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Building Science

Grading the Installation Quality of Insulation

How to tell the difference between perfectly installed insulation and a lousy insulation job

Image 1 of 4
The right way to do it. Fiberglass batt insulation installed to RESNET Grade I quality.
Image Credit: Energy Vanguard
The right way to do it. Fiberglass batt insulation installed to RESNET Grade I quality.
Image Credit: Energy Vanguard
Fiberglass batt insulation installed to RESNET Grade III quality. Installers didn't cut notches for electrical junction boxes, causing plenty of compression and incompletely filled areas.
Image Credit: Energy Vanguard
Fig. 1: These diagrams from the HERS Standards show the effect of gaps on the installation grade.
Image Credit: RESNET
Fig. 2: These diagrams from the HERS Standards show the effect of compression and incomplete fill on the installation grade. There is no diagram for Grade III.
Image Credit: RESNET

Six years ago, RESNET published a major revision of the HERS Standards, officially named the 2006 Mortgage Industry National Home Energy Rating Systems Standards. One important new feature in the standards was the grading of insulation installation quality. Before this change, R-13 insulation installed poorly (as shown in the second photo, below) was equivalent to any other R-13 insulation, including insulation with impeccable installation quality (as shown at the top of this article).

If you know any HERS raters, you’ve probably heard the debates about the various types of insulation and the poor quality of installation they sometimes see. (OK, you’re right; it’s more than just an occasional sighting.) In fact, the Green Curmudgeon’s suggestion that maybe we ought to consider banning batt insulation has generated a discussion that won’t die.

So, how exactly does this system of grading the installation quality of insulation work? Raters have been doing it for six years now, so it’s time for the rest of you to find out what they’re doing. First, a brief overview:

A home energy rating includes energy modeling to see how energy efficient a home is or will be. Most HERS ratings have been done for new homes that are seeking to qualify for a program’s label or certification, such as the ENERGY STAR new homes program. The rater gathers all the information about the building envelope, the heating and cooling systems, ventilation, water heating, lights, and appliances. The data go into the energy rating software, which does the calculations and comes up with estimated annual consumption and costs as well as a number called the HERS Index.

The three grades: I, II, and III

The R-value of the insulation in all the insulated building assemblies (walls, ceilings, floors) can have a big effect on the results. Now that raters put a grade on the installation quality, it helps the rater develop a more accurate energy model of the home. When a rater goes in and looks at the insulation, they’ve got to record each assembly as having a Grade I, Grade II, or Grade III insulation installation quality.

Note: This protocol applies to all types of cavity insulation, not just fiberglass batts. Spray foam, cellulose, and mineral wool can all be installed with gaps, compression, and incompletely filled areas, and they can all be installed well.

Grade I is the best. This means that the insulation is installed according to the manufacturer’s instructions. It completely fills the cavity in the case of air-permeable insulation and also is encapsulated on six sides (with an exception for IECC climate zones 1-3). It’s cut around electrical junction boxes, split around wires and pipes, and generally not compressed.

Grade II is second best. There’s some allowance for imperfections in the installation but overall, it’s still not too bad. The HERS Standards say a Grade II installation can have “moderate to frequent installation defects: gaps around wiring, electrical outlets, plumbing and other intrusions; rounded edges or “shoulders”; or incomplete fill…”

Grade III is the lowest grade. It has “substantial gaps and voids.”

The energy rating software models these three grades differently. When the rater enters Grade I, the software calculates according 100% of the cavity insulation having the R-value entered. When the rater enters Grade II, the software models the cavities as having 98% of their area insulated to the given R-value and 2% uninsulated. For Grade III, 95% of the cavity area is calculated with the given R-value and 5% is treated as uninsulated. (The reason for these particular numbers should become clear to you below.)

