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Foil faced ridged foam board near boiler?

MaineGuy80 | Posted in Energy Efficiency and Durability on

We had an energy audit performed last week and they suggested that we spray foam our basement walls as that is causing heat loss.

I’m not sure that I want to go that route or even worry about it, however, I am wondering if this product would be 1) Better, 2) Safe around our Oil boiler (it’s a ground model, about 8-12″ from the wall. Here’s the product: Johns Manville (Common: 2-in x 4-ft x 8-ft; Actual: 2-in x 4-ft x 8-ft) AP Foil R-13 Faced Polyisocyanurate Foam Board Insulation —

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  1. Expert Member
    Peter Engle | | #1

    Rigid foam board like the one you suggest can be used to insulate your basement walls. Polyiso foam can absorb water, so you don't want it if the walls are frequently damp. It is best to also hold it an inch or so off of the floor in case of flooding.

    As far as its location near the boiler, this type of foam does require a "thermal barrier" if it is used in an exposed location, so it would have to be covered with 1/2" drywall or something equivalent. Other than that, it should be fine within a few inches of the boiler. The boiler should have a label on it showing the allowable clearance to combustible materials. For these purposes, the foam and/or the drywall covering would be considered "combustible." If there's no label on the boiler, you can look up the installation manual online for most reasonably recent boilers.

  2. MaineGuy80 | | #2

    These walls don't have water problems coming through- at least they haven't in the 10 years we've been there. I'll check the boiler label.

    With that said about the cement walls, it's safe to put right up against it? I was not planning on putting anything in front of it, instead leaving the foil exposed. If this requires the thermal barrier, how does spray foam differ?

  3. Expert Member
    Dana Dorsett | | #3

    As long as the code clearances to combustibles are met for the exhaust venting and manufacturer's installation instructions clearances to walls are met, as long as there is half-inch wallboard over the polyiso would be fine.

    Vent clearance requirements depend on venting type:

    Can you share manufacturer & model number of that boiler?

    1. MaineGuy80 | | #8
      1. Expert Member
        Dana Dorsett | | #9

        See the clearances info on the bottom of page 3:

        Looks like zero clearance to combustibles is fine for everything except the front side for that boiler. They suggest nine inches from sides & rear for reasonable access.

  4. MaineGuy80 | | #4

    I will get that information tonight.

    The vent is a direct vent that goes through a sidewall. I would not be insulating around that, only the cement walls around the oil boiler. I would not put OSB over it, so if that is a code or rule I just won't bother. I was planning on gluing the foam to the wall and taping it- then just leaving it.

    1. Expert Member
      Dana Dorsett | | #5

      > I would not put OSB over it, so if that is a code or rule I just won't bother. I was planning on gluing the foam to the wall and taping it- then just leaving it."

      While more fire-safe in general than other types of foam, in most states only a fire-rated polyiso product can get away without a timed thermal barrier against ignition from a code point of view (whether it's next to the boiler or not.)

      OSB doesn't cut it as a thermal barrier but half-inch wallboard does.

      JM AP Foil polyiso is not a fire rated foam board.

      Dow Thermax is:

      1. MaineGuy80 | | #6

        Interesting, the auditor recommended just 2 part spray foam around the boiler and not to cover it up.

  5. Expert Member
    Peter Engle | | #7

    Spray foam contractors routinely reduce their costs that way, but it doesn't meet code. Spray foam also requires a thermal barrier. Here too, 1/2" drywall works fine. Most spray foam manufacturers have also qualified "intumescent paint" that works as a thermal barrier. The stuff is easy to apply, but expensive. If the spray foam installers had to install code-required levels of foam and code-required thermal barriers on every job, they would install much less spray foam because it would be even less cost competitive.

  6. Expert Member
    BILL WICHERS | | #10

    Johns Manville makes a product called “CI Max” that is polyiso rated to be left exposed just like Dow Thermax is. I’d use one of these two products around the boiler for some extra protection even if it isn’t required by your local inspector.


  7. MaineGuy80 | | #11

    Just to clarify, Zephyr7 and Dana, are you saying there should be a fire barrier only around the boiler or along the entire basement wall?

    1. Expert Member
      Peter Engle | | #12

      There should be no exposed foam anywhere in the basement. All foam requires some form of fire protection. 1/2" drywall works fine, so finished basement walls are OK. If you are considering using rigid foam and not finishing it, then the Dow Thermax or the JM CI Max should be used. They have aluminum facers that provide the thermal protection required by code as well as providing a more attractive and durable finish. Both come with either white or silver surfaces. Spray foam insulation on basement walls requires either 1/2" drywall or a manufacturer-approved fire retardant paint coating (intumescent paint).

