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Polyisocyanurate sheet as top attic insulation layer for radiant barrier

Gshps4all | Posted in Building Code Questions on

Does the building code permit me to place polyisocyanurate sheets as my top attic floor insulation layer? I realize that might not be ideal because of the R value roll-off at low temperatures relative to other insulation choices, but I want the radiant barrier, the rigid sheet will make the floor walkable, and our winters are getting milder, shifting our energy use more towards cooling, which, unfortunately, is a trend that, as GBA readers know, will likely continue in the coming years.

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  1. Expert Member
    Akos | | #1

    There is a common misconception that radiant barrier seems to be this magical thing that drastically cuts your heat gain. This is definitely the case in older homes with no insulation, but once you add in insulation, radiant barriers simply don't matter. An up facing radiant barrier will do even less over time as it will be covered in dust at which point it stops working as radiant barrier.

    Your money is best spent adding an extra inch or two of insulation to the attic floor. If you want an attic floor, the best is to add new joist running perpendicular to the existing ceiling joists, fill the whole thing with fluffy insulation and install plywood as the walking surface.

    As with any energy improvement project, the biggest return is not on increasing insulation but air sealing. A bunch of extra fluffy insulation does nothing to stop air leaks from the house into the attic, these need to be sealed up with canned foam or caulk. The main culprits are light fixture device boxes and wall top plates. This not fun work but relatively easy if you can move the existing insulation out of the way.

    1. charlie_sullivan | | #2

      This is the right answer. I'll add that a vapor impermeable layer on top of thick insulation could also result in condensation problems on the underside of that layer.

      I think the confusion people have is that they think that radiant heat is somehow traveling through the roof, insulation and ceiling, with the insulation doing nothing to stop it. The truth is that radiation from the sun is already stopped by the roof. There is radiation from a hot roof down to the top of the insulation, but it's stopped by the first cm or two of insulation, and there's no need for an additional radiant barrier layer.

      People know that other kinds of radiation, X-rays and radio waves, go through walls. But thermal radiation does not. You need special exotic materials to allow thermal radiation through, just as you need special materials (such as glass) to pass light. It's harder to find materials that pass thermal radiation--ordinary glass blocks it.

    2. Gshps4all | | #4

      Thanks for your feedback. I was surprised that all three responders were so strongly opposed to radiant barriers. Here are my thoughts:

      If an attic is well ventilated, then an effective radiant barrier (infrared mirror) on the attic floor insulation top can help maintain that surface closer to the ambient temperature rather than the much higher roof temperature. Without the reflective layer, the infrared radiation from the roof will bring the top of the insulation substantially closer to the roof temperature making the driving temperature drop across the insulation stack substantially higher.

      For example, if the internal temperature is 75F, the ambient temperature is 90F, the roof temperature is 140F, and the top of the insulation is either 100F or 120F depending on whether an effective radiant barrier is used, then the thermal conduction is (100-75)/R versus (120-75)/R or 80% higher without the barrier implying that an 80% thicker insulation stack would be needed without the barrier.

      The radiant barrier has to be facing the roof to work. I agree that means that dust will be a problem. Most folks won't want to climb into their attic periodically to dust their thermal mirrors, so the question is how quickly dust will accumulate and how rapidly the dust will degrade performance. I haven't found any data on that, but I can understand your skepticism that the infrared mirrors will function well over long periods of time. I just want to argue that they could be an effective component if maintained properly.

      Would the fire code permit the placement of metal-faced polyiso as the top layer?

      1. Expert Member
        DCcontrarian | | #5

        I think the reality is you won't see that big of a temperature difference with a radiant barrier.

        All of the heat that hits the roof has to go somewhere. It's either going through the ceiling or out the vents. In order for less to go through the ceiling, more has to go out the vents. In order for more to go out the vents, the attic air temperature has to be higher. But if the attic air temperature is higher ... more heat goes through the ceiling.

        I just don't see a scenario, with or without radiant barrier, where the surface temperature of the insulation is much different from the air temperature of the attic.

        1. Gshps4all | | #6

          It is critical to have good attic ventilation. You're correct that the roof is going to try to shed its heat. It can only do so through conduction, convection, and radiation. If the thermal mirrors reflect its radiation back at it, it will conduct its heat through the air adjacent to the roof. As the air heats up, it will rise and exit the ridge vent through convection. The resulting low pressure sucks ambient-temperature air in through the soffit vents which will flow over the top of the insulation because that is the coolest spot while the air that was over the insulation will rise to replace the air exiting the ridge vent. This circulating convection airflow, in combination with efficient infrared mirrors, will keep the insulation near ambient temperature. Just as dust reduces the efficiency of the radiant barriers, a failure to keep the soffit vents free of insulation and debris, and the installation of a ridge vent that has too narrow and twisted airflow paths will allow heat and moisture to get trapped in the attic air.

