“R-38 attic insulation is all you need in Phoenix”

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energystarinsulationlevelsI routinely recommend R-50 to R-60 attic insulation retrofits, even in hot climates. I heard it again a couple weeks ago. “R-38 attic insulation is all you need in Phoenix.” Or Miami. Or Sacramento. Or Las Vegas.

The context was retrofitting existing homes, in particular, those that have already have 3-4 inches of insulation (not R-38 already). And the premise was that it isn’t cost-effective to raise levels to more than R-38.

That’s silly. Is R-38 a magic number? Or does R-38 represent 12″ thick fiberglass batt? And since you’re blowing in insulation, do you have to hit a number that was determined yesteryear based on the R-value of a batt? It is true that DOE used to have charts that recommended only R-30 in climate zones 1-2. But even those charts have been updated (although they still precisely and miraculously determine that optimal levels match r-values of typical fiberglass batts.)

I asked that question. The answer was that we could round to R-40. But apparently I was missing the point–insulating to R-50 or even R-60 as I had mentioned just isn’t cost effective. It doesn’t make sense.

Or does it?

I’ll answer my own question. Yes, if the attic construction allows it, and you’re upgrading the attic insulation anyway, pushing to R-50 (or even R-60, but let’s take this one step at a time) is very reasonable.

Let me give some of my assumptions. First, it gets hot in attics in the summer–we’re talking about hot climates here after all. The indoor/attic delta-T rivals a cold January day in Burlington, Vermont, albeit with the gradient in the opposite direction. Insulation slows heat transfer. And slowing heat transfer out of a hot attic into cooler living space is a good thing.

Second, if we’re talking adding existing insulation, you’re airsealing the attic plane before you add insulation. (You are air-sealing, right?) This is labor intensive and it represents a big part of the cost of upgrading an attic. Other costs include marketing, sales, and getting the truck and crew out to the address. Yes, you’re onsite already. That being the case, the additional cost (not the price, that’s a separate issue) to add an extra R-10 is small.

Don’t believe me? Do the math. Let’s assume a 1,500 s.f. attic. You’ve already prepped the attic. The truck is in the driveway. The tarps are laid down. You’ve hung plastic in the daughter’s bedroom where the attic hatch is. You’ve air-sealed. And you’re set to add insulation to get up to R-40. How much does it cost to bump that up an extra R-10? We’re looking at incremental costs only because we’ve already covered everything else.

You’ll have to use your own numbers, for insulation cost, coverage, labor costs, lbs/hour of fiber your machines can move, etc., but it will look something like this. If a 25lb of cellulose costs $8 and gives me about 100 s.f. at R-10, I’m going to need 15 bags to cover that 1,500 s.f.  attic. That’s $120 of material. Now, a Krendl 2300 is rated to move 3000 pounds of in an hour. I’ve got 25 lbs x 15 = 375 lbs to move. The guys are already crawling around the attic blowing, remember, so all they need to do is stay a bit longer in each position–and not too much longer if you’ve got a good blower. Let’s call it an extra 30 minutes. (Don’t like that? Figure out your number.) Two guys, each at $75/hour (that’s what you pay your insulation techs, right?) for 30 minutes, that’s…$75.

Now I add the materials to the labor and I’m looking at $200. Remember, I’m talking cost, not price. But on an incremental basis it doesn’t cost that much.  If you’re focused on cost-effectiveness, make sure you crunch the right numbers for that incremental R-1o. And if your customers care about comfort, resilience (how cool does the house stay in August, or January, when the power goes out?), or even taking the extra step to save a bit more energy just because they want to (they drive a Prius and want to buy solar) be sure to factor that in, too.

Now, the math looks very different if the customer has R-38 already, and you’re not going to otherwise be in the attic doing things like air-sealing. But if you are there, taking the extra step often does make sense. You don’t have to stop at R-38. It will never be cheaper to bump that up to R-50 than while you’re there already pushing insulation through a hose into the attic. So do it now.

Next, let’s talk about adding wall insulation. In Phoenix.

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About the Author:

Mike Rogers is the President of OmStout Consulting. A nationally recognized expert in residential energy-efficiency, he works with contractors and programs to scale sustainable market approaches to improving homes. More on Google+


  1. Nick  September 24, 2017

    Hi Mike, do you recommend doing a blower door test before and after attic air seal? Would this be worth the cost and wise to do before adding insulation? Thanks

    • Mike Rogers  September 29, 2017

      Hi Nick, Yes I do. A blower door test in most cases is quick and easy. It allows you to better understand the extent of the issue beforehand. And it allows you to measure your effectiveness afterwards, something that might be of interest to the customer, but also something that should be of interest to you. Knowing how you did helps you figure out what to improve.