The two criteria for assigning the grade

When the HERS rater is inspecting the insulation installation quality, they look at two criteria:

  • Missing insulation
  • Compression and incompletely filled areas

Missing insulation. When a cavity in a building assembly has insulation installed in a way that leaves gaps, that affects the amount of heat that flows across the building envelope. More heat will pass through assemblies that have gaps. (If you want to see just how much a little bit of missing insulation can hurt performance, check out this calculation for a ceiling with an uninsulated attic hatch.) The more gaps there are, the worse the grade it gets. Here’s how the HERS Standards relate missing insulation to grade:

  • Grade I: “Occasional very small gaps are acceptable.” In another place, the Standards say, “if the exterior sheathing is visible from the building interior through gaps in the cavity insulation material, it is not considered a ‘Grade I’ installation.” 
  • Grade II: Up to 2% missing insulation
  • Grade III: Between 2% and 5% missing insulation

If you’re wondering what happens when you do an inspection and find that more than 5% of an assembly is missing insulation, the answer is that you have to break out the uninsulated part and model it separately. If it’s new construction, you’ll probably be telling the builder to finish insulating, but in existing homes, sometimes you have to model the uninsulated part.

The illustration below (image #3), taken from the HERS Standards (Appendix A, pages A-11 to A-16), shows visually what the rater should be looking for.

Compression and incomplete fill. Compression is a common problem with fiberglass batt insulation because the batts are often not cut to the proper size for the cavity.

  • Grade I: Up to 2% of the area can have compression or incomplete fill. If a spot is incompletely filled, it must be depleted no more than 30% to attain Grade I. In other words, no more than 20 square feet of each 1000 square feet can have this problem, and even those 20 square feet must be filled to at least 70% of their intended depth.
  • Grade II: Up to 10% of the area can have compression or incomplete fill and again must be filled to at least 70% of their intended insulation depth.
  • Grade III: Unspecified. I take this to mean that any compression or incomplete fill that lies outside the bounds specified for Grade I or Grade II would garner that assembly a Grade III.

The illustration below (image #4), again taken from the HERS Standards, shows what these conditions would look like.

Making the grade

Most of the time, assigning a grade to an insulation installation isn’t really so hard. Sometimes, you look at it and see immediately that it’s Grade I. Other times, it’s immediatley obvious that it’s Grade III. The difficulty comes in when you’re on the boundary between I and II or between II and III. That’s when you might need to dig a little deeper and get out your measuring tape. If you want to learn more about this, download a copy of the HERS Standards (link below) and read pages A-11 through A-16 in Appendix A. If you’re a home builder or insulation contractor working with HERS raters, it’s important to know exactly what they’re looking for.

Putting a grade on the installation of insulation and doing inspections before drywall goes into new homes were two of the biggest changes that RESNET and ENERGY STAR introduced six years ago. New homes that have gone through this process have gotten a lot better as a result.


Download the HERS Standards: 2006 National Mortgage Industry Home Energy Rating System Standards (pdf)

Allison Bailes of Decatur, Georgia, is a RESNET-accredited energy consultant, trainer, and the author of the Energy Vanguard blog.


  1. Expert Member
    ARMANDO COBO | | #1

    You didn't learn your lesson... JM is coming after you!!!

  2. GBA Editor
    Allison A. Bailes III, PhD | | #2

    I think I did! << Response to Armando Cobo
    Armando, I think I did learn my lesson, and that was that it's OK to show brand names in blog posts, even when their products aren't installed well. In fact, that's the key. If the problem is the installation, not the product itself, then they've got nothing to come after me for, right? And showing that the installation problem occurs whether it's JM, Guardian, OC, Certainteed, or any of the others proves the point. They can all be installed poorly...and they can all be installed well.

    But your point is well taken, Armando. I certainly considered the consequences of posting such a photo. Then I decided it was just fine.

  3. Expert Member
    ARMANDO COBO | | #3

    I was kidding...
    ;-))) ;-))) ;-)))

  4. wjrobinson | | #4

    PHDs.... grading.....
    PHDs.... grading..... insulation.... installation.

    Maybe December 21 is the end.

    (I have to add for those that don't know, I think our GBA Doc is da man. A great asset and aid to all if us building green yet fretting over all the details that can derail us easily.)

