Garages are considered unconditioned space by the International Residential Code (IRC). However, there are some circumstances where insulation is needed in the garage ceiling. There are specific requirements that must be adhered to when insulating the garage ceiling in these cases to prevent moisture and mould problems in the future.
Living Space Over A Garage
When living space is present over a garage, the ceiling of the garage directly under the living space must be insulated. Vapour barriers need to be applied to the conditioned side of the floor which is in the living space. This can be achieved by laying a 4 mil polythene vapour barrier across the garage ceiling joists before the plywood floor is installed for the living space over the garage, or the living space floor can be painted with vapour retardant paint which is much easier to perform.
The minimum insulation requirement for garage ceilings with living space above them is R-19 fibreglass insulation. However, Energy Star Homes recommends that a minimum of R-38 be installed between garage ceilings and living space floors. This will also aid in sound attenuation when cars pull into the garage.
If the garage is going to be heated, IRC requires that a 4 mil polythene vapour barrier be installed on the ceiling prior to insulating. Garage attic insulation requirements are the same as a house with a minimum insulation R-value of R-38.
Types Of Garage Ceiling Insulation
For garage ceilings that have living spaces above them, closed cell spray foam insulation is the most energy efficient option. The spray foam insulates and air seals the living space from the garage space. It only takes 3-5 inches of closed cell spray foam to totally insulate the garage ceiling, as the R-value per inch of closed cell foam is R-7. This will give you the opportunity to use fibreglass batts to sound proof the living space from the garage if desired as there will be a lot of space to play with.
Cellulose insulation is ground up newspaper that has borates and sulphides added to it for fire, pest, and rodent retardency. Cellulose insulation is blown in and gets into all the nooks and crannies that fibreglass insulation cannot get into. The R-value per inch of cellulose is R-3.8 per inch.
Fibreglass insulation can be blown in or be in batt form. Fibreglass is the most common insulation and is made of spun glass fibres that use the air, not the material as the insulator. Fibreglass has an R-value of R-3.5 per inch.