With some research on wall/floor/roof insulation on buildingscience.com I have decided to do an exterior wrap insulation. Check out this article on wall systems in the Pac Northwest. Modern construction uses studs every 16" or 24" with insulation (batt, blown in or something subsequent) in-between the studs. This creates thermal bridges in the insulation wherever the stud or joist or rafter is. With advances in exterior wrap insulation and super sealing, innovative builders and scientists are discovering new systems, and have been for decades! With the goal of building a portable, light, super efficient home, I'm going make it transparent what I use, why it is used and how much it costs.
I am using poly foam board insulation for the exterior wrap. There are three kinds of poly foam that are commonly used as insulation: Extruded Polystyrene (XPS), Expanded Polystyrene (EPS), and Polyisocyanurate (Polyiso). A cost benefit analysis that should be conducted for each type of insulation and and consideration of the climate and the assets of each material. I have decided to use a non-foil faced polyiso insulation. I am not using the foil faced because the foil acts as a moisture barrier and I am using a particular permeable housewrap for my moisture barrier. If moisture gets into the wall space I want it to be able to pass through the wall instead of getting trapped in by an impermeable material.
The Polyiso offers the highest Rvalue at the lowest cost, so I'm going with it due to budgetary constraints. There is information showing that the Rvalue of polyiso decreases in temp. extremes. Check out the article here. I am not overly concerned about this because of the consistent mid-range temperatures in Washington.
Rockwool and cork are other less toxic options, but they were out of my price range so I did not do much research on either material.
Let's start with the floor, there is the trailer frame foundation with house wrap as the first, exposed layer. I want to have a permeable membrane, so when water gets in it can escape. House wrap will act as the barrier to the outside and to the road, which will take some abuse and will require cleaning after moving. To hold down the house wrap I will screw down a 2x2 wood sill plate around the perimeter of the trailer. Then fill the space with a continuous layer of 2" foam insulation on top of the house wrap (taping all the seams), then sandwich the foam with 3/4" plywood subflooring. I'll use self tapping screws to secure the subfloor to the frame and hold the 2" foam insulation and house wrap in place.
I'm going to frame with 2x2s and 2x4s on their side. This will make for thin lightweight walls. I will use batt insulation between the studs, 1/2" plywood sheeting and house wrap just like contemporary construction. Then I will add 3" of insulation board with taped seams to provide a continuous layer. Furring strips will sit on top of the insulation and they will be screwed through the insulation into the plywood and stud. The furring strip will hold the foam insulation onto the house and provide a 1" air gap space in the wall and a nailer for cedar shingles.
The roof will be framed with 2x4 rafters, batt insulation in between the rafters, 3/8" plywood capped with a special roof housewrap. 4" of foam will sit on top of the plywood, held down by furring strips. The furring strips provide a 1" air gap space and a nailer for the metal roof.
In conclusion: I do not think it is necessary to do an exterior wrap on the house, it is such a small footprint that it will be easy to heat no matter what. I am curious to do an exterior wrap because I've been reading about it and never had a chance to experiment with it. Instead of discovering my learning curve on a job for a client, I thought I would iron out the bugs on my little house project and see for myself how effective this technique for insulating is. My plan is to leave out the heating system in the trailer for now and see how it maintains in the temperate western Washington climate.
I have my floor, walls and roof plans, I've sourced materials and after a nice long coastal hike with