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Class A fire-rated standing-seam metal roof assembly for an unvented attic

Stephen Watts | Posted in Green Building Techniques on

I have a few questions about standing seam metal roof assemblies.  First, Class A fire rated assemblies appear to require metal roofing directly over Class A rated underlayment over the roof deck, with no ventilation space between the metal and underlayment.  Under these conditions, does it matter whether the underlayment is vapor permeable or impermeable?  Second, would the following roof assembly (from top to bottom) be likely to work?: standing seam metal roof, underlayment, plywood, R-60 vapor-permeable insulation (e.g. 15+ inches Rockwool), smart vapor retarder, sheetrock, unventilated attic (or living space if cathedral ceiling). Note that I prefer not to use foam, if possible.

House would be built to pretty good house (near passive house) standards.  It is single-story, rectangular in shape, 1350 sq ft floor space, with an insulated concrete slab.  Walls would be vapor permeable (but airtight) with R-40 insulation.  Roof slope would be 8/12. Solar PV panels would be mounted on most of the standing seam metal roof on the south side.  Roof structure would probably be I-beams (cathedral ceiling), or trusses (unvented attic).

House will be located in northeastern Oregon at 3300 ft elevation in climate zone 5.  Annual precipitation is 24 inches, with 48 inches of snow.  Historic low temperature is -30F, and historic high temperature is 110F. Average winter temperature is around 25F and average summer temperature is around 70F. Temperature range between day and night is typically 40 degrees in the summer and 15-30 degrees during the rest of the year.  Air humidity is low in the summer and moderately low during most of the rest of the year. There are ~6800 heating degree days.

Thanks, Steve

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Stephen,
    Q. "With no ventilation space between the metal [roofing] and underlayment, ... does it matter whether the underlayment is vapor-permeable or impermeable?"

    A. No, because this type of roof assembly won't have any outward drying, regardless of whether the underlayment is vapor-permeable or vapor-impermeable.

    Q. "Would the following roof assembly (from top to bottom) be likely to work: standing-seam metal roof, underlayment, plywood, R-60 vapor-permeable insulation (e.g. 15+ inches Rockwool), smart vapor retarder, sheetrock, unventilated attic (or living space if cathedral ceiling)."

    A. No. If you are installing mineral wool insulation between your rafters, building codes require that you include a ventilation channel between the top of the insulation and the underside of the roof sheathing -- unless you are willing to either: (a) install an adequate thickness of rigid foam above the roof sheathing, or (b) install an adequate thickness of closed-cell spray foam on the underside of the roof sheathing (in other words, the flash-and-batt approach). For more information, see this article: "How to Build an Insulated Cathedral Ceiling."

    1. Stephen Watts | | #2

      Martin,

      Thank you for taking the time to respond to my questions. I have read the excellent "How to Build an Insulated Cathedral Ceiling" article several times. What I am searching for is a good roof design that has a reasonable chance of survival during a wildfire. The ways I was trying to address that was to not use foam and limit the opportunities for fire to enter the attic and roof.

      What would be a good (approximately R-60) roof design for the Pacific Northwest (east of the Cascades) to maximize ability to survive a wildfire? Standing Seam Metal Roof is preferred due to ability to mount solar PV without roof penetrations, but is not a hard-and-fast requirement.

  2. Expert Member
    Akos | | #3

    Stephen,

    Lot of standing seam metal roof can be installed directly over cross battens or Z bars. If you can move a good portion of the 15" of mineral wool the above your roof sheathing/air barrier between the Z bars, could be possible to have a foam free roof without venting.

