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Shiplap in lieu of sheetrock – Questions regarding fire safety

Jabberwocky79 | Posted in Building Code Questions on

I don’t know if this is correct category to place this question in, but here’s the deal:

Building a living unit in a section of a post-frame building. The exterior walls are constructed using the PERSIST method, with Grace Ice & Water shield covering the OSB, 1″ foam on top of that. The stud bays will be insulated with poly-iso cut and cobbled, 3″ thick.

We will probably be using sheetmetal for the ceiling. But my question is about the walls. We would really like to use pine shiplap boards in lieu of sheetrock. I realize that shiplap is leaky, but I’m not terribly concerned with that due to the air sealing efforts on the exterior. My concern is with fire safety/ code considerations. This building is not in any jurisdiction and therefore not subject to inspections or approvals, but I realize those things are in place for a reason. Can someone give me some advice on concerns with this practice, particularly in regard to fire safety considerations?

Many thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, your approach isn't really the PERSIST approach. The classic PERSIST approach requires that there be no insulation on the interior side of the Ice & Water Shield -- all of the insulation should be on the exterior side of the Ice & Water Shield.

    You didn't mention your climate zone. In theory, your use of Ice & Water Shield -- with most of the R-value on the interior side of the Ice & Water Shield -- raises condensation concerns, especially in a cold climate. For more information on this issue, see these two articles:

    Getting Insulation Out of Your Walls and Ceilings

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    In most cases, your choice of cut-and-cobble rigid foam on the interior would probably prevent condensation problems on the interior side of the Ice & Water Shield layer. However, cut-and-cobble is not a foolproof air barrier. If any interior air migrates through cracks in your cut-and-cobble system, you might have moisture problems -- especially if you are located in a cold climate.

    So it's not a very robust design.

    Concerning the question of whether pine boards are an acceptable thermal barrier, I'm fairly confident that they aren't. It would be safer to install interior drywall, and then to install the boards on the interior side of the drywall. For more information on this issue, see Thermal Barriers and Ignition Barriers for Spray Foam.

  2. Jabberwocky79 | | #2

    Sorry Martin, I'm in climate zone 3. Oklahoma to be exact. Perhaps that allays your concerns about the wall system?

    I've read all of those articles more than once prior to starting this project. It is my understanding that given my climate, 1" thick exterior poly-iso will be sufficient to prevent condensation on the interior side of the wall. Incidentally, the only reason I'm not putting thicker foam on the outside is because I simply don't have room. As mentioned, this is a retrofit, and as such, the roof overhang doesn't come out far enough to permit a thicker exterior wall, which would end up sticking out past the overhang - not good!

    Although this isn't the classic PERSIST method, I still feel that it most accurately describes my approach to the exterior. As for the interior stud bays, it's not that I love to cut-and-cobble, but because I have lots of left over poly-iso from the roof insulation phase, I might as well use it.

    That brings me back to my original concern. We love the look of shiplap, but our desire to use it is not merely for appearance. We don't have the budget for professional drywall insulation (I spent it all on insulation and air sealing, hee hee) and such is beyond my expertise. I can, however, install shiplap myself. Installing both is simply out of the question.

    After reading the article concerning thermal and ignition barriers again, I have a few thoughts:
    1) Since the article focuses on dealing with spray foam, I'm not sure what truly applies to my situation and what doesn't. Would you classify poly-iso as the same or no?

    2) If 3/4 inch plywood is generally accepted as an equivalent to other 15-minute barriers, would not 1" shiplap be the same? Is the difference because of all the air gaps? Or because it's located on the wall instead of the floor?

    3) I noticed the allowance for 15/32" tongue and groove boards on the ceiling as well... Again, why is this concession made? Is it location?

    4) I've seen 6 mil plastic flame retardant sheeting advertised and stocked in building supply stores. It wasn't mentioned in the article, I would assume for a reason. What is this and is it a bunch of bunk? (Even if it is worthwhile from a fire retardant standpoint, my fear was that it would act as a vapor barrier and prevent the walls from drying to the inside).

