View Full Version : Is blown in fiberglass insulation safe?

Walter Denton
04-27-2011, 07:53 AM
We have 6 inches of cellulose insulation in our attic. After air and duct sealing we are going to add another 12 inches or more of additional insulation.

Been getting quotes on both blown-in fiberglass and cellulose. Is fiberglass OK? Been reading that it is about 20 to 30 percent spun glass. What is the remaining 70 to 80 percent. Air? or other stuff?

If this was your home.......what would you do?


04-27-2011, 10:56 AM
Hello Walter.

Fiberglass is almost as dangerous as asbestos if it becomes airborne. Even if it works the best.

I would think that the cellulose would be safer even if the R factor is less.

You have to consider whatever you use may come threw cooks and crannies, like electrical outlets and the such.

I would use cellulose insulation, and maybe put more to reach the R factor that you want.

That is just what I would do in my house, I am no expert. But I do understand about Lung Caner.

Welcome to Terry Love's Forums. They are Great, Thanks to Terry and the good people here.

Have a Great Day.


04-27-2011, 12:15 PM
Unless it's a new-school ultra-fine blowing wool like JM Spider or Certainteed Optima blown at the right density it's real-world performance will be underwhelming, particularly in an open attic blow. Low density fiberglass loses SIGNIFICANT R value in a horizontal application where the cold side is up (like your attic in winter), since air can convect relatively freely. When it's way below freezing in the attic (say 0F) it can be delivering only half or 2/3 of it's rated R for the installed depth. Cellulose (even at ~1.5lbs nominal open-blow density) is far more air-retardent, and the R value remains stable (it actually rises very slightly, but don't count on that in your modeling.)

Under a hot 125F roof deck in the blazing sun again, cellulose will outperform most fiberglass. Fiberglass is semi-translucent to infra-red, and the aborption into the fiberglass layer makes the HOTTEST part of the fiberglass an inch or so into the fiberglass, where it rises well above the attic air temp. So in effect, you're insulating against a higher temp, with an inch or two LESS insulation. Celluose absorbs all of the radiated heat in the first 1/8" and re-radiates most of it back. The hottest part of the cellulose is at the top surface, where it gives up most of it's heat to the attic air, and runs at only a modest elevation above attic temp.

The only real advantage fiberglass has over cellulose in an attic application is R/lb. Cellulose is inherently denser, and if you have skinny joists or the insulation is being supported by extra thin sheet-rock with joists on 24" centers or something the additional ~1.2-1.8lbs per square foot of that 12" cellulose (over 2lbs/ft with your existing cellulose counted) vs. roughly half that for fiberglass. If loading isn't an issue, go with cellulose, and insist on "stabilized formula" if it's to blown initally to 12", or "borate only" dry blow goods with sufficient excess to stabilize at 12" over the next decade's worth of settling. The manufacturers spec out what that is based on the blower settings and the product- typically for 12" of settled depth you'll need 13.5" on day 1. Fiberglass installation has a similar factor, but it's easier to screw up and over-fluff fiberglass (to ill effect) than cellulose.

If you go with fiberglass, going higher-density (1.8lbs/cubic foot) new-school goods with a "blown in blanket" mesh on top pretty much eliminates the fluffing, sagging, and convective loss issues, as well as the IR-translucency problem. But BIB is a more expensive way to go. It's usually cheaper to just heap on more cellulose and forget about it if you have the space. The tricky part is getting the R-value up at the soffits where you might have to thin it out to maintain attic ventilation. It's often worth the money to use closed cell spray foam (R6/inch) or stacked cut'n'cobbled rigid XPS (R5/inch) sealed with 1-part foam on top of the studawall plates, and to form the chutes between the rafters for maintaining ventilation. If 18" thins out to 7" at the edges, the thin wedge of insulation at the edge starts to dominate the heat transfer. An 18" of settled cellulose is good for about R65, but 7" (as with 2x8 joists) would only hit ~R25, and every square foot at the edges would pass ~2.5x as much heat as a square foot of area out in the center. It's enough to show very distinctly in IR imaging. But 7" of 2lb closed cell foam would be ~R42-R45, which isn't nearly as large a differential.

04-27-2011, 12:32 PM
Hello all.

That was very nice Dana. That will give Walter something to think about.

You gave some very good information. Thank You. Keep up the good work.

Enjoy your day.


Walter Denton
04-27-2011, 10:18 PM
Many thanks for the info. I am leaning 90% for cellulose. Read some MSDS sheets on loose fill fiberglass and it gives me pause for thought.

