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Fiberglass Is Still the Number One Insulation for Home Builders

The Home Innovation Research Labs (HIRL) does an annual survey of home builders to find out what they’re doing.  The results of their 2019 survey of the homes built in 2018 are out now, and they recently published an interesting article about the breakdown of different insulation materials used by home builders.  Before we talk about the implications, let’s see what they found.

Here’s the pie chart of different insulation materials builders are putting into single family homes.  (The SF in the title below stands for “single family.”)

As you can see, fiberglass is the dominant insulation material.  71% of all the insulation used by the 1,600 builders surveyed is fiberglass, 52% of it in the form of batts and 19% blown.  According to the article, fiberglass has held fairly steady at that level for the past few years.

Since this material makes up more than two thirds of the insulation market for new, single family homes, this is a good time to review the problems, myths, and best practices associated with fiberglass.

Problems with fiberglass insulation

There’s really only one problem with fiberglass:  poor installation.  OK, it also can be really itchy if you get it on your skin and it’s a lung irritant if you’re working with it and not wearing a mask or respirator.  So wear proper protective gear when you’re installing it or crawling around in a space where it’s exposed.

But the installation problem is well documented here and many other places.  Since 2006, there’s been an insulation installation grading protocol, with grade 1 being the best and grade 3 being the worst.  What I’d like to see HIRL add to their survey is how much of all that fiberglass installed each year is installed to grade 1, how much to grade 2, and how much to grade 3.  Most of it, however, never gets assigned a grade because that’s not required for the typical insulation inspection.

So what’s the effect of grade 3?  If you insulate a standard 2×4 wall with fiberglass, the average R-value drops about 12% when you install to grade 3 compared to installing it to grade 1.  Yeah, when you go from R-11.8 to R-10.3, you still have a much better wall than older homes with no insulation in the walls, which would be about R-3.7 due to the other materials in the wall.  And you won’t see much of a hit from that on your monthly bills.

But those little bits add up.  Grade 3 insulation installations can cost thousands of dollars in heating and cooling costs and put a lot of extra carbon into the atmosphere over the life of the home.  And for a lot of that insulation, you get one chance to do it right before the house gets remodeled or torn down.

Myths about fiberglass insulation

I love a good myth as much as anyone, and there are some doozies out there in the world of fiberglass.  Here’s my number one all-time favorite:

Myth:  Fiberglass-insulated homes are too leaky, resulting in high energy bills and comfort problems.

Whoa, there!  Yes, fiberglass is an air-permeable insulation and many houses where it’s been installed are definitely too leaky.  But this is like blaming your stale, moldy candy bar on the ingredients rather than the fact that the one you bought came wrapped in tissue paper.

A properly designed and installed building enclosure needs to have well thought-out control layers for heat, air, and moisture.  Fiberglass is a perfectly fine control layer for heat, but it’s not going to stop air movement.  That’s the job of the air barrier.  You need both.

Myth:  Fiberglass insulation causes cancer.

In the late 1980s, the National Toxicology Program and the state of California declared fiberglass insulation a possible carcinogen.  That was based on early studies that later were shown not to be accurate.  In 2011, both removed fiberglass insulation from their lists.  Read more about it in this National Insulation Association article.

Oh, by the way, fiberglass is second only to cork as a healthy insulation material.  See the table in Lloyd Alter’s article on the HIRL survey to see how it ranks.

Myth:  Blown fiberglass attic insulation loses half its R-value because of convective loops.

I heard this one for many years before I finally wrote about it.  Yes, there was a study from the early 1990s that showed blown fiberglass in attics could lose 50% of its R-value when the attic temperature dropped.  The problem was the way the insulation was made, and the fiberglass industry fixed the problem.  See my article for more details.

Myth:  Compression in fiberglass insulation is a bad thing.

This is another persistent one, and I have to admit that I fell for it for a while, too.  The truth is that compressed insulation does have a lower R-value than that same insulation expanded to its full thickness.  Here’s what I wrote in my article on this topic in 2017:

Compression isn’t the problem. Incompletely filled cavities are a problem. Gaps are a problem. But you can compress fiberglass insulation as much as you want. The North American Insulation Manufacturers Association (NAIMA) has a little two-page document about compressing fiberglass insulation (pdf). Here’s what they say:

“When you compress fiber glass batt insulation, the R-value per inch goes up, but the overall R-value goes down because you have less inches or thickness of insulation.”

As long as you’re filling the cavity and getting the R-value you want, compression doesn’t matter.

Best practices for fiberglass insulation

If you have any say in the design or installation of the insulation in your home, here’s a bit of guidance for you.  I’m focusing on fiberglass here but the same things apply to most of the alternatives, too.  I’ll break it down by what kind of assembly is being insulated, starting at the bottom.


