David I agree with Harold. I recently had icynene foam installed in a house and where the walls were open, because I had to replace the siding, it covered great. But, on the closed walls I removed 3 to 5 boards on the inside for them to put the foam through and the coverage was poor. I had to call them back to fill in and I've since found more voids. The 1 1/2 inch holes will never do. The only thing I would add to what Harold said is that when I studied up on this subject I found that if you insulate your exterior walls you should include a vapor barrier, in your case on the inside. You may accumulate moisture in the walls and rot out the structure of your house without it. My undersanding is that celulose does not offer a vapor barrier like the foam does. That's why I went with the foam.