Replies

1. GBA Editor
   Brian Pontolilo | Jan 27, 2020 10:44am | #1

   Hi Giovanni.

   Just to be clear, there is no sheathing or roofing underlayment. Is that correct?

   The 1x4s are run perpendicular to the rafters. Is that correct?

1. 
   Cz4a_Giovanni | Jan 28, 2020 12:17am | #2

   yup, that is the case. perpindicular.

1. GBA Editor
   Brian Pontolilo | Jan 28, 2020 11:38am | #3

   I'm always reluctant to say to much about atypical buildings in design, construction, and use. But if you treat this like a vented, insulated cathedral ceiling, you could use some of the rigid foam to make vent baffles between the rafters, then fill the cavity with fibrous insulation for R-value, then use more rigid foam, continuous along the underside of the rafters to mitigate thermal bridging.

   In this arrangement and your climate and given the high-humidity activities you plan to do in the workshop, I think you'd want to make the ventilation baffles from the most vapor-permeable material you can, maybe EPS foam, and use the more vapor-closed rigid foam for the continuous insulation. In this way, you'll have a warm-side vapor retarder and an assembly that dries to the outside.

   You should do an impeccable job air sealing the interior side of the assembly. You can install the continuous rigid foam air tight with caulks, sealants, and tapes. If you don't install continuous interior rigid foam, install an airtight drywall ceiling before the tongue-and-groove ceiling (the paneling will not be air tight). And manage humidity with a dehumidifier or depressurize the building during these activities (you could simple put a box fan in a window blowing air out during canning, for example).

   Again, because this is not a common situation, I'm not positive about this approach and hope that if any other GBAers have experience with a building/use like yours, they will chime in.

1. 
   Cz4a_Giovanni | Jan 28, 2020 12:43pm | #4

   Brian,
   thanx much for your insights, on helping me think through this. Totally understand your initial reluctantcy, but any and all insights will help me think through the various options.
   Been a huge fan of the siga sicrall tape, and likely will use it for air sealing in this scenario.
   Any more thoughts, confirmations, etc. would be great as ya'll have time. many thx
   Giovanni

2. 
   Cz4a_Giovanni | Jan 28, 2020 01:12pm | #5

   Btw, say i obtain the EPS board foam, to dry to the outside, is a thinner piece advisable, say a 1 in verses a 2 in of the EPS, (since the thickness can affect density, and thus permeability/or permeance...before i layer a fibrous insulation for example...)
   Or in other words, put more simply...what proportion of the sandwiched layers ought be higher permeability to lowest permeabilty?..If i basically have 5 inches within the interior rafter cavity...after the air channel...say, maybe 2.0 inches of material with highest permeability facing outward, and the remainder 3.0 inches inward the least permeabilty Interior? or would something more like 1 to 4 inch ratio....work similarly?
   ( i do realize i am limited in making this a VERY cosy place, due to lower R-levels in gen). My idea is do my best to seal internally while preventing trapped moisture from foolish decisions on my part).

1. GBA Editor
   Brian Pontolilo | Jan 28, 2020 03:32pm | #7

   You are correct that the thicker the rigid foam the less permeable. In the case of the site-made ventilation baffles, use as thin of material as you can that won't deform. Seal it in place with caulk, sealant, canned spray foam, and/or tape. This way you will promote proper ventilation and maximize drying potential.

2. 
   walta100 | Jan 28, 2020 01:53pm | #6

   Of course you will cover the foam with something with a 1 hour fire rating as require by code? If for any reason there will be no inspection making you cover the foam, I say you would be crazy not to as the fire rating will give you the time needed to escape before the fumes from burning foam kill you.

   Be sure find and read both of Martin’s articles on how to insulate a pole barn.

   In my opinion every cent you saved and more by building a barn will be spent in a failed effort to make it be a high performance house.

   Walta

1. GBA Editor
   Brian Pontolilo | Jan 28, 2020 03:33pm | #8

   Here is the article on how to insulate a Pole Barn: How to Insulate a Pole Barn

3. 
   Cz4a_Giovanni | Jan 28, 2020 05:29pm | #9

   Brian, yup, had looked through some of those various barn articles. Good guidance. i suspect since my scenario had somewhat slightly different conditions, and materials i had that werent the ideal...( range in permeabilty..) specifically in 4a... i couldnt determine necessarily an ideal way to proceed. (I am a bit too detail oriented, but not necessarily a perfectionist). Different perspectives help.

4. 
   Cz4a_Giovanni | Jan 29, 2020 06:10pm | #10

   Any more tips or suggestions from the Pro's?