    Akos

  3. Tim R | | #4

    To get the Class A rating on a Metal roof you need a listed assembly. GAF Versasheild is a fiberglass underlayment that gets an assembly a class A rating. Unfortunately the ICC report indicates it requires a vent roof assembly.
    The other option in DensDeck that is a gypsum panel that gets the roof assembly the Class A rating. I did not look to see if it can be used with an unvented assembly

  4. Roger Berry | | #5

    For a good general visual of embers and how they ignite parts of a house, I would refer you to the IBHS video at this link - https://www.youtube.com/watch?v=IvbNOPSYyss I would also suggest going to these links for more insights. https://disastersafety.org/wildfire/wildfire-videos/
    https://disastersafety.org/wildfire/preventing-fire-damage-other-roofing-tips/ With that said, I chose to follow Martin's advice and put 8+ inch thick nailbase down and insulated with batts inside to keep it vapor open to the interior. Locally, it is required to have 100% coverage with high temp ice shield under metal roofs. In this case a clip type standing seam, unvented. I will let you know in (hopefully) more than 30 years if I have any moisture issues. Everything seems fine so far. The installer knew how to detail for fire conditions so there are no points where the embers can collect in significant amounts and of course it is unvented so no access to interior spaces.

    In consider wildfire risks, metal roofs need to be detailed carefully. It seems to me that any attempt to vent under the roof would be creating avenues for embers. If the air is going to flow for venting, even screens may not be enough. Keeping vegetation well back from the house and proper distances for combustible structures like fences and outbuildings from the main house are just a few of the well documented "fire smart" choices. Google wildfire councils for your own state or Colorado and California. Lots of good materials and guides are available.

    I would however, like to mention that metal roofs are pretty noisy in heavy rain and especially hail. We only average 12" of precipitation per year so the total time listening to the drumming is not too bad. Worst is post cloud burst when the drips from the upper roof drop onto the next one down - right outside the bedroom window. Water torture at its finest. We can't have gutters due to the icing here generally tears them off.

    I also am a bit doubtful of the value of venting given that it provides condensate on the bottom of the metal panels to collect as it slides down and hits the battens. I seem to remember an article about this is being a big issue in Texas in the humid parts. In addition, as Martin often notes, the magic air arrows don't always cooperate and actual flow rates can be much less than expected. Since metal re-radiates the heat it absorbs equally toward the roof deck and sky, little real gain is had by the relatively small air gap provided by battens.

    Before you get too far on the PV option, do have the roofer and the PV installer you use provide written assurance that the clips the rack use are compatible or at least not destructive. The snap lock types of metal roofing with the concealed roof anchors might need more per panel to accommodate the increase wind loading from panel uplift. Also the PV panel rack mount clips could compress the roof seams in such a way that they prevent proper expansion relief. At a minimum this could cause twisting of the panel much like oil canning. And if you do go with a panel on batten system for venting, consider how many feet will be stomping around on the panels. Not sure what kind of span is used for them, so denting might happen if someone gets careless.

    1. Bryce Nesbitt | | #19

      For ember safe vents for metal panel roofs, see "Vulcan vent". It's a honeycomb with intumescent paint that seals up if it gets hot enough.

  5. Stephen Watts | | #6

    Roger,

    Thank you for your comprehensive response. The ember video is sobering given that they are only simulating 10-20 MPH wind. On the other hand, fiber-cement siding and class A roof appear to perform well. Currently being in a wildfire zone in S. Calif, I am well aware of best practices to reduce wildfire risk (although I have to say that my current house would probably not survive a wildfire). There have been no wildfires within a mile of the build site for >30 years, but there have been many wildfires in the region, and a lightning strike started an 80ft tall larch on fire on the property a few years ago (which was quickly put out by the fire department before it spread)

    The use of nailbase certainly simplifies installation, but if you have to replace the sheathing, it seems like you will probably also have to replace the foam. I guess that's a pretty low probability event and dwarfed by whatever caused the problem. The sheathing is also encapsulated, so if it gets wet it will stay wet. However, I suppose that if there is a problem that causes the sheathing to get wet, ventilation is unlikely to make much of a difference.

    My planned roof is a simple gable without multiple levels, however rain will drip a few inches off of the solar PV panels onto the roof. Downpours are not common in NE Oregon. I might consider use of something like DensDeck to reduce both fire risk and noise.

    I do understand the necessity for ensuring that specific PV mounts are compatible with specific standing seam metal design. I am over 3 years away from the actual build, so I haven't made a choice of specific components.

