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Building Science

Does Fiberglass Attic Insulation Really Lose R-Value?

An Oak Ridge study from the early 1990s has led to a lot of confusion

Loose-fill fiberglass attic insulation was shown to lose up to half its R-value in a study from the early 1990s. [Photo credit: Energy Vanguard]

When you get into the world of building science, it’s inevitable that you’ll hear about the Oak Ridge study proving that fiberglass insulation loses about half its R-value. Maybe, though, you’re a homeowner who’s been told about this problem by an insulation contractor warning you not to put fiberglass insulation in your home. Or perhaps this article you’re reading now is the first time you’ve heard of the issue.

However you came to it, though, it’s important to understand what that Oak Ridge study showed and how much of what I said above is actually true.

The Oak Ridge study

First, let’s look at the actual research paper on this study (something that many of the people who cite it haven’t done themselves).  Titled Thermal Performance of Fiberglass and Cellulose Attic Insulations, the paper describes the research done by Kenneth E. Wilkes and Phillip W. Childs at Oak Ridge National Laboratory (ORNL) in the early 1990s.  (Download their paper along with the technical bulletins from Owens Corning and Johns Manville described below.)

They set up an attic test module that simulated temperature differences across an insulated attic floor.  You can read all the details in the paper — the sketch from their paper is shown below — but basically they put a whole roof and attic assembly into big chamber and measured the R-values of three insulation types:

  • Loose-fill fiberglass
  • Fiberglass batts
  • Loose-fill cellulose
A sketch of the test chamber used to study attic insulation in the Oak Ridge study (Image credit: Oak Ridge National Laboratory)
A sketch of the test chamber used to study attic insulation in the Oak Ridge study (Image credit: Oak Ridge National Laboratory)

What they found is that the fiberglass batts and loose-fill cellulose performed as expected at the whole range of temperature differences.  The loose-fill fiberglass, however, showed a significant reduction in R-value as the attic got colder and the temperature difference got larger.

In fact, the loose-fill fiberglass lost 35% to 50% of its resistance to heat flow at temperature differences of 70°F to 76°F.  The loss of R-value started at a temperature difference of about 32 F°.  The temperature below the ceiling drywall was held at 70°F, so the R-value started dropping when the attic temperature was reduced to 38°F and lost 35-50% when it got to 0°F and below.

In looking at the data, the researchers saw a pattern that led them to suspect convection within the insulation as the culprit.  They did some calculations and further experimentation and concluded that was indeed the case.  The further experimentation they did was to put a covering layer over the top of the loose-fill fiberglass.  The two they tried were (1) a polyethylene film and fiberglass blanket combination and (2) R-19 fiberglass batts.  Both eliminated the convection and the reduction in R-value.

If that were the end of the story, the lesson would be to avoid loose-fill fiberglass for attic insulation or use it with a covering layer.  But that’s not the end of the story.

Questions raised by the study

If you read the paper and think about what they did and what they found, a couple questions might occur to you.

  1. Why would loose-fill fiberglass and fiberglass batts behave differently in an attic?  They’re made with the same material and were of similar density in the Oak Ridge study.
  2. Is fiberglass made and installed now the same as it was back in the early 1990s when they did this research?

The answers are related, so let’s see what two fiberglass manufacturers have to say.

Not all fiberglass is the same

The Oak Ridge paper doesn’t say what brand of loose-fill fiberglass insulation they used, but at least two fiberglass insulation manufacturers have written technical bulletins about their product and shown data about measured R-values under conditions similar to those used in the Oak Ridge study.

Density of fibrous insulation materials is certainly an important factor.  But as I wrote in the last section, the fiberglass batts and loose-fill insulation used in the Oak Ridge study were of similar density.  The batts were 0.46 to 0.48 pounds per cubic foot (pcf) and the loose-fill ranged from 0.40 to 0.56 pcf.  So density doesn’t explain the discrepancy.

What does explain it, according to Owens Corning and Johns Manville, is chunk size.  (The industry calls them nodules or tufts, not chunks, I guess because they don’t want the image of installers blowing chunks in all those attics.)  Fiberglass batt insulation is one large chunk with a lot of glass fibers bonded together.  In the early ’90s, Owens Corning loose-fill fiberglass was made by taking their fiberglass blanket insulation and cutting it into little cubes.  This information is given in their technical bulletin titled Fiber Glass Loosefill Attic Insulation Performance in Cold Climate Conditions, which you can download from the link at the bottom of this article.

Johns Manville has a similar technical bulletin (IST09-005) titled Convection in Fibrous Attic Insulation, also included in the download link below.  They don’t say how they were making loose-fill fiberglass in the early ’90s, but they do say they used the research results to “establish design specifications for all of Johns Manville’s loose-fill fiber glass attic insulations to improve winter thermal performance.”  That led JM to “maintaining an appropriate nodule or tuft size, which decreased the permeability of the installed insulation.”

The issue with chunk size was explained well in the OC bulletin:  “The bonded cubes did not nest well, leaving voids of relatively large air spaces and allowing R-value depleting convection to occur.”  That’s why the older loose-fill insulation had a problem with convective loops.  And it’s why the modern loose-fill fiberglass product doesn’t.

Fiberglass industry research on R-value

After changing their product, Owens Corning has measured the R-value versus attic temperature.  The graph below shows the results.  The dashed lines are the claimed R-values for four different thicknesses, the solid lines are the measured R-values in their study.

Research data from Owens Corning showing R-value versus attic temperature (Image credit: Owens Corning)
Research data from Owens Corning showing R-value versus attic temperature (Image credit: Owens Corning)

As you can see, the measured R-value is above the labeled R-value for each thickness and at temperatures all the way down to -40°F.  Also, note that the R-value increases as the temperature drops until you get below 0°F.  For the R-30 insulation, the R-value keeps rising all the way down to -20°F.

