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Am I asking for trouble?

user-1137156 | Posted in Energy Efficiency and Durability on

I may need to switch to a 2×6 load bearing exterior wall as my outer wall cannot be load bearing but had to be changed to a “Larsen” like truss. To maintain about the same total wall depth I’m considering an inner 2×6 load bearing wall sheathed on the exterior with OSB and filled with mineral wool r23 and a modified Larsen truss 2×6 wall outside. I’m envisioning the trusses as frames of 1×2 with pieces of Thermoply on one side at both ends and filled with mineral wool (about r19). Should I stagger the studs? or put them inline? Staggering the studs gives a bit higher effective r value but at a bit more condensation risk to the OSB at the cold sripes where the outer “larsen” stud is. With half the r value on each side of the OSB (between aligned studs) I’ll need to keep the interior dew point below 30 f to be safe at -10f outside, which the ERV should be able to do. Is half the R on each side of OSB too risky?

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  1. GBA Editor
    Martin Holladay | | #1

    1. You mentioned outdoor temperatures of -10 degrees F, but you didn't tell us your geographic location or climate zone. It would help if you did.

    2. If you are trying to keep the OSB layer above the dew point during the winter -- not strictly necessary, but evidently that's what you are thinking when you are asking, "Is half the R on each side of OSB too risky?" -- you need at least 36% of the wall's total R-value on the exterior side of the OSB in Climate Zone 6, or at least 43% of the wall's total R-value on the exterior side of the OSB in Climate Zones 7 and 8. (More information here: Combining Exterior Rigid Foam With Fluffy Insulation.) In other words: You don't have to worry about the ratio.

    3. In any case, these ratios were developed for walls with rigid foam, not walls that are vapor-open on the exterior side of the OSB.

    4. Larsen trusses are incredibly strong if they are built according to John Larsen's recommendations. They are often fastened to the top of the wall at the top plate and the bottom of the wall at the rim joist. In other words, there is nothing wrong with staggering the studs. More information here: All About Larsen Trusses.

  2. user-1137156 | | #2

    Thank you! I'm in central Kentucky near Lexington , climate zone 5, I think, I'd rather be safe than sorry and I've seen several days of -10f temperatures. After reading the article I'm not sure if what I'm planning is properly called a "Lasen" truss It will be the outer stud of a double stud wall, bear no roof load, cantelever outside the "foundation, the total wall will have one layer of sheathing ,located in the middle. There will be plywood top and bottom plates connecting the two lines of "studs . nailing the outer studs to the OSB will not be necessary as the inner cord of the ? truss is over the foundation
    Regarding staggering the studs: With staggered studs, at each outer stud location, the insulation inside will e r23, outside the Thermoply web is about r10 so at the Thermoply web the ratio r outside to total r drops to about 30%. While with aligned studs the ratio of r outside the OSB to total r is over 57% at the Thermoply web.. The difference in whole wall r is about 1, 41 vs 40, being risk averse I'll switch to aligned, at this point it only involves editing my Sketchup model.

  3. EthanT | | #3

    It's funny how my brain works because I really do better with drawings of the walls than with long written descriptions. It would really help me if you posted a picture of even a very rough sketch of your wall build up.

  4. user-1137156 | | #4

    Here is an annotated scan of a printout from Sketchup.

  5. Expert Member
    Dana Dorsett | | #5

    All of Kentucky is in US climate zone 4A.

    Having half the R outside the structural OSB sheathing has HUGE dew point margin at that layer in your climate!

    What is the non-structural sheathing on the exterior going to be?

    Despite the occasional -10F temperatures during Polar Vortex events, the 99th percentile temperature bin in your area is about +10F. Only ~87 hours of the year are colder than +10F.

  6. user-1137156 | | #6

    Thanks Dana, I'll still do my heat loss calculations based on -10 f. Starting at the outer edge of the trusses and going outward will be 1. permeable house wrap, 2 Greenguard DC14 (XPS drainage mat), 3. Ambrico EZ wall (substrate for thin brick, 4. 1/2" thick bricks.

  7. Expert Member
    Dana Dorsett | | #7

    There's no point to the permeable housewrap if you're covering it with 1.1 perm DC14 (less vapor open than cold OSB) , and an even less permeable sheet steel Ambrico EZ wall. It needs a vented air gap or a mesh-type rainscreen between the housewrap & drain mat to have a path to drying toward the exterior.