    You and Martin make a good argument for use of foam for an unventilated roof, and the recommended method is now at the top of my list. We'll see if there are any other reasonable options in the next few years.

  6. Roger Berry | | #7

    Stephen W81,

    An addendum for clarity on the roof structure chosen. The house is "SanteFe-esque" to keep marital happiness. The result was a much more complicated roof situation than I really wanted to deal with. The low slope areas behind the parapets are done with a PVC fabric membrane over two layers of fiberglass mat material rather than DensDeck gypsum. The fire rating comes from the fiberglass not the PVC. There are no scuppers, as the entire down slope side is open with, as noted, no gutters due to the icing conditions.

    One of the lower roof planes that is metal receives all the runoff. It covers the kitchen bump out and down there the noise is well muted by the nearly 20" thick distance between the metal roof and the interior drywall. Unfortunately, the master bedroom window opens right onto the same metal roof, which means (like last night) all the drippy noises can be very tedious. Not my finest design choice.

    As for moisture concerns, I finally had to roll the dice and pray that our locally dry alpine environment would provide me cover. I am well aware of all the SIP issues in roofs and did not want to recreate the same failure risk piece-meal. The primary decking on the rafter/trusses is 5/8 which was then weathered in with a non-permeable synthetic roof roll. (sorry no scraps to look at for brand) That kept the house dry as work inside continued. Once weather cooperated we got the insulation panels in place, foamed, sealanted, etc then completely covered with HiTemp Grace watershield. Plus of course, the metal roofing detailed to protect against embers.

    So yes, we did create a big sandwhich bag around the nailbase/foam with the two non-permeable skins. I am gambling that if one starts dry and doesn't have bulk leaks, the rather sad 7/16ths skin on the nailbase will carry me through until I am long gone and the grandkids will have to cuss my name for the repairs I might have created. Even if the nailbase did fail some how and it was necessary to peel everything back, I should have a sound primary roof deck to work from. No bets on how the flashing details would need to change.

    Vapor intrusion still lurks in the shadows, but here the average humidity in the summer can be quite low. And we have a long drying period. The roof, being non-vented, shouldn't really be seeing much of any exchange of air. The primary roof deck is vapor open to the interior, albiet through 11" batts and 5/8 drywall. Besides, with the heavy outsulation layer the primary roof deck (and all my wall sheathing) the lowest temperature the sheathing should attain is in the mid 50's. The entire primary structural wood framing and sheathing shouldn't be subject to temperature driven moisture loading. Your own environment sounds similar even if you get twice the rainfall we do.

    I am currently working out the details for my offspring's house and I am trying not to do nailbase again. Partly because of the relatively thin skin they come with. Not much grip for the screws on the panel clips. This is why I suggested getting the roofer and PV supplier on the same page. Wind load here can be quite high, so the mix of panels on standing seam could stress the clip anchors far more than normal duty. Other problem I want to avoid is using 10" screws to hold down the nail base. Contrary to some discussion on this forum, the screws do an excellent job of conducting heat, exact percentages be damned. I have photos I can share regarding this. Before I forget, adding DensDeck means longer screws and a pretty large expense.

    If Martin wants to act as go between, I would be glad to keep in contact off forum and let you know how my thinking on unvented roof insulation tactics develops.

  7. Stephen Watts | | #8

    Roger,

    Screws seem like a big problem with all thick external insulation. It seems like it would be fairly difficult to screw through 8" of insulation and reliably attach to a 1.5" wide rafter, since only a little deflection would result in missing the rafter entirely. I'm curious how you addressed that issue. However, I'm not sure that screw length would make much difference in heat loss when the extra length is through insulation.

    Another solution that I have considered is replacing 8" of foam with 8" of Gutex wood fiber board (R-29). As long as total R-value of the roof is less than R-72, that should work. That would allow the entire roof assembly to dry to the interior (in theory). The downside is that the cost is probably double the foam solution. So, when I actually have to fork over the money, my decision may be made for me.

