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Best insulation for a barn that will be converted to a workshop

zumbo | Posted in Energy Efficiency and Durability on

I have a 2 story 24′ X 36′ barn built on a concrete slab. It is constructed with 6×6 posts, 8′ apart. The exterior is board and batten. There is no insulation and no interior wall; i.e. the interior walls are the board and batten siding. Although it is well constructed, there are a few areas were moisture could enter the building. What would you recommend to insulate this type of building?


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  1. Expert Member
    Dana Dorsett | | #1

    One rough-cut approach:

    Buy a truckload of reclaimed roofing iso from some outfit like Insulation Depot and a sheet of 1" EPS.

    Cut some 2x2" spacer blocks from the EPS to screw & glue to interior side of the board & batten on a 24" o.c.

    Tack up housewrap to the posts roughly flush with the spacer blocks, then cut'n'cobble a rough fit of the roofing iso to the post edges, then put up interior furring screwed to the posts, and through-screwed to the board & batten (through the spacer-blocks if you can), onto which you can mount the interior gypsum. Use can-foam to seal the edges to the posts.

    With the 1" air gap on the exterior wind-blown moisture that makes it to the housewrap can still dry to the exterior.

    How thick the foam needs to be to make sense depends on your climate and how warm you intend to keep it in there when unoccupied/occupied, how you intend to heat & cool it, etc, and how air-tight and well insulated you make the rest of it. You can insulate between rafter beams in roughly the same way, but allow a 2" air gap, and add soffit & ridge venting if it doesn't already have some.

  2. jklingel | | #2

    Just a thought. Is it feasible to frame between the 6x6's and put a Weather Resistant Barrier on that, then frame again for a double-stud or single stud wall inside that? It is bass-akwards and extra wood, but you'd have all the components of a "normal" house that way and a huge rain screen.

  3. GBA Editor
    Martin Holladay | | #3

    Clearly, if you have board-and-batten siding on a barn with posts spaced at 8 feet, the walls must also include horizontal nailers every 2 feet. That being the case, you don't need the 2"x2" spacer blocks proposed by Dana.

    You can install a rigid product between the posts, attached on the interior side of the horizontal nailers. This could be Zip System OSB with taped seams, or it could be 2-inch-thick foil-faced polyisocyanurate with taped seams. This layer acts as your exterior drainage plane. The horizontal nailers supply the air space between this layer and the siding. Be sure to air-seal the perimeter of these panels.

    Once you've done that, you are free to insulate on the interior of these panels any way you want (by erecting stud walls and filling them with insulation, for example, or by installing more layers of rigid foam).

  4. Expert Member
    Dana Dorsett | | #4

    Good point on the probable existence of horizontal nailers. I've seen it done a few different ways- hopefully it's dimensioned lumber and at a reasonable spacing, but if it's rounds or on random spacing one could always install your own 2x stock to make the interior wall flatter.

    Virgin stock foil-faced iso is about 3-4x the cost of reclaimed roofing foam per unit R. It can also be cheaper than bargain-basement batt + studs solution. If this is only an occasional-use building normally not fully conditioned, note that R12 roofing foam (or 2" foil faced) outperforms any fiber-insulated 2x4 studwall, and R18 (3" of foam) outpeforms any fiber-insulated 2x6 studwall. The economics of doing anything more than 2" of virgin stock iso probably aren't there unless it's heated & cooled on a continuous basis, but if using reclaimed goods R20+ isn't unreasonable. (And it's greener to re-use rather than going with virgin-stock.) Even with the cost-adder of a housewrap it's a pretty good way to go.

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