Blown-in cellulose is going to be far superior to rigid foam board as the best attic insulation in your home.
- Most residential roof assemblies do not facilitate large rigid panels very well in that there are many small and often odd-shaped spaces created by the ceiling structure.
- This is why you see rigid panels being used primarily in commercial roof assemblies that tend to be flat.
If your attic is like most, you will see that cutting and inserting rigid panels to every space in your attic without leaving significant voids is impossible. Even if you could fill all of the areas, there would certainly be voids around the edges of the panels where they contact wood.
Blown-in insulation, on the other hand, inherently is able to get into every space that it is sprayed into and not leave any air voids.
This will give you a far better-insulated ceiling, not allowing as much radiation or air infiltration.
Cellulose can move around
The big downside of cellulose is that it has the potential to move around in your attic since it is a loose material.
- If you do a good job of air-sealing the large voids, this should not be a problem.
- Make sure you caulk any voids around the walls and anywhere else that you see as a possible air infiltration point before you spray in the cellulose.
The other area that we see cellulose moving around in attics is near the attic entry, usually caused by the puff of air that moves up there when you pull down the attic door/stairs. Make sure to take a peek up there once a year and spread the cellulose back into any voids that may have been created.
Buying and installing cellulose
It is a very cost-effective way to insulate your attic.
For more information:
Read Ian MacLeod's Q&A "What is the safest and most effective insulation for our attic crawl space?"
Also, read Steve Saunders's Q&A "Is three inches of batt insulation enough for my attic?"