2 Ways to Get the Best Insulation in Your Home

19 Comments Read/write comments

loaa 2 insulation blown cellulose complete coverage 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 2010 I wrote an article called Flat or Lumpy - How Would You Like Your 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 below. Flat beats lumpy.

Lumpy blown insulation in an attic

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.

Uniformly installed R-30 blown insulation in an attic

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 Flat or Lumpy article, 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.

The same amount of  blown insulation in an attic, but installed non-uniformly

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 2x4 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", you get about R-10 there instead of R-13. If the rest of the wall is right at 2" thick, your average R-value in the cavities is 12, not 13. You're not getting what you paid for.

Even if you're not building a new home, you can still take advantage of this property of insulation. If the insulation in your attic is lumpy, as in the second photo of this article, you can save energy and probably enhance the comfort of your home by spreading it out uniformly. Remember: Flat beats lumpy.

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, 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. Here's the RESNET diagram, with the dark areas representing gaps in insulation.

RESNET insulation installation grade diagrams showing gaps

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 let me say something about compression here. It's not the most heinous sin to commit with insulation. Sometimes it's unavoidable. If you put R-19 fiberglass batts in an enclosed 2x6 wall, for example, they will be compressed. That happens when you put a 6.25" batt in a 5.5" space. The result is that the R-value per inch goes up and the total R-value goes down. That R-19 batt yields R-18 in that case. (See my article on compressed insulation for more.)

Grade I installation quality with fiberglass batt insulation

And yes, Grade I is possible with fiberglass batts, too. I've seen it done a few times, as in the photo above 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:

  1. Insist on having it installed to a minimum thickness, not an average thickness.
  2. 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).

Insulation is important. And for a lot of it, you get only one chance to get it right for the three or more decades until the house gets gutted. It's good to understand how it works.


Related Articles

Flat or Lumpy - How Would You Like Your Insulation?

Attic Stairs - A Mind-Blowing Hole in Your Building Envelope

4 Types of R-Value

The Layers and Pathways of Heat Flow in Buildings

How to Grade the Installation Quality of Insulation


NOTE: Comments are moderated. Your comment will not appear below until approved.


What does "random fill" mean in relation to spray foam?

Allison: What are your thoughts on using unfaced glass fiber batt insulation vs. paper-faced or foil-face batts? I claim that the paper or foil face is only used so that the installer doesn't have to touch the glass fibers and that it is ineffective as a moisture retarder or air barrier and probably results in "lumpier" insulation. Also, it makes it harder to inspect to see if you have Class I installation.

Paper (aka Kraft) faced insulation is fine because the paper performs as a vapor retarder. I've never seen foil-faced batts in the South and I can't image why you'd need them unless it met some weird code requirement.

Aside from being able to easily pull out unfaced batts for inspection of insulation around wiring and behind outlets, I don't see any substantial difference in being able to determine the grade of install between paper-faced and unfaced batts.


I agree with you, Roy, for all the reasons you stated. Unfaced batts are definitely the way to go.

JC: If you look at the above photo with unfaced batts, it is easy to view the installation quality without pulling out anything. If you were to look at a comparable photo with paper-faced, I don't see how you could inspect it without pulling out every batt. As far as the paper acting as a vapor retarder, I have problems believing that, since you have an unsealed seam around every stud space.


JC, the key part of your comment is "looks like." Here's the full photo I took of that insulation:

Grade III installation of fiberglass batt insulation with a kraft paper facing

As you can see, the installers left a gap that provides a view of what that insulation looks like behind the facing. (I don't know why they left the gap.) Here's a closeup of the gap:

Grade III installation of fiberglass batt insulation with a kraft paper facing, closeup

It's clear they left voids in all four of those lower cavities. They didn't split the insulation around the pipe in the left bay. It looks like they did chink some insulation behind the junction box, but overall, this is grade III. But if all you see is the paper facing, you're right. It "looks like" grade I.

The facing seems to be only for the convenience of the installers. I prefer unfaced batts and that's what I used when I gutted and remodeled my bathroom two years ago. Here's what grade I looks like there:

Unfaced fiberglass batt insulation, grade I installation

You still have to pull the insulation out to make sure it fills the cavities completely but you don't have to tear any paper facing to do so.

Maybe, maybe not. I would claim that it depends on the batts behind the paper, not the paper itself.

Trickery with your cropping of photos !!! ;)

The full picture is too funny. I bet their rolls were of a fixed length and they didn't have enough insulation to fill in the gaps. Reminds me of a time when I saw sheetrock crew cover a void like that on an exterior wall of a luxury townhome.

This raises another question. If you are using unfaced batts (which I think that we all agree now are preferable), what do you do when there is an electrical cable (Romax) crossing the stud space? The cable is typically in the middle of the wall cavity. Do you pull the insulation behind the cable, go over the top of it, or slit or split the insulation so that it is on both sides of the cable?

I still remember installing these batts in my youth. For each house, that was the most miserable week of my life -- 1 day of installation and 6 days of itching afterwards.

I believe you should cover unventilated attics and outsulation for a complete picture.


Armando, if I were going to provide a complete picture, I'd have to discuss the partial differential equations for heat and moisture flow (below) and talk about all kinds of things that would take attention away from the main point of this article.

wufi partial differential equations heat moisture flow

Diff E. Q!

