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I’m getting two different opinions about foam insulation in the attic

rus54 | Posted in Energy Efficiency and Durability on

Some say spray the roof deck-allowing the attic air to become conditioned which is great for the H&A ducts in the attic. Others say spray the attic side or the ceiling to allow the attic to vent. I don’t understand this one since the attic air is now unconditioned air and the ducts a
have only a wrap of insulation..I’m confused. Sounds like there are two schools of thought going on. Are both ways OK? Is one a lot better than the other? Help!! I even heard that the house would be too tight and allow for stale air to remain in the house if you spray the roof deck..I need answers.

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  1. wjrobinson | | #1

    Rus, why are you considering spray foam?

    Have you considered the cost verses the savings verses the risk and when adding insulation there is much more or less or different that can be done.

    Say you spend $5,000 on spray foam. And then the home uses $200 less fuel or you turn the heat up higher or the AC down more and you save not increasing your costs but have more comfort. That may be worthy. Only you know all your facts.

    save the planet

  2. Richard Beyer | | #2

    Read before you leap...

    Also understand if this is a retrofit application you may need to downsize your HVAC equipment if you air seal your home with any insulation. Your blower door tester can verify this for you based on the test results. This topic can easily become extensive so I'm sure when Martin steps in he can direct you to the proper site where this is discussed on GBA.

  3. GBA Editor
    Martin Holladay | | #3

    Here are two articles you might want to read:

    Keeping Ducts Indoors

    Creating a Conditioned Attic

  4. GBA Editor
    Martin Holladay | | #4

    In case you're not clear on this issue, it's worth pointing out there there are two types of attics:

    1. Vented unconditioned attics are basically outdoors.

    2. Unvented conditioned attics are indoors.

    If there are any ducts in your attic, it's always better to have the second kind of attic -- an unvented conditioned attic -- so you aren't wasting heat during the winter or losing cool, conditioned air from your ducts during the summer.

    Once you bring your attic into your house, you don't have to vent it. (For more information on attic venting, see All About Attic Venting.)

    One last point: if you want to create a conditioned attic by adding insulation between your rafters, you don't have to use spray foam. You can use other types of insulation in this location.

  5. charlie_sullivan | | #5

    Underlining Martin's last point: Spray foam is very expensive for your wallet and for the planet (the blowing agent is more than 1000X more potent for the greenhouse effect than is CO2). In some cases, it's hard to find another good option, but in this case there are much cheaper options that work just fine.

  6. user-1041981 | | #6

    Just curious what the other options are which work just fine? I'm having my roof deck spray foamed to a depth of 4" and then 8" of blown-in fiberglass. I don't think there is another option to the spray foam (besides cobble-and-fit rigid foam) in order to meet IRC 2012 Section R806.5 (see Table 806.5). Since I'm in Zone 5, I need R20 of air-impermeable insulation - hence the 4".

  7. Expert Member
    Dana Dorsett | | #7

    Mind you Table 806.5 is predicated upon an R49 total-R. If you go higher-R on the total, you need to go proportionally higher-R on the foam, so pay attention to the R/inch on both the foam & fiberglass. (For climate zone 5 the foam has to be at least ~40% of the total R or higher.)

    Typical 2lb polyurethane runs ~R6-6.5/inch and about 1.2 perms @ 1". That would give you an honest R24 to work with , allowing you as much as R36 for the fiber layer, which means you're probably good to go with 8" of any fiber insulation.

    But where does that 8" come from? The true depth of 2x12 milled timbers is only 11.25". With 4" of foam you'd have only 7-1/4" of room for the fiber. At R4/inch (1.8lb density Spider or Optima) that's R29. Add that to the R24 foam and you're at R53. If you did only 3" of R6./inch foam (R18) you'd have enough foam to deal with R27 in the fiber layer, and have 8.25" of room to play with for the fiber which would need to be about R3.3/inch to not have dew-point issues. But if you installed 3" of an R6.5/inch product (R19.5) you could install up to R29 in there, and a damp-sprayed cellulose product could work. That would put you right at the IRC 2012 R49 code min, but use 25% less of the climate damaging foam.

    There are a few 2lb foam products out there that are blown with water instead of HFC245fa that comes with the climate-hit. Icynene's MD-R-200 runs about R5.1/inch, and has a vapor permeance of about 1 perm @ 4", which would be fine. They have a similar product that is slightly denser, MD-R-210, which is only R4.9/inch, but is significantly lower permeance. (Don't confuse it with their R7/inch MD-C-200, which IS blown with HFC245fa.) A regional player in upstate NY called Aloha Energy has proprietary 1.8lb density water blown foams that are lower permeance and about R6/inch thermal performance.

    With 4" of MD-R-200 you'd have R20.4 of foam, which could accommodate R31 of fiber. With 7.25" of space for fiber and 1.8 lb fiberglass that would give you R29 of fiber, R20.4 of foam, putting you a hair above code min, with less damage than using an HFC-blown foam. The trick is to find an installer near you that has experience with 2lb water-blown Icynene foams.

    An even better solution is to put R12-R13 rigid polyiso on the exterior of the roof deck and R8.4 Type-II EPS above that (which is 4" of foam), then put R30 fiber on the interior. That delivers sufficient exterior-R for dew point control a the roof deck, but also puts R20 over the thermally bridging rafters, and would thus outperform code-min from a thermal point of view.

    The reason for the dual foam types is that polyiso falls off a performance cliff when the average temp through the layer is under 30-35F (it peaks at 50F mid-foam temp), whereas EPS gains performance with ever falling temperatures. The 2" of EPS keeps the 2" of polyiso warm enough at your mid-winter average temp to perform at about R10-R11, while the EPS rises from R8.4 to something like R9-9.5. The combination yields a flatter performance curve with temperature, and keeps the total foam stackup performing at R20 or so without having to add more thickness. The EPS + polyiso stackup outperforms 4" of polyiso from a dew-point control aspect, despite somewhat lower performance when its warmer outside.