    Gutex also has an interesting vapor permeable EIFS over wood fiber board solution for walls with a European 90-minute fire rating. Same issue with cost plus probable building inspector skepticism in rural northeastern Oregon.

    I would be interested in keeping in contact to see how things go and what new tactics you develop.

  8. Expert Member
    Malcolm Taylor | | #9

    Stephen,

    I don't have a lot of practice, but I found it difficult to consistently hit studs through 3" of foam. For timber framing I have a jig for drilling holes that keeps them plumb or level. I wonder if something similar might help with screwing through thick foam?

  9. Stephen Watts | | #10

    Malcolm,

    I have always had difficulty eyeballing things straight and level, and attaching long screws through thick insulation seems particularly difficult. I had thought of a jig, but I am wondering what other people are using. Thick insulation on the outside of the building envelope is common enough that it seems like this problem should be solved... but is it?

  10. GBA Editor
    Martin Holladay | | #11

    Stephen,
    Q. " I had thought of a jig, but I am wondering what other people are using."

    A. See "A Timber Frame House for a Cold Climate — Part 3."

    In that article, which is illustrated with photos, Rob Myers wrote, "Each screw was predrilled and countersunk using a small jig to ensure that the angle was consistent and the hole was straight. Predrilling the strapping was especially important for the roof, because the strapping and foam are both thicker. If you simply try to drive the screw without a pilot hole, then the slightest variation in grain direction will move the screw off axis and it will miss the rafter. I also inserted all screws before lifting the piece to the roof — it is much easier to work on the ground whenever possible rather than hanging off the roof."

  11. Roger Berry | | #12

    I will reach out to the builder and see what he says about how they handle long screws. I must admit that was the one day I wasn't on site so everything got buttoned down before I could watch. I did check the underside of the decking for just the reason discussed and amazingly only two misses showed up, and very near misses at that.

    I mentioned the heat loss via screws only because I am trying to figure out a three layered foam process with the middle layer being narrow foam strips between battens. The battens would hold the first layer and provide anchorage to the sheathing sitting on top one last layer. Might be feasible monetarily, might not. Possible advantages over shooting 10" long screws might be lower overall cost of screws, reduced thermal losses , and a nice wide target for the second sheathing to hang on to. Downside will probably be too much fussing (like cut and cobble)

    Simply using the same nail base or a polyiso version may still be the most practical. I have also thought about various guide jigs for long screws, but getting a crew to use it might be a bridge too far. Martin, as usual, might have the easy answer, just set them neatly before you go up on the roof. You could pre-chalk line the bottom sides to verify if the screws are pushed through straight.

    My snow dot pictures of the screw heads melting light snow falls are currently trapped on my older computer, which is temporarily comatose. I will try to get access to them and post in the future. I have hesitated before simply to avoid a brouhaha. Maybe someday I will try for thermal imaging for a guesstimate of losses.

    Before I forget or the thread is lost to time. Look for the vertical load charts some of the long screw manufacturers offer to assuage building inspectors. Some are not easily convinced that furring strips over foam aren't going to droop under load. It helps if the engineer (if required) signing off on the plans has the information as well. I was very fortunate to have a good inspector that wants better insulated houses to be built.

  12. Stephen Watts | | #13

    I wonder if the overall theory of an unventilated roof with sheathing encapsulated between impermeable roofing/underlayment and foam is for the the sheathing to be a sacrificial layer. The rest of the roof structure would be protected from water/vapor issues. If that is so, perhaps it might be better to go with inexpensive roofing (e.g. asphalt shingles) and replace both the shingles and the sheathing (if necessary) every ~25 years?

    On second thought, the Gutex option I mentioned earlier might not be appropriate for an unventilated roof because the condensation layer would fall within the wood fiber during cold weather. Examples of use of Gutex for roofing insulation all seem to utilize ventilation between Gutex and roofing.

