Cellulose or Fiberglass Beneath Flat Roof?

We’ve narrowed our search for replacing our old B.U.R… It’ll be either another B.U.R. or a modified bitumen. Final decision will be based on the price spread between the two, and what recoatings will cost prorated over the life ( 20 yrs. + we hope ) of the system. ( We have two bids in hand, and will be getting at least one more. )

Meanwhile, we’re researching what kind of insulation to install underneath. The roof has 4 air vents ( aprox. 4 x 9 in. ) along both sides of the long axis, and the space between the bottom of the roof and the top of the joists varies from 18 in. at the high end, to about 8 in. on the low side. Total area is about 12 squares.

Without removing everything, there’s no way to install batting or to spray foam between the joists and on top of the ceiling. So, that leaves us with two choices: Blowing in cellulose products, or blowing in fiberglass products.

When the roof was replaced on the small ( 700 sq. ft. ) rental we rehabbed next door, we went with fiberglass since our roofer had a preferred vendor they worked with who only installed fiberglass. The price was reasonable, and that roof had fewer, smaller, vents. It seemed to work. Our tenants stayed comfortable all summer, and even had to turn the A/C down because it was getting too chilly. ( It didn’t hurt that we had the A/C unit cleaned & serviced for the first time in decades. The 4 story Cottonwood in the neighbors back yard had, over the years, deposited a 3/4 in. blanket of “cotton” over the interior. Yuck! )

Our home is another story. We want to get the maximum R value we can, and that probably means some sort of cellulose product. Or does it? On paper, it has a higher R value, but what about “real life” situations?

Are there any significant differences between different cellulose products
either in R value, ease of proper installation, or longevity?

We heard a sales pitch years ago that said fiberglass was inert, not friendly to critters or bacteria, and couldn’t absorb water and cave-in the ceiling if there was a leak. (not an issue with a new roof that we intend to service properly. ) All of that sounds good, but staying warm & cool inexpensively is more important to us now, and probably will remain that way for the foreseeable future.

What would you suggest?

Thanks for your time and attention.

i did some limited insulation work and i can tell you that we blown in cellulose as well as installed the rolls of fiberglass. you can achieve the same rvalue with both. the difference to achieve the rvalue is signigicant though. you buy a roll of fiberglass, it says the rvalue on the package. you do blown in insulation, for lack of a better word, there is a meter that measures the density of the insulation in a given area right through the wall. Anything less is just a guess and the blown in is not as accurate. Knowing the kinds of roofs ive seen in the world, i’m betting theres a lot of contractors that go by eye and ear when they blow it in and dont want to spend the $5000 (i think i heard is the cost) on the meter to do it right. its like when i teach a guy how to do a scratch n’ patch on a BUR and you tell them ten thousand times how important the membrane is to the tar, yet they decide to test it and the roof leaks and they wonder why. The insulation installer feels theres some guy in a tie with a machine busting his balls, if you do have the right equipment. then the guy usually just tells the installer which bays are inadequate and doesnt check again, so what does the installer do? BTW, cellulose is a way of recycling if that matters to you. It’s merely chemically treated shredded newspaper. Fiberglass obviously can’t be broken down and reused as easily in todays world.

Scratch N Patch is what I call BUR repairs, also.

Anyhoo, like S&G said, how does one go about making sure the cellulose, or even the fiberglass is blown in properly if you go through a few little holes? How does this meter work?

I’m not a scientific guy. The meter measures the volume in the walls right through it was my understanding of it. Every time the job was inspected, there were some bays that needed to be redone, and it was no big deal when there were only a few, so I took it that it was expected… just sucked when it was a tough one, like aluminum siding over it and it was all put back together. I personally never took it like the guy was saying it was “wrong”, I just accepted that the meter is part of the process, though I’m sure I was more gracious than some of the installers… they paid me fifteen dollars an hour to do that, which to a seasoned roofer was like sitting around doing nothing, so it didn’t bother me that they told me to do it again, it was like free money IMO.