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Community and Q&A

Planning a dual-purpose residential post frame

Dave Sorenson | Posted in Project Management on

Or pole barn if you prefer… I’m in the process of planning, have tried to scour through pertaining threads. I am choosing post frame for combination of cost to build and to minimize thermal bridging. My goal is to find the best compromise between cost of building and energy efficiency, in a similar fashion to a Pretty Good House. Building planned to be 48’ x 64’ x 14’, with 20’ x 48’ of that to be living space. I am planning slab on grade with radiant floor for both the living area, and the shop/garage. Living area will be air conditioned in the summer with mini-splits (zone 6/7A), shop/garage will not. Obviously I need to treat this as 2 separate but connected building environments.

My initial plan is:

Living Area Exterior Walls
R3.85 R-Tech 1” Foam between girts and siding, taped and sealed
R7.7 R-Tech 1 ½” Foam fit between girts
R3 ¾” Foil Faced Polysio between columns, face toward inside, vapor barrier
R25 8” fiberglass in wall between columns

Shop/Garage Exterior Walls – 3 large doors, hard to enclose, not sure about air/vapor barriers?
R3.85 R-Tech 1” Foam between girts and siding, taped and sealed
R25 8” fiberglass in wall

Roof/ceiling blown cellulose, R50 if possible, looking at 6/12 raised heel scissor or parallel chord trusses as I would like to have a loft in the living area, and additional storage in the shop/garage

I would appreciate any comments/advice with regard to my plan. Also, I would expect the wall separating the 2 spaces to be 2×6, 24” OC, with fiberglass batt insulation. Do I need more insulation separating the 2 environments? I would also like thoughts on vapor and air barriers in this wall which separates the 2 environments.

I am neither a builder nor an architect, but have a basic knowledge of construction.

Thanks,

Dave

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Dave,
    The fundamental problem you face is creating a tight air barrier for your post frame building. It sounds like you will be using a 1-inch-thick layer of exterior rigid foam as your air barrier. That might work, but it won't be very robust, and rigid foam has been known to shrink.

    All of the other insulation layers in your wall aren't continuous. They are broken up into small rectangles, with lots of seams that offer opportunities for air leakage.

    A much better plan would be to put most of your wall's R-value -- perhaps R-20 to R-40 -- on the exterior side of your posts, installed as a continuous layer of rigid foam. This would require you to frame up between your posts, of course, and to install exterior sheathing before you insulate -- negating any cost advantages to your framing choice.

    In short, post frame buildings are hard to air seal and insulate.

  2. Dave Sorenson | | #2

    All of the other insulation layers in your wall aren't continuous. They are broken up into small rectangles, with lots of seams that offer opportunities for air leakage.

    My thought was that this was broken much less than traditional stick built construction, and even advanced framing at 24" OC.

    A much better plan would be to put most of your wall's R-value -- perhaps R-20 to R-40 -- on the exterior side of your posts, installed as a continuous layer of rigid foam.

    Other than Larsen Trusses, Double Wall construction, or SIPS, how would you accomplish large R-Values of insulation without any breaks? Looking for ideas that are not expensive to build, and something that is conducive to DYI participation at various levels. I'm willing to settle for a Pretty Good House, but must be at a pretty good price. Since we are in planning stages, it's a good time for this discussion from my standpoint.

  3. KEVIN ZORSKI | | #3

    Dave - Pretty much everyone who partakes of this site, when considering new construction, wants what you want. Going post and beam is a more expensive and time consuming route to this end. That's why it's not as popular as stick builds. But if you really want the looks of post and beam, then you have to weigh in that aesthetic choice. Otherwise, no need to re-invent the wheel. Double wall, or foam/ mineral wool "boards" over sheathing over stick build are the current "ways to go". These 2 methods, with all their options and permutations are well figured out in terms of detailing ( windows, doors, air and water sealing). If you really want the looks of post and beam, then investigate what people are doing to achieve a well insulated and sealed shell. I imagine this entails thick foam ( or mineral wool boards) installed over the frame/ sheathing, or some kind of really thick SIP panels (SIP panels more likely). Another option I have built in the past is a stick frame( double wall or insulated sheathing) with exposed timber floor joists - giving that post and beam look. Best of luck to you with your exciting project. Once you decide on your overall approach, this is a great site for questions about all the details.

