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When is it necessary to use foil-faced insulation?

ebt111 | Posted in General Questions on

I am insulating an exterior wall (8″ CMU with direct applied 1/2″ stucco) with 1 1/2″ rigid closed cell insulation which is placed on the inside face of the CMU in a furred out (1 5/8″ metal framing) interior wall. can I use regular closed cell insulation or is this an application which is better suited for a foil face closed cell rigid insulation.

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  1. user-2310254 | | #1

    Where are you located? The metal framing is interior to the foam, correct? Are these above-grade walls? What type of foam are you using?

  2. ebt111 | | #2

    We are located in Tallahassee, Florida. The metal framing is interior; the foam is within. We are above grade. We are using Extruded polystyrene close cell rigid foam insulation.

  3. Expert Member
    Dana Dorsett | | #3

    What means "...foam is within..." in 'merican? Interior to the furring cavities?

    If the furring is thermally bridging the foam it's a waste of expensive (environmentally & financially) foam, due to the extremely high thermal conductivity of the metal. The best stackup would be:

    outdoor air | stucco | CMU | 0.5-1" air gap | continuous rigid foam board | steel furring | wallboard | latex paint | indoor air

    If you skip the air gap there is some potential for ground water and rain moisture wicking up to the rafters & ceiling joists, but with deep roof overhangs the risk diminishes, since the rain wetting is limited. With the air gap with weep holes at the bottom of the cavity and correspondent vents to the exterior just under the eaves the CMU will dry in both directions, reducing the risk to any wood that's in contact with it. If skipping the air gap, rigid foam can be glued to the CMU, taping the seams, or an HFO blown closed cell foam would still work.

    Extruded polystyrene is one of the most environmentally damaging foam products in common use, due to the HFC blowing agents used, and as it loses it's blowing agents over time it's performance drops to about R4.2/inch (down from the labeled R5/inch). Polyisocyanurate is blown with much more benign hydrocarbons (typically pentane variants), at about 1% of the impact of HFCs, and it's somewhat higher R/inch.

    If sprayed foam is used, search out an HFO-blown type (there are now several). Over the past couple of decades post-Montreal Protocol the industry standard blowing agent for site-applied closed cell foam has become HFC245fa, a powerful greenhouse gas (about 1000x CO2). In recent years several vendors have come out with HFO1234ze blown foam, which has a very low greenhouse potential (less 3x CO2). It's also somewhat higher R/inch, and has a dramatically lower a fire-risk during curing than HFC blown foam.

    A coworker of mine recently insulated his basement with 3" of continuous HFO blown closed cell foam behind a 1-5/8" steel steel framing, with nothing inside the framing cavities. The steel was installed first 3" from the foundation wall, and the foam was applied later, using the steel as the depth gauge, which all went up very quickly.

  4. ebt111 | | #4

    The Contractor is asking for a substitution of spray foam (Specify Heatlok XT high Yield by Demilec) in lieu of rigid board. I am considering yes to the substitution, but want to be sure I am not creating a moisture problem since there is no air space in this assembly. Let me know what you think. Based on you comments blown seams to be the better solution.

  5. GBA Editor
    Martin Holladay | | #5

    We still need a clarification. When you say there is "no air space in the assembly," do you mean that the steel studs are tight to the CMU wall, with no air space?

    What stage of construction are you at? Is the CMU wall complete, with the steel studs already in place? Or are you still in the planning stage?

  6. ebt111 | | #6

    The steel studs are tight to the CMU wall. the proposed blown insulation substitution would be blown on to the CMU inside face (1 1/4" thick) in the cavity of 1 5/8" steel stud framing. 2/3 of the CMU is in place and 1/3 of the framing along the exterior walls. Between the blown insulation and the 5/8" gypsum board finish is about 3/8". Can that be consider air space? I think not. Do we have the making of a problem?

  7. GBA Editor
    Martin Holladay | | #7

    The main problem is that your concrete block wall and your steel studs are both conductors. So you have lots of thermal bridging through your wall assembly, with no attempt at a thermal break.

    It would be much better to have a continuous layer of foam insulation between the CMU wall and the steel studs. Even better (as Dana proposed) -- although more difficult to build -- would be an air gap on the interior side of the CMU wall, and then a continuous layer of foam insulation, before the steel studs are installed.

    And better still -- the best of all options -- would be a continuous later of insulation (perhaps EIFS) on the exterior side of the CMU wall.

  8. ebt111 | | #8

    I don't think the exterior wall will be getting very wet there is a 3' over hanging roof (3 stories high). Would it help to have a strip building paper between the studs and the CMU serve as a thermal break; or not let the studs touch the CMU (1/4" gap). I am assuming The spray foam is not considered a conductor. I am limit to what I can change in this assembly. Also would a insulation board work better than a spray foam in this situation. Let me know.

  9. Expert Member
    Dana Dorsett | | #9

    With the extremely high conductivity of the very short 1-1/4" path through the steel and the high surface contact of the steel stud edge the net R value of the ~R8-ish foam + R0.000008-ish steel averages out to about R2.5-R3, which is a RIDICULOUSLY high $/R. A continuous sheet of 1/2" polyiso between the steel and the CMU would deliver the same or better overall performance.

    To hit IRC 2015 code min with a CMU wall in US climate zone 2 would only take R6 on the interior if it were continuous (rather than thermally bridged by steel framing), or R4 continuous insulation on the exterior. An inch of Type-II (1.5lbs per cubic foot nominal density) on the exterior covered with an EIFS type stucco would be R4.2, which would make it. An inch of foil faced polyiso (R6-R7) on the interior side (taping the seams with foil tape) would also get you there, but would require pulling the already-installed steel framing and moving it in an inch. Depending on local labor costs even with moving the steel it may still end up being cheaper than 1-1/4" to 1-1/2" of sprayed closed cell foam (and would be twice the performance.) The exterior inch or more of EPS is probably the cheapest solution overall.

    If the top of the foundation CMU wall will be topped with a wood plate supporting the trusses/rafters/ceiling joists, install a good capillary break material such as Grace Ice & Water Shield or an EPDM sill gasket between the CMU and wood, which would protect the from wicking any rain or ground water moisture that might end up in the CMU walls. Ice & Water Shield is "self healing", and would re-seal around any penetrations made for hurricane reinforcement anchors etc. into the CMU.

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