Rigid foam insulation is one of several options for insulating a home. It is relatively inexpensive compared to other types and is fairly easy to install. Consider using this insulation in the basement, between floors, ceilings or even in the walls. You generally glue and nail it when applying it to ceilings and other locations, to ensure that it doesn't come loose later on.
Measure the space between the ceiling joists. Transfer those measurements to the rigid foam insulation. Mark the measurements. Use a utility knife to cut the insulation along the marked lines so that it will fit the space between the joists.
Squirt construction adhesive in a zig zag line in between the joists and let it sit for a few minutes to get tacky.
Place the insulation between the joists and hold it there a second or two until it adheres. Repeat the process between each joist. If there are obstructions such as light fixtures, measure out the opening and cut it out with a utility knife. Make the opening about 3 inches wider on all sizes than electrical items like light fixtures. This is to protect against fires.
Slip washers onto nails and then hammer the nails into the rigid foam insulation. The nails will add a little more stability to the insulation if the glue doesn't hold. The washers will keep the nails from tearing through the insulation.
Squeeze a tube of caulk along the seams between the boards and the joists to seal them tight. Press and slide your finger along the bead of caulk to flatten it out and seal it to the joist and insulation.
You may also attach the rigid foam to the outside of the joists but this will require you to install furring strips over top of the insulation so that you can hang the drywall. Glue the boards to the joists so that the panels are at a right angle to the direction of the ceiling joists. Apply glue along the joists and wait the three minutes before hanging the insulation panels. Afterwards nail the panels to the joists to ensure they are secure. You don't need as many nails, since you will nail over furring strips later.