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A few questions on advanced framing strategies

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Danny Kelly | Posted in Green Building Techniques on

Getting ready to start a house and will be using advanced/OVE framing techniques. 2×6 wall with 1” DOW SIS sheathing. Seems like all of the published literature on advanced framing always shows headers attached to the king studs with header hangers in lieu of a jack stud. I am thinking of moving the header up into the floor system (Band Joist) above and not even have a header directly above the window. Can hang the second floor joists on the hanger if there is not enough room on the 2×6 plate – on the second floor, the header can be up at the ceiling joist band. Seems like a no brainer – will allow more room for insulation. We already remove the headers in non load bearing walls so why not remove it in the bearing walls as well. Anyone have any reasons why this would not be a good idea?

Insulated headers – seems like everyone has different opinion on this – have seen the rigid foam used as the insulation on the inside, outside and the middle of the header. I would think putting it in the middle would compromise the strength of the header. Seems to me like it makes the most sense to put it on the exterior. Anyone have any pros or cons of the different locations?

The last few houses I have used advanced framing on have been brick. This current house is siding so need to think about blocking/deadwood for exterior trim and siding to be secured properly. Seems like at each corner and around each window will need to install a 2×4 sideways (or maybe rip in half if trim is not too wide) to serve as a nailer. Seems like we will putting back a lot of the wood we are trying to save by having to do this. Anyone know of a better way?

Not sure if it matters for this topic – but in NC – 3A

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Replies

  1. Riversong | | #1

    Danny,

    You can use band joists as headers and, in some cases, the single band member will be sufficient to carry the loads, but you have to do the engineering.

    Floor joists perpendicular to header/bands have to be on hangers so that their tributary floor load is carried on the header and not the top plate. Since the hangers will need to be below the bottom plane of the joists, you will want to place the header/band to the inside of the top plate. You can either place a secondary header member on the inside with the primary continuous band in the normal position on the outside (with foam in between) or move the entire band joist to the inside, hang all joists in hangers and add additional foam outside the band. Or you can leave the band joist on the outside of plate and notch the joists over the windows to make room for hangers.

    Most OVE buildings rely on sheathing to provide nailing for exterior trim and siding ends, but with SIS you'll probably need solid wood nailbase and longer siding/trim nails to get proper embedment.

    Since the minimalist framing connections of the OVE system, I believe, rely to some extent on the sheathing to help tie it all together and provide additional structural support, be sure to follow SIS nailing guidelines around window and door openings (3" oc nailing at edges).

    Do you know the thickness of the structural material on the SIS panel and what it's made from? Dow seems a little secretive about that.

  2. Dylan Eide | | #2

    Concerning the moving of headers to the rim board, I have often wondered the same thing. The entire point of a header is to transfer load around the window to the supporting studs. Why have an 1 1/2" of rim board only to have a doubled header below. So long as the rim board is the correct size for the load, why not double the framing members at that location by adding one piece that spans the window versus placing two more pieces lower down?
    A similar notion I had, while admittedly adding cost to the initial construction, would allow for greater flexibility and easier expansion/remodeling and that is to double the entire rim board along the load bearing walls. It should act as a continuous header across the load bearing wall. Thus replacing the need for structural window or door headers entirely, except for very large openings. This would allow for future owners to easily reconfigure windows and doors as needed and/or desired without extensive structural reconfiguration.

    Robert beat me to a better explanation

  3. Riversong | | #3

    The reason we use headers below the top plate for door and window openings in walls that bear floor loads is because the top plate(s) is supporting the ends of the joists - the end-grain nails from band to joist are not sufficient to transfer those loads.

    So a band joist can serve as a header only if the floor joist loads are transfered to the band joist on hangers. This complicates floor construction and may create drywall problems, and doubling the entire band joist makes thermal bridging more problematic while also requiring more lumber than headers would.

  4. User avatar
    Danny Kelly | | #4

    Thanks guys - seemed like a simple idea and I didn't think I had invented it, but not sure how this has never been on any advanced framing details I have ever seen.

    Robert - I'm not sure what it is made of but reminds me of the old Thermoply product. Have not used it in a while but as far as I remember, it was probably 1/4" thick. The first time I used it, was a little leary on it being structural so installed let in 1x4s for additional lateral stability but pretty impressed at how rigid it actually was after installing.

  5. Allan Edwards | | #5

    There is a thread going on over at JLC about SIS and someone posted this about the thickness of the structural material:

    "This stuff is insulation laminated to a thin compressed fiber board. The insulation is closed-cell polyisocyanurate with a facer and is considered a vapor retarder and air impermeable. The board used for SIS is only .113 in. thick and for SIS Plus it is only .135 thick."

  6. User avatar
    Danny Kelly | | #6

    Yeah - about 1/8" may be closer - they do have a 1/2" product so that makes a little more sense.

  7. Allan Edwards | | #7

    I am about to start a new home with SIS panels and I wonder what the cost and performance difference is between Zip Wall and non-structural foam board vs. structural foam panels (SIS).

  8. Riversong | | #8

    pretty impressed at how rigid it actually was after installing.

    Even interior drywall is credited with a minimum of 150 lbs of lateral resistance per linear foot, so it's all cumulative. But I think it makes more sense to use a structural sheathing that is also a nailbase for trim and siding, then deal with insulation and weather barrier separately.

