Insulated ceilings within the thermal envelope of a building and located in an area governed by the 2018 International Residential Code must satisfy certain requirements. For complete details, which may vary from location to location, click here for the reference in the code book.
Ceilings without Attics
There is no attic for a ceiling when the ceiling also doubles for the roof. This occurs with cathedrals and the sloped part of upstairs attic rooms. The minimum requirement is R30 as long as its properly installed over 100% of the ceiling and extending down over the top plate of the wall at the bottom. The trick to this is that R30 requires a minimum cavity of 8 1/4″. Just because insulation is labeled R30 does not mean it can be compressed into a cavity of smaller size. So even when rafters are smaller than 8 1/4″ in depth, they must be furred down to meet the specifications.
Ceilings with Attics
While R38 is the minimum thickness, R30 bats may be used as long as they cover 100% of the ceiling and extend down over the top plate of the wall at bottom. The most popular and cost effective method is to use loose filled cellulose. This will need about 13″ of initial fill to allow for settling.
There should be clear markers measuring the depth of the fill facing the access opening. The access must be protected by a physical barrier of some type to keep the loose filled material away from the opening. The opening itself must also meet the same insulation requirements according to 2018 International Residential Code.