  2. Marie  March 25, 2016

    Mr Rogers. I have an emergency situation. I am in Phoenix and need to have all of the fiberglass removed from my attic. Would you please recommend the two types of insulation I should research to replace the fiberglass? I have appx 1700 sq ft. Wood frame and stucco construction. Thank you Marie. March 25 2016

    • Mike Rogers  March 26, 2016

      Hi Marie– It’s hard to prescribe a solution without knowing more and seeing your home. Why do you need to remove the fiberglass?

      Personally, I like cellulose insulation (only cellulose treated with 100% borate fire retardant). But spray foam and fiberglass work well, too. One very important thing is to air-seal (see https://www.energystar.gov/ia/partners/publications/pubdocs/DIY_Guide_May_2008.pdf) BEFORE you re-insulate.

      In Phoenix, there is a good chance that the ducts for your air-conditioning are in the attic–this is a great time to have them inspected and sealed and insulated if necessary.

      And if you’re removing the insulation damaged because of some problem with the home (roof leaks, pest entry), be sure to fix that problem first.

      You might also check with APS for any available rebates to help you pay for this.

      Good luck.

    • Marie  June 10, 2016

      I have to remove the batt (?) Fiberglass insulation to reveal potential foul play. My home is constantly bombarded by microwave frequency, tested and verified
      I appreciate your recommendations and will heed your advice
      Do you know if the fire retardant has the potential of releasing an odor? I ask because I had to return two beautiful new mattresses because I could smell the fire retardant the manufacturer used.
      If you can recommend a reliable and fair licensed contractor, I appreciate your input
      Thank you, again

    • Marie Raygoza  June 13, 2016

      Mr. Rogers, I just reviewed the link containing comprehensive instructions regarding sealing and insulating air leaks in the attic prior to installing attic insulation. Thank you for this valuable information.
      I plan to continue living in my home for two more years. And I understand the long-range value of sealing and insulating air leaks. Two questions, will I achieve the full benefit of this extra effort and cost in my two remaining years in residence? And about what percent of the total cost of the insulation job should a reputable contractor charge for the up front preparation keeping in mind I do not have water damage etc and this wood frame and stucco construction is tract builder quality built in 1996.
      Is it fair to say that even though a roofer includes the inspection and sealing prep work in the bid that I need to keep an eye on their work as that effort can easily be short cut?
      Again, can you recommend a name or two of licensed roofers in Maricopa County?
      Thank you. Marie

      • Mike Rogers  July 6, 2016

        Hi Marie – great question, and unfortunately without seeing your house in particular, there’s really no way I can give you an answer. You’ll have to have someone take a look and help walk you through the math.

        Regarding contractors in your area, I’ll suggest a good place to start is with APS and contractors participating in their rebate program. https://www.aps.com/en/residential/savemoneyandenergy/rebates/Pages/house-of-rebates.aspx.

        A couple points. Air-sealing is often at least as expensive as the insulation itself. But as you know, it doesn’t make a lot of sense to add insulation without air-sealing. You can ask your contractor to guarantee a specific reduction in air-leakage which can be verified with a blower door.

        And while a contractor is in the attic, make sure you get the ductwork properly sealed. In fact, if I lived in Phoenix, and if I were moving soon, I’d start with the ductwork first–if you have leaky ducts, that impact can be huge and immediate both in terms of dollars and comfort (and dust and pests and…)


  3. Aryona  February 18, 2016

    The contractor is soppused to give you a certificate that can be submitted with your taxes and a cost with your deduction listed.I got a new AC unit and was provided with that info.

  4. Dennis Brachfeld  January 4, 2014

    It is the law of diminishing returns. Depending on the home, the extra $200 bucks, could be more efficiently spent, on conditioned space air sealing, and passive attic ventilation, high and low vents.
    After retrofitting 34,000 homes, I’ve learned that; Every home is unique and deserves individual inspection and detection, or I may just be “venting”!

    • Mike Rogers  January 4, 2014

      “Every home is unique and deserves individual inspection and detection”
      That’s a totally fair point, Dennis. And in some homes it won’t make sense to go further so you can apply that $200 to something else. Keep in mind as you’re weighing things out that the $200 is a one-time opportunity while the truck is in the driveway, the set-up has been done, the tarps are laid down, and the guys are already in the attic. Later, it will cost you much more because you’re have to redo all the other stuff (not to mention covering overhead, marketing, sales, etc.)

      We agree in saying that in some cases adding more insulation doesn’t make sense. Of course, that’s not the same as saying adding more insulation–more than R-30 or R-38 in Phoenix–never makes sense. “all you need is R-38” applied as one-size-fits all thinking isn’t doing much analysis (and I’m not implying that’s what you said).

  5. Frank  December 14, 2013

    I agree with you totally. And the savings should be concider over many years not 5,7 or even 10. I did it in my own home and actually am more like R-60 Plus and the savings is far more than the REM file and formulas suggested.

    I see the same thing with spray foam. the first inch is “X” and the second and a every inch after that is stil “X”. not “Y”. no wonder so few people opt for spray foam on a per cost bases. The same principle applies. Once the set up is done, it is done. This is one of the few construction jobs where this is true.


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