R-value

• The thermal resistance of insulation is measured in terms of its R-value. This number represents a material's ability to resist heat transfer, and the higher the R-value the more effective the material is for insulating. If a material is compressed or installed incorrectly the actual R-value will typically be lower than the specified rating.
  
  

Materials

• Fiberglass batts are among the most affordable materials used to insulate garage doors. To keep fibers from making a mess look for batt insulation faced with foil or vinyl. Batt insulation has an R-value of between 3 and 4 per inch of thickness. Rigid foam panels are a more expensive option, but offer about twice the thermal resistance of fiberglass. Either of these materials can be cut to size and fastened to the door using adhesive or double-sided tape.
  
  Most garage door manufacturers sell pre-fabricated kits designed for insulation. These kits typically contain pre-cut insulation that will fit your specific door type. Because these kits tend to provide more complete coverage of the door, they will usually be more effective than other forms of insulation.
  
  

Weatherstripping

• Many homeowners will be surprised to see the amount of unsealed space around a typical garage door. Air can leak into the home from under the door or along each side where the door meets the walls. To fully insulate the door, these areas should be sealed using a weatherstripping kit designed for garage doors. This generally includes a threshold which is nailed directly into the ground, as well as a neoprene or vinyl seal that adheres to the sides of the door. A door bottom seal or sweep can also be screwed into the bottom of the garage door. Neoprene varieties tend to seal more completely than nylon brush models, but are also subject to cracking in very cold temperatures.