There are three types of rigid foam panels used in building insulation. Expanded polystyrene (EPS) is a basic white foam panel much like that found in product packaging. It is the cheapest type of rigid foam but it also has the lowest R-values -- its ability to block heat -- so it is not often used for insulating. Polyisocyanurate (ISO) is the most expensive rigid foam barrier. This type of rigid foam panel always has a backing and has the best R-value. An in-between rigid foam option is extruded polystyrene (XPS), which is the most commonly used foam in the construction industry. An easy way to install it is by using nails with 1-inch diameter plastic caps. The nails should be long enough to penetrate the wood studs by at least three quarters of an inch.
Hook the tape measure to the end of the wall and measure across 48 inches. Check if the 48 inches falls on a stud. If it does not fall on 48 inches, move the tape down to the closest stud and take that measurement. Transfer that measurement to the rigid foam panel and mark it. Snap a line the length of the panel at your mark. Cut the rigid foam panel to the correct width with the saw.
Measure the height of the wall with a tape measure, from the top of the foundation to the double top plate. Standard stud frame walls will be 8-foot-1-inch while the subfloor and floor joist will add more height to the face of the house. This means that you'll use a full 8-foot section of foam panel with an addition piece cut to fill in the gap at the top of the 8-foot foam panel.
Hold the panel up to the stud wall. Flush up the outside edge to the outside corner of the structure. Check that the bottom edge of the rigid foam panel is flush to the foundation line.
Secure the rigid foam panel by hammering plastic cap nails through the foam into the studs. Space perimeter nails 12 inches apart. Hammer nails every 16 inches inside the field -- or the face -- of the rigid foam panel.
Measure from the top of the installed 8-foot panel to the double top plate of the house, the section of housing that was not covered by the full panel. Transfer this measurement to a rigid foam panel and cut to height. Place the custom cut section of panel on the top of the 8 foot section and secure it to the studs in the same nailing pattern as the full 8 foot section.
Continue laying full panels across the lower portion of wall and filling in the small sections at the top of the wall with custom cut pieces. The width of all panels should be 48 inches since most stud framing is 16OC. 16OC means that you secure each stud every 16 inches. The panel’s width will stay at 48 inches since 16 multiplied by 3 equals 48. So each panel will end at the third stud.
Measure and cut custom sections of rigid foam panels when you encounter windows, doors or any other object that hinders the rigid foam path across the building.
Once you have finished the installation process, follow through by applying insulation tape to all seams to make the structure air tight. Center the tape over the seams and overlap it when you start a new seam.