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Building Science

Advice for Getting the Most Out of Your Insulation Contract

Hint: The same amount of insulation doesn’t always give you the same R-value

The cellulose insulation in this attic was installed at a uniform depth.
Image Credit: Energy Vanguard
View Gallery 6 images
The cellulose insulation in this attic was installed at a uniform depth.
Image Credit: Energy Vanguard
Lumpy attic insulation doesn't perform nearly as well as the same amount of insulation installed uniformly.
Image Credit: Energy Vanguard
You could take a given amount of insulation and install it uniformly in an attic to yield an average R-value of R-30.
Image Credit: Energy Vanguard
Or you could take the same insulation and install it non-uniformly in a way that yields only about half the R-value of a uniform installation.
Image Credit: Energy Vanguard
RESNET insulation installation grades are based on two factors. This diagram illustrates the effect of missing insulation on grade.
Image Credit: RESNET
This Habitat for Humanity project in Nashville, Tennessee shows Grade I installation quality with fiberglass batts.
Image Credit: Energy Vanguard

I love insulation. It’s a wonderful thing because it saves energy. It makes buildings more comfortable. And it’s pretty inexpensive considering how long it lasts (or should last). I get asked a lot for my opinion on the best insulation to put in a building and my answer is straightforward: A well-installed insulation is the best. I like fiberglass. I like cellulose. I like spray foam. I like mineral wool. I like blown, sprayed, batt, and rigid insulation.

Yeah, different materials have different properties, with their advantages and disadvantages. But if it’s installed well and protected by good water and vapor control layers, it should do its job for a long, long time.

So, what are my two ways to make sure you get the most out of your insulation? Both have to do with installation.

1. Request a minimum thickness

Way back in 2011 I wrote an article called “Is There a Downside to Lumpy Attic Insulation?” I refer to it now and then but it’s important enough to make it the highlight of this article. The point of the article was that if you install insulation uniformly, as in the lead photo above, you’ll get much better performance than from insulation installed (or later disturbed) like you see in the photo reproduced as Image #2 at the bottom of the page. Flat beats lumpy.

In that article, I showed an example of an attic done two different ways. First, you insulate the attic uniformly to a thickness that gives you R-30 everywhere. You can’t do this in a typical attic because the roof framing doesn’t give you enough space over the eave walls to get full thickness so you’d have to do something like use raised-heel trusses. But we’re going to assume here that you get full thickness everywhere because that’s what you should be doing, even if it’s not required by code.

In the other scenario, I looked at what happens if you take the same amount of insulation and install it so that you have enough thickness for R-10 on one side of the attic and R-50 on the other side. Your first guess may be that the average resistance to heat flow would be R-30 since 10 and 50 average to 30.

But you’d be wrong. In the article about lumpy insulation, I showed the calculation and it comes out a lot less than R-30. In fact, at R-17 it’s about half. That means you have almost twice as much heat flow even though you have the same amount of insulation.

So, rule number one is to make sure your insulation contractor isn’t selling you on average thickness. That means they’re getting away with selling you less R-value.

Where this matters the most is when you have less thickness of insulation. For example, if you’re using closed-cell spray polyurethane foam, you’re getting an insulation with a much higher R-value per inch than many of the other insulation types. So you usually get less thickness.

In a 2×4 wall where you need R-13 to meet code, spray foam contractors usually install two inches of closed-cell spray foam. Since it’s usually rated at about R-6.5 per inch, that means if, say, 25% of your wall has only 1.5 inches, you get about R-10 there instead of R-13. If the rest of the wall is right at 2 inches thick, your average R-value in the cavities is 12, not 13. You’re not getting what you paid for.

2. Request Grade I installation quality

In addition to making sure you get the thickness to achieve the R-value you’re paying for, you should also make sure the insulation is installed in other ways that ensure it achieves maximum R-value. RESNET created an insulation grading protocol back in 2006 and certified home energy raters have to use that protocol for every rating they do. When they’re inspecting a house, they have to determine the R-value for each insulated assembly and also the grade (Grade I, II, or III). Grade I is the best, Grade III the worst.

The protocol is based on looking for two things. First, the amount of missing insulation determines what grade it might be. The RESNET illustration is reproduced as Image #5 below. The dark areas represent gaps in the insulation.

Officially, Grade I means essentially no gaps, Grade II can have up to 2% gaps, and Grade III can have no more than 5% missing insulation.

The other factor is compression and incomplete fill. The insulation might fill the cavity completely from side to side and top to bottom but still have a reduced R-value if it’s compressed or doesn’t fill the cavity completely from front to back. (I’m thinking of walls when I use those directional terms. Adjust as necessary for ceilings and floors.) I wrote a thorough explanation of the grading protocol back in 2012, so check it out for more detail.

And yes, Grade I is possible with fiberglass batts, too. I’ve seen it done a few times, as in the photo below (Image #6) from a Habitat for Humanity project in Nashville.


When you get insulation, you want to make sure you get the full R-value you’re paying for. Do these two things:

  • Insist on having the insulation installed to a minimum thickness, not an average thickness.
  • Insist on Grade I installation quality.

By the way, if you read the manufacturer’s instructions for installing insulation, they generally align with Grade I installation quality — so you’re not really asking for anything special here.

This certainly isn’t all there is to getting a good insulation installation. Before you ever get to the installation part of the job, way back in the design phase, it’s a good idea to see what you can do to eliminate thermal bridging and make sure you can get full thickness everywhere (as with raised-heel trusses).