Allison, my company has long advised it is NOT OK to partially fill a wall cavity with spray foam. Many bad things can happen - and in fact your above analysis does not take into account that you will have large thermal gradients within that hollow cavity if not filled with addition insulation of some other type. You can't ignore the science that the framing itself can have significant influence on the temperature of the interior cavity - especially where using 2x6 at 12" oc (which seems to be popular where I live). The backside of the sheetrock does not receive anywhere near the full benefit of the intended insulation. We have done thermal studies on this. It's easy for anyone to do.

BUT ON ANOTHER SUBJECT ENTIRELY (sorry for the rant) - I used to live in a home with an attic that had blown cellulose insulation. I bought it new. The attic was tough to get to so I maybe only went up there once a year to check on things. By the time we sold the home I had started work in the building industry and so I noticed that the blown cellulose had "aged" - by that I mean it had settled significantly, and was dirtier than the nice clean product I had seen when we bought the product. I'm sure humidity (from inside and out) weighs it down at ALL TIMES of the year. In retrospect I'm wondering if I should have been going into the attic every year and with a wire rake I fashioned out of a pole and coat hangers (it looked so bad I did this before we moved so it wouldn't fail inspection) trying to re-loft the insulation. What do the manufacturers and installers say about this? We still recommend blown cellulose often where budgets can not use our roof panels, but I still wonder about the need for lofting over time.

I used to teach a heat transfer class and we would analyze heat transfer through a wall to compare the effects of various mechanisms of heat transfer. If you have a vertical 3.5" cavity filled with still air, the R-value is about 21. This is a lot better than the typical R11-R15 that you get with 3.5 inches of insulation. So why bother with insulation? Because the air will not be "still". If you have about a 30 F temperature difference across that air space, the R-value drops to about 3 due to natural convection in the air space, and that is assuming that you have blocked long wave radiation with a perfectly reflective surface on one side of the cavity. If you don't have that reflective surface, you will only have about R1. So why do we put insulation in the wall cavity? It stops natural convection and blocks long wave radiation. Since the only significant heat transfer mechanism with insulation in place is conduction, you do want a material with a high R-value, but again, its main value is to block natural convection and radiation.

The homes that I build in coastal North Carolina consistently achieve a blower door test well below 2 ACH (NC code is 5 ACH max). What I tell my clients is that heat moves in three ways: conduction, convection and radiant. We try to address all three in some manner but the real kicker is that convection trumps all else (IMHO). If air is moving through the house envelope anywhere you are wasting you money on the other two. So our first energy dollar is always spent on sealing the envelope first. If I can spend it on closed cell stray foam and get the benefits of stopping convection, reducing conduction, AND dramatically increasing the structural integrity (hurricane resistance) of the building, that is money well spent.

Thomas, just to be technical, which is a bad habit of mine, energy loss through due to the bulk movement a fluid from one place to another is "advection", not convection "Convection" is a combination of "conduction" (heat transfer from a surface to a moving fluid) and "advection" (bulk movement of the fluid). For my wall analysis, I was ignoring any bulk movement of air (advection) through the wall cavity. I will agree that sealing the envelope is important too, but I don't think that I would say that it "trumps" all else.

So hear is another question (or two): Does spraying foam insulation in the wall cavity sufficiently seal the wall assembly, or do you still need a properly installed air barrier? I would still think that you need the air barrier to handle leakage paths that don't get spray foam. If that is the case, then does spray foam make a house with a properly installed air barrier any tighter? I don't think so, so I would go with the most cost-effective cavity insulation regardless of its air sealing properties.

As for spray foam having structural benefits, that makes sense. Are there any studies that show that to be true? Does using spray foam allow for other cost savings such as thinner sheathing or reduced wind bracing?

"Does spraying foam insulation in the wall cavity sufficiently seal the wall assembly, or do you still need a properly installed air barrier?"

- Versus other forms of insulation closed cell spray foam (ccSPF) if applied correctly will easily fill in air-leaks, but IMO it's poor judgment to solely rely on interior applied ccSPF as your air-barrier. Note: You could apply ccSPF on the exterior and it would work fine but it's expensive (https://buildingscience.com/documents/insights/bsi-048-exterior-spray-foam).

"As for spray foam having structural benefits, that makes sense. Are there any studies that show that to be true? Does using spray foam allow for other cost savings such as thinner sheathing or reduced wind bracing?"

- Yes there are studies. Correctly applied ccSPF will not readily compress. Peruse around the BSC website.

My company provides closed cell polyurethane foam structural insulated panels and the closed cell foam - even sprayed in a stud framed structure - will absolutely make the structure stronger. And it air seals at the same time (so one of the earlier commenters was not correct when saying it would not help air sealing). Of course air sealing is secondary to wall R values - that's well established science according to ASHRAE. But back to the strength thing - the great thing about closed cell spray foam and wood is that the entire structure becomes rigid, but slightly elastic. We call this "rebound". We have tested panels to over 200 lbs/sqft on 10' span and after an hour of loading the panel will immediately rebound to zero deflection after weight removed. We had structures that were the ONLY surviving structures in a particular neighborhood on eastern side of St. John in USVI after more than 5 hours of 185+ winds in Hurricane Irma - all other stick frame and CMU buildings were destroyed either entirely or significantly. That was a pretty good testament to the strength you can achieve by having a structure held together by closed cell foam - either panelized like our system or stick frame, the polyurethane "glue" is bonding everything together.

Add new comment