Johns Manville has done similar research with their loose-fill fiberglass attic insulation.  The two graphs below show their lower and higher density products.  They present the R-value results in relation to 100% of the labeled R-value rather than just R-value, but it’s essentially the same graph as the one above.

Research data from Johns Manville showing R-value versus attic temperature (Image credit: Johns Manville)
Research data from Johns Manville showing R-value versus attic temperature (Image credit: Johns Manville)

For both densities, the R-value starts at 100% with an attic temperature in the low 40s Fahrenheit and rises as the temperature drops.  The difference is that the low-density R-value hits a peak of nearly 125% at a temperature of about -15°F and then begins to drop, hitting about 107% at -40°F, whereas JM’s high-density (0.9 pcf) loose-fill fiberglass keeps rising all the way down to -40°F.

Where does loose-fill fiberglass attic insulation stand now?

To summarize, researchers at Oak Ridge National Lab found that loose-fill fiberglass insulation in the early 1990s had a problem.  As the attic temperature dropped, so did the R-value.  It happened only with the loose-fill fiberglass insulation they tested, though.  Fiberglass batts and cellulose didn’t show that problem.  As a result, fiberglass insulation manufacturers took a good look at their product and found that by using unbonded material in smaller chunks, the problem went away.

Sometimes people (usually those who sell other types of insulation) will refer to the Oak Ridge study as proof that fiberglass doesn’t work at all, ever, in any circumstances.  That has always been an exaggeration because the flaw was found only in loose-fill fiberglass used in horizontal installations on an attic floor and the study looked only at cold attics, not hot attics.

Now the manufacturers say they have eliminated the problem altogether by improving their product.  You can choose not to believe them, of course.  They do have a vested interest, after all.  But I believe their claims of having changed the product and that their research proves they’ve gotten rid of the problem.  If I find some independent research corroborating the OC and JM data, I’ll be sure to share it here.

I’ve taken heat for supporting rigid foam and spray foam industries and I’ve taken heat for attacking fiberglass batt installations (even getting a letter from one manufacturer’s lawyer).  I’m sure I’ll get some flack now for supporting fiberglass now, but it’s important that we understand the facts.  A lot of what people know about the Oak Ridge study is something they heard from someone who heard it from someone else who heard it from their boss who talked to someone who learned about this at a conference in 1994.

I’ll close with the immortal words of the late US Senator Daniel Patrick Moynihan:  “Everyone is entitled to his own opinion, but not to his own facts.”

Click here to download the ORNL paper and OC and JM technical bulletins (pdf).


Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


  1. charlie_sullivan | | #1

    Thanks. I knew that that the conclusion from that study was narrower than they way its often quoted, but you've made me realize that I too have been overstating it.

    Worst example: I once heard a contractor claim that, at cold temperatures, a fiberglass bat in a wall performs worse than an empty cavity.

    1. GBA Editor
      Allison A. Bailes III, PhD | | #7

      Charlie, I've heard a lot of exaggerated claims based on this study but this is the first time I've heard the one about an empty cavity being better than one insulated with a fiberglass batt. Wow!

  2. JIM BAERG | | #2

    Thank you for the much needed clarification. The local insulators I use will appreciate not using cellulose in lids. The conclusion seems to be that air currents WITHIN the FG insulation zone are much better controlled with the reformulated product. Those currents being primarily driven by a temperature differential. I'm still wondering about the comparative performance of FG and cellulose when larger pressure drives are present. This would be when wind or stack effect create air currents through gaps in the ceiling plane. I just did an audit yesterday on a new house with R-60 FG in the lid. The IR scan while depressurizing the house showed lots of air coming out the recessed cans and down into the wall cavities through the top plates. I've often specified R-30 cellulose top dressing over existing FG in existing attics when it is too difficult to completely air seal the ceiling. I wonder if this technique is still justified.

    1. GBA Editor
      Allison A. Bailes III, PhD | | #8

      Jim, I think cellulose is better than fiberglass when there's air leakage because it's more dense. Bruce Harley published an article a decade or so ago about air leakage in cellulose-insulated homes being lower than in fiberglass-insulated homes. Owens Corning, however, says their product beats cellulose in walls when installed at 2.5 pounds per cubic foot. I haven't seen much research on what happens to insulation performance when air moves through it. Building Science Corporation's Thermal Metric Project was going to tackle that but sadly, the project ended before they got there.

      I think your recommendation of R-30 cellulose over existing fiberglass should help the performance.

    2. user-689847 | | #9

      Excellent points Jim. How should this information affect our treatment of homes with existing blown FG insulation installed in the 80's and 90's? And is there a difference in performance between contemporary blown fiberglass or cellulose, taking into account initial cost and changes in thermal resistance over the long term.

  3. yamaguchimartin | | #3

    Interesting. This was the first I heard of this. My recently built home has R72 loose fill fiberglass on the floor of the attic. I recently had some thermal imaging done for an energy audit and the attic looked very warm even though it was pretty cold outside at the time - about 20°F. So far, so good.

    1. henryrose | | #4

      LawrenceMartin, if your loose fill FG is resting on the attic floor, shouldn’t you be hoping for a cold attic in winter with the insulation preventing house heat from rising into the attic?

      1. yamaguchimartin | | #5

        Yes, that is correct. I should have pointed out that my thermal imaging was done from the interior. So, I wanted it to appear warm which indicates that the ceiling is holding the heat in the room.

        1. henryrose | | #6

          Got it, thanks.

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