    It's silly to design for the calculated heat load at -10F when the outside design temp is in postive digits, and that's MORE true for a high-R house than with a code-min house. In a high-R house the thermal mass inside the house and in the walls deliver a longer thermal time lag- it has to be -10F for a number of hours at a significant heating input shortfall before the interior wall surfaces drop by more than a degree or two. (The time constant varies by the particulars, of course.)

    If you do that you will be prone to sub-optimally oversizing the heating equipment to the point of lower efficiency and lower comfort. So go ahead and calculate the load at -10F, but also calculate the load at the 99% outside design temp (+10F, at the airport in Lexington.) Most heat load calculations will overshoot reality, and the step sizing of equipment isn't on a continuum- the equipment sized to cover the calculate load at +10F with any margin at all may already cover the calculated load at -10F, and will in most cases will cover the ACTUAL load at -10F. Any shortfalls can easily be made up with a modest amount of resistance electric heating in the rooms that matter.

    Heat load increases roughly linearly with the temperature difference between indoors & outdoors. If you use 70F as a indoor design temp (+68F is code minimum), at +10F there is a 60F temperature difference, and at -10F there is an 80F difference. So an oversize factor of only (80F/60F=) 1.33x has you covered.

    But in 95% of the cases even 1.2x oversize factor from the calculated load at the 99th percentile temperature bin would not result in a comfort problem even in a code-min house. Your house is looking like overall performance will be more than 1.5x code min.

    Equipment step sizing matter. If you design for the load at -10F the load itself is 1.33x the 99% load, but the next step size to cover the -10F would have to be even higher. At even 1.2x overzing for the -10F load you'd then be at 1.6x oversizing, and at 1.4x above the -10F you'd be closing in on 2x oversizing at the 99% load.

    ASHRAE recommends no more than a 1.4x oversize factor for the 99% heat load (which in your case would still have margin at -10F). AFUE for fossil burners are tested at a presumed 1.7x oversize factors, and while that's not an efficiency hit, with hot air furnaces it's a comfort hit, due to the wind chill that comes with the higher cfm, as well as the higher noise factor.

  8. user-1137156 | | #8

    My understanding of the DC14 is that it allows drying and serves as the vented rain screen because of the textured inner surface. Are You saying this is insufficient and an additional air gap needs to be built in?.

  9. user-1137156 | | #9

    From the Greenguard web page.
    "DC14 Drainage Mat was designed following rain screen principles to allow ample space for drainage and drying between exterior wall cladding and wall sheathing. With drainage channels on both sides, it permits easy drainage for water and allows air movement between the cladding and sheathing to help keep the wall dry. It is ideal behind exterior claddings that are closely attached to the wall sheathing, such as traditional stucco, stone, thin brick veneer, and siding. Because DC14 is made of extruded polystyrene (XPS), it does not absorb moisture and is not a food source for mold."

  10. EthanT | | #10

    Jerry, thanks for posting the image. I still can't 100% visualize your wall but maybe I'm dense. My main concern with Thermoply is that the perm rating is very low. Why are you using the Thermoply?

  11. GBA Editor
    Martin Holladay | | #11

    I think that Dana's point about the Greenguard DC14 is that the DC14 can act as your water-resistive barrier (WRB), so that you don't need two adjacent WRBs (the DC14 and the housewrap).

    I think that Dana is right, but it can't hurt to contact Greenguard and ask a technical representative whether the DC14 product complies with code requirements for a WRB.

  12. user-1137156 | | #12

    My reasons for using Thermoply are it's thickness (under 1/8"), it's low cost (less than OSB) adequete strength. You are correct that it has low permeability But In my use of it that doesn't matter as it is never installed parallel to the wall surface

    DC 14 is approved for use as a WRB when installed over wood sheathing but I've found no indication of use without sheathing so I've added the "housewrap" . This may, indeed, be unnecessary.

