Build Your Own House

Adding Attic Insulation

Summary: Adding attic insulation prevents heat from escaping from your house in the winter or getting trapped in the attic in the summer, making it hard to keep your house cool.


Is there a certain depth of insulation that I need in my Arizona home?

Can I measure the depth with a yard stick to know if it is the proper amount of blown insulation in the attic?

I believe our builder used blown cellulose insulation.

I was wondering if I could add the pink rolls of fiberglass insulation on top of it.

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.

Attics cause more heat gain and heat loss than anywhere else in a house except windows. Attic insulation R value is important so add attic insulation and save energy.

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

Cellulose insulation 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-38 value.

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.

Obtain bids from several insulation contractors for the R-value you need.

If you want to install it yourself, obtain instructions and safety precautions from the insulation manufacturer.

Carefully follow their instructions and be sure to check your local building and fire codes.

I’m not sure I would try putting fiberglass insulation (rolls) over the cellulose. The weight of the fiberglass insulation would most likely cause compression of the cellulose rendering it less effective.

Good Luck, Carl Heldmann 2017