    In conclusion, I don't treat building regulations flippantly, but I'm primarily concerned with following the IRC when the IRC aligns itself with common sense. I'm just wondering if this is one of those cases or not? My main goal is to be fairly certain that if my wife flames a cooking pan our entire house isn't going to go up like a matchbox ;)

  3. GBA Editor
    Martin Holladay | | #3

    The reason that PERSIST builders can get away with the use of Ice & Water Shield, which is a vapor barrier, is that all of the insulation is located on the exterior side of the Ice & Water Shield.

    In your case, you have chosen to build a different type of wall. You aren't building a PERSIST wall; you are building a wall with a thin layer of exterior rigid foam. For that type of wall, you don't want to use Ice & Water Shield. You want to use a vapor-permeable layer like housewrap. If you want the housewrap to be a durable air barrier, choose one of the European products designed for this purpose, and tape the seams.

    Using polyiso doesn't mean you don't need a thermal barrier. Building codes require rigid foam to be protected by a thermal barrier in the same way that spray foam insulation needs to be protected.

    Q. "We don't have the budget for professional drywall insulation (I spent it all on insulation and air sealing, hee hee) and such is beyond my expertise."

    A. I can't solve your budget problems, but I'll simply point out that you don't need a professional installation. All you need to do is fasten the drywall to the studs, and install one layer of paper tape embedded in mud. This is called "fire taping." You are allowed to do a sloppy job, because no one will ever see your work once you install the shiplap boards.

    Q. "If 3/4 inch plywood is generally accepted as an equivalent to other 15-minute barriers, would not 1-inch shiplap be the same? If 3/4 inch plywood is generally accepted as an equivalent to other 15-minute barriers, would not 1-inch shiplap be the same?"

    A. Good question. Ordinarily, this ruling would be made by your local building code official. You may be right that most building officials would approve the use of shiplap boards for this purpose. Frankly, I don't have enough experience to make a ruling on this issue -- and in your case, all you really care about is safety. It's your call.

    Q. "I've seen 6 mil plastic flame retardant sheeting advertised and stocked in building supply stores. It wasn't mentioned in the article, I would assume for a reason. What is this and is it a bunch of bunk?"

    A. I've never heard of any type of plastic sheeting that complies with the requirements of a thermal barrier.

  4. Jabberwocky79 | | #4

    Okay Martin, the lightbulb has gone off. I understand now what you were saying about the wall system. If the fact that it didn't apply to PERSIST walls was mentioned in the article about calculating the minimum thickness of exterior foam, I must have missed it.

    So that being the case, what I have now is a situation. I could use your help to fix it, and it'll probably have to be a trade-off between doing it the perfect way, or way that's "good enough considering."

    I have already applied the Grace, including the window flashing, and as you can imagine, that would be extremely difficult to remove at this point. The only walls that have Grace applied are the west wall and the south wall. The north wall utilizes the existing metal siding, and therefore I've retrofitted sort of a rainscreen system from the outside in, but there's no Grace. The east wall separates the apartment from the shop area, which is already insulated and heated.

    Going back to the west wall. It has a 6x4' window, an 8x7' sliding door, and a 5x7' french door, so the actual 'wall' with Grace applied is approximately 50%. As mentioned, the overhang is such that I can't install foam more than 1" thick.

    The south wall also has Grace, however it is underneath a large section of lean-to roof from the existing building. There's no problem adding thicker foam there and building a true PERSIST wall. I have attached a picture so it's easier to visualize.

    So we are only talking about 50% of one exterior wall on the 'hot' side (also wind-driven rain side) of the building that is a concern. Other than ripping it all off, what are your suggestions, if any?

    Now as, for the subject of sheetrock. I think I would feel most comfortable, given the situation, with installing a sheetrock "firewall" on the cooking side of the kitchen using the method you described, and shiplap over that. I am comfortable with just the shiplap on the rest of the apartment. Fire safety aside, since the exterior/interior walls are interrelated, wouldn't the shiplap be a good choice considering the Grace problem? Obviously we need the wall to dry to the inside, and leaky shiplap would insure that would happen, correct? I suppose it works both ways - allowing moisture ingress and egress.