      1. MaineGuy80 | | #14

        Even if the foil is facing out? I'm just confused about that as the foam side faces the cement.

        Looking further at the report, they did say to spray it with intumescent paint.

        1. Expert Member
          BILL WICHERS | | #15

          Foil-faced polyiso typically has foil facers on BOTH sides. The products like dow Thermax and johns Manville CI Max have ONE side with thicker foil that is rated to be left exposed. If you’re using one of these two products, you must have the fire rated facer facing towards the inside of the structure.

          If you’re using “regular” foil faced polyiso, it doesn’t matter which side faces which way, but you’ll need to cover it per code with something like drywall.


          1. MattJF | | #20

            Standard thermax is silver aluminum 1mil both sides:

            There are no restriction I have seen on which side may face out to qualify as a thermal barrier. The fiberglass in the foam is what improves the fire performance.

            These are the other variants available that would be applicable to basements:
            Themax White Finish 1.25mil white, .9 mil aluminum

            Thermax light Duty 1.25 mil white, 1.25 mil aluminum

            Thermax Heavy Duty 4mil white, 1.25 mil aluminum

    2. Expert Member
      BILL WICHERS | | #13

      Code is that ALL foam needs to be either covered (usually by at least 1/2” drywall), or be rated to be left exposed. The two rated products mentioned here have a thicker facer on one side and have been tested and approved to provide the necessary fire resistance. The idea is to keep the foam from igniting and spreading a fire around the structure.

      Sometimes local building departments won’t require the fire rated materials (you’d have to check with your city) in some situations, and if that’s the case for you then I’d recommend you put on the fire rated material anyway around the boiler just for some extra safety. Safest is to use the fire rated material everywhere.


      1. CMObuilds | | #23

        As someone else mentioned, it is not the facers that provide the barrier, it is the fiberglass in the foam. The thicker facer versions are just for durability and are very expensive. Standard Thermax typically used in residential basements has 1 mil facers on both sides. Label side out is for inspectors to confirm the product installed if left exposed.

  8. MaineGuy80 | | #16

    Thank you everyone, I will probably just leave the cement exposed. Is this fire retarder paint safe to have in our house once cured?

    1. Expert Member
      Peter Engle | | #17

      It is, but painted spray foam is not pretty. One of the advantages of the Thermax or Ci Max is that the aluminum facers are pretty rugged, and they come in white. If you tape the seams with their white aluminum tape, you get a serviceable wall surface that can suffice as your finished wall in utility areas. Thermax even comes with textured aluminum surfaces (think old 'fridge door surfaces) that are reasonably attractive. Their Ag-board has rugged enough surfaces for powerwashing. For a one-stop wall material, it's not bad.

      1. MaineGuy80 | | #18

        The Thermax product would cost about $1240 for the product alone- not sure about shipping. I'm trying to find a local vendor. I like the idea of a smooth wall vs the spray foam. I would probably get a Dow spray foam kit and spray on top of it (I already sprayed my rim joists).

        Our walls don't get moisture, atleast since we've been there. I shouldn't be worried about that with this product either?

        1. MattJF | | #19

          The Home Depot pro desk can order Thermax.

          Check craigslist for reclaimed foam. These guys have had thermax seconds in the past

          The facer on standard thermax is only 1mil, so don't expect super durability. But it can work. I would put something else up if the space will be used as a shop or kids play room. There are versions with thicker facers, but I don't have experience with them.

          Another option would be reclaimed foam. Last price I got was 3" foam for $20/sheet. 23/32 sheathing counts as a thermal barrier. Basic 23/32 OSB runs around $20/sheet. You can screw right through the OSB to hold the foam in place.

          Place 1" or 1.5" strips of EPS foam on the floor under the polyiso to keep it away from any water.

          1. MaineGuy80 | | #22

            Good ideas- What about placing 1/4" plywood over foam? Assuming it's against the code since it's combustible.

          2. MattJF | | #24

            You need a thermal barrier, which is 1/2" drywall or 23/32nd sheathing unless using Thermax or similar rated foam. Thermax with 1/4" plywood would work.

            The one exception to this is rim joist are allowed up to 3.5" of foam without a thermal barrier per IRC 314.5.11. Check how your local code has adopted this provision.

  9. Deleted | | #21


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