  8. GBA Editor
    Martin Holladay | | #8

    Q. "Just curious what the other options are which work just fine?"

    A. If you include a ventilation channel between the top of your insulation and the underside of your roof sheathing, you can use a wide variety of insulation materials. For more information, see:

    Creating a Conditioned Attic

    How to Build an Insulated Cathedral Ceiling

  9. Richard Beyer | | #9


    In my opinion, I would not recommend that manufacturers foam to anyone. I would be more than happy to send you a sample of what they claim is good foam 4 years after installation if you dare to smell it? It smells so chemically bad the CT General Law Committee at a hearing in Hartford Connecticut would not go near it. I handed it over to the Commissioner of Consumer Protection and he pushed his chair back so fast you would have thought I was handing him a skunk. If law makers fear it, why would I give this manufacturer a recommendation?

  10. wjrobinson | | #10

    Richard, Dana mentioned two manufacturers and several different foam formulas. I have had open cell water blown Icynene installed but not the more dense or closed cell formulas. The 1/2 pound open cell has no smell once cured. It barely smells when standing right behind the installer as he sprays it. One can take a piece of that foam and bury ones nose in it and no smell. I also have been in a closed cell huge home where the 4,000sqft cathedral ceiling was closed cell sprayed, no smell.

    That all said I now prefer to not use spray foam but if I do, I have a sub that does good work spraying Icynene that does not have an odor once placed.

    So what foam are you referring to Richard and is your knowledge first hand?

  11. Richard Beyer | | #11

    AJ it's in my house, I'd say it's first hand.

    Take your foam and place it in a mason jar. Let it stand in the sun for a few hours. Open the jar and then tell me that foam does not smell. Spray foam leaves a finger print in a homes air quality test.
    This may explain this...


  12. wjrobinson | | #12

    Richard, I ask again, what exactly foam is installed where in your home? Do you heat with natural gas? Why did you retrofit your home with spray foam? Did you hire the lowest cost installer? How long had your installer been in business? Did you talk to other satisfied home owners who had done the same as you had done to your home? Why haven't you sold your home and moved to a home with no spray foam? Are you presently in a law suit against the spray foam installer and the maker of the foam?

    As I note often I am no longer promoting the use of spray foam myself. I also feel the best solution to high energy costs is to sell one's home and move to a home that costs less to own. Smaller homes, better climates, natural gas availability are ways to reduce costs. Spending $30,000 on a home to save money to me seems stupid but who am I to say that but that is how I feel. Same with solar panels, if one buys $30,000 dollars worth of panels, to save a few dollars per month it sounds to me like it's time to put the two piles of money on a table in dollar bills and observe the difference in size of the piles!

    Since the invention of advertisement marketers have been training all of us that we can "spend money. to save money." And the crazy part is we all do so! We spend money to save money! "Buy a new furnace and save! Buy all new windows and watch the savings roll in!

    Anyone who truly wants to save, to start with can lower the tstat setting. We did years ago and now prefer a much lower setting than those of you that set your temps in the 70s even high 70s. You can heat less of your home. One can sell a huge home or any home and live in a small home.

    Lastly, Richard, I have the most experience with 1/2 pound open cell Icynene sprayed in new construction and not occupied after the spraying except to finish building for 6 months. I have checked the foam for smell and water moisture over the years due to the bad outcomes posted by others here and I can say there are no problems. The foam is fine as far as I can tell without hiring a team of scientists and lawyers. The company now has 6 spray foam rigs that did the install. They have done thousands of projects. Thousands. When I asked my friend at the company about problem installs they said they had one spray where they sensed no issues but in the end I think they did something either removed it or covered it or something.

    Richard, what did you have sprayed? Where? Why are you still in the home? Are you a contractor?

    And yes I have often played with a piece of the foam sprayed for me, I have buried my nose in it several times. There is no smell period. None. I have asked many others to test. No one can smell a thing in the pieces of open cell spray foam I have. But who knows I may be lucky to have a great install company or a bad nose and friends with bad noses.

  13. Expert Member
    Dana Dorsett | | #13

    I'm not a big fan of spray foam in general, but if foam water-blown is less damaging.

    My preferred solution is rigid insulation above the roof deck.

    Full disclosure: There are some parts of my home that are insulated & air-sealed with half-pound Icynene LD-R-50, and I'm OK with that. The installation was not great- I've definitely seen better, but I've seen worse. There was some lingering odor for the first ~4-8 months in places where it was not covered by drywall, but nothing to speak of thereafter. I have never used MD-R-200, but would consider it where the project constraints made reasonable on a price/performance or other basis. It's considerably more expensive than rigid foam solutions.

  14. Richard Beyer | | #14

    Dana, Water blown or not they both require isocyanate (MDI). Both are toxic regardless of brand as AJ is asking. Open cell or closed cell they both permeate odor and their own finger print in air quality analysis and that includes sprayed on or off ratio, regardless of manufacturer. AJ the installer I hired was not based on price. They employ +/- 140 men and women in the tri-state region.

    Dana and AJ.. When you think there's no odor there are chemical compounds in your indoor air directly linked to the foam insulation which is unlike any other insulation. Sometimes it's what you can not smell which should concern you.

    Dana, 4 to 8 month's? Your product is without question off-ratio according to text published by industry. The odor technically should not be detectable after 72 hours.
    AJ almost all of your questions I can not answer per a court ordered gag order until the trial is over.

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