What is Fiberglass Attic Insulation?

If you have childhood memories of being in your attic and thinking the walls and roofing looked like they were covered in cotton candy, then you’re a part of the vast majority of Americans who grew up with fiberglass insulation. An attic mainstay for several decades now, fiberglass remains the go-to choice for an attic insulation installation for a whopping 85% of American homeowners.

What exactly is fiberglass insulation? What forms is it available in? And why has it become such a standout in the world of attic insulation amongst all the other insulation types? Read on for everything you need to know about the cotton candy dead ringer that has found a home in countless attics.

What is Fiberglass Made of?

Fiberglass is a series of immensely delicate fibers composed primarily of—get ready to thank Captain Obvious—glass (typically the translucent kind you see in windows and drinking glasses).

Here’s how fiberglass gets made:

  • Step One – The glass gets heated until molten.
  • Step Two – This melted glass is combined with other eco-friendly materials like sand and limestone and then woven into strands.
  • Step Three – The fiberglass strands are then fortified with different resins, which also allows them to be customized into various shapes.

How is Fiberglass Installed in Your Attic?

Speaking of fiberglass shapes, there are two ways for a professional installer (or an ambitious DIY-er) to hook your attic up with fiberglass insulation:

  • Batts – By far the most common form of fiberglass insulation, these are the large rolled-up sheets held together by either reflective aluminum foil or paper acting as an adhesive vapor barrier. Batts make for the easiest installation process, as one sheet not only covers a whole bunch of real estate but also is designed to be the same size as standard joists.

Yes, their size does make them a hassle for attics with multiple obstructions like pipes or HVAC units. But for new buildings or gut renovations that leave plenty of open space, batts are the optimal choice.

  • Loose-Fill – What if you want to reap all the benefits of fiberglass insulation but your attic is littered with obstructions or has awkward spacing between joists? That’s where loose-fill, blown-in fiberglass comes to the rescue.

Loose-fill fiberglass comes in chunks that can be purchased in bags and is applied by way of your installer using a large blowing machine to fill in the gaps of your attic space as needed. These machines make it easier for insulation contractors to work in attics that have a low ceiling or don’t offer much space for maneuvering around.

Loose-fill is also a sensible option for older homes with attics that have prior insulation in place.

Why is Fiberglass so Popular?

Like Kleenex, Band-Aids, and Google, fiberglass is such a popular choice with consumers that the word “fiberglass” itself has become virtually synonymous with “attic insulation.”
It’s easy to see why. Fiberglass presents homeowners with numerous benefits, such as:

  • Cheap and Easy – Fiberglass is a highly accessible commodity that can be found at just about any home improvement store. Whether you purchase it in batt or loose-fill form, fiberglass is extremely easy to carry and unpack. Best of all, it’s dirt cheap, which enables you to boost the insulation level in your attic without having to break the bank.

But don’t let the low price tag fool you: fiberglass is far from being a “you get what you pay for” product. Fiberglass insulation cost may be cheap, but you get the bang for your buck. Naturally adept at trapping pockets of air to keep your home warm in the Winter and cool in the Summer, fiberglass can hang with the best when it comes to thermal performance (more on this later).

  • Saves You Money Over Time – According to industry professionals, fiberglass conserves 12 times as much energy as it uses to prevent airflow. This unique capacity for storing energy can help you axe your utility bill by up to 40% annually! These energy savings make this type of insulation a no brainer. And as an additional perk, fiberglass also conserves nonrenewable fuel sources, making it among the eco-friendliest insulation materials on the market.
  • Insane Longevity – Fiberglass is an insulator you can set and forget. When installed in an attic that maintains ideal conditions, fiberglass will last for a full 100 years before it needs to be replaced.
  • No Shrinkage – Unlike some attic insulation material, fiberglass doesn’t shrink over time. When properly installed and kept dry, what you see on day one is what you’ll continue to get.
  • Fire-Resistant – Fiberglass is non-flammable, which gives you invaluable peace of mind knowing that you can use it to line your wooden joists without having to worry about it setting the wood on fire.
  • Moisture-Resistant – Fiberglass is naturally resistant to the spread of moisture, which by extension makes attics with fiberglass insulation much less vulnerable to the spread of mold and mildew. Thus, fiberglass is a particularly smart choice if you live in a region with a notably humid climate.
  • Acoustic Boosting – Fiberglass is a top performer when it comes to absorbing sound waves. With fiberglass insulation in place, outside noise will remain outside of your home. As an added bonus, fiberglass also reduces the transmission of inside noise. No matter how obnoxiously loud the rumblings from your HVAC unit may be, homes with fiberglass installed in the attic will enjoy peak acoustics.
  • Discourages Infestations – Fiberglass may look like cotton candy, but it’s completely unpalatable. This means that your attic isn’t likely to become the popular neighborhood spot for insects, rodents and other unwelcome instigators of infestation.

Is There a Downside to Fiberglass?