  4. Malcolm Taylor | | #4

    Dave, Kevin has offered you good advice. The problem with timber frame is that you need to infill between the structure, and what you infill with is often sufficient to carry the loads, making the timber frame redundant.

  5. User avatar
    Reid Baldwin | | #5

    Dave,

    In my area, most stand-alone storage building use pole-barn type construction, presumably because that is cheaper. Most residential buildings use stick-frame for the reasons Kevin and Malcom mention above. I am guessing that you reasoned that, since 2/3 of your building is a shop, pole-barn would be cheaper overall.

    I am building a house with an attached airplane hangar. Since most stand-alone hangars are pole-barn type construction, I looked into mixed construction. My builder didn't want to go that way. He said that he doesn't need to have an engineer sign-off if he uses conventional residential construction, but would for alternative construction. I am wishing I would had pursued it a little further. We ended up getting an engineer to look at the hangar portion anyway and he specified things that added significant expense around the large hangar door. Maybe mixed construction would have ended up cheaper - maybe not. Mixed construction would require a contractor comfortable with both techniques which could be hard to find.

  6. Bob Irving | | #6

    Or, you could buy 3-4" recycled sheet foam and install it on the outside of your girts and posts, using the girt cavity for wiring, and install rough pine boards facing the interior space. Then strap over the foam and install your siding on that. Inexpensive and pretty fast. The issues are going to be the transition to the roof, and the joint at the floor. Since a pole barn does not normally have a continuous foundation, you'll need to find a way to get an air-tight, insulated transition at the floor/wall.

  7. User avatar GBA Editor
    Martin Holladay | | #7

    Kevin Zorski and Malcolm Taylor,
    I'm guessing that you are both confusing post frame buildings (commonly used for barns and warehouses) with post-and-beam buildings (commonly used for high-end homes). These are different systems.

  8. User avatar GBA Editor
    Martin Holladay | | #8

    Dave,
    Q. "Other than Larsen trusses, double wall construction, or SIPs, how would you accomplish large R-values of insulation without any breaks?"

    A. If you want an inexpensive high-R wall, forget about SIPs. Most builders who have looked into this issue choose either (a) a double stud wall insulated with dense-packed cellulose, or (b) a 2x6 wall with one or more layers of exterior rigid foam.

    Neither of these options requires a post-frame building. These systems are structural without any additional posts.

  9. Dave Sorenson | | #9

    Reid, you're spot on. I don't require the same level of energy efficiency and air/moisture control in the larger part of the building, and pole barn construction would be the choice for that portion of the building. Based on that, the initial goal was to find a way to insulate and control the smaller 1/3 of the building as a living space. I still believe this is the way to go, unless I want to build 2 totally separate buildings, which is also not desirable from an economic position.

    Looking at the suggestions here, it appears that one option would be a double wall in the living area, adding a stud wall inside of the pole barn shell. I have a lot to chew on now, and appreciate all the feedback here.

  10. User avatar GBA Editor
    Martin Holladay | | #10

    Dave,
    You wrote, "It appears that one option would be a double wall in the living area, adding a stud wall inside of the pole barn shell."

    That is, indeed, one option. If you go that route, just remember that you need to include a good detail for an exterior air barrier. That's the tricky part.

  11. Malcolm Taylor | | #11

    Martin wrote: "I'm guessing that you are both confusing post frame buildings (commonly used for barns and warehouses) with post-and-beam buildings."

    The same problems apply. You end up with bays in the range of six to eight feet. These need infilling. You can do it horizontally on both sides and call them girts, or vertically between the poles and call them studs. In both cases the same amount of wood could support the structure. Unless you are using some panelized infill, like SIPs, I don't see the point.
    Pole buildings do make sense for uninsulated storage buildings where shear and air sealing aren't issues, so no sheathing or interior girts are required.
    It's also worth noting that when used for anything beyond storage, their structure falls outside the ambit of many codes and must be engineered.

  12. Dave Sorenson | | #12

    This technique looks entirely possible, using bookshelf girts:

    https://www.greenbuildingadvisor.com/video-superinsulating-home-rigid-foam

    Key to the conversation is that this is only needed in the living portion of the building. The shop/garage portion, 2/3 of the total building, can be finished as a typical pole barn would, with girts on the outside of the columns. Also key, is that while the cost of materials is a concern, but so is the cost of labor.

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