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Insulation on Exterior walls & Codes

GBA Editor | Posted in Green Building Techniques on

OK… I’ve been reading 100s of responses using exterior rigid foam (from 1″-8″) and then installing nailing strips for cladding, and all the gurus are going to this technique. May question is this allowed by code??? If I’m not mistaken, any cladding should be nailed to the stud at least 1″. I understand that QC is a big component but when you have 1″-8″ of rigid foam in between the nail strips and the stud and you are trying to “hit” the stud with a nail gun… well, I’ve seen the real world apps and I wouldn’t bet my life on it. Also, wouldn’t the weight of the cladding (any cladding) pull down the nails or screw since there is no structural support from the rigid insulation? I know in NM the code officials are on the fence about allowing this technique with stucco and possible be written in the 2009 State Code as NOT allowed. Any thoughts????

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    Hi Armando

    I believe the intent of the code is that when the cladding is used as a shear panel the nails loaded in sheer should penetrate an inch into the studs per the nailing schedule. Siding is typically only nailed well enough to hold it to the building and it's main purpose is really to just keep sun and rain off the house wrap.

    Our nailing schedule for stucco lathe is 6" OC each way. For that to have 1" nail penetration it would have to go on 1" sheathing or the studs would have to be 6" o.c. neither of which would be practical. Similarly, cedar shingle siding is typically stapled to 1/2" OSB.

    My understanding is that most of these exolation foam assembly's have the rain screen nailing strips attached with long screws, not shot on with nail guns, and it seems they're getting their sheer from metal straps, not OSB, so it would be pretty obvious if you missed a stud as the screw wouldn't draw up. I can see that it would be an issue if you sheathed a house with OSB for sheer and then tried to shoot a screw through a thick layer of foam, through the OSB into the stud.

    My engineer feels as you do about the screws creeping over time when penetrating a thick layer of foam. My sense is that when the strips are attached to the sheathing and drawn up tight they would need to pull out to sag so they are likely to stay snug enough for wood siding but an unforgiving siding like stucco or even fiber cement would likely be a problem. I sheath everything 100% OSB for hurricane resistance so getting the strips pulled in snug to the studs through a thick layer of foam would be a royal pain.

    The word of the code and the code as enforced are, of course, very different things from day to day and county to county. I hope you're doing well down there. I'm feeling ready for spring right about now.

  2. homedesign | | #2

    I think the ginourmous nails and screws are a Not -So-Good solution.
    100's or 1,000's of penetrations in the outsulation and WRB.
    degrading the outsulation (I have photos)
    Not to mention the structural issues that you bring up.
    Architecturally it just looks "disturbing"

    I know Martin WILL disagree...I think it is a PROBLEM

  3. Riversong | | #3

    all the gurus are going to this technique

    not all the gurus

    Many of the acolytes, to be sure, but not those who follow their own inner guidance (or grumpy old code officials).

  4. Armando | | #4

    Thank you all for your point of view... In NM I'm using 3C stucco>Stuccowrap>1" dow bd>7/16" OSB>BIF>5/8" Rock and in Dallas I'm using Cement Bd Siding>Housewrap>1/2"dow bd>7/16" OSB> BIF>5/8" Rock; all with advanced framing. However more and more I'm questioning IF I can/should use thicker rigid bds.
    my understanding is that BSC may do a study of this, I was hoping to see if anyone else feels the same way. Maybe I should ask this same question to all those guys building PASSIVHAUSES... but I'm looking more for code answers than "trial & error" answers.

  5. Armando | | #5

    Good points Michael. I guess your engineer and I have same bothersome thoughts. Thank you.