    Almost without exception, anything designed to perform several functions performs none of them as well as a single-function element.

  9. Brett Moyer | | #9

    Why hasn't anyone talked about the permeability of SIS?

    According to Dow's website:

    As tested per ASTM E96, Procedure A, the perm rating of STYROFOAM™ SIS™Brand Structural Insulated Sheathing is approximately 0.03 perms.

    A Class I vapor retarder with an R-value of only 3-5.5 is concerning.

  10. Riversong | | #10

    Because the question was about headers.

  11. Brett Moyer | | #11

    Oh, I thought the thread was about "A few questions on advanced framing strategies" using a 2x6 wall and SIS as sheathing.

  12. User avatar
    Danny Kelly | | #12

    Allen - I have looked at zip system before and is probably superior as structural sheathing but I am choosing the DOW SIS for additional R-Value and a thermal break and keep my wall somewhat minimal. With a 2x6 wall, dense packed insulation and 1" rigid foam - I can get an R-28, more than double our code minimum here. Both manufacturers say they can be used as a WRB - but I personally do not want to rely on tape as my WRB for the life of the structure so consider them equal in that sense. With both products, I would install felt or housewrap on top of them. Both products are about equal as an air barrier as well from what I can tell.

    Brett - before using the DOW SIS for the first time I was concerned that my wall could not dry to the outside and we had a discussion on this forum (a little over a year ago) - I'll see if I can find it. In outr mixed humid climate, solar vapor drive is a problematic force and the low perm rating will help prevent that issue. With a good HVAC system and proper ventilation, drying to the inside should not be a problem.

  13. TJ Elder | | #13

    Shear strength of dissimilar materials isn't necessarily cumulative. It depends on stiffness, the stress vs. strain behavior of each material. The most elastic material can end up carrying all the load after the less elastic materials have reached their limits and failed. Thus, at the limits, gyp board will crack and carry no load, leaving plywood to carry the entire load.

    Looking for some backup on this point, it seems the rules about this have been relaxed recently under IBC for structures that only resist wind load.

  14. Riversong | | #14

    Shear strength of dissimilar materials isn't necessarily cumulative.

    IRC Bracing Methods, Relevant Test Data, and Recommended Design Values for Wind Bracing Analysis of Conventional Wood-Frame Homes

    Prepared by Jay H. Crandell, P.E.
    Prepared for ICC Ad Hoc Committee on Wall Bracing

    Construction Requirements for IRC 2006 Bracing Methods (R602.10.3)

    Minimum Interior Finish (inside face of braced wall)
    • ½” Gypsum wall board
    • Vertical or horizontal unblocked installation
    • 8”oc nail spacing or alternatively pairs of nails at 12”oc; or 12”oc screw spacing for 24”oc stud spacing or 16”oc screw

    Method 1 – let in bracing
    Based on tests of similar walls with GWB only (addressed later), with 1x4 brace only, and with both combined, it appears that for additive purposes the GWB provides about 250 plf (ultimate) and the brace provides about 1,500 lbs (ultimate) when combined. Thus, as a representative fully-restrained 1x4 Method 1 ultimate shear strength value (including ½” GWB interior finish) can be determined as follows: 250 plf x 8 ft + 1,500 lbs/brace = 3,500 lbs per Method 1 brace (ultimate).

    Method 4 – fiberboard sheathing
    As before, it appears that the strength of the GWB interior finish is additive and, thus, a characteristic ultimate shear value could be determined as follows: 500 plf (fiberboard) + 250 plf (GWB) = 750 plf.

    Method 5 – gypsum sheathing
    Two-sided Applications:
    • GWB (4”/8” fastening per gypsum sheathing) + GWB (8”/8” fastening per finish)
    Ultimate shear value = 415 plf + 250 plf = 665 plf
    The value of 415 plf is less than that reported in the literature, but it facilitates an additive approach that results in a total shear value for the construction that does generally agree with the test data

  15. User avatar
  16. Mojave Disaster, 3B | | #16

    Hello, pros.

    I'm bumping this thread, which began with a headers discussion, because I'm a couple of weeks away from installing a window in 12" concrete block in my basement, right up under the rim joist. It'll be a new opening, and I'd like to size the header properly so I can also insulate well. I'm intending to do the install (but not cut the hole) myself. The person cutting the hole normally does the installs too, and he says he normally puts *two* 4x6 posts in as a header. I'd like to be able to balance safety and insulation a little bit better than that.

    Are there special weight considerations for installing in concrete block that I need to be aware of, even if I'm right up under the rim joist? Because apart from that detail, I don't think there's a lot of weight for me to worry about. The span isn't crazy--it's 33". This wall, gable end, is perpendicular to the joists, but the wall that the center beam actually rests on is a bumpout--parallel to this wall in which I'm putting the opening, but not the same wall. And in terms of the rest of the load above, the second story and the roof bear only on the exterior walls and not on the center beam. So, I don't think there's an exceptional amount of weight above this proposed window opening.

    So how do I calculate or closely approximate how large of a header I'll need under these conditions, but not drastically oversize it? Even if I did a single 4x6 header and matched the depth of the sides to that, considering I'm in 12" block, that'd still give me a remaining 6" of extra insulating room before I even get to the interior face of the block wall, which sounds fine to me. Fair? Still drastic? Or insufficient?

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