Insulation is a very important component of our tiny house construction project. We used foam core in the base of the trailer (6″ thick) and will use it again in the ceilings (4″ thick).  We are also using it selectively in sections of the walls where we wish to provide maximum protection to plumbing and drain lines. It has an insulation value of R-5 for every inch of thickness. For a 3-1/2″ wall that translates to R-17.5.  Using sheets of 2″ and 1-1/2″ material allows us to snuggly sandwich plumbing lines.  We also use foam core in angled framing corners an area often overlooked.  Around the shower stall we were able to fit a full 4″ of foam core for a toasty R-20.  We hope the future owner of this tiny house will appreciate the level of care that went into creating such an energy-efficient living space.