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. dugg | | #1

    Excellent article, you had mentioned a situation that I wasn't fully aware of.
    That said, though....shouldn't we also be on the lookout for "air fluffing", as well? I'm under the impression that a way to get around that potential, is to inform the insulation installers via the installation contract/drawings, that there WILL BE A 3rd PARTY INSPECTION of the blown-in insulation by weighing multiple 1 cubic foot samples, and comparing them to the manufacturers own weight specifications. Since the GC is ultimately responsible for all sub work, the installers would most like toe the line, rather than to skimp...jus' to save a few bucks in their pockets. In all fairness, there are many very reputable insulation installers, but..... ;-)

  2. GBA Editor
    Martin Holladay | | #2

    Response to Douglas Harrison (Comment #1)
    If you hire a contractor to install loose-fill fiberglass insulation in your attic, you might need to worry about fluffing. That's another reason why you might want to specify cellulose instead of fiberglass -- cellulose can't be fluffed.

    Here's what I wrote about the issue in an article from the December 2010/January 2011 issue of Fine Homebuilding:

    "The reputation of blown-in fiberglass has been hurt by concerns about the deliberate “fluffing” of fiberglass by some unscrupulous insulation installers.

    "A series of investigative reports in the late 1990s indicated that most installers of loose-fill fiberglass were mixing too much air with the fibers during the blowing process, a practice called “fluffing.” (Unlike loose-fill fiberglass, cellulose insulation can’t be fluffed.) Whether done deliberately or accidentally, fluffing benefits insulation contractors by reducing the cost of their materials. Homeowners suffer, however, because fluffed insulation has a lower R-value than properly installed insulation.

    "Fluffing is specifically prohibited by the federal R-Value Rule. Insulation contractors are legally required to follow manufacturers’ label requirements dictating the amount of material that must be installed—the “bag count”—per square foot of attic. Wary builders or homeowners may wish to perform their own bag-count calculations—not a simple matter, unfortunately—and collect empty bags at the end of an insulation job."

  3. greenhouse437 | | #3

    Watch Those Eaves....
    After a 5.5" closed cell under-roof attic insulation I found that while the main roof underside was insulated well--even covering the joists to reduce thermal bridging--it was the eaves area that didn't do so well with the infra-red test. As far as I know all the old fiberglass insulation had been removed from about the fascia inward a few feet. That should have allowed a complete fill of the closed-cell foam right to the fascia and in and around any of the 'birds mouth' areas. However I believe by either carelessness or lack of skill air pockets were left in there that made the job --in the view of a subsequent professional inspection--less than optimum. The problem is that this work--like chimney work--gets done without the homeowner being able to inspect in real time generally due to safety requirements and wanting to keep out of the worker's way. It is not an easy job. But had I to do it over again I would have donned the safety gear and watched at least that part of the process. I also wondered, if, in the dense eave and soffit-fascia area where so many joists and other wood come together, if some of the 'leakage' the infra-red sees is more likely thermal bridging from all the wood than from air pockets.

  4. AndyKosick | | #4

    Demand More Evidence
    I've been on a soap box for a couple years now trying to get builders and homeowners to demand more of insulation contractors in terms of verification, in attics and existing wall fills especially. It is so easy to take pictures now and you can buy a thermal camera that will do basic insulation inspection for the price of the smart phone it attaches to. With tens of thousands in equipment in the truck or trailer, there's really no excuse anymore. Builders and homeowners should be demanding pictures of the insulation work done in attics, IR of wall cavity fills, and not hiring contractors unwilling to have this done. (I'm looking at you injection foam) Unless you've contracted with a third party to do this, get a requirement of evidence in the contract up front. With wall fills, we find missed cavities with IR on every job we do and make sure they're filled before the insulator leaves. Remember that our contractor knows we're checking and is doing their best. I also regularly find low insulation in attics of new builds where the contractors know we're checking. I honestly think the process of making a record of the install might improve performance.

  5. dugg | | #5

    Response to Martin Holladay - ref: Comment #1 on AIR FLUFFING
    Point well made!
    While I had noted in the photo above, that it was, in fact, cellulose, i was mentally fixed [at the time] on fiberglass loose blow-in, since that's my current choice with my future home build. While I plan on using wet cellulose in the walls, the unconditioned attic will be loaded up with 20" of fiberglass to reach an R-value of 60. Initially felt that I would use cellulose, but was shy of using it, after reconsidering a possible water leak....due to new construction, new roof install, etc. If I recall, fiberglass was lighter per cu. ft, and is relatively unchanged if a trickle of water passes through it..
    Thankx for re-setting my memory banks

  6. mikeF2023 | | #6

    Nice article. My concern is related to unvented attic assemblies that are sprayed (on underside of sheathing/truss) with closed cell foam. Your other articles (about wall assembly and not using more than 1: of closed cell foam) speak of being able to dry to the interior so the sheathing can dry. But by spraying a couple lifts (approx 4-5") in the attic doesn't that greatly impede the ability for drying to the interior? I like to unvented attics, but now I'm concerned about keeping the roof assembly integrity at a maximum. I would think the science of allowing things to dry to the interior for walls would also apply to attics?

  7. GBA Editor
    Brian Pontolilo | | #7

    Hi Mike.

    Because closed cell spray foam is vapor-impermeable and air seals the roof, there is little chance of an unvented, closed-cell-insulated roof assembly getting wet from moist air infiltration or vapor drive. However, many builder avoid this type of assembly because they believe roof leaks may go undetected until severe damage is done to sheathing and framing (and because of the high global warming potential of most closed cell spray foam). Anyway, have you read this: How to Build an Insulated Cathedral Ceiling? It will probably answer your questions.

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