating a sloped roof is a lot more involved (and expensive) than insulating a vented attic flat. Often cathedral ceilings are designed as a purely aesthetic feature, without consideration to the impact on the project budget to insulate it properly. Note that some low-pitched cathedral ceilings actually have attic flats above them, making them a cost effective alternative to steep-pitched ceilings, which require the advanced thermal enclosure assemblies detailed below.
Doing a cathedral right requires establishing a robust air control layer and enough R-value (thermal insulation) to reduce heat loss through conduction and to mitigate ice-dams. There are generally two approaches to cathedral ceilings: hot roof and vented roof. A hot roof does not have any venting. With the right assembly, this works very well. A vented roof maintains a narrow ventilation cavity under the roof deck (sheathing). This is a must if the cathedral ceiling air and vapor control layers are not robust or if there is not a complete vapor barrier established (cellulose or open cell spray foam).
Principals of a high performance cathedral or vaulted roof assembly:
Continuous insulation on top - Addressing heat loss through the rafter framing is the best approach. The more R-value on the top of the roof the better, with minimum around R20 in our climate.
Closed cell spray foam works - As a vapor and air barrier, at over 3” , a 7-9” install of closed cell spray foam makes a robust approach. At around $1/board foot, this is costly.
Batts are very risky - The old school approach is a thick fiberglass batt with a vented cavity behind it. This is a recipe for ice-dams, failing blower door tests, and moldy sheathing. Code allows it, but this route requires a well-executed interior smart air/vapor barrier or extremely careful air-sealing detailing.