Even the best attic insulation products will have their share of drawbacks, and fiberglass is no exception. Some of the fiberglass disadvantages include:

  • It’s useless if it gets wet or even damp. If your fiberglass insulation does get wet, you should have it inspected and fully dried by a professional immediately. Thankfully, the fact that fiberglass neither holds nor retains water makes it fairly easy to dry out.
  • It’s a headache to customize fiberglass batts. But again, cutting batts down to a specific size is typically only relevant in older homes with an attic space that has multiple obstructions, awkward spacing or low clearance.
  • As is the case with the majority of insulation materials, it presents some safety hazards. The consequences of touching or inhaling fiberglass during the installation process range from an itchy rash on your skin to serious damage to your respiratory system.
  • Though fiberglass doesn’t shrink, its ultra-light nature makes it prone to irregular compression, settling and sagging, all of which will hurt its R-value (and jack up your monthly energy bill) over time.

Wait, what the heck is R-value?

In any discussion about insulation, the term you’re bound to keep hearing is “R-value.” An insulation material’s R-value is basically its effectiveness in resisting airflow. The higher the R-value, the better the material is at trapping air.

The R-value that you should try (or in some cases, might be required by state law) to maintain in your attic all depends on the climate in the region where you live. The US Department of Energy created this helpful guide for R-value recommendations by region. Generally speaking, ideal R-values are as follows:

  • If the climate in your region is so hot that playing a round of golf during the day could be life-threatening, then the R-value of your attic should fall somewhere near R25 and R30
  • If you live in a region that has virtually perfect weather year-round (looking at you, San Diego), keeping your attic’s R-value at about R30 should do the trick
  • If you live in a region where scraping ice off of your car’s windshield during winter mornings is as routine as a cup of coffee, your attic’s R-value should be at least R49, and perhaps as high as R60
  • In all other regions, the R-value in your attic should land somewhere between R38 and R49

What is Fiberglass’s R-value? And How Does it Compare to Other Insulation Material?

Not all fiberglass is created equal, as it can vary greatly in both density and thickness, the two key factors in determining R-value. The R-value of the cheapest quality fiberglass can check-in at as low as 2.0. For a slightly higher cost, however, specially designed medium and high-density fiberglass will have an R-value as high as 4.3.

Let’s take a look at how fiberglass stacks up against the competition in the R-value category:

  • Fiberglass: 2.2 – 4.3
  • Cellulose: 3.2 – 3.5
  • Mineral Wool: 3.7 – 4.2
  • Cotton: 3.4 – 3.7
  • Spray Foam: – 3.5 – 7.0

The Battle of Fiberglass vs Spray Foam

Chances are you just did a double-take when you noticed spray foam’s potential to reach an impressive 7.0 R-value. But before you go searching for spray foam installers on Yelp, it’s important to note that foam insulation, for all of its advantages, is expensive.

Additionally, it’s not only extremely complicated to install, but also potentially hazardous to your health if installed incorrectly. In fact, the risk and potential fallout of installing spray foam improperly are such that most home improvement and construction companies choose not to offer it.

Fiberglass doesn’t reach the same R-value heights as spray foam, but the money you save on both the material itself and the fast, simple installation process makes fiberglass the most cost-effective insulation option out there.

The Oak Ridge Myth

In the 1990s, a study was conducted at the Oak Ridge National Laboratory in Tennessee to test the thermal qualities of various insulation materials, including fiberglass. The results of the “Oak Ridge Study” revealed that the R-value of loose-fill fiberglass decreases significantly in the face of cold temperatures, especially in situations when temperatures dropped rapidly.

In the wake of the study, many people accepted the results as fact, and others still misinterpreted these results to mean that all fiberglass performs poorly in cold temperatures, regardless of design. And while these results won’t mean much to homeowners in San Diego, they have understandably caused homeowners who reside in frigid climates to balk at the idea of purchasing fiberglass.

Recent follow-up studies, however, have revealed that the results of the Oak Ridge Study are misleading. The diminishing returns of the loose-fill fiberglass used at Oak Ridge didn’t stem from the fiberglass itself, but rather the now outdated way in which the fiberglass was installed at the time. The process of blowing in loose-fill fiberglass has since been perfected, which means you can count on fiberglass to maintain its R-value no matter how cold it gets outside.

Count on Attic Construction to Install Fiberglass in Your Home

Ready to get started and insulate your attic with fiberglass? Attic Construction is ready to help!

Our expert team prides itself on offering outstanding customer service in answering any specific questions you may have about the pros and cons of fiberglass insulation, and then either installing it in your attic for you or walking you through the DIY route.


With its unrivaled low price, hassle-free installation requirements, dynamic thermal performance and unique ability to continue helping you slash your energy bill over time, fiberglass gives you the most bang for your buck of any insulation material.

At Attic Construction, we consider ourselves the fiberglass of home improvement services, offering you the best service at the lowest price. Give us a call for an estimate and to set up your next attic insulation installation today.


1) “Recommended Home Insulation R-Values” by the US Department of Energy

2) “Everything You Need to Know About Insulation’s R-Value” by editorial staff at Today’s Homeowner with Danny Lipford

3) “Does Fiberglass Insulation Really Lose R-Value” by Allison Bailes

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