Had not considered the weight of adding cellulose on the sheet rock and having it bow out. Will take this into consideration.

04-28-2011, 07:54 AM
In an open attic with rafters & joist it's pretty easy to improve the loading capacity of thin joists by creating a truss structure with either plywood or light timber webbing and plywood or metal gussets at the joints. Beefing up the capacity of gypsum is tougher unless you can double-layer it, re-finishing the ceilings of the rooms below.

What is the depth/span/species/spacing of the joists, and how thick is the ceiling gypsum? With 16" o.c. joists, 5/8" gypsum can handle the weight of 18" of celluose, but at 24" spacing or half-inch gypsum hold the line at 12" depth or provide other support. The depth/span/species/spacing determines the static loading limits of the joists.

A lightweight floor above the joist with 1/4-3/8" OSB to handle the load of the portion of the insulation above the joist-tops can also be detailed to serve as a primary air-barrier to reduce overall air infiltration levels in the house. Otherwise, prior to installing the additional insulation, detailing the ceiling gypsum as an air barrier is a valuable first step. Most ceilings leak air pretty badly unless detaied during construction, due to myriad electrical & plumbing & flue penetrations, etc. (BTW: On any flue penetrations buy some R11 or higher rock wool batts and wrap the flues with it to seperate the flue from the cellulose/fiberglass in an insulating but code-compliant manner.) If air-sealing as a DIY project, use large window fans to pressurize/depressurize the room(s) below to find leakage at the ceiling. Sometimes partition-walls sans-top plates, balloon framing, or plumbing & electrical chases form air by passes from even lower levels. Air sealing is by far the most cost-effective envelope efficiency measure to take in most homes. Some insulation contractors offer whole house air sealing as a service, and provide before/after calibrated blower door testing numbers (usually denominated in air-changes-per-hour at 50 pascals pressure, or ACH/50) to prove they met the reductions contracted for. If your inintial ACH/50 number is over 5 it's usually pretty easy to cut that by at least 30-40%. Under 2 can be tough to knock much off as a retrofit, but building new to 1.5 or less is fairly straightforward if the primary air barrier is defined in the plan, and the contractors follow the necessesary detailing. Under ~1 ACH/50 mechanical ventilation is usually necessary to keep humidity levels down and indoor air quality up. (Under 1 is nearly impossible to achieve cost-effectively in retrofits.)

Other pebbles on the cellulose side of the cellulose^fiberglass balancing scale is that rodents will nest in fiberglass, but not-so-much in cellulose. The borate fire retardents used in cellulose while not very toxic to mammals, is an eye irritant to us warm-blooded critters. Borates also kill the gut flora of ants/wasps/termites and other wood boring insects necessary for processing the wood fiber into usable sugars, eventually killing the host insect via starvation. It'll will often also kill the nest, since weakened not-yet-dead wood boring colony insects are usually killed an eaten by their nest-mates, further distributing the borate within the nests.

04-29-2011, 06:54 AM
If your attic has blown fiberglass now, adding cellulous will pack it down to nothing. But you dont...so the choice is clear.

Opt for cellulous and add much more than just 12”. It will settle a lot and why not add extra now so you never have to worry about the R value level again.

04-29-2011, 08:00 AM
If your attic has blown fiberglass now, adding cellulous will pack it down to nothing. But you dont...so the choice is clear.

Opt for cellulous and add much more than just 12”. It will settle a lot and why not add extra now so you never have to worry about the R value level again.

Adding ~1.5ls per square foot does NOT pack fiberglass at any legitimate density by more than ~10% (6" might drop to 5.5", that's it.) The R value of the compressed fiberglass per inch would go up slightly, but the total R of that now-thinner layer would be slighly lower. But it's only the total depth of new + old that is relevant.

Blown fiber manufacturers publish specs as to how deep the initial blown depth & density has to be in order achieve a particular settled depth & R value, eg:


http://www.greenfiber.com/images/technicaldocuments/10122009105621PMPM-6.3-197%20Rev%20A%20INS760LDU%20Attic%20Card.pdf (http://www.greenfiber.com/images/technicaldocuments/10122009105621PMPM-6.3-197%20Rev%20A%20INS760LDU%20Attic%20Card.pdf)

For a foot of settled depth for cellulose intitial blows are typically 13-13.5".