    I believe that Martin has suggested that you could use rigid mineral wool board (e.g. Roxul Comfortboard) in place of foam, although almost never utilized except on flat roofs. This would result in a more fire-resistant assembly than foam while maintaining vapor permeability throughout the roof.

  13. Roger Berry | | #14

    Stephen W81,

    I think it might be worth considering the dynamics of fire and embers on houses a bit. I went with EPS foam nailbase on the roof and EPS materials for the walls with two exceptions. One place on a roof due to height limitations, the other in the garage. The garage is lined with polyiso partly due to lower delta T concerns over R factor and partly to act in line with the firerock drywall during a fire. The X class drywall is the primary fire barrier, but if a fire should break out in the garage, the polyiso won't collapse or melt when the gypsum heats up.

    Mind that a fire inside a garage is a very different beast than a fire outside, especially in regard to a roof. A garage is typically full of fuel items leaning against the walls so the burn intensity is much different than outside. If one adds in the explosive effects of flashover fire balls in an enclosed space, it becomes pretty clear that EPS might hit it's melting point behind the gypsum. Then the support for the drywall is compromised and who knows how it will fail or allow flame penetration. There may be videos out there.

    On a roof there should be no flame supporting fuels. Hence the video of the test burn showing why cleaning your gutters and valleys of debris is one critical step. The second critical step is not having nooks and crannies for embers to pile up in. The individual heat value presented by individual embers is pretty small. Without additional fuel and oxygen they aren't going to do much. Think of being near a campfire when someone stirs the fire. Embers fly up and probably land on clothing or skin with out instantly bursting you into flames. Much the same for a roof.

    Blowing hot embers onto additional fuel mass in the gutters or inside your attic are more relevant to fire hazard than most embers falling on a non-combustible roofing surface. I won't matter if the exterior or interior insulation is rockwool or foam once the embers have snuck inside the attic through a vent. The piles of embers that can collect in dormer pockets can become "hot pockets" which will ignite the siding or trim materials more readily than a metal roof. (That of course assumes the metal roof is detailed correctly). You have already beaten that problem by having a simple gable roof.

    Something that is often overlooked however, is the skirt where the siding stops and the foundation begins. Depending on conditions, this area can behave just like a ember catching pocket on a roof with gable windows. Good detailing with fire resistant/proof materials needs to be applied here as well. If not, embers swept by winds across the ground can pile up against the house. Vulnerable materials will ignite and carry the fire into the wall behind the siding. Hardiboard won't help if there is a ignitable bit of furring peeking out. Bay window bump outs are even more likely to be sealed with simple thin plywood.

    I guess the main thought here is metal and fire rated shingle roofs are lesser weak points in most cases. The insulation under either is not likely to see an ignition point temperature without a lot of fuel sitting on top of the roof. If a flame front is situated 30' or more from a structure, surfaces have a hard time reaching ignition point. ( Fire-nados render this moot) This is why wood fences are not supposed to be run up to the house wall. Outbuildings or piles of wood also should be well clear of the house. The propane tank on the grille next to your wall is a big risk factor.

    You will still need to consider and design for moisture control whatever you go with, but I think detailing against ignition points will be more important should a fire ever arrive.

  14. Stephen Watts | | #15

    Roger,

    I generally agree with your comments. I believe the art of building a house is figuring out when the design is "good enough" and what trade-offs are most appropriate. In contrast, a science/engineering-only approach can probably solve almost any problem, but it may not be practical or cost effective.

    It seems to me that best practice roof and wall design for reducing water vapor issues requires ventilation, whereas best practice fire resistant design requires no ventilation. I have observed that fire resistance gets very little attention in cold climate green building design, possibly a result of geographic concentration in the northeast U.S. (at least it seems that way to me).

    There are wall ventilation solutions that appear to support Class A fire rating (e.g. VaproMat, HydroGap). These solutions provide minimal ventilation compared to strapping and open air space, but in a well designed wall with an effective interior vapor retarder, minimal ventilation might be sufficient to ensure no mold on fiber cement siding. I have been unable to identify any roof ventilation solution that retains a Class A fire rating (the best I have seen is Class C (e.g. Cobra)).

