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Community and Q&A

Drop chord truss, a.k.a. dropped heel truss

Fred H | Posted in Green Building Techniques on

I’ve been designing a 1467 sq ft, single-story house for Vermont, which is zone 6. The simple energy modeling I’ve done makes it clear that sealing is an important issue. We’re intending to use SIP’s with trusses, both installed by the SIP panel company. So the ceiling seal seems the most problematic. But the same issue exists with other approaches, and we may need them if they’re significantly cheaper.

I’ve spent some time reading everything I could find on ceiling sealing here and on BSC. I’ve reached the conclusion that only the two extremes exist. The normal practice is to seal with the wallboard; the average effectiveness of this is evidenced by the many articles for homeowners on how to fix these leaks. Certainly, it can be done right. But it’s difficult and finicky; I’d like to find an approach that’s easier — so it’s less likely to be done poorly. The other extreme is for a passivhaus, where extremes seem to be the norm. We can’t afford this. I’ve found what seems to be a middle ground, and I’d like to expose it to criticism.

A drop chord truss is a horizontal chord below the normal bottom chord (which rests on the house wall to support the truss). This results in another chord perhaps a foot lower than the bottom chord. The air seal, and the ceiling can be attached to this — and not involve the interior walls in any way — and the insulation placed on top of it, with full depth to the inside edge of the wall. Except for seams, only sealing to the outside walls is necessary. The only disadvantage seems to be the cost of building a taller outside wall; as an energy heel is our alternative, some of the costs apply to both..

I found it best expressed and explained in a short section on page 13 of a building manual by the Alaskan Housing Finance Corporation: Some detail is shown in a Foard Panel document: A slightly different use is shown in: Everything else I found is redundant with these three brief mentions.

Thanks, I’m looking forward to comments.

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  1. GBA Editor
    Martin Holladay | | #1

    As a courtesy to our readers, I am copying an illustration from one of your links.


  2. GBA Editor
    Martin Holladay | | #2

    You can certainly specify drop chord trusses if you want. But I fail to see any advantage compared to raised-heel trusses.

    Traditional advice calls for attic insulation to cover all of the top plate of the exterior wall. That's certainly easier with a raised-heel truss than a drop chord truss.

    With careful detailing, you can make sure that the insulation at the top of the insulated wall is so well installed that the eaves will perform just as well as they would if you installed raised-heel trusses. But this region is susceptible to air leakage and ice dams, so I'd stick with raised-heel trusses unless you have a compelling reason not to.

    With either type of truss, the necessary work to make sure that the ceiling drywall performs as an air barrier is the same. The only problem with your approach is that you need some type of careful blocking between the studs, carefully caulked or sealed with spray foam, to connect your ceiling air barrier to the air barrier at your wall sheathing. Another advantage to raised-heel trusses, in my opinion.

  3. Fred H | | #3

    Thanks for the answer, Martin, and for posting the illustration. But I'd like to quibble with some of your answer.

    Covering all of the top plate of the exterior wall is necessary to prevent a gap in the insulation when the insulation is only above the top plate. In this circumstance, a raised-heel truss is necessary to get full insulation depth across the top place. But with a drop-chord truss, the insulation is only *below* (the top of) the top plate -- so covering it doesn't seem necessary.

    For your ice dam point: It seems to me that there's the same space between the insulation and the roof with both raised and dropped heel trusses if they are all specified to the same criteria. You'd specify the raise/drop to be the insulation depth, adjusted for the basic heel height determined by the roof pitch, plus an additional space (an inch?) to provide the needed ventilation. So it seems to me that there's no intrinsic difference between the two heel types from this point of view.

    I hadn't considered the need for blocking, etc. as I was primarily thinking of SIP's. A more conventional alternative was purely cost-driven in my mind, and fuzzy. Your blocking, etc. point makes me lean more strongly towards SIP's.

    With SIP's, your concern about blocking/calking/etc. the studs doesn't seem to apply as there are no studs.

    The inside of the SIP wall (OSB, already an air-seal due to the construction of the wall) would seem to be an ideal surface to which to attach the air barrier. As the air barrier can be installed before the interior walls, we'd avoid the need to seal at the top of the interior walls, which -- for our design -- have twice the ceiling-to-wall length of the periphery of the SIP walls. This seems to reduce the need for this type of seal by 2/3, reducing the likely leakage due to errors by the same amount. And, thinking of the latitude, if there is any truss lift in very cold weather the air seal won't be affected.

    I very much appreciate the time and effort that you bring to this and all the other questions.

  4. GBA Editor
    Martin Holladay | | #4

    I assume you don't want to install roof SIPs -- only wall SIPs. Is that correct?

    If you want to use SIPs for your walls, you are correct: the interior OSB facing on the SIP walls is an air barrier, and you can seal the ceiling air barrier to this OSB layer easily (for example, with tape).

    Again, I fail to see the advantage of a drop chord truss over a raised-heel truss. Either type of truss will work, with exactly the same air sealing details at the ceiling. In your case, however, the SIP order will be more expensive, since your wall SIPs will need to be 16 inches taller with drop chord trusses than they would be with raised-heel trusses. I'm sure that the SIP company will charge you more for the extra 16 inches.

  5. Fred H | | #5

    Martin, thanks again,

    Yes, only SIP walls. We don't want the additional vertical space.

    Yes, they will be more expensive if they are taller; about $315/inch for our small house, based on a preliminary quote.

    I don't think they need to be 16" deep. For a 9/12 pitch roof, at the inside of the wall the top of the top chord is 10.81" above the wall. We're looking for R50 (R30 walls), so need 14.2" of cellulose plus an inch for ventilation for 15.2". 15.2" - 10.8" = 4.4" or 5" drop, for an additional SIP cost of $1575.

