Pros and Cons of Insulation for Attics

Shutterstock/ Ungvar

What You Need to Know About Attic Insulation Types

Looking for the best attic insulation? That answer depends on your definition of “best.” The best performing way to insulate an attic doesn’t always align with the most cost-effective way.

First, let’s discuss the most common types of insulation available for attics: fiberglass batts, cellulose, loose-fill fiberglass and spray foam. More options exist, but these four materials make up the vast majority of installations. The term “R-value,” when used below, refers to the material’s ability to resist the transfer of heat and measurement looks at “per-inch” units. The higher the number, the better.

Interested in increasing the R-value in your attic? You can save energy with blown in insulation.

Fiberglass Batt Attic Insulation Pros and Cons

Fiberglass batts offer the easiest way to add attic insulation or  just about any area of your home. However, typically batts offer the worst insulation performance for any job. But grabbing a bundle at the store and rolling it out takes little effort, so people use them frequently.

The image below shows an atrocious installation found at a two-year-old home. The residence stood in an upscale neighborhood and yep, this passed the city inspection.

I won’t even discuss R-value, because fiberglass batts don’t belong in an attic.

Pros and Cons of Cellulose Attic Insulation

Cellulose — recycled, ground-up paper with boric acid added for insect control and fire resistance — offers a better attic insulation option than batts.  If you choose to install cellulose yourself, you can buy the insulation in bags from your local home-improvement store. If you buy enough, they’ll probably let you use an insulation blower for free. Don’t try to buy a single bag and spread it out by hand for spot-insulation: It’s way too densely packed.

The cellulose insulation method creates lots of dust, but it gets the job done. If you hire a pro, they use wet-spray cellulose, which adds a small amount of water to help control the dust and to slightly increase the insulation value per inch.

Cellulose provides an R-value of approximately 3.5 per inch. While it doesn’t actually create an air barrier, the dense material stops most air movement. That helps control frost in attics. Not completely, of course, but it does a pretty good job. The same cannot be said for fiberglass.

If you check with the Cellulose Insulation Manufacturers Association, they’ll assure you that cellulose is definitely your best choice for insulation.

Pros and Cons of Loose-Fill Fiberglass Attic Insulation

Loose-fill fiberglass seems to dominate attic insulation in new-construction homes and has an R-value of approximately 2.5 per inch. Like cellulose, you need a big machine to blow it in. You can’t simply buy it in bags and spread it around yourself.

Older fiberglass is itchy and bothers your skin and lungs way more than the newer stuff.

Pro tip: Don’t even think of doing any type of insulation work without wearing a respirator.

A widely publicized study conducted by Oak Ridge Laboratories in 1991 said that loose-fill fiberglass attic insulation lost a lot of its insulation value once temperatures dropped below 20 degrees, making loose-fill fiberglass an inferior product when compared to cellulose. I contacted Andre Omer Desjarlais at Oak Ridge Laboratories about this issue, and he said: “This was true 20 years ago but all fiberglass manufacturers have changed their products appreciably since then and this is simply no longer an issue.”

I also contacted several insulation manufacturers about this. They said the same thing and sent me some great information to back it up. Loose-fill fiberglass attic insulation still experiences convection, but not nearly as much as old fiberglass used to.

If you check with the North American Insulation Manufacturers Association, they’ll assure you that fiberglass or mineral wool is definitely your best choice for attic insulation.

Spray Foam Attic Insulation

Spray foam offers the best attic insulation performance available. Choose from two types: closed-cell and open-cell, aka 2-pound and ½-pound, respectively. They sport insulation values of approximately R-6.5 and R-3.6 per inch, respectively. When installed properly, both types of insulation fill all of the nooks and crannies in a space and make for a perfect air barrier. When air can’t move through it, you have zero heat transfer through convection.

With closed-cell foam, 2 inches of thickness also serves as a moisture barrier. Because of this and the higher R-value per inch, most foam insulation used in Minnesota is closed-cell. To tell the difference between the two, try poking it with your finger. You can easily poke a hole in open-cell foam, but not closed-cell foam. That stuff is way too hard. Here’s why spray foam can regulate your house temperature.

The big downside to either type of spray-foam insulation lies in cost. It’s expensive stuff, and it shouldn’t be installed by a DIYer. Of course, that’s not to say it can’t be done, it just shouldn’t be done. It requires expert use of a spray foam gun. Professionals already have a hard enough time getting it right.

The image below shows a botched spray-foam installation at the rim joist of a new-construction home that I inspected.

One drawback: Spray foam insulation requires the off-gassing of toxic poisons. I’m no expert on that matter, so I won’t discuss. Some people express concerns about it, so do your own research. After conducting my own research, I concluded that I was comfortable putting it in my own home.

About the Author:

Rueben Saltzman is the president and owner of Structure Tech Home Inspections