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Q & A: Can I add blown-in cellulose insulation on top of preexisting loose-fill fiberglass insulation?

Question by Ron B: Can I add blown-in cellulose insulation on top of preexisting loose-fill fiberglass insulation?
I just moved into a house and need to add insulation to my unfinished attic. Presently, there is approximately four inches of loose-fill fiberglass insulation, but I would like to increase the R-value to 49. I’d rather use cellulose insulation versus fiberglass, because I heard mice and rats use fiberglass as nesting material, whereas cellulose contains chemicals (boric acid, for example) that repels such creatures.

Best answer:

Answer by imaramblins
While I’m not an insulation contractor, I did have the mouse problem in my old house. I actually saw them run up my outside brick facade, and between a gap in the facia board, and the brick. That’s how I found their entry point, and could deal with the problem.

If you currently don’t have a mouse problem, you shouldn’t have to worry. Just make sure all possible entry points (facia boards, soffits, gable vents, etc. are tight, caulked, etc.), and you should be good to go. No worry of nesting, if they’re not there to begin with. Make sure your pad is tight, but ventilated properly, and blow it in baby!

Good luck!

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2 Responses to Q & A: Can I add blown-in cellulose insulation on top of preexisting loose-fill fiberglass insulation?

  1. Jim S

    I agree that there’s no problem blowing cellulose on top of fiberglas. I don’t agree that Lowe’s and Home Depot have crappy products. Go to the retailer and discuss what you’re trying to achieve. They can help you decide how deep you want the insulation. Their machines are just fine except they must be used close to the application point because they’re not as powerful as the truck mounted machines the pros use. That can be messy.

    Now, let’s address the rodent access issue. Mice can get in any opening the size of a dime! If your attic has proper ventilation (soffit and ridge or turbine vents, etc., or gable vents on both ends) you should be ok. You need airflow through the attic because it eliminates moisture in the winter and heat in the summer. So, look around your home for gaps between facia boards and brick, gaps between trim and roofing, etc., and fill them with steel wool. Don’t block any intended vents. This will not only keep mice out, but squirrels and other little critters as well.

  2. wearehere

    You currently have an R-11 to 13 avg. in your attic. An R-38 is more than enough to fix any attic problems. 13-15 inches of a good brand cellulose over the existing will make you happy. Fiberlite Technologies out of Joplin Missouri is a great product. Don’t go to Lowes and rent those bogus machines and buy their crappy product. You can hire a pro to do it and save money. You can also get a Federal tax energy rebate for part of the work.

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