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Kit Cabin Exterior Insulation and Weather Barrier Design Questions

SustainableTinyHomes | Posted in General Discussion on February 21, 2020 12:53pm

I am a complete novice and I am assembling a small lincoln log kit-cabin wood structure in Los Angeles, CA to which I am planning on adding exterior roof and wall insulation.  The interior of the wood structure will be exposed, so all weather/water/air/vapor sealing will be on the exterior of the structure and it has been painted on the outside. Based on my research, I feel that Poly-iso rigid foam insulation will be the best fit for the roof and the walls to achieve an R-value of R-38 for the roof and R-20+ for the walls.  I plan to use corrugated metal for the roofing system and I feel that Joe Listubek’s Perfect Wall and Perfect Roof designs will serve the needs of the structure.  

 

This is the kit cabin: https://allwoodoutlet.com/TINY-HOUSES-SMALL-AND-MIDSIZE-CABIN-KITS/Allwood-Chloe-123-SQF-kit-cabin-FREE-SHIPPING

 

 


I have several design and materials questions. 

 

Roof System:

 

Corrugated Metal Roof Atop Wood Sheathing, Weather Barrier, Poly-Iso Rigid Foam Insulation, Plus Rain Felt? and 1” x 4” Wood Rain Screen (Based on Perfect Roof System with Green Building Advisor Recommendation of a Roofing Felt Layer Atop the Rigid Foam Poly-Iso Insulation)

 

From bottom to top of roof system – Wood Sheathing (slats), ½” Ply-Wood Sheathing, Weather Barrier (Water, Air and Vapor Barrier), Poly-Iso Rigid Foam Insulation 3 x 2” or 2 x 3” Rigid Foam Insulation, Synthetic Roofing Felt or (Breathable Membrane), Diagonal 1” x 4” Wood Strips, Corrugated Metal Roofing

 

Weather Barrier: 

Do you still recommend the following water, air and vapor barrier(s) for the roof?

 

For the roof: Carlisle WIP 300 HT

 

It is my understanding that the Siga Vent SA acts as both a water, air and vapor barrier for incoming weather but that the wrap also allows water and vapor to “breathe out” if condensation or reverse vapor drive occurs.   I just wonder if you these “newer” wraps offer significant advantages and if you have experience working with them.

 

Is reverse vapor drive a concern? If so, how do you deal with it? I believe that the Siga Vent SA acts as a water, air and vapor barrier for incoming weather but that the wrap also allows water and vapor to “breathe out” if condensation or reverse vapor drive occurs.   I just wonder if you these “newer” wraps offer significant advantages.  

 

Do you wrap the foam?  Do you use another wrap or tape for the edges? Wig-Luv type tape?

Does this cause a problem with condensation or reverse vapor drive?

 

Would any of these above options cause a problem with condensation or reverse vapor drive? If so, what are your suggestions to mitigate this? 

 

What is the purpose of the Synthetic Roofing Felt?  Is this the best product for this application or do you recommend another product?   

 

 

For the back exterior wall and half of the side walls:

 

Corrugated Metal Siding Outside Wood Sheathing, Weather Barrier, Poly-Iso Rigid Foam Insulation, Plus Rain Felt? and 1” x 4” Wood Rain Screen

 

From inside to outside of wall system – Structural Wood Siding, Weather Barrier (Water, Air and Vapor Barrier), Rigid Foam Insulation 2 x 2” Poly-Iso Rigid Foam Insulation, Felt Paper or Wrap ( Potentially Breathable Membrane), l 1” x 4” Wood Strip Rain Screen, Corrugated Metal Siding.  

 

Do you still recommend the following water, air and vapor barrier(s) for the walls?

 

For the walls: Carlisle CCW 705 40mil Peel and Stick – Air/Water/Vapor Barrier

 

If not, what manufacturer and product do you recommend?

 

Thank you for your time.

 
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Replies

  1. mpergiel | Feb 21, 2020 01:48pm | #1

    Seems overkill. What is the purpose of such strong sealed insulation on a building that is basically a 10x12 room that has 130 sq feet of r3 windows? ISO/SPX foam basically negates any vapor exchange. Opening one door will probably cause nearly a full air change.

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