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Community and Q&A

Add batts above ceiling to Icynene on roof deck?

rmrswims | Posted in Energy Efficiency and Durability on

We had our attic treated with 5 -6 inches of Icynene (an open-cell foam product). The team vacuumed out the very aged blown in cellulose that was in the attic.

While we have seen our bills go down, I am wondering if we might be even more comfortable with some simple rolls of fiberglass insulation added between the ceiling joists. Even though our bills have gone down, I hate the thought of sending heat up into the attic.

Opinions? Thank you for your time 🙂

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  1. GBA Editor
    Martin Holladay | | #1

    We need more information.

    1. Where are you located? (Or what is your climate zone?)

    2. Was the Icynene installed between the rafters (on the underside of the roof sheathing) or between the joists (on the attic floor)?

    3. Just to be sure that there is no misunderstanding: are you proposing to install the fiberglass batts between the roof rafters or between the joists (at the attic floor)?

  2. rmrswims | | #2

    I am in central Indiana (zone 5?). Icynene was installed between the rafters under the roof sheathing.
    I was thinking of adding batts between the joists on the attic floor. Thanks for asking!

  3. GBA Editor
    Martin Holladay | | #3

    Since you posted your question, I note that you have posted a similar question on a different page (Open-Cell Spray Foam and Damp Roof Sheathing).

    As I noted in my answer to your question on that page, "If you are in Climate Zone 5 -- most of central and northern Indiana is located in Climate Zone 5 -- then you need to install a vapor retarder on the interior side of your cured spray foam to prevent interior moisture from diffusing through the foam and accumulating on your cold roof sheathing.

    "While building scientists used to recommend installing vapor-retarder paint on the interior side of the cured foam, further research has shown that vapor-retarder paint won't work in that application.

    "To protect your roof sheathing from moisture, you need to install a layer of gypsum drywall on the interior side of your cured foam, and then you need to install at least one layer of vapor-retarder paint on the gypsum wallboard.

    "For more information, see Joe Lstiburek Discusses Basement Insulation and Vapor Retarders."

    If you have spray foam insulation installed on the underside of your roof sheathing, you can install additional insulation on your attic floor if you want, but there are some caveats -- and some experts warn that it's not a good idea. To learn more about that issue, see Flash and Batt in the Roof.

  4. rmrswims | | #4

    I did post the question outright on another page, but found this string that was recently active so thought I'd try to reach out that way. Anyway. . .

    Ouch - I don't see how to accomplish that task of hanging drywall in the roof deck- the Icynene is hanging from there like upside down lemon meringue pie. At any rate, I guess I should wait on the batts or skip them altogether

  5. GBA Editor
    Martin Holladay | | #5

    The easiest way to add drywall to the interior of your roof assembly would probably be to first install 1x4 strapping, 16 inches on center, screwing the strapping through the bumpy insulation to the rafters. Of course, this method would require you to shim the strapping as necessary to make it co-planar.

  6. rmrswims | | #6

    I can't just paint the Icyene with Drylok? :)

  7. wjrobinson | | #7

    Rachel, I have two homes doing fine with just vapor barrier paint spray applied on open cell. There are literarily thousands of home as such with no problems. Martin is quoting what is now considered best method but Martin has zero actual experience.

    Your home may be fine as is. A core into the foam at the upper part of your roof to check moisture is the most I would do to allay any lingering concerns you may have.

  8. wjrobinson | | #8

    Martin, every home with spray foam applied to the roof underside has drywall that is painted and is an air barrier and ignition barrier, the drywall layer of the ceiling.

    That layer has been working for a decade or more for me and many spray foam companies in NY anyway.

    Adding drywall after the fact or even planned for in an attic is rediculous. Cost, how to do such, getting a real air right install rediculous.

    I totally disagree with your advice as one who does this type of construction verses one who writes about such.

  9. Expert Member
    Dana Dorsett | | #9

    Vapor barrier latex sprayed onto open cell foam performs at about 5.0 perms, and order of magnitude more vapor permeable than it's ~0.5 perm rating when painted onto wallboard.

    This higher permeance isn't a problem for the east/south/west facing roof pitches of a roof with dark shingles, but is potentially a problem for north facing or heavily shaded roof pitches. It's also not much of a problem for roofs with plank sheathing.

    It IS a problem on north facing pitches when the sheathing is OSB (aka "mold-food") , especially those with "cool roof" shingles designed to reject solar gains.

