Cellulose vs. Fiberglass in an Attic

I live in a 1940's cape house in PA with little insulation in the attic space. I am talking to contractors about installing blown-in insulation to bring the space up to R38. I've heard contractors bash both cellulose and fiberglass for this attic application, dependent upon which they offer of course.

What blown-in product is better for an attic space, cellulose or fiberglass? Why?

Thanks!

 

Fiberglass probably has an edge on cellulose simply because cellulose will usually settle under its own weight over time, whereas blown fiberglass remains fluffy. The advertised R value of insulation depends on the insulation remaining fluffy, not compressed. As insulation compresses, it loses r value.

Some cellulose guys can mix a glue of sorts into their product, which helps lock it in so it doesn't settle. That would be a good thing to ask... assuming they will tell you the truth and not bs you.

Here is a link to a page with some good maps and charts if it helps you. I have no connection to the company, just thought it might visually explain the difference. Insulation R Value Chart - --- don't pay attention to the reference in the first paragraph where cellulose seems superior... go down to the bottom chart and compare the "attic" values, which take gravity into consideration. Both are quite similar... so unless you are getting the best possible blown fiberglass they are almost dead even.

Whomever you choose, air sealing the penetrations in the attic prior to blowing the insulation would be a must, imo.

 

X beat me to it, this was what I was going to advise.

 

If this is really a cape, small attic space up top, slopes on each side with side attics, maybe a dormer or two, then the r-38 up top is only a small part of the insulation picture.

Capes usually suffer from poor insulation on the slopes and no blocking under the knee walls.

Describe what you have.

Bud