  6. GBA Editor
    Martin Holladay | | #6

    There is a technical document — "Wall Bracing Guide" from the Foam Sheathing Coalition — that answers some, but not all, of your questions. In case you're interested, it's here:

  7. Armando | | #7

    Thank you Martin... I'll have good reading for awhile. AC

  8. Armando | | #8

    I'm talking about the structural integrity for cladding ATTACHMENT with 2", 4", 6" or 8" of rigid insulation "sandwiched" between the nail strips and the OSB bracing panels. Does the weight of the cladding pulls down the nails/screws on the foam? I imagine the thicker the rigid foam the worse the condition. What type of fastner should we use (ie. material, thicknes & lenght) and how deep into the stud should it be applied (assuming IT IS NAILED to the stud)?
    Thank you, AC

  9. GBA Editor
    Martin Holladay | | #9

    If you haven't seen it yet, check out Thorsten Chlupp's article:

    Chlupp wrote, "We provide nailing for siding by installing 1x4 furring or strips of 3/4-inch plywood over the foam, fastening through to the studs with long screws. We space the screws about 12 inches on-center and make sure we penetrate at least 11/2 inches into the framing. These long fasteners have to be mail-ordered; we use Wind-Lock W-SIP screws (800/872-5625, and FastenMaster HeadLok and OlyLog screws (800/518-3569,"

    I have read several accounts of builders who have asked their engineers to perform calculations showing that siding can be adequately supported on vertical strapping screwed through foam. These engineers have given the thumbs-up to the method, although I don't have access to the calculations on which this conclusion was based.

    If your building inspector or engineer is skeptical, you may need to pay for an engineering analysis.

  10. Armando | | #10

    That's an excellent article not just in rigid foam installation but in other installations. I think this applies to everyone using exterior rigid foam no matter what climate zone. I'll take it to the engineer. Maybe the ICC should look at this application.
    Thank you, AC

  11. Garth Sproule | | #11

    If I remember correctly, Thorsten Chlupp also recommends installing the screws at an upward angle of 5 degrees or so?? If the rafters are vertically stacked on the wall studs, it is also possible to "hang" the furring strips from the rafters using an appropriate metal hanger.

  12. Riversong | | #12


    If you or your engineer or inspector balks at hanging both siding and windows on 8" or 10" screws, another option is to install an extended 3/4" PT plywood subsill out to the thickness of the exterior foam. This will offer some support to the foam and cladding, while introducing only a minimal thermal bridge (probably far less than the hundreds of heavy-gauge steel screws penetrating the insulation), and still creating a thermal break at the primary sill. You'll just have to do a good job air-sealing between the subsill and the foundation (and, of course, a capillary break).

  13. Riversong | | #13

    "Thorsten Chlupp also recommends installing the screws at an upward angle of 5 degrees or so"

    Clever. Pre-sagging the fasteners to reduce shear. But I don't think 5° is going to make any difference. Now if they were at 45°, you'd be good to go, but that might require 15" screws and some way to bury the head!

  14. Armando | | #14

    Thank you all. I've also decided to email ICC and my buddies at El Rey-Parexlahabra stucco in ABQ for their imput. I do have more questions about attaching the wire mesh for 3C stucco and stone veneer. I'll keep you posted.

    It's funny... the more answers I get the more questions I have; and I hope I'm not making a mountain out of a small hill.


  15. Riversong | | #15

    " I hope I'm not making a mountain out of a small hill"

    You don't have moles in NM?

  16. Armando | | #16

    We have too many KOYOTTES, wey...

  17. joseph ferut | | #17

    Mfrs. of cement siding require "engineering" with rigid more than 1"; cultured stone is only 1/2" (so much for the dew point!). An answer could be to adapt the veneer anchors used in masonry construction. Hohman & Barnard make the "X-seal" , a c-shared anchor that fastens thorugh 1/2" to 4" insulation directly to the stud. It's shape should reduce/eliminate the movement of the faster.

    It should be an easy adaptation for veneer stone, and with a little ingenuity could be used for firring strips and siding installation.

    Joseph Ferut, Architect, Cleveland, Ohio

  18. Armando | | #18

    Thank you for your information Joseph. I found several fastners in their website but I'm going to contact them tomorrow for more clarification on fastners for stucco and stone veneer. They do have a 6" & 9" fastener, but I wonder if it would be allow for these applications.
    I do have 3 other sources checking this for me... I'll post their findings for all to read.

  19. jklingel | | #19

    On the exterior foam: IMO, it is terribly expensive when you look at the $/R, at least for DIY folks. The Cold Climate Housing Research Center has what they call a REMOTE wall like that, but to build it is beyond my means. Double wall is a fraction of the cost.

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