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Use a household cleaner and some paper towels to clean off any dirt, grime, and grease on the inside of your garage door. Wipe it down, swapping out paper towels whenever they become dirty. Use a garden hose to spray away any remaining dirt or residue, wipe away the excess water, and leave the door to dry for 3 to 4 hours.  Make sure your door is completely clean and dry before you begin installing insulation. Any dirt or grime on the inside of the door will prevent the insulation from sticking, and any will be trapped behind the insulation once it’s installed. You could also use a pressure washer to clean the garage door. Test it on a small area first to make sure it doesn’t damage the door, before spraying down the entire surface. Use an old towel or a rag to wipe away excess water before leaving it to dry completely for 3-4 hours. The dimensions of your door will be a key factor in determining which insulation kit to buy, or even how many kits you need to buy to get the job done. Use a tape measure to measure the height and width of your garage door, noting down all of the dimensions as you go. If your door is broken up into panels that are separated by a metal railing, measure each of the panels individually rather than measuring the whole door. These should be available from your local hardware store or online and will come with the fiber batting and retaining pins needed to insulate the door. Purchase a kit that comes with enough batting to cover your garage door.  Depending on the size of your door, you may need to purchase two insulation kits. If you have a double garage, for example, it’s likely you’ll need multiple kits to insulate the whole door. Purchase the insulation kit with the highest R-value for the best insulation. If you’re unsure about which kit to buy for your door, feel free ask someone at your local hardware store for assistance. You may be able to buy the individual parts of the kit independently, but this will often be more difficult than purchasing the right kit. Ask at your local hardware store to look into your options. For each of the panels on your door, mark two points in the middle of the panel and roughly 12 inches (30 cm) from each side. Peel the adhesive backing off of the retaining pins in your kit and stick them onto these points, pressing in firmly to make sure they adhere fully.  The retaining pins will be long plastic or metal spikes on a wide and flat base that can be stuck to your door. They will help hold your insulation in place. The pins don’t need to be lined up perfectly, as long as they are roughly evenly spaced and divided across each panel. If you don’t have panels on your garage door, use the sheets of batting insulation as a guide to break your door up into different sections. Install your retaining pins in the same method, but spread out over these sections rather than across prebuilt panels. Unroll the insulation batting from your garage door kit and lay it on a flat surface. Use a pencil or pen to mark out the dimensions of each of your garage door panels, before cutting the batting down to size with a craft knife. Work one panel at a time until you have enough trimmed batting for the entire door.  If your door is without panels and you installed the retaining pins based on the size of the batting, you shouldn’t need to trim it down at all. For extra safety, wear cut-resistant gloves while working with the batting. Lift one piece of batting up and press it onto the retaining pins in one section or panel of the garage door, making sure it is straight and centered. Tuck the edges of the batting into the metal railings surrounding the panel to help keep them held in place. Repeat this process until all of the insulation is in place.  If your batting is only covered in foil or plastic on one side, make sure it is that side that is facing towards you when installing the batting. This will both make the finished door look better, as well as improving the insulation. If you don’t have metal railings surrounding the panels or sections of your door, you can simply press the batting onto the retaining pins to hold it steady as you continue working. Use your hand to feel underneath the batting and locate the retaining pin sticking out. Press down on the batting around the pin and use a craft knife to cut through the batting. Make 2 small cuts across the tip of the pin, roughly 1 inch (2.5 cm) in length to create a small cross that the pin can stick through. If there is foil or plastic on both sides of the batting, make a deeper incision so that the pin can push through both layers of protection and emerge on the other side. The fasteners should come with the insulation kit and will be metal or plastic sleeves that slip over the retaining pins to keep the insulation in place. Press a fastener onto each of the pins, pushing down until it clicks into place.  The fasteners will stop the batting from falling out, as well as softening the tips of the retaining pins. Different kits will have different retaining pins and fasteners. Check the manufacturer’s instructions for your specific garage door insulation kit for advice on how to attach your fasteners. If there are any edges on the batting that are popping out of the metal railings on the door, or that can’t be tucked under any railings, you can use aluminum foil tape to keep them from coming loose. Run strips of tape over the loose edges to secure them together, or to hold them to an exposed fitting on the door itself. Aluminum foil tape will hold the batting place and help insulate the gaps between each piece of batting. It should be available at your local hardware store or online.

Summary:
Clean and dry the garage door. Measure the height and width of your door. Purchase a garage door insulation kit. Install the retaining pins across your door panels. Cut the batting to the size of your door panels. Press the batting onto the retaining pins. Make a cross-shaped incision over the pins. Secure the batting in place with a fastener. Use aluminum foil tape to hold any loose edges.