    22 plus years of education.... grading..... insulation... installation.

    This may just be the century of the Chinese. But It's not going to be, we all will do grade 1 installs so the PHDs can really help us in other areas if building science.

  5. user-980774 | | #5

    Do builders actually still
    Do builders actually still use batts in walls? (other than production builders)
    Or more specifically, do any builders that give a $hit about quality and energy use still use batts?

  6. user-1140531 | | #6

    Batts are capable of
    Batts are capable of achieving as high of a level of energy efficiency and quality as any other type of insulation.

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

    Response to Richard Patterman
    The protocol I described here is for all types of cavity insulation. It doesn't matter whether you use fiberglass batts, blown fiberglass or cellulose, spray foam, mineral wool, or something else. For the purposes of a home energy rating, you have to put a grade on it. Sorry the article wasn't clear on this topic, but I've just done a little edit to help clarify.

    Now, to answer your question, yes, a lot of builders who aren't doing production work do still use fiberglass batts. Some even do it well, like the Habitat for Humanity group in Nashville that did the job in the photo at the top. If you install them well and do all the proper air sealing to keep air from moving through them, they can work quite well.

  8. user-1140531 | | #8

    Regarding the inspection
    Regarding the inspection rating of insulation:

    Regarding the criteria of compression and incomplete fill, I assume compression would only apply to fiberglass batts, and fill would apply to all types of insulation. Incomplete fill of any insulation always lowers the insulation performance of a cavity.

    But why are compression and incomplete fill linked as though they were equivalent causes of insulation deficiency? Compression of fiberglass will increase performance up to a point.

    Similar to the issue of compression with fiberglass batts, is the issue of density with cellulose and blown fiberglass. How do raters determine that the proper density has been achieved with these materials?

  9. GBA Editor
    Allison A. Bailes III, PhD | | #9

    Response to Ron Keagle
    Ron, no, compression affects all kinds of insulation - batts, blown, and sprayed. When compressed, the density and R-value per inch go up, but the total R-value goes down. For example, the standard R-19 fiberglass batt is 6" thick. When you put it in a closed 2x6 wall cavity at 5.5", the actual R-value drops to about 18.

    I wasn't involved in creating the standard for insulation installation grading, so I don't know what their reasoning was in lumping them together, but they're kind of related.

    Insulation density isn't part of the grading protocol for HERS raters. Yes, it's certainly important for performance, but as Building Science Corporation is showing with their ongoing work on the effectiveness of insulation at different temperatures and air leakage rates, the whole R-value thing is way more complex than we're capturing in our energy modeling tools.

  10. wjrobinson | | #10

    Mentioned before somewhere,
    Mentioned before somewhere, continuous insulation which is at the same R value is one of the best ways to insulate and achieve low energy demands. In some climates thermal mass is very important to take advantage of day verses night temperatures.

    Standard batts between studs with no air sealing is how most homes were built up to like ten minutes ago and is the worst way to insulate. Why? because it is air leaky, and the R value is not constant and continuous.

    That is my understanding anyway and refer details to the PHDs. And these details are where we need our PHDs to stay focused on.

  11. user-1140531 | | #11

    Early insulation woes

    I would say that a common approach up until recently was fiberglass batts (often poorly installed) between the studs with air sealing, but air sealing that was often damaged and/or poorly installed. But aside from the inadequate air sealing, I think the larger problem was simply inadequate insulation R-value. The consensus was that 3 ½” stud was adequate for insulation space, and with anything thicker; the payback was not worth the extra cost.

  12. wjrobinson | | #12

    Ron, I have built for
    Ron, I have built for decades. I have no clue what you do. I have no clue where you get your opinions from in your posts. There are two builders near me that have done very well for decades building complete six sided continuously insulated homes. They out perform stick and batt homes period. Joe L and others here have posted some great info regarding the science behind poorly performing building assemblies that have R levels that are NOT uniform. Search this site, they exist. I am not sure I could find the threads, possibly. BSC discusses much of our differences too.

