In Between Joists
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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.
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Squirt construction adhesive in a zig zag line in between the joists and let it sit for a few minutes to get tacky.
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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.
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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.
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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.