    Open cell foam doesn't do so well on north facing pitches with OSB in zone 5 without any sort of vapor retarder. See Table 3 of this document:

    But with a spray-applied ~5 perm vapor retarder it's not terrible. Take this document as a bedtime story for the next couple of weeks:

    In particular see Figure 10 which shows the moisture levels in the OSB simulated over several years in a Minneapolis (zone 6 climate, colder and more susceptible than yours), and the verbiage below it that reads:

    "Because the ocSPF more easily tracks interior RH, a comparison using only ocSPF was
    completed. Figure 11 shows the effect of increased RH on a system using ocSPF and a 5-perm
    coating with a 0.10% rainfall leak. With elevated interior wintertime RH of 40%, the plywood
    sheathing MCs rise above 25%, but are not sustained for a significant period of time. If interior
    RH is kept below 35% in this climate zone, this system can operate safely. "

    The 5-per coating they're discussing is described in the paragraph preceding figure 10:

    "5 perm coating was modeled, as this represents the actual effective achieved perm value based on BSC experience with spray-applied Class II vapor retarder coatings."

    That Class-II vapor retarder coating is precisely "vapor barrier latex" paint.

    So, if you keep the interior RH at 35% or less in the middle of winter (not hard to accomplish, but monitor it), you'll be fine using vapor barrier latex sprayed onto the o.c. foam as the vapor retarder, even on the north side of the house, even with OSB sheathing.

  10. GBA Editor
    Martin Holladay | | #10

    I am not advocating the use of spray foam. Spray foam is an expensive insulation that carries a small but real risk of problems with lingering odors. Moreover, spray foam needs to be protected from ignition to reduce the risk of fire.

    These are real issues, and every builder or homeowner needs to consider these issues before deciding whether or not to use spray foam.

    Rachel asked for advice about her house, which already has spray foam installed.

    When it comes to protecting spray foam from fire or ignition, the first requirement is to satisfy your local building inspector. Beyond that requirement, it's up to every builder or homeowner to decide whether they are comfortable living in a house with exposed foam in the attic. Many experts prefer cured foam to be protected by gypsum drywall.

    Concerning the risk of moisture accumulation in roof sheathing that has been sprayed with open-cell spray foam: the risk is real in cold climates, and many building scientists have made moisture measurements to prove that the risk is real. Again, I'm providing advice, backed up by data. Once you've read my advice, it's up to you to decide whether to follow it.

  11. rmrswims | | #11

    I have a bunch of questions.

    1) Most of the answers here focused on vapor issues - does that mean you agree with Martin that adding fiberglass rolls in my ceiling joists is not a great idea? I thought keeping more warm air from going to the attic would help to prevent condensation/vapor issues. Which leads to my next thought.

    2) Apparently the focus is on indoor moisture is that coming from showers and cooking for the most part. Is that correct? Would it help to put boxes over the can lights in bathrooms and the kitchen to prevent moisture from moving that way?

    3) My house was built in 1960 (a hip roof ranch - spraying paint over the foam in those low corners sounds next to impossible!) Not sure what the sheathing would have been in that time period - any guesses?

    4) Around here humidity is a summer issue, but the focus seems to be on winter - from the house is my understanding. Does the summer humidity not matter much?

    5) My understanding is that Icynene is a foam that helps prevent the spreading of fire. Look a this video:

    6) Dana - thank you for the articles. I will try to spend more time absorbing them, but as a lay person my eyes are crossing after reading for awhile. I did go to the Icynene page and found that it is <5% on water absorption. Not sure where that fits in the buildings in your article.

    7) AJ - thank you for giving me mental permission to core out a bit of the foam and check the state of things. Do you think I can (or should) do that on an annual basis?

    Thanks again for all your thoughts and time - you are a bunch of smart people!

  12. GBA Editor
    Martin Holladay | | #12

    Q. "I thought keeping more warm air from going to the attic would help to prevent condensation/vapor issues."

    A. Indeed, that would help. But fiberglass batts are useless for air sealing. If you want to prevent warm air from entering your attic, you need to seal air leaks in your ceiling. This article explains what you need to know: Air Sealing an Attic.

    Q. "Would it help to put boxes over the can lights in bathrooms and the kitchen to prevent moisture from moving that way?"

