Cabin Pier Floor System – GreenBuildingAdvisor

Hi all,

I am helping a friend design some details for a 16’x12′ shed-roofed cabin in Northern MA (Climate Zone 5A). He is planning to do it half-DIY, and sub out the work he does not feel experienced enough to do. Essentially, this cabin will be a temporary home while his family gets settled and starts to plan a “main house” on the property. The important part of all this is that the cabin does not have to any code requirements due to its small footprint, based on the local jurisdiction. At least that’s what I’ve been told.

He wants to do a pier foundation with a floating floor. My two questions are regarding the structural integrity and the airsealing sequencing and detailing of the floor assembly. See attached sketches – they are rough and not to scale but you get the idea. 

1. Are (9) 14″ sonotubes with a 2’x2’x1′ ftg. strong enough to support an 18′ shed roof 2×6 wall structure? 

2. The idea with using TJIs as a floor system was so that you could lay down beads of sealant on the bottom flanges, drop in pieces of 3/4″ PT plywood, screw and glue and insulate from the bottom up due to low ground clearance. Is there a better way to do this?

2a. I am concerned about moisture going underneath the system. Perhaps ZIP with the WRB facing down would be an OK substitute if PT ply is not enough?

3. Is there a concern of moisture buildup in the floor system by not having any outboard insulation?

4. Since this is a secondary structure, is a 2×6 cavity insulated (maybe HD Fiberglass or Rockwool) wall well airsealed w/ taped ZIP and rainscreen really enough, or would one highly recommend that this assembly be modified to have some ext. CI?

Any and all comments and tips or resources to make this detail better are highly appreciated. Trying to suggest the best options for my friend whose building experience is limited.