    Here is a link to the plastic sheeting I was mentioning: - Again, not that I think it is a viable solution, but like you I find this stuff interesting.

    Thank you for your help.

  5. GBA Editor
    Martin Holladay | | #5

    You wrote, "If the fact that it didn't apply to PERSIST walls was mentioned in the article about calculating the minimum thickness of exterior foam, I must have missed it." This sentence is a head-scratcher. First of all, what is "it" in the phrase, "it doesn't apply"? Second, why do you keep talking about PERSIST walls? Your house doesn't have PERSIST walls.

    You have come up with a wall design that breaks the rules, because it has an exterior side vapor barrier. That doesn't mean that you have a disaster on your hands, although it's really too bad that you didn't ask for advice before you purchased and installed the Ice & Water Shield.

    You have two things going for you: (1) You live in Oklahoma, where winters aren't as cold as they are closer to the Canadian border, and (2) You plan to install rigid foam rather than fluffy insulation between the studs.

    To limit the chance that interior air will migrate toward the sheathing layer, you need to do an impeccable job of air sealing when you perform the cut-and-cobble part of the job. If you do that, you'll probably be OK.

    If you ever build another house, don't make the same mistake again.

  6. Jabberwocky79 | | #6

    When using the term "PERSIST," I think you understood my words to mean 'the complete wall system of the exterior walls of the building' when what I actually meant was "The exterior side of the walls" (as in the wall design from the OSB-out). I have made reference to the PERSIST system when describing my exterior because I figured it was more succinct than saying "I'm wrapping Grace to OSB, then applying a layer of foam, taping the seams, adding 1x4 strapping, and finally sheathing it with siding."

    A lot of the advice I have utilized to good effect has come from reading articles on greenbuildingadvisor. Unfortunately I thought that the article "calculating minimum thickness for exterior foam sheathing" applied to any type of wall. Apparently it does not. That's no one's fault but my own... but until now I had no reason to think that I didn't understand it, therefore did not feel the need to ask. The reason I thought it would be fine to add interior insulation is because of that article, which talks about interior insulation of various types.

    Here's the opening sentence: "If you plan to install exterior rigid foam on the walls of your house, how thick should the foam be?" And later it says "If you are building a house in one of the warmer climate zones — zone 1, 2, 3, or 4 (except for 4 Marine) — you don't have to worry about the thickness of your foam. Any foam thickness will work, because your sheathing will never get cold enough for “condensation” (moisture accumulation) to be a problem."

    The crucial difference between PERSIST and other exterior foam designs is the Grace, as you have mentioned. But it was the concept of applying rigid foam to the exterior that made me mistakenly interchange the concepts.

    Going back to mitigating the mistake, based on what I currently understand, I could have three possible solutions:

    1) Rip off the Grace and apply vapor-permeable house wrap instead, and continue with the exterior foam as planned.

    2) Leave the Grace, build a [TRUE] PERSIST wall, which will require some clever flashing details at the top and would look a little funky in the end.

    3) Leave the Grace, continue with exterior foam, insulate the interior, paying attention to detail, and then pray.

    I'll have to think about it ;)

  7. GBA Editor
    Martin Holladay | | #7

    You wrote, "Unfortunately I thought that the article 'calculating minimum thickness for exterior foam sheathing' applied to any type of wall. Apparently it does not."

    Actually, it does. You have the correct amount of exterior foam sheathing. Everything is just fine when it comes to the rigid foam thickness.

    The problem is that you decided to install a wrong-side vapor barrier. As I said, I don't think it's the end of the world in your case, for the reasons I mentioned. (And by the way, you aren't going to be able to peel that stuff off. Once it's on, it's on.)

  8. Expert Member
    Dana Dorsett | | #8

    The cut'n'cobbled 3" polyiso is a waste of foam due to the thermal briding of the framing. Even though at center-cavity it's R18, the 3" path through the framing is about R3.6. The framing is only ~25% of the total wall area, but it's 5x as conductive as the cavity foam, so more than half the heat transfer is through the framing(!). With R15 rock wool or fiberglass full depth cavity fill the path through the framing rises to ~R4.2. That's only a 17% improvement in that high conductance path but sufficient to make up for the 20% reduction in center-cavity R, and the thermal performance is roughly the same. But with fiber cavity fill the sheathing has a drying path.