In practice the intial blow on cellulose goods will usually be installed slightly higher density than the min-spec, as will the settled R, unless the installer has intentionally "fluffed" it- bumped the air volume setting above the manufacturer's recommendation (rarer these days than 15 years ago, but I suppose it can still happen). To verify it competent installers will usually count the bags and do the math on the weight, not just measure the initial blown depth. If it takes 10% more material to achieve the specified depth than the manufacturer's specs indicated it's still cheap insurance against customer dissatisfaction. The extra cost to the installer for10% more material is mostly "in the noise" of the setup & break-down labor. Most will staple up cardboard depth gauges at various places within the attic and blow to the manufacturers prescribed initital depth raking it smooth, only counting the bags later as a sanity-check. You can of course set up your own depth marks if you like.

Total settling of properly installed cellulose is less than 15% in 25 years (at which point even dry-blown is completely stable), and the reduction in total R value is less than 10%. If "stabilized" wet-spray cellulose is used, the moisture activated adhesives in the mix maintains the intial blown depth over time (and keeps wind in the soffits from moving it around in a storm, which can happen in highly ventilated very windy locations.) Whether dry blown or wet-sprayed, insist on "borate only " or "sulfate free" goods, since aluminum sulfate fire retardents can corrode metals if it ever gets wet (it's banned for this application in some countries, but not in the US, and some manufacturers still use it.) All stabilized or wet-sprayed goods are sulfate-free, and can be dry-blown, if you like. Better installers would never touch the sulfated goods, but much of the stuff sold in box stores for DIYers contains sulfates. It's not a disaster if you have some in your house (it's not worth paying to remove & re-install), but there's not even a 2% upcharge for going with borate-only material.

04-29-2011, 08:22 AM
Adding ~1.5ls per square foot does NOT pack fiberglass at any legitimate density by more than ~10% (6" might drop to 5.5", that's it.) The R value of the compressed fiberglass per inch would go up slightly, but the total R of that now-thinner layer would be slighly lower. But it's only the total depth of new + old that is relevant.

While great at pointing out specs and recomendations...what I stated is fact. I was warned about this verbally, I have done it, and watched first hand what occurs. (not in my own home)

Adding 12" of cellulose on top of blown fiberglass will crush fiberglass to less than half. The 6" in this case would become less than 3" when the job is complete and in a few years assume less than that.

In my own home, I have added 18+ of fiberglass blow in over 12" of exsisting. It has compacted over just 2 years to 18" on average.

Real world doesnt always follow whats written on paper.

04-29-2011, 09:13 AM
If 6" of fiberglass was crushed to 3" with a foot of cellulose the original fiberglass installation was fluffed! Unlike fluffed cellulose, fluffed fiberglass will settle & stabilize at a density FAR below spec.

I'm a bit concerned about your 18+12= 30" of fiberglass settling to 18" in just 2 years too- something just isn't right. A wet-sprayed new-school fiberglass wool like Spider is dimensionally stable for the same reasons wet-spray cellulose is. Old-school low density fiberglass blowing wools will settle some, but it should be nothing like losing 40% of loft in 2 years (even when applied over a 12" fluff-job that gets crushed to 6".) The loss of R at the temperature extremes of fluffed fiberglass is huge- far more than with standard density batts (which are pretty lossy.)

But the total detph of fg + celluose determines the total R. It's more than OK to "crush" fluffed fiberglass to a density where it actually has SOME air-retardency, and the compressed K-value will be similar to that of the cellulose at that point.

04-29-2011, 10:51 AM
Since it's only been 2 years, it might be worth getting an ajustment from the installation company that installed the 18" overblow.

More on fluffing & coverage chart disparities: http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/00/001112.html

The installers I've dealt with locally set up the blower per the manufacturer's spec, then blow to the recommended initial depth measured with depth gauges, and yes, it always ends up being more than coverage charts indicate. But IIRC it's not 25% more, as the author of that article was indicating. I'm not sure if that's because the charts have gotten better or the blower settings have been tweaked (or both). But it's good to both measure initial depth and count bags (=weight) to calculate initial density, which will be what determines the ultimate settled depth. With celluose there will be some regional variation based on climate (the seasonal cycling of humidity in the fiber causes creepage), but it slows dramatically with density.

05-03-2011, 01:28 PM
Since it's only been 2 years, it might be worth getting an ajustment from the installation company that installed the 18" overblow.

This is my own home and DIY. Thats why I know exactly what the numbers were and are now. It is what it is.