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Corrugated metal indoors (as ceiling/wall panel)

Don J. | Posted in Green Building Techniques on

Dear experts, I need some advice.

Instead of drywall we want the ‘barn’ look. We want to install these wavy corrugated metal panels on the ceiling and perhaps walls.

If you’ve been to Joe’s Crabshack, that’s what we want it to look like, roughly.

are there any concerns using galvanized steel like that indoors?
how about air quality?
fire hazards?

thank you so much

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  1. GBA Editor
    Martin Holladay | | #1

    Steel roofing will not create a fire hazard. Your only potential problem might occur if you are building in a hot, humid climate, and if you expect to use an air conditioner for a significant percentage of the year. If that's the case, you want to be sure that the building doesn't have so-called "reservoir" siding (e.g. brick or stucco) with a vapor-permeable sheathing -- a combination that will set you up for inward solar vapor drive problems.

  2. Don J. | | #2

    thanks so much for answering.
    Do you see any issues for air quality? Would galvanized steel react with something in the air?
    should we paint it?

    one more thought: we have foam boards underneath which are supposed to be covered with a fire barrier like drywall. would steel still be fine?

  3. Expert Member
    Dana Dorsett | | #3

    The galvanizing isn't the issue, the vapor-barrier aspects of the material is. If you air-condition the room to below the outdoor dew point, (== all summer long in FL) you'll have condensation on the metal inside the wall cavity, creating mold conditions inside the wall, and you'd inevitably end up with high mold-spore counts in the indoor air.

    If you air-seal the foam so that none of he cavity air reaches the steel the mold issue subsides, since the other side of the foam would be above the outdoor dew point (at least most of the time). But steel on it's own isn't a recognized ignition barrier, but if you used fire-rated polyiso (some versions of Thermax) for the foam you'd be good to go.

    This issue isn't to be ignored- if the foam boards are polystyrene (EPS or XPS), they would actually melt where they were in contact with the metal in the event of a fire, dribbling flaming liquid plastic liquid out the bottom. Polyiso never melts, even when fully engulfed in flame, and has a higher kindling temp than polystyrene, which is how it's possible to make a fire-rated version with only thin-metal facers. But not all polyiso meets spec without a separate ignition barrier- you have to be sure to get the right stuff.

  4. GBA Editor
    Martin Holladay | | #4

    You wrote, "Steel on its own isn't a recognized ignition barrier."

    I disagree. In sections R314.5.3 and R314.5.4, the IRC defines an ignition barrier as one of six permissible materials: 1 ½-inch-thick mineral fiber insulation; ¼-inch-thick wood structural panels (e.g., plywood); 3/8-inch particleboard; ¼-inch-thick hardboard; 3/8-inch-thick gypsum board; or corrosion-resistant steel having a base metal thickness of 0.016 inch.

  5. Expert Member
    Dana Dorsett | | #5

    Martin: R314.5.3 and R314.5.4 are relaxed specifications for crawlspaces & attics that have no ignition sources, not the ceilings & walls of occupied spaces.

    The prescriptive thermal barrier on foam for occupied spaces is half-inch gypsum or (or better).

  6. GBA Editor
    Martin Holladay | | #6

    I know the difference between an ignition barrier and a thermal barrier. You wrote, "Steel on its own isn't a recognized ignition barrier." But it is.

  7. Expert Member
    Dana Dorsett | | #7

    OK, I'll bite: S'plain me how he can meet code with the steel smack up against polystyrene?

    Are we being a bit too particular about the semantics, mayhaps?

    Under IRC 2006 the more narrowly defined ignition barrier is allowable in the limited cases of attics & crawlspaces. Elsewhere the thermal barrier described in 314.4 is required:

    Under IRC 2009 & 2012 the section numbers and some of the verbiage have changed, but the requirements, not so much:

    The distinction is only meaningful if we were talking about an attic or crawlspace with limited access, and limited air communication with the conditioned space. But sure, galvanized steel meets the narrower IRC definition of "ignition barrier", but will not meet code here on it's own.

    If the semantic distinction was your point, I'll accept it (mea culpa). But it would be important to spell it out for Don J., since the implication might have been taken that metal on plastic meets code without the gypsum, when it actually doesn't (which was and still is my point.)

  8. GBA Editor
    Martin Holladay | | #8

    For code geeks like me, the distinction between an ignition barrier and a thermal barrier is important. It seems you wrote "ignition barrier" when you meant "thermal barrier." No biggie -- but when we all use the correct vocabulary, misunderstandings are avoided. My guess is that the next time you want to refer to a thermal barrier, that's what you'll call it.

  9. James Morgan | | #9

    Fire barrier apart, I'm surprised nobody's mentioned 'air barrier'. Galvanized tin is not one, neither is foam board unless it's carefully taped. A layer of taped and mudded drywall under the tin will be more reliable, longterm. You don't need to finish it.

  10. Don J. | | #10

    interesting discussion! I learned during my dissertation days that sometimes it's best not to read too much in books and go hands on instead ;-)

    OK, here's what we have and what we are planning to do:
    We have XPS installed between rafters with a 1/2 to 1 inch gap, filled with canned 'great stuff'. totally sealed, open at the top where a tunnel is formed which connects all rafters to the gable vents. it works better than I thought. The entire system is equipped with hygrometers all over (under roof deck, above XPS) and RH values are acceptable at all times, observed over 12+ months.