    Bottom line. spending the foam budget on thicker exterior foam would make far more financial sense (and would make the assembly more moisture resilient), but even 1" is more than sufficient for dew point control.

    Using a "smart" variable permeance vapor retarder such as MemBrain (now available through box-stores) or Intello Plus behind the ship-lap detailed as an air barrier would be cheap moisture accumulation insurance, but not absolutely necessary for thermal performance if only high-density fiber insulation is used, and fitted correctly. There is another rationale for the smart vapor retarder/air barrier: It keeps friable fiber insulation from getting into the room air, since the ship lap is not air tight.

  9. Jabberwocky79 | | #9

    Thanks for the added insight Dana. You hit on something that I hadn't thought of but could employ easily. We used Intello Plus in our roof insulation system, and I have enough of it left over that I could probably cover the west wall that has been the subject of debate.

    I already have the foam, which was also left over from the roofing phase, so when you speak of financial sense, just know that "financial sense" would ultimately mean using what I have already purchased rather than purchase more items.

    Rather than cut-and-cobble the foam into the stud bays, I agree that it would be more robust to simply install it on the outside. That would really open up a whole new can of worms however, since the wall would now stick out past the roofing overhang. I'm sure it's nothing that some aluminum flashing and some careful attention to detail couldn't solve, but appearance-wise it would look very counter-intuitive.

    I really think my best approach to this situation is to continue with the 1" exterior foam, which is going to minimize thermal bridging through the studs, cut 'n' cobble the 3" poly-iso in-between the studs, sealing the edges with spray foam. I will add a layer of Intello Plus across the interior wall face before installing the shiplap.

    Sound good enough?

  10. Expert Member
    Dana Dorsett | | #10

    The Intello doesn't really buy you anything when you have impemeable or low-permeance foam on the interior side of the wall sheathing. It only makes a difference when the cavity fill is air &/or vapor permeable.

  11. user-6184358 | | #11

    I have worked on a burnt out condo. The interior cedar burned to the studs and damaged studs and roof framing. Where there was 1/2" drywall, no framing was damaged,even though the drywall was covered with cedar. Installing drywall will possibly help limit the damage from an interior fire. In California and in the IRC fire sprinklers are required in new residential construction. They are there because the save lives and limit property damage. So, at a minimum is fire sprinklers then a far second is complete drywall.

  12. Jabberwocky79 | | #12

    Dana, would not the Intello be added insurance just in case the poly-iso is not completely air tight? I was just thinking that cut 'n' cobble isn't the most foolproof method of air-sealing, especially after the typical shifting that wood framing is prone to.

  13. GBA Editor
    Martin Holladay | | #13

    You're right that attention to airtightness is essential. Either Intello or the Airtight Drywall Approach should improve the airtightness of the wall assembly, but only if installation details are attended to with care, and only if you have a plan to address air leakage at electrical outlets and other penetrations.

  14. Jabberwocky79 | | #14

    Martin, any advice on the best way to seal Intello Plus to the electrical boxes? E.g. Cut an 'X' in the Intello and fold the edges into the box... cut a square out and caulk around it.... you know... that sort of thing?

    Also, is there any harm in using spray foam around the electrical boxes? I've seen this suggested on YouTube videos, but I was a little concerned. I guess it's no different than having rigid foam or other types of insulation right next to them?

  15. GBA Editor
    Martin Holladay | | #15

    The traditional solution is to purchase airtight electrical boxes. Most airtight electrical boxes (you can Google the term) were developed in Canada for use with a polyethylene air/vapor barrier, so they should work with Intello.

  16. Jabberwocky79 | | #16

    Yes, I have looked at airtight boxes before, but what I remember seeing were boxes with a skinny flange with a foam-like gasket around it - I pictured that being a good base to compress against drywall, but not for taping Intello to. But I found this:
    And the wider flange without a gasket seems much more appropriate for this application. I will be going this route. Thanks

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