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Most cost effective for insulating existing stick framed cabin roof?

user-975232 | Posted in Energy Efficiency and Durability on

Cabin in Blue Ridge Mountains of VA originally built as tractor shed 35 years ago. Plan to later build primary home netzero super insulated with best practices studied in this forum but for now trying to minimize cost on this part time cabin of 450 sq ft.

2×4 walls with black board sheathing, plywood at corners, original cementuous siding…didn’t realize that stuff was around 35 years ago but it’s original.

Roof is galv metal on top of 2×6 rafters with 1×6 strapping with 4″ of space between straps – the rascal sucks heat out this time of year.

Focusing on ceiling at this point…walls should be easy. Must keep cathedral as there is a sleeping loft. Would love to spray foam the whole thing. Although it’ll probably be around another 50 years, I don’t want to spend that kind of $ for something that’ll be used 2-3 days per month. Flash and batt such a small project means a diy spray kit which is 3 times per bd ft as premade panels that I can tight fit and can foam seal and then batt.

I’ve also considered encapsulated batts straight against the strapping with hot roof and 1″ foam board on bottom side covered in 1/2″ drywall but I imagine that may create a potential mold situation??? How about I spray leftover latex enamel on the wood components prior to this method of insulation…got a large supply free.

Feedback please….more time than sense & cents so excessive labor on my part is welcomed to save $

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  1. GBA Editor
    Martin Holladay | | #1

    Your problem is that your roof has no air barrier. I wouldn't waste my time installing batts between the rafters.

    If I were doing the work, I would first install a layer of OSB on the underside of the rafters as an air barrier. Tape the seams with high-quality tape. Then install rigid foam, as thick as you can afford, followed by gypsum drywall. Sort of like a site-built SIP.

  2. user-975232 | | #2

    Thank you so much for your reply Martin

    Please correct me if I'm wrong - it appears from your answer, that the first
    thing is to fulfill the air barrier need with something that has the ability
    to pass moisture and dry out any humidity/condensation from within the
    insulated rafter cavity. If so, how about using solid thin carboard similar
    to that used in backing picture frames in place of the OSB - I have discarded
    rolls of that where I can back tack it up to the strapping and then seal the
    edges with Covalence Adhesives 617020 Foil Tape - have numerous partial rolls
    of that as well.

    Next fill the rafter space with Roxul which actually offers greater R value
    than open cell foam boards at a lower $.

    From there create a thermal break with, as you state, as much foam boarding
    below the rafters as I can afford or in my case justify , tape the
    seams and drywall over.

    Does this direction appear to you to fulfill your recommendations on a budget
    or am I failing to recognize any particular issues here?

  3. user-975232 | | #3

    Photo of project at hand

  4. GBA Editor
    Martin Holladay | | #4

    1. You wrote, "Must keep cathedral as there is a sleeping loft." I think the floor joists for your proposed sleeping loft are seriously undersized, too far apart, and already sagging -- before anyone even installs a mattress.

    2. If you want to install thin cardboard between your rafters in hopes of creating an air barrier, go ahead. I don't think it's going to be airtight, however. A good air barrier material should (1) be stiff, (2) be long-lived, and (3) have few seams. I think your proposal to use thin cardboard fails on all counts.

    If you really want to achieve an air barrier on the underside of the skip sheathing, you probably can, but it would be fussy work. I would use OSB or plywood, not cardboard. You'll have to spend a lot of time sealing the perimeter of each piece, with caulk, spray foam, or high-quality tape. Because most wood-framed buildings move with changes in humidity and temperature, though, I think that such an air barrier would deteriorate over time.

  5. user-975232 | | #5

    Sincere thanks for the concern Martin - good eye, bad camera

    No fear - you're right, they do look as if they are sagging...guess it's the cheap camera angle - they are actually properly crowned and there is a support wall mid- span behind the camera supporting the loft with much beefier joists and the load is carried to an existing set of piers. The ones in the photo are simply in place to prevent the outer walls from moving out.

    I was figuring no seams since the card board is on a roll but taping all edges would be a bear, guess I'm thinking it would be the same taping as OSB, only I wouldn't have any seams at the 8' mark. OSB would be easy to can foam though.

    You've got me to thinking about life span and the cardboard probably would deteriorate from the heat produced by the sun hitting the roof.

    Thanks again....oh and do you have an opinion on the Roxul/Rock Wool material?

  6. GBA Editor
    Martin Holladay | | #6

    Roxul rock wool is a perfectly good insulation material. Since the material is air-permeable, however, it should always be installed in conjunction with an interior air barrier, an exterior air barrier, or (ideally) two air barriers.

  7. user-975232 | | #7

    You've sold me Martin - sure appreciate all of the help.

    OSB on bottom of strapping
    Foam all edges - foam ends of where OSB panels meet and tape seams
    Fill cavities with Roxul
    1" of insulating boards on bottom of rafters taped at seams creating a thermal break - always top quality tape - more depth of insulation if I can find a great $....2 layers of 3/4 t&g foam panels staggered seams would be pretty awesome
    5/8" drywall since it's 2' OC

    * Ready for my next challenge in the Spring?

    Cabin has nice solid floor but only a slingle layer of solid lumber with several gaps between slats. I'm pretty trim but doubt I can slither under the cabin as it's pretty close to the ground - about 14" at the best part and decreases to only 9". I'm contemplating using a push stick to cover the ground with heavy plastic to minimize moisture rise but it's going to be nearly impossible to tape the seams so I'll use a single large sheet and cut only to get it around the middle support column. I recon the good side is that the soil in that spot is fairly sandy thus the ground is powder dry every time I've reveiwed it. From there I'll overlay a second sheet over the seam and tape anything I can reach.

    Next is to 2" foam board insulate the perimeter of the foundation, come back inside, roll out and tape rosin paper on top of the existing floor, then 1" of foam board taped then 3/4" OSB screwed clean through the foam to the subfloor. I plan to simply water based stain and poly the OSB as the finish flooring.

    I have the 3/4" OSB under a shelter on stack sticks airing out as much of the formaldehyde as possible over the next several months.

    See any issues with this approach?

    Should I put in automatic temparature controlled crawl space venting?

  8. GBA Editor
    Martin Holladay | | #8

    The standard solution to a tight crawl space is to dig out the crawl space to make it deeper (for better access).

  9. user-975232 | | #9

    Ooops, I didn't give enough details. Too close to the riverbank. Although no signs of it ever having been flooded, I'm concerned that an excavated crawl space would invite trouble.

    And as tenaciously as it is anchored...cost more to raise it than to tear down and totally rebuild.

    Wish I'd originally built it, but alas I'm working with it as is.

    Back to the drawing board

    Next oh great guru?!?

  10. GBA Editor
    Martin Holladay | | #10

    I think your proposed floor insulation -- 1 inch of rigid foam -- is too thin. That's only R-4 to R-6.

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