I believe the exterior walls of my cape have R9 fiberblass insulation. When I replaced my bath tub I removed the old, thin insulation behind it and thought I could read R9 on it but it was very difficult due to the age. I replaced it with R13 as necessary. My next project I plan to remove the exterior shingles and reside the house. And I thought it would be a good time to blow insulation in before I re-side. My question is can i blow in insulation even if fiberglass insulation already exists?