  1. Don’t use fiberglass batts to insulate most floors.  It’s practically impossible to get a grade 1 installation with them.  If the floor joists are open to below (as over a crawl space), move the building enclosure to the walls and ground/slab by encapsulating the crawl space or use spray foam, which won’t fall down.
  2. If you’re insulating a floor over a garage or over outdoor air and the cavities will be completely closed, fill the cavities with blown fiberglass.
  3. If there are relatively few obstructions in the cavities and the joists are dimensional lumber rather than I-joists or open-web trusses, batts can work.  Again, it’s best to fill the whole cavity.
  4. If you do use batts to insulate floors that are open on the bottom, make sure they are in contact with the subfloor at the top of the cavity and as secure as possible.  You’ll also need to check them regularly because of this thing called gravity.


  1. Both batts and blown can work fine if you fill the cavity completely.
  2. With batts, cut around junction boxes and put the cut-out piece behind the box.
  3. Split batts around wires.
  4. Cut batts around pipes, blocking, and other obstructions.
  5. Use unfaced batts whenever possible.  They’re easier to install properly as well as easier to inspect.
  6. Put an air barrier on the attic-side of kneewalls.  (Or eliminate kneewalls by encapsulating the attic.)


  1. Blown insulation is better for filling the joist cavities.
  2. When installing batts, follow the guidance above for walls when it comes to fitting the batts in the joist cavities.
  3. Choose batts that are the same thickness as the depth of the joist cavities.  To get more R-value, add another layer of batts, running them perpendicular to the joists, or blow insulation on top.  (There’s a reason most attics have all blown insulation, not batts or a combination of batts and blown.)
  4. Make sure the blown insulation reaches the minimum thickness necessary for the design R-value.  Some contractors like to deliver average thickness, but that can result in significant under-performance of your insulation, as I described way back in 2010 when I wrote, Flat or Lumpy – How Would You Like Your Insulation?

The big takeaway

I’ve written many articles about insulation, a lot of them about fiberglass or applicable to fiberglass.  The most important thing I want you to take away from this article today is that you need to understand fiberglass — or any insulation material, really — to get the most out of it.  As I wrote above, the main problem is poor installation.  Learn the truth behind the myths so you can dismiss them.  And then follow the best practices I outlined.

Fiberglass is a perfectly fine insulation material with a bad rap because of all the sins committed — or said to be committed — in its name.  Learn the truth, do it right, and you’ll be healthy, wealthy, and comfortable…or at least a bit closer!


Related Articles

How to Grade the Installation Quality of Insulation

Flat or Lumpy – How Would You Like Your Insulation?

2 Ways to Get the Best Insulation in Your Home

4 Types of R-Value

The Layers and Pathways of Heat Flow in Buildings


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Earlier this year, one last piece of light density residential fiberglass insulation bearing formaldehyde-based binder rolled off a production line somewhere in the Midwest United States. Then the line stopped, and a new binder came on line.

As of October 2015, every fiberglass insulation company in the United States and Canada has phased out the use of formaldehyde-based binders in lightweight residential products.

Formaldehyde is a human toxicant with a long history of use in residential insulation. [2] Phenol formaldehyde remained the industry standard binder for the next seven decades.

In 2002, Johns Manville moved first, shifting to an acrylic binder in its lightweight fiberglass insulation. The Healthy Building Network (HBN) used the Johns Manville example (and the introduction of Bonded Logic's formaldehyde free cotton insulation batts) to support a credit in the Green Guide for Health Care (GGHC) in 2003. GGHC rewarded the use of insulation products without formaldehyde.

Other manufacturers resisted changing. They claimed that phenol-based formaldehyde resin released very little formaldehyde and that the interior walls protected occupants from emissions. They pointed to indoor air quality certifications to prove the products' safety.

HBN Policy Director Tom Lent debunked this myth in a January 2009 memo. Testing procedures, like the California 01350 protocol used by leading indoor air quality certifiers, were "not particularly protective for long-term exposure to formaldehyde emissions," he noted. [3] Tom pointed to evidence from the industry's own 01350-based testing that fiberglass insulation using phenol formaldehyde emits significant levels of formaldehyde even through wall assemblies for at least several months after it is installed and would never meet health protective standards in homes.

"Alternative insulations without formaldehyde-based binders that pass the 01350 test are available now and even more competitive products could be available rapidly if encouraged with a clear market signal," he wrote.

The signal from the green building community grew brighter. The Living Building Challenge's Red List, introduced in 2007, banned the use of products with added formaldehyde, and was gaining traction. Our Pharos Project database launched in 2009, and disclosed the binders used in most insulation on the market. A month later, Perkins + Will, a national leader in green building design, introduced its own precautionary list, including formaldehyde.