  13. user-1137156 | | #13

    While the My " Larsen" like truss is thermally a bit better than a plain 2x6 it is far from cost justifiable!!
    My goal was a whole wall r of 40 and with 2 2x6 lines of studs 24" OC and r46 in the cavities the clear wall r of just studs is r 40 switching to staggered studs and doing the same calculation studs only with them staggered says r43.25 instead of r40, where did i get the idea that staggered stud weren't a big thermal win? Staying with the staggered layout and calculating in the inner wall's top and bottom plates and the tie plates I'm still at r41.97 + 1 for the DC14. The window " frames" which must be aligned may be where the complex stud is worth while the door frames will need to be aligned 2x6'.
    It'll take me a while to calculate the rest. I'm still concerned about condensation risk with staggered plain 2x6.

  14. GBA Editor
    Martin Holladay | | #14

    Your latest comments are a little hard to decipher. But for those who are designing a double-stud wall, the usual advice is to leave a gap between the two rows of studs. When this gap is filled with insulation, the question of whether to stagger the studs becomes moot -- because you've got some R-value in the gap that breaks the thermal bridge.

    In your case, you seem to be designing a double stud wall without the gap.

  15. user-1137156 | | #15

    Yes, I'm designing a double stud wall with no gap. That is why the staggered studs are such a big thermal win. And also why the staggered studs are a significantly greater condensation risk. I won't have insulation between the stud lines but can make the outer studs better insulators a wood 2x6 outer stud is about r 6 7/8, my specific insulated truss stud, insulated with r6/in foam is r21.3 . or using much cheaper r4/in foam r16. Using either insulated truss stud almost completely eliminates any thermal advantage from staggering the studs and essentially eliminates the increased condensation risk as well. Aligned should be a bit easier to build (one layout) but staggered with the lower cost foam is almost exactly as good as inline with the expensive foam. Staggered with he lower cost foam still has 41% of total r outside at the outer stud locations so it is low condensation risk as well. Adopting a staggered layout with the low cost foam insulated studs compared to inline wood studs raises the clear wall r of just stud lines from r40 to to r44.02 and may justify the added complexity while the more expensive foam only gets r 44.39 and is clearly not justified.

  16. Expert Member
    Dana Dorsett | | #16

    With no rigid sheathing between the DC14 & compressible-shift-able over time rock wool cavity fill counting on the back texturing of the DC14 to provide truly adequate ventilation path seems imprudent to me. A less compressible mesh solution between the DC14 and rock wool (or housewrap air-barrier) seems like it would be more reliable.

    It's not really possible to get to R40 whole-wall without foam in a ~foot thick double studwall and only 11" of rock wool, and thin foam. Clear-wall numbers are always going to be much more optimistic than whole-wall calculations. When it's said & done you'll probably still be ~R35-ish whole-wall with a double 2x6 approach. See cases #4, #5, and #10 in this document (# 5 particularly) :

    At the same wall thickness it'll be in the target range if the exterior studs are 2x3s rather than 2x6. (Use finger-jointed 2x3s, if flatness matters, or you don't want to spend a lifetime shimming up the thin-brick steel to be perfectly flat. They're not very expensive, but usually have to be ordered in advance.) Insulating from the structural 2x6 layer out using full dimension 24" wide 2" thick soundproofing rock wool batts (designed for 24" o.c. steel framing) to fill in behind the 2x3 layer, and 23.25" wide 2.5" (or compressed 3") soundproofing batts designed for wood framing.

  17. GBA Editor
    Martin Holladay | | #17

    As an overall comment, your struggles to re-invent the wall are getting complicated. You may want to read this article: How to Design a Wall.

  18. user-1137156 | | #18

    First you said there is no point in house wrap, now you say it's needed I agree and that's why I designed it in. A double 2x6 is not what I'm planning! The inner wall is 2x6 with insulated box headers. The outer wall is 2x6 size frames of 1x2 partially sheathed on one side with Thermoply and the 1 1/2" x 4" space between the 1x2's filled with foam ( could as well be mineral wool) as shown above. All of the outer wall's window framing will use the foam filled 1x2 framed "lumber". The only places where there will be wood spanning the full 11 7/16" depth of the wall are the exterior door frames, end blocks of the insulated box at the top of windows and doors and the 1/2" plywood "tie" plates. Case 5 of the referenced document is closest but they start out with a lower cavity r 43 vs 46, my calculated clear wall r is also about 3 greater than theirs, I may not hit r 40 but it'll be pretty close.

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