Construction adhesive is useful for a multitude of purposes on a job site, and one of those purposes is to attach rigid foam insulation board to plywood. Not spreading adhesive all over the board saves adhesive and still results in a good bond. Only one blob of adhesive is needed about every 12 inches. When you press the board against the plywood, the blobs of adhesive will make good contact with the plywood surface and adhere securely.
Using screws in conjunction with constructive adhesive provides a strong bond between the insulation board and the plywood. Screws also act as clamps to hold the foam board and the plywood together while the adhesive dries. The screws must be long enough to penetrate both the foam board and the plywood, but not long enough to stick out the other side of the plywood. Screws that are 2 1/2 inches long are appropriate to attach 2-inch thick foam board to 3/4-inch thick plywood.
Nails are adequate to secure insulation board to plywood in some situations, but they are not as good as screws. Because nails don't have threads, movement and changes in weather can cause them to work their way out of the wood over time, leading to gaps between the insulation board and the plywood. Using a couple of nails, however, can be the easiest way to hold in place small bits of insulation board inserted in gaps or between other foam boards.
Using washers in conjunction with screws and nails is important. Because the surface of foam insulation board is porous, the head of a nail or screw will go right through the foam board without a washer. Washers for foam board don't have a hole in the middle; they are simple steel discs that measure about 2 or 3 inches in diameter. A nail or screw is driven directly through the disc, through the foam board and into the plywood. The disc distributes the pulling pressure of the nail or screw over a wide area, preventing it from pulling through the foam.