The market received the signals and changed. In late 2008, Knauf Insulation released its EcoBatt fiberglass insulation. [6]

Guardian Building Products was the industry's last holdout. It continued to use formaldehyde-based binders until Knauf Insulation bought the company in 2014. Knauf closed a couple of Guardian plants, and converted the remaining two to formaldehyde-free technology. [7]

HBN examined formaldehyde pollution data for thirty residential fiberglass insulation factories in the US and Canada over the past ten years. The reports confirm the reformulation. As the graph below reveals, these factories released nearly 600,000 pounds of formaldehyde in 2005; by 2014, these releases dropped by 90%, to about 60,000 pounds. With the conversion of the former Guardian plants to formaldehyde-free binders, any releases going forward will come from the production of some higher density batts, which are mainly sold on the commercial/industrial market.

This means real relief for human health, from factories to homes across North America. Workers and people living near the plants will no longer be exposed to formaldehyde air emissions. And in people's homes, residents will no longer be exposed to formaldehyde emissions from new insulation. [8]

A well-informed marketplace, not federal or state environmental regulation, drove this change. From HBN's first marketplace victory in 2003 (when we campaigned to end the use of arsenic preservatives to treat wood intended for most residential uses) to this year's demise of phthalate plasticizers in vinyl flooring, we've seen building material composition change when consumers become aware of health hazards and manufacturers respond with innovative chemistries. [9]

In these cases, and many more in the works, informed consumers and visionary manufacturers are leading where regulators have been slow to act. Transparency and innovation are working.

Research for this article was supported by the BlueGreen Alliance Foundation. The BlueGreen Alliance Foundation is working to identify insulation products with the highest amounts of recycled content, and lowest amounts of toxic content. Also this year, HBN has researched insulation as part of the new Quartz Project--see its Common Product Profile for batt insulation--and as part of the Optimizing Recycling collaboration in our report Post-Consumer Cullet in California.


[1] Formaldehyde is a carcinogen and causes asthma and developmental harm. These resins impact indoor air quality, which led green building rating systems to reward the avoidance of formaldehyde, through red lists and rating systems. See Pharos Project,

[50-00-0] FORMALDEHYDE, last updated October 1, 2015,

[2] "Bakelite [phenol formaldehyde] has been found very satisfactory for use in the present invention," reads Owens Corning's patent filed in 1938. See Owens Corning Fiberglass Corp, Patent US2252157A: Insulating bat, July 26, 1938,

[3] At the time, the 01350 standard did not reflect the current science on the toxicity of formaldehyde. The 01350 standard also was designed for well ventilated offices and schools and did not yet apply to residential use of products where lower ventilation rates lead to higher, more hazardous concentrations of emitted formaldehyde. The standard was revised in 2010 to reflect a lower formaldehyde threshold and a procedure was added to address residential uses. See Tom Lent, "RE: Formaldehyde emissions from fiberglass insulation with phenol formaldehyde binder," January 16, 2009,

[4] Healthy Building Network, Product Comparison: Insulation: No Added Formaldehyde Batt Insulation, May 2008,

[5] CertainTeed, "CertainTeed Brings New Sustainable Insulation™ To Market From California Manufacturing Facility," press release, April 1, 2010,

[6] Owens Corning, "Owens Corning Launches EcoTouch(TM) PINK(TM) FIBERGLAS(R) Insulation With PureFiber(TM) Technology," Yahoo News, January 12, 2011,

[7] Joey Viselli, "Knauf Insulation Completes Acquisition of Guardian Insulation" (Knauf Insulation press release), November 8, 2014,

[8] Unfortunately, a complete understanding of the net benefits of the replacement binders is elusive. Fiberglass manufacturers still have not fully disclosed these products' compositions. From what we know of the replacement binders, human health concerns do not appear to be as significant as those posed by formaldehyde. HBN researchers examined patents and other technical documents describing these resins and methods for their manufacture. They found that the binders now used most commonly in the industry are thermoset polymer resins consisting of a polyol-polyacrylic acid copolymer, a sodium hypophosphite catalyst, and a silane coupling agent. Some substances potentially used in these formulations pose human health hazards in their unreacted forms. See Quartz Project, "Common Products Database: batt," 2015,; CertainTeed Insulation, Sustainable Insulation Unfaced and Kraft Faced Batts (Environmental Product Declaration), 2013,; Knauf Insulation, EcoBatt Unfaced Insulation (Environmental Product Declaration), 2013,; Owens Corning, EcoTouch Foil Faced Insulation (Environmental Product Declaration), 2014,; and Johns Manville, Patent US8865816 B2: Formaldehyde-free binder compositions containing metal-ion crosslinkers and products made there from, 2014,

[9] Bill Walsh, "Removing Arsenic from Building Materials: A Success Story," Healthy Building News, March 12, 2013; Jim Vallette, "The End is Near For Phthalate Plasticizers," Healthy Building News, July 9, 2015,

Fiberglass Is Still the Number One Insulation for Home Builders

Residential Fiberglass Insulation Transformed: Formaldehyde is No More


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