    (Another calculation approach: 9/12 pitch creates a 3/4/5 triangle, so the vertical of the sloping 2x4 is 3.5*5/4 = 4.375". Adding the butt cut of 0.25" gives the minimum heel of 4.625", increase across the wall = 8.25*9/12 = 6.185". So 4.625 + 6.185 = 10.125". 15.2 - 10.8125 = 4.3875. Same result.)

    To me, this seems a reasonable cost to avoid sealing more than 300 feet of interior wall-to-ceiling. Further, since the required sealing is just tape -- and all done at once -- I have more confidence in the quality of the work.

  6. GBA Editor
    Martin Holladay | | #6

    I don't get it. Why can't you seal the seam between the drywall ceiling and the SIP exactly the same way with raised-heel trusses as you would with drop chord trusses?

    Tell me more about your plan to "to avoid sealing more than 300 feet of interior wall-to-ceiling."

  7. Fred H | | #7

    Thanks for pushing me. I'll upload some cross sections and a house layout -- probably tomorrow -- in enough detail that we can put this to bed.

  8. Fred H | | #8

    Martin, I'm writing this at a detail that I'm sure you don't need. However, I've gotten so much from others on this forum that I wanted to try to make it understandable to all.

    For R50 we need 14.2" of blown cellulose; adding 1" for ventilation, we have an insulation depth target of 15.2". (I won't be mentioning ventilation again until the end, when I'll round up to make sure there's enough.)

    My starting point for this discussion is the heel height. The diagram below was copied from RoofTruss dot ca. (The small bit of vertical at the left of the bottom chord is the "butt cut", which I referred to in an earlier posting. It is generally 1/4".)

  9. Fred H | | #9

    The heel height for our 9/12 roof is 4.625"; heels for various pitches are readily available on line.

    With blocking at the end of the bottom chord, we can fill to the heel height -- that is, the bottom of the roof sheathing that is on the top chord.. So our 15.2" is reduced to 15.2 - 4.625 = 10.575".

    Next, I'll discuss filling above the top of the heel. Our R30 SIP wall is 8.25" thick; think of the space above the heel as a right triangle with a 9/12 pitch. Happily, this is equivalent to a 3/4 pitch, indicating that we are working with a 3/4/5 right triangle. This will allow us to avoid using trigonometry and simplify the explanation. The triangle below was copied from a site and then modified with the addition of the DE line. (I apologize for the orientation reversal and my inability to show a 3/4/5 triangle.)

  10. Fred H | | #10

    AB is the width of the top of the SIP wall, B is the top of the heel, and AC as the height of the space above the inside wall. Note that AC is (side) three, AB is four, and BC is five of the ABC triangle. In my earlier posting I thought that I could fill to the top of AC, but that was wrong. Your "ice dam" mention pointed the way: DE, the vertical height of D above AB is the shortest distance from warmth to cold. So this is what we need. All of the above triangles are 3/4/5 triangles, so we can use that to calculate DE. Here are the steps:

    1. Since we know AB is 8.25" (four) then AC (three) is 3/4 of 8.25 = 6.185"
    2. Using the ACD triangle, AC is five = 6.185", so AD is four => 4/5 x 6.185 = 4.95"
    3. Using the ADE triangle, AD is five = 4.95", so DE is four => 4/5 x 4.95 = 3.96"

    We've gained an additional 3.96" of insulation depth. So our 10.575" insulation thickness is reduced to 10.575 - 3.96 = 6.615". Rounding this up, we have a needed heel height/drop of 7".

    For the permutations of how to implement the 7" I'll use trusses from Medeek dot com, who writes a truss plugin for SketchUp -- and a truss designer; I'm confident in his layouts.

  11. Fred H | | #11

    Above is a conventional 7" raised heel. From this I can see that you were right. We can apply the air seal and ceiling direct to the bottom chord of each truss (I hadn't realized that). The only "con" is that bracing is required because the heel is greater than 5.25".

  12. Fred H | | #12

    Above is a truss without a raised heel, but with a 7" drop. The con is the additional 7" of SIP wall height needed -- as you mentioned. It would work fine but cost too much.

  13. Fred H | | #13

    Above is a truss with a 5" raised heel with a 2" drop. The heel falls within the 5.25" limit so no bracing is needed. The walls are 2" higher ($630), but we've saved the bracing cost. I don't know which alternative is more costly.

    It's clear to me that I wasn't thinking in enough detail yesterday. Thank you for questioning me.

    The next issue is the amount of wall-to-ceiling sealing. Below is a layout of our planned house.

  14. Fred H | | #14

    The green walls are the outer walls, the inside perimeter totals 160'. The purple lines touch the ceiling, requiring sealing if we don't have a monolithic air seal layer. The length of the lines totals 128' (less 4' for both ends of a chase across the center room); both sides must be sealed so the total is 248'; I propose to not seal the wall-to-ceiling joints of the inside walls; the painters can hide visible cracks.

    I'm sorry about the length of this reply, but it was the only way I could think of to properly reply to your questions. Again, thanks.

  15. GBA Editor
    Martin Holladay | | #15

    Whether or not you install the drywall on the ceiling of the whole house before or after the partitions are installed is up to you. It's exactly the same amount of work for raised-heel trusses as for drop chord trusses.

    Your complicated math on raised-heel trusses is both confusing and unnecessary. All you have to do is to go to a truss manufacturer with your house plans and say, for example, "We want raised-heel trusses with 18 inches of vertical height between the outer edge of the outside walls and the top of the truss," and the engineers at the truss company will take care of all of the rest of the details.

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