    Do you build? Where? What? For how long? I need a reference to discuss any further in any serious vein.

  13. user-1140531 | | #13

    I thought I was agreeing


    I thought I was agreeing with your comments preceding my last post. So I have no idea what you are taking issue with.

    You said, “Standard batts between studs with no air sealing is how most homes were built up to like ten minutes ago and is the worst way to insulate. Why? because it is air leaky, and the R value is not constant and continuous.”

    I took that to mean that you were being critical of that practice.

  14. wjrobinson | | #14

    Ron, air sealing batts is a
    Ron, air sealing batts is a baby step in the right direction. There is more that can be done and the latest codes call for such. And there are just a few builders that use JUST continuous insulation.

    Please post if you are a builder or just an interested homeowner, physicist or.. and the zone you live could be added past your last name by editing your account. It helps others when someone posts often to remember all this. And it does alter what we all yap about. Some live in high humidity, some in a cold climate... some in Hawaii... lucky few...

  15. user-2094615 | | #15

    Ad Hominem?
    I'm also curious about Ron K's. background and qualifications. However, no matter what he does or who he is he deserves answers based on facts not on his personal experience or place in life. Agreed?

  16. user-1140531 | | #16

    My Context

    Thanks for your comments.

    My approach to this is simple. I judge people's ideas by what they say about them, and not by the credentials they say they have. I don’t claim to have all the answers, and if somebody disagrees with what I say, I would be happy to listen and consider their point of view.

    I am not a builder. Earlier, I did remodeling work and also was an excavating contractor. My current vocation is mechanical design for product development as a contract service. I design products ranging from off road equipment to home appliances. I have a lot of interests and ideas outside of my actual paid work, and one of those areas has been home building and design. I built one superinsulated house in 1985.

    Then I got very busy with the mechanical design field, and stayed focused on that. In the last few years, the bottom has fallen out of the product development work, so I have been rethinking what I did with building this house. I am looking at what worked well and not so well. Basically, I have become really enthused about house building projects again. So I am starting with a clean sheet of paper to optimize superinsulated house design.

    In the 1980s, I was following all of the energy efficiency trends, and my superinsulated house was the result of that interest. In a way, that era was kind of ahead of its time, but now I sense there is a lot of growing interest in those ideas. There is a sort of convergence going on right now.

    In revisiting the superinsulated house concept, I am really amazed at how much controversy there is about the execution of details. So I come here just to hear what people think and to throw out a few ideas to see what others think.

  17. GBA Editor
    Allison A. Bailes III, PhD | | #17

    Passions run high!
    Ron, when I got into this field nearly a decade ago, I thought building science and green building are pretty objective fields. I figured that people would disagree on some things, but I was a bit surprised to find the high level of passionate discussions that people in this field have. I think you captured it perfectly with your statement: "I am really amazed at how much controversy there is about the execution of details." Don't worry, though. There's plenty of room for folks like you, too.

    Speaking of superinsulated houses, will you be going to the Passive House conference in Denver this month?

  18. user-1140531 | | #18

    Lots of different options
    Thanks Allison. I won’t be going to the Passive House Conference, but I will be very interested in the latest developments.

    In the course of my superinsulation revival over the last two years, I have talked to a lot of material manufacturers and suppliers. Many times, the person who answers the phone has a friend or relative who has built some type of experimental, energy efficient house, so they have a story to tell.

    Back when I built my house, I was reading a book called, Super-Insulated Houses and Double Envelope Houses by William A. Shurcliff. Shortly afterward, another book came out called THE SUPERINSULATED HOME BOOK by J.D. Ned Nisson and Gautam Dutt. I did not acquire that book until about a year ago. That book champions superinsulation as a system that can be applied to virtually any type or style of house. That universality was seen a big plus for selling the mass appeal of the superinsulation concept when it was first emerging.

    I am interested in blending superinsulation with a specific group of house attributes that complement it. That includes things like simple roof geometry and a compact, non-sprawling layout.

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