©2013 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
Unless the homeowner opts for spray foam , the insulation choice usually comes down to fiberglass vs. cellulose. So what are the advantages and disadvantages of each one? How are they similar and how are they different?
At first blush, itchy pink fiberglass and fluffy gray cellulose seems quite different. But the two types of insulation wouldn’t be competing for the same share of the housing insulation market if they weren’t in many ways alike.
Cost: First off, both cellulose and fiberglass are inexpensive. Among the wide range of available insulation materials — including XPS foam board , EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest., polyisocyanurate, rock wool , and spray foam — cellulose and fiberglass are, inch for inch and square foot for square foot, the least expensive.
Of course, prices can vary from contractor to contractor, and may vary if you have access to a special deal, but in general, fiberglass batts and cellulose are usually the cheapest insulation options.
Ease of Installation: Fiberglass has become the most popular insulation in the world because it is effective (if properly installed) and inexpensive. Contractors and do-it-yourself folks don’t need special training or equipment to install it. (However, inexperienced installers often do a sloppy job of installation, reducing the effectiveness of the insulation job.)
When installed in an attic, blown-in cellulose requires about 3 molecules more effort. The job requires an insulation blower  and 30 minutes of training from the guy at Lowe’s. (An important note: I am talking about blown-in cellulose here… not damp-spray, netted, or dense-packed cellulose.)
R-value: Fiberglass batts and cellulose deliver comparable R-value  (between 3.5 and 3.7 per inch). This can vary based on many factors, including settling, wind-washing or outside temperature, but in general the R-values of the two products are similar.
Air leakage: Both types of insulation help retain heat, but neither one can act as an air barrier . Both cellulose and fiberglass allow air to pass through and need to be paired with an air barrier. The effective R-value of fiberglass can be particularly affected by air flow.
Moisture: Neither insulation is a fan of moisture. Both cellulose and fiberglass can retain large amounts of moisture. Because of their high air permeability both can dry out very quickly.
Cellulose and fiberglass are fibrous insulation which can easily trap moisture. If paired with a vapor barrier  in a high moisture environment like your basement … it can be problematic.
Wind washing: Lastly, when blown onto the floor of an attic with vented soffits, both loose-fill cellulose and fiberglass are susceptible to wind.
A strong gust of wind will blow loose insulation all over the attic. With Maine’s stiff sea breezes, I’ve seen several houses whose insulation is blown entirely to one side. Loose-blown insulation requires properly sealed and blocked eaves to prevent wind washing.
Air leakage: It's true that neither insulation is an air barrier. Neither cellulose (even when dense-packed) nor fiberglass meet any technical standard for an air barrier. However, cellulose will slow air flow whereas fiberglass does not.
When dense packed into a wall cavity, cellulose prevents most air flow. Even loose-fill cellulose slows some air movement.
Flammability: Fiberglass and cellulose have different issues with fire and flame spread. Fiberglass is spun glass; it won’t burn at any normal temperature. Under direct flame, it will simply melt. However, most fiberglass batts are faced with kraft paper which most certainly will burn.
Cellulose is ground-up paper. Very early cellulose-style insulation was quite flammable. (I mean, c’mon — it’s paper.) During the '70s and '80s, cellulose couldn’t shake a bad reputation stemming from (possibly apocryphal) stories about insulation fires.
Modern cellulose is heavily treated (about 15% by volume) with boric acid, borax nitrate or ammonium sulfate. [Editor's note: see Daniel Lea's 3/8/2012 comment on this point below.] These chemicals aren’t harmful to people, but are very effective flame retardants and help reduce pest issues. Modern cellulose manufacture has sufficiently high production standards that product quality and flammability are no longer issues of concern.
Ease of installation: Anyone can insulate a wall with fiberglass batts. It’s just a matter of cutting around electrical outlets, slapping the batts into the wall cavities and stapling the facing to the studs. Unfortunately, a fast, sloppy installation usually results in voids or imperfections. Proper installation of fiberglass batts is slow, meticulous work (which is why most fiberglass batt installations are fast and sloppy.)
Fortunately, it's harder to do a sloppy job with cellulose, although installing cellulose in a wall requires special equipment like high-powered insulation blowers which are a sight more powerful than the Geo Metro versions you can rent at the Big Box store. Also, unless you like blowing out your finish drywall, more than a little experience is helpful.
Embodied energyEnergy that goes into making a product; includes energy required for growth, extraction, and transportation of the raw material as well as manufacture, packaging, and transportation of the finished product. Embodied energy is often used to measure ecological cost.: Embodied energy is the sum of energy required for a project or material. Fiberglass has a much higher embodied energy than cellulose insulation. Fiberglass is glass that is melted and spun into fibers like cotton candy. There are fiberglass brands which use recycled content but more often they use new raw materials.
Most cellulose brands use a high recycled content and the production process (shredding paper and adding fire retardant borates) uses much less energy.
Extreme cold: Last, the two types of insulation react very differently in extreme cold. During very cold weather — the type of weather sometimes seen in Minnesota or Maine — heat is quickly stripped from fiberglass insulation, and the R-value of fiberglass insulation drops. Cellulose doesn’t suffer as acutely from this problem.
So should you insulate with cellulose or fiberglass? Well, I've included a photo showing insulation being installed at my house. (Click Image 2 below to see the photo.)
We dense-packed the wall cavities with cellulose. I choose cellulose because of the better air sealing, and comparable R-value for the same price. Should you insulate with cellulose or fiberglass? It depends on the project at hand.