If you plan on finishing your attic then you cannot install it directly.
How to install insulating foam board in an attic.
Remove the old insulation boards carefully without causing any damage to the walls.
Step 2 install insulation boards.
Your board must leave at least inch gap between the board and the roof.
If it you need to spread it use a brush to help cover and smooth out the glue.
If it fits proper with no space between on either side of the board pick it up and put a thin layer of glue on.
Next you will need to place the foam board insulation between each joist as a test.
You can use a foamboard adhesive to glue it in place.
Remove any existing pieces of insulation that might have been covering the attic walls until now.
Put the board between the rafters.
No matter how much insulation you add to your home s walls floors and ceilings you re throwing your money away if you don t insulate your attic.