    I have experimented with vapor diffusion and in reality 2" XPS offers no drying to the interior at all in practical terms. Yes I know the perms rating states it should pass about a teaspoon of water per sq yard per day but you can forget that, it's theoretic and has no relevance. Besides we now have 4" XPS in 80% of the a vent at the top is a good safeguard

    From experience I can tell you forget taping XPS when used in this particular application. XPS sucks as nothing sticks to it well, not even canned spray foam sticks really good. Canned spray foam sticks really good to wood and fingers, hair and eyes ;-)
    However, I am very pleased with the results as there's no chemical smell, it was relatively quick to install and it brings a lot of advantages over all other types of insulation commonly used. It wasn't cheap however but we expect it to last and offer very good mold and water resistance.

    Now, we don't like drywall, there is no plywood in our house either.
    I did the mistake of using joint compound when mudding the cement boards in our finished porch and after over 18 months we can still smell it. In the kitchen I used tile thin set instead and it worked great without odor. I guess we were dogs in our previous life.

    We want the following benefits and hence thought metal panels are best so far:
    * moisture/water resistance
    * can be removed and reinstalled if need be without much effort / damage
    * low maintenance
    * there are no seams that open up in case rafters expand/contract
    * no painting necessary
    * light weight
    * relatively cheap
    * easy to install
    * no taping/mudding necessary
    * no mold since we'll be leaving vent gaps at top and bottom (it's for a cathedral kind of ceiling)

    The next best thing I can come up with is cement board at 1/4 inch but it wouldn't have all of the above benefits.

    1. So what to do next?
    2. Is there a different material we should consider?

    3. Use a special paint as fire protection? I heard it costs a lot though....

  11. GBA Editor
    Martin Holladay | | #11

    James is right -- you still need an air barrier. Expansion and contraction of your rafters (due to humidity and temperature changes) will eventually open up cracks at the edges of your rigid foam panels. If you don't want to use gypsum wallboard -- by far the easiest and cheapest option -- you could use MemBrain or one of the European membranes sold by 475 or the Small Planet Workshop.

  12. Expert Member
    Dana Dorsett | | #12

    At 2" most XPS is running 0.6-0.8 perms- a real class-II vapor retarder. At 1" it's usually a bit over 1 perm, always under 2 perms (any density or manufacturer), unless it has a facer (most thicker goods don't.) At 4" it's about 0.3-0.4 perms, which is comparable to asphalted kraft facers on batts. Stuff can still dry through that, but it's slow, for sure.

    Type-II EPS on the other hand is about 2.5-3x as vapor permeable at any given R-value, and 1lb nominal density Type-I EPS is about 4x as permeable, which is sometimes a very useful feature. (EPS has 99% less lifecycle global warming potential than XPS too, which is a real issue at higher R values using XPS.)

    Even at 5" (R19.5) Type I EPS is about as permeable as 1" (R5) XPS, and as such there are no real issues with installing it smack up tight against the roof deck in 2x6 rafter bays, at which point you can worry a bit less about the long term integrity of the air seal when can-foam at the edges. The roof deck still dry at a reasonable seasonal rate to the interior, and since it's a minimal Class-II vapor retarder, no other interior vapor retarder would be required.

    Using a flexible "Windows & Doors" version of can-foam gives you better long term integrity on assemblies (like windows and door frames) that are guaranteed to have dimensional shifts over time. It would be the preferred type for sealing cut'n'cobbled foam in framing bays, but not absolutely essential.

  13. Don J. | | #13

    Thanks Dana
    I would like to add that vapor diffusion is a two way street.
    When we have a metal roof, we usually talk about letting materials 'dry towards the interior' (in the case of an unvented roof) however winter moisture goes through the insulation in the winter, and the wood structure absorbs it.
    The winter moisture in my house (DC area) going through 6" of Icynene caused the wood planks to reach 30% moisture content. Unlike all expectations and "book knowledge" and Icynene statements we have about 'perms', the wood did not dry fast enough. I took the foam down in late spring after many many hot dry days with RH lower than 35%. Foam was installed as spec by vendor.
    On the opposite side I had another rafter which was sealed with just one 2" XPS sheet. The plank's moisture content after opening was around 15%.
    To play safe, I decided to open all XPS sheets at the top and vent them outdoors (through gable vents). The point was to keep the wood as dry as possible, even though 15% MC isn't too bad.

    I think the ideal design when you have a metal roof is using a vapor barrier with a vent gap in the middle. Or a ridge vent if you have one and seal interior 100%, 0 perms.
    I would definitely not consider again having a design that relies on drying towards the interior only without ventilation under the roof deck. You need to have an efficient escape path for moisture somewhere.

    Another reason is the relatively high hygroscopicity of open cell spray foam. Unlike what SF installers and vendors publicize, the foam holds on to a lot of vapor at night and dumps it during the hot day temps. To verify this phenomenon, place a piece of spray foam along with a hygrometer/thermometer, seal in a transparent plastic bag and place in the sun. You will see the RH remaining high as temp increases, opposite of what you would expect from a non-hygroscopic material when heated.

  14. Expert Member
    Dana Dorsett | | #14

    The vapor permeance of half-pound open cell foam like Icynene is at least an order of magnitude higher than most 1.5-2lb closed cell foams. Half pound Icynene runs about 8-11perms at 5" (depending on which version), whereas even crummy 0.75 lb EPS is still about 1 perm at 5". It's in the specs:

    That is a huge difference on how fast the roof deck can load up in winter, but is fine from a "rest of the year" drying rate point of view.

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