          1. Expert Member
            DCcontrarian | | #7

            Except there's no evidence that what you're describing actually happens to any great extent. Here's an alternative hypothesis: the foil reflects more radiated heat into the attic. This causes the temperature in the attic to rise, leading to more heat moving through the ceiling due to conduction.

            The foil coating on polyiso insulation raises the r-value by 0.5. There's really no reason to believe it does any more in the roof than it does in a wall.

  2. Expert Member
    BILL WICHERS | | #3

    Polyiso alone won't make the floor "walkable". Polyiso isn't all that strong structurally, just like other rigid insulation. You'd need some kind of plywood or OSB to act as a structural floor to be walkable.

    You'd be better off just adding more loose fill insulation, assuming this is the usual vented attic with an insulated floor which is what I think you have here. I wouldn't put a lot of effort into optimizing cold climate homes for warm weather, thinking things will get warmer in the future, either. The reason for this is that insulation works "both ways", so more insulation for the heating season will also help you with cooling season. Radiant barriers in reality tend to be about R1 worth of insulation, which is the equivalent of a bit more than a quarter inch worth of loose fill cellulose insulation. The polyiso part of the polyiso would be doing much more for you than the radiant barrier properties of the foil facer.

    Regarding absolute temperature changes, we've seen near record colds in my area in the past few years, with this year being milder. I wouldn't count on the heating season going away any time soon. Insulate the usual way for your climate zone and you'll be in good shape. If you want to save more on cooling costs, add more loose fill insulation to what you already have and you'll save money for cooling AND heating.


  3. Gshps4all | | #8


    R value is a figure used to measure the ability of insulation to block conductive heat flow. A radiant barrier, by definition, is blocking radiant heat flow not conductive heat flow so it does not have an R value.

    It sounds like you are implying that the radiant barrier is adding heat to the attic. The barrier is simply reflecting heat away from the ceiling that the roof has already brought into the attic. The roof will heat up the air touching it and that air will rise along the roof pitch and exit the ridge vent if it is allowed to flow freely. It is physics at work and it is the reason why attic ventilation is placed at the lowest and highest points of the attic. The purpose of the radiant barrier over the insulation is to keep the roof from being able to radiate its heat to the insulation during the convection cooling process. If the IR mirror is good and the ventilation path is open, the process will and does work.

    1. Expert Member
      DCcontrarian | | #9

      "R value is a figure used to measure the ability of insulation to block conductive heat flow. A radiant barrier, by definition, is blocking radiant heat flow not conductive heat flow so it does not have an R value."

      If you go to the store and buy polyiso insulation, if it has a foil facing the listed R-value is 0.5 higher than if it doesn't. This is true of all thicknesses.

      "It sounds like you are implying that the radiant barrier is adding heat to the attic. "
      This is not at all what I'm saying. Rather, I'm saying that the amount of heat entering the attic is constant, and the radiant barrier doesn't reduce it or remove it. There's basically two places that heat can go -- either out the vents through convection, or into the ceiling through conduction. In order for the amount of convection to increase, the attic temperature has to increase. And if the attic temperature increases, the amount of conduction is going to increase as well.

      The more important point is that in an insulated attic, radiation between the roof and ceiling below is not a significant source of heat transfer. Four different people on this thread have told you that, yet you keep insisting it is. I don't know why you even came here seeking advice when you keep rejecting the opinions offered, but instead insist that things must work the way you think they must work because you think they must work that way.

      So please, do this instead: Go to a Home Depot or Lowes and buy a sheet of foil-faced polyiso. Since all you care about is the foil facing you don't even have to get a thick one, you can get a half-inch thick piece for about twenty bucks. Then get an infrared thermometer gun, you can get one on Amazon for about 20 bucks. Wait for a hot day, then go up into your attic and put down the sheet of polyiso and take some readings with the gun.

      Report back what you find.

      1. Expert Member
        BILL WICHERS | | #10

        Careful there DC, IR heat measurements are unreliable on shiny surfaces like foil. You can usually get around that by putting a sticker on the shiny surface and measuring the temperature of the sticker.

        Since I'm probably the only one here that has actually made some real measurements of this on a real building (albeit a commercial one with a flat roof and no attic), I can say that a "radiant" barrier (I used white paint) most certainly does work -- if it's on the EXTERIOR of the assembly, so that it reflects the heat back into the atmosphere. If you put that same barrier inside, you get very nearly no benefit from it, which I would attribute to basically the process you've already described.

        Radiant barriers used INSIDE of attics gain a teensy bit of R value, usually said to be around R1. That's it. Nothing magic, and certainly not something able to replace conventional insulation.


      2. Gshps4all | | #11

        Let's back up a minute.

        First, my question had nothing to do with radiant barriers. My question was: "Does the building code permit me to place polyisocyanurate sheets as my top attic floor insulation layer?" No one answered the question so I repeated the question in my first response. Still no one answered the question.

