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).