House being built where the garage is under living space.
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 mold 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. Vapor 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 polyethylene vapor 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 vapor retardant paint which is much easier to perform.
The minimum insulation requirement for garage ceilings with living space above them is R-19 fiberglass 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.
Heated Garage
If the garage is going to be heated, IRC requires that a 4 mil polyethylene vapor 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 fiberglass 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 sulfides added to it for fire, pest, and rodent retardency. Cellulose insulation is blown in and gets into all the nooks and crannies that fiberglass insulation cannot get into. The R-value per inch of cellulose is R-3.8 per inch.
Fiberglass insulation can be blown in or be in batt form. Fiberglass is the most common insulation and is made of spun glass fibers that use the air, not the material as the insulator. Fiberglass has an R-value of R-3.5 per inch.
Tags: living space, garage ceiling, closed cell, garage ceilings, R-value inch, spray foam, above them