If all the work is done carefully, you can effectively insulate a floor over a cold space using fiberglass batts.
39+ Insulate Floor Over Unheated Space
Pictures. Insulating any floors above your unheated garage will help reduce the energy costs for the above conditioned space. When insulating floors over unconditioned garages, use the following techniques:
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If the air distribution is in the attic space, then consider insulating the rafters to 4. T cover the entire earth floor with sealed 6 mil polyethylene (recommended for all crawl spaces). Guidance for insulating suspended timber floors.
The result of this crawl space encapsulation process is a clean, dry crawl space that isn't affected by moisture in the soil or in the outside air.
Traditional insulation will still allow not only for air movement into the space, but also the odors from your garage and moisture. In unfinished attic spaces, insulate between and over the floor joists to seal off living spaces below.* 1a. Fiberglass batts are the preferred insulation under floors because they can be easily fastened between floor joists. Stapling fiberglass batts between joists to the underside of your home's floor presents a number of problems.