        Second, neither in my original question nor in any follow-up have I stated or implied that I would be substituting a radiant barrier for conventional insulation. In my original question I said I would, "place polyisocyanurate sheets as my top attic floor insulation layer."

        Third, while insulation manufacturers may be permitted to equate a radiant barrier with a certain R value, as Bill notes, the ability of a radiant barrier to block heat transfer is going to vary dramatically depending on the conditions in which it is used. You are assuming that the air temperature within the attic is constant. If there is convection, that is not correct. Hot air is at the roof surface and cooler air is entering at the eaves so a temperature gradient is established between the insulation and the roof. The infrared mirror of the radiant barrier increases that gradient so the roof stays hotter and the insulation top surfaced cooler than if it were not there. The effect will be greater in climates where the delta between roof and ambient temperatures is higher.

        We are not the first to argue over the effectiveness of radiant barriers ( In this case, the radiant barrier would be adding zero cost because it is an integral part of the polyiso sheet, so if it provides any energy savings, that is a net positive.

        1. Expert Member
          DCcontrarian | | #12

          The topic you chose for your posting was "Polyisocyanurate sheet as top attic insulation layer for radiant barrier" so sorry if we interpreted that to be about radiant barriers.

          And even though you insist that your question is not about radiant barriers, you keep insisting that radiation is a significant source of heat transfer in an attic. It isn't.

          Moving on, the purpose of attic venting is to eliminate moisture which otherwise would accumulate under the roof. That vapor comes from within the house. The way an insulated ceiling should be constructed is that the ceiling itself should be at least a vapor retarder to keep the moisture out of the attic, and the insulation above it should be vapor open to allow whatever vapor does get through to dissipate into the attic.

          It would be a bad idea to put an impermeable layer over the top of the insulation. That would trap moisture in the insulation and potentially lead to rot and mold. Different polyiso products have different levels of permeability depending on the facing, the unfaced product is actually pretty permeable. So the answer would depend upon exactly what product you're proposing.

          If you use an impermeable insulation on the outer layer, the building code requires that it be thick enough relative to the rest of the insulation that the inner face of the insulation never get cold enough for condensation to occur. Exactly how thick that is depends upon your climate zone. The code also requires that the ceiling assembly be somewhat vapor open to below , so that if moisture does get in there is some chance of it getting out again.

          By sealing the existing insulation from above you would in effect be creating an unvented roof. In general vented roofs are easier and more reliable. Converting a vented roof to unvented is not really something you want to do.

          1. Gshps4all | | #13

            Great, thanks for this information!

          2. Expert Member
            BILL WICHERS | | #17

            I would note that as far as I know, only foil faced polyiso can act as a radiant barrier, due to the foil facing. ALL types of polyiso that are more vapor open use some kind of facer that will NOT act as a radiant barrier, typically kraft paper or a fiberglass mat. You basically have to choose between "free radiant barrier from the foil facer", or "have some vapor permability due to a fiber facer". You can't have both with any product I've seen.


        2. jollygreenshortguy | | #15

          You wrote - "First, my question had nothing to do with radiant barriers. My question was: "Does the building code permit me to place polyisocyanurate sheets as my top attic floor insulation layer?" No one answered the question..."

          I totally understand your frustration. I was curious about the issue when I read your question and a bit frustrated myself, after reading the entire comment string. Just now I did a bit of researching and as best as I can tell, you should be able to have exposed polyiso in your attic under specific conditions. Whether you meet those conditions I can't know. But here is the relevant IRC 2021 code section: R316.5.3 Attics

          edit - Having re-read the code section as well as related code sections, it looks to me unlikely that your case would meet code. The polyiso would need to be part of the "roof assembly" rather than the attic insulation.

    2. Expert Member
      Michael Maines | | #18

      Response to #8: I just need to note that despite common misunderstanding, R-value testing measures all forms of heat flow, not just conductive heat flow. R-value testing is determined by the ASTM C518 testing method which measures the rate of heat flow through the material, between two plates that are 50°F different in temperature. Martin wrote about it here:

  4. BirchwoodBill | | #14

    I would not assume any climate prediction, I,e., getting milder. Wait another 100 years….

  5. Gshps4all | | #16

    jollygreenshortguy thanks for the code link. R316.5.3 states that the barrier material is not required if "the foam plastic insulation has been tested in accordance with Section R316.6" and R316.6 lists a set of specific tests "or fire tests related to actual end-use configurations." PIMA Technical BUlletin #104 on the website states that polyiso is approved for open installation in roof systems which, as you note, is not good enough. I recall that long ago I purchased a version of RMAX that was fire-rated for open installation in walls. One would think that, if the insulation was approved for open interior wall installations, it could also be used in attics, but unfortunately I haven't been able to find the specific product now.

    I guess it is immaterial because of the moisture-control problems that DC_Contrarian described.

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