Attic Insulation

Attic Insulation


From Lori:

"Is there a certain depth of insulation that I need in my Arizona home, in other words can I measure the depth with a yard stick to know if it is the proper amount of blown insulation in the attic.

The builder used, I believe, blown cellulose and I was wondering if I could add the pink rolls of fiberglass on top of it? PS If so how do I know how much I would need -- I think I am supposed to have at least R-38 value in my roof.

Thanking you in advance, Lori"

Hi Lori,

Yes, there is a certain depth of attic insulation that you need in your home.

As you correctly stated, you need at least an R-38 value in your attic. (Insulation Calculator)

Cellulose provides a thermal resistance of R-3.2 to R-3.8 per inch.

So, using the more conservative R-3.6 per inch, you would need 11.9 inches to have an R-38value.

Cellulose settles with time, more so than rock wool or fiberglass, so you might be a little shy on R value by now.

If you need to add depth, I would consult with a professional insulation contractor as to the best way to do it and how much it would cost.

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Obtain written cost estimates from several contractors for the R-value you need. Don't be surprised to find quoted prices for a given R-value installation to vary by more than a factor of two.

If you want to install it yourself, you should try to obtain instructions and safety precautions from the insulation manufacturer. Carefully follow these instructions. You should also check your local building and fire codes.

I’m not sure I would try putting Fiberglass (rolls) over the cellulose. The weight of the fiberglass would most likely cause compression of the cellulose.

Good Luck,
Carl