    A. Yes, as long as you followed best-practice recommendations. For more information, see Recessed Can Lights.

    Q. "Around here humidity is a summer issue, but the focus seems to be on winter - from the house is my understanding. Does the summer humidity not matter much?"

    A. Moisture problems in homes can occur in winter -- when warm, humid interior air can condense on surfaces that are cooled by outdoor air -- or in summer, when warm, humid outdoor air comes in contact with air conditioning ducts. The vigilant homeowner never sleeps, summer or winter.

  13. Expert Member
    Dana Dorsett | | #13

    The indoor moisture drives affecting roof deck isolated to high humidity events/locations like showers and kitchens. Average wintertime indoor air typically has a dew point higher than the average winter temperature of the roof deck, so moisture that finds it's way from the interior space to the roof deck stays there until the weather averages take the roof temp higher.

    Direct solar gains on the roof the warms up the roof deck considerably even when the outdoor temps are still cold, which raises the average temp, lowering the moisture content on the sunny side of the roof. But there isn't much solar gain on the north facing pitches, which is why the problems are far more likely to occur on north facing or shaded roof decks.

    The dew point of 68-70F/35% RH indoor air (typical average humidity for reasonably tight, not super-tight houses in climate zone 5) is about 40F. Most climate-zone 5 locations have January mean temps in the 20s, with whole-winter averages in the high 20s or low-30s. Your shaded & north facing roof decks will average temperatures well under that 40F dew point average for the interior air, and can collect quite a bit of moisture even though 5-perm paint, but not enough to be a guaranteed disaster. But half-pound Icynene is rated at 11 perms @5.5"- it allows more than twice as much moisture if left untreated.

    In 1960 most roof decks were done with 3/4" plywood, which is quite a bit less susceptible to mold/rot issues than OSB, but not nearly as robust to this kind of moisture cycling as ship-lap or other planking.

  14. wjrobinson | | #14

    Lots of blah blah blah lions and tigers and theories oh my. The hot air in this thread including my own is driving moisture from my fingertips as I type.

    Look, those of us that have used open cell foam have had no issues for the most part. I have no problems at the builds I used it in. None. Dry as a bone. OSB was used though a despise the stuff. Dana is right boards and plywood handle moisture much better than OSB. I also now prefer to not install an entire home with spray foam. There are many ways today to insulate a home quite a bit better than the days of just two decades ago.

    Rachel, Martin answered your questions well in his last post. As to doing a core check you could do it one time and be done with that if all seems fine and dry. Hardwood flooring installers have moisture meters that could be borrowed to check the moisture of the roof sheathing. An eyeball and nose check too. If you see no evil then you should be fine. IMO I think this thread is raising unfounded alarms for your particular home Rachel.

    Separately from Rachel's needs, YES we now think closed cell is safer to use where I live than open cell but where I am we still install it on a roof plane with no drywall in contact, the drywall fire barrier being the ceiling. There are a very few places where this may not be allowed but most codes do allow the foam to be exposed in an attic that has limited access etc and also at the rim joist in a basement. Rachel, your home is done so none of this applies to you at this point IMO, please no lawsuits!

    Today in 2014 i now prefer 2' of cellulose on an attic floor after ceiling the drywall air barrier well and less use of spray foams. Also to use less energy one should live in a smaller space. Simple. Many ask here how to fix old homes too. Financially to me it makes way more sense to sell a problem home and then buy a solution. There is no financial gain to be had super insulating any existing home. Does not pencil our verses starting from scratch. But we all make decisions based on emotions as much as logic... the fun part of life.

    Rachel, I think you should do... some ceiling plane sealing and call it a day. The neatest way to cover a recessed can light posted here at GBA somewhere is to foam glue Styrofoam cooler over the light. One of my favorite tidbits picked up here at GBA. I also need to acknowledge that the two best that post here are Martin and Dana. I learn something from everyone of their posts.

  15. rmrswims | | #15

    Styrofoam coolers! Love it :) I will definitely look into this.

    Considering sun exposure on the north side of the house- suddenly the hip roof looks good - the low angle enables the north side to get more sun than a steeper roofline. So I feel a little better about my hesitation to go spray dry lock paint on the north side.

    Just one more question - if fiberglass insulation has an "R" value - but does not stop air flow - what does it do - block radiant heat from flowing?

  16. GBA Editor
    Martin Holladay | | #16

    R-value is an indication of resistance to heat flow, not resistance to air flow. It's useful to have both an air barrier and a thermal barrier (insulation).

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