Reinsulating 1988 Attic – Foam plus Blown in Cellulose or Blown in Fiberglass
Hi All. I’m looking at reinsulating a large attic (roughly 1800 SF) in a house built in 1988. The existing insulation is blown in fiberglass. I’m going to remove all of the existing insulation, as we have had a rodent problem (rodent sh!t everywhere) and have also had significant moisture issues which have degraded the original insulation. Typical of 1988 construction, the air sealing is horrible, with stuff like 4 non-sealed recessed cans installed in the master bathroom (including one in the shower), absolutely no air sealing around any other penetrations into the attic, such that in the dead of winter (Minnesota) I’ve seen 1/4 – 1/2″ of frost on the underside of my roofdeck (which, by the way, is a very tall and steep 12/12 pitch). Lots and lots of stack effect.
So, the plan is to remove (industrial truck mounted vacuum) all of the existing fiberglass, cut out all of the existing poly vapor barrier (using that term loosely) between the ceiling joists, and then spray 1 1/2″ of closed cell foam (roughly R10) on the entire ceiling, after boxing in the recessed cans in the master suite. My contractor then wants to blow in fiberglass, but almost everything I’ve read suggests cellulose is the better way to go, especially since I’ve had a rodent problem.
So, not intending to touch off a recurring holy war, what say you all regarding the current state of debate on blown in cellulose versus blown in fiberglass, from the perspectives of R value, economic value, and rodent control?
Replies
We're down here in Byron, house built in '76. Was originally insulated with about 4" of cells (the builder was cheap). Over the years I've sealed off most of the penetrations (worst were the drop ceiling over bathroom and soffits in kitchen), but we never had bad ice in the attic. A couple of years ago we had another foot or so of cells blown in -- had to dig around to find someone doing cells, since most are doing fiberglass. But I definitely wanted cells since there are a few spots I couldn't practically air-seal.
Never had a rodent (or bug) problem in the attic, just in the garage -- and I've been careful to be sure that the firewall between the two is well sealed with some metal flashing in critical spots.
I don't know why folks doing cellulose are so hard to find. I suppose in part it's because DIYers can do it, but to blow a medium-sized attic to a reasonable depth would take about 3 days using a rental blower.
Cellulose
is going to perform quite a bit better than FG in a loose fill application. I would consider going more than 1-1/2" on the foam, unless the installer is sure he can avoid any thin areas.
I love the idea of sealing the whole attic up first with the spray foam. Forgetting the air sealing seems to be a common mistake when adding insulation to attics.
R-value fiberglass 2.1 to 2.7 per inch. Cellulose is around 3 per inch.
Economic value: As far as I know the cost isn't too different.
Rodent control: supposedly cellulose will repel rodents. Although I have seen evidence of rodents in both cellulose and fiberglass blown-in attics
All-in-all I have always preferred the cellulose. Its easy to install, better R-value, and less "scratchy".
I have to disagree with the time taken to blow cellulose in a medium sized attic as being three days. This may be the case with box store rental units, but I recommend renting a unit from a tool rental place. The are much faster and most moderate attics can be easily blown in by two people in less than a day.
I have to disagree with the time taken to blow cellulose in a medium sized attic as being three days. This may be the case with box store rental units, but I recommend renting a unit from a tool rental place. The are much faster and most moderate attics can be easily blown in by two people in less than a day.
I doubt that you can find anything larger at a tool rental place in our neck of the woods. And, in any event, you can't DIY it nearly as quickly, cleanly, or efficiently as with a truck-mounted commercial unit.
Blowing equipment
Is there any difference in the blowing equipment between cellulose & fiberglass? Do they generally use the same machines?
Be extremely careful on the removal. Lots of really nasty (and potentially deadly) bugs can be caught from rodent doo doo.
I have blown a lot of attics DIY with the big box freebie machines and have never spent more than a half day give or take doing it. I did get a machine one time that I had to stop and unplug the hose a couple of times which was a pain but it was just a matter of getting used to the machine. I've also dense packed with freebie machines and not had problems. As long as you have a good machine guy and he and the guy blowing can communicate well, I never had major problems and can't fathom taking more than a day.