    The relatively low temperature melting point of EPS makes me nervous, but your point with respect to the likelihood that this would be an issue in a well-constructed roof in a typical survivable wildfire is well taken. It's too bad that FoamGlas is no longer available for residential construction in the U.S. It doesn't solve ventilation issues, but it resolves any potential fire issues, especially for above-ground portions of foundation exterior insulation.

    So, the bottom line is that it still appears that the solution that Martin advocates and you have implemented is preferable. I am curious if there are any data available for the probability that encapsulated roof sheathing resulting from this design will last for say 25 or 50 years without moisture issues?

  15. Roger Berry | | #16

    I'm only three years into finding out whether moisture will be my downfall. I do know that Joe Lsterbuik (wrong spelling) freely posts pics about a failed roof he did many years ago. Can't remember where I saw it, but most likely via a link from GBA where I have lurked for years. He does explain why it failed so spectacularly and what he did to correct it. Try googling his name (spelled correctly) with home office as secondary term.

    I think the key to unvented roofs, which have been promoted on GBA , is not allowing air exchange to occur through the foam layers, whatever type used. Edge sealing with tapes or foams and sealing the side edges of the insulation layers serves to limit or eliminate active air exchange. If all the assembly is done under dry conditions and encapsulated with non vapor permeable barriers (in my case Grace Ice shield Hi Temp on top) I don't see why there would be much occasion for moisture to accumulate. Think of a dried sponge inside a baggie. Not much will change unless bulk water makes its way in.

    Since wood will naturally accumulate more moisture as the temperature falls, the outer layer of sheathing holding the roofing material is the most likely reservoir for moisture introduced by screw and nail penetrations. The Grace is gummy and should seal the screws holding my roof clips. The alternative synthetic roofing papers may not pass water vapor, but they won't be as secure against water sneaking down the threads of a screw. That said I did use a synthetic roofing paper on the primary deck sheathing before putting the foam layer on. I am risking things a bit here as any water vapor that makes it through the dry wall and 11" of fiberglass batt to the underside of the sheath will still have an easier path back out to "free air" going back to the living space than through the plywood roof sheathing and the impermeable roofing paper.

    Because the primary roof sheathing is so far from the yearly condensation temperature points, there is little inherent absorption force on the wood elements. An indoor swimming pool would not be a good idea for this situation, but for normal homes it should be fine. At least I keep telling myself that.

  16. Rob Hunter | | #17

    Can someone summarize the conclusions from this thread? So many variables I'm lost - and I have (I think) the same circumstance as Stephen Watts: building in California, standing seam "Class A" Taylor Metals metal roof panels over 5/8" OSB with a TBD slip sheet between the metal and the sheathing. Unvented roof, which means either 6-7 inches of closed cell foam or (more likely) 10" of open cell foam.

    Seems like the slip sheet/underlayment should be one of the following: 30# building felt, a synthetic membrane like Tyvek Protec or Grace Tri-flex, a "fire blanket" like VersaShield, or a cementitious board layer like DensGlass or Type X drywall.

    Huge range of costs for this list, from a few cents per sf to several dollars per square foot. I did a quick search on the site to see if there was a good prescriptive article but must have missed it. Advice, please?
    Thanks,
    Rob

  17. GBA Editor
    Martin Holladay | | #18

    Rob,
    Roofing underlayment (traditionally, asphalt felt -- more recently, sometimes synthetic roofing underlayment) is required by code.

    I'm not too familiar with slip sheets, so I did some research. According to information from the National Roofing Contractors Association, "NRCA also recommends a slip sheet be installed over the underlayment for metal panel roof systems. A slip sheet is a layer of smooth building paper, such as rosin-sized or unsaturated building paper. Its purpose is to protect the underlayment from damage, as the panels can adhere to and tear the underlayment."

    I've installed a lot of metal roofs, and I've never installed a slip sheet. But smooth building paper is cheap, so it seems easy enough to add if you feel it is necessary.

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