How To Install Foam Board Insulation Under Siding

Justin Ross

Today, many homes are making use of what is known as "double wall" insulation. In this system, insulation batts are positioned between the studs and joists on the inside of the home, while rigid foam board insulation is applied over the exterior sheathing. Once this is done, whatever type of siding you choose to install can then be applied over the foam board. The following will show you the steps to take in order to install your own foam board insulation.

What You Will Need

  • Foam Board Insulation

  • Polystyrene Wrap

  • Measuring Tape

  • Hammer

  • 3/8 Inch Galvanized Nails

  • Utility Knife

  • Sealing Tape

  • Reciprocal Saw

Step 1. Use a measuring tape to measure your exterior walls. Based on these measurements, buy enough sheets (either 4 x 8 or 4 x 9) of rigid foam board insulation to cover the exterior of your home from the roof line to the sill plate. You usually want foam board that is 3/4 to 1 inch in thickness, but you should check with local building agencies to get the recommendations for your area.

Step 2. Start at one end of your house and begin applying the panels to the walls. Align the edge of the first panel with the corner of the wall and then start nailing the panel to the wall with 3/8 inch galvanized nails. When you're hammering the nails through the foam board and into the studs, they should be positioned two feet apart on each stud. While you do want the nail heads to be flush with the foam board, avoid driving the nails in so far that you indent the foam. The nails have to be long enough to penetrate the foam you're using and go at least three quarters of an inch into the stud.

Step 3. Make sure that the panels fit together tightly. It's best to choose foam board panels that have a tongue and groove, since this allows you to slip one panel into another. When you get to areas where there are doors and windows, use a reciprocal saw to cut through the insulation from inside your house. For any exterior electrical panels, faucets or other exterior wall features, use a utility knife to trim away the insulation.

Step 4. Once you have finished covering your home with the foam board insulation, use tape designed for this purpose to seal any seams between the foam board panels, door and window openings and wall corners. The tape should run in a continuous strip vertically and horizontally along all the seams. Any tape placed along the edges of doors or windows can be folded over to fit inside the wood framing.

Step 5. After installing the foam board insulation, it's important to also install a polystyrene wrap (following the manufacturer's instructions). You can now start applying your siding. Make sure the nails you use for your siding are long enough to penetrate the foam board and the siding and embed themselves in the studs.

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