What insulation can I add to my attic?
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greg1258
| Posted in Energy Efficiency and Durability on
I currently have blown-in loose-fill fiberglass in my attic. Do I have to add the same product, or can I use cellulose?
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Replies
Greg,
Yes, you can add cellulose insulation on top of your existing loose-fill fiberglass. Not only can you -- you should.
Adding cellulose on top of loose-fill fiberglass insulation is the best way to overcome the inherent problems of loose-fill fiberglass (especially the degradation in thermal performance due to convective loops in fiberglass). A cellulose "cap" not only warms up the top of the fiberglass, cutting down on convective looping, it also reduces air leaks through the fiberglass.
It takes at least 3" of cellulose over-blow to "restore" the full R of the fiberglass layer. If you add 6" or more the weight of the cellulose will compress the fiberglass a bit, but that gives the fiberglass a higher R/inch (if a lower overall R of the fiberglass layer by itself.)
The overall performance will be a function of the total settled depth of the cellulose + fiberglass, which will average about R3.5 per inch over a wide range of temperature conditions (some manufacturers claim R3.7/inch, which it probably does in a lab-test blow), as long as there is at least 3" of cellulose over the fiberglass. Less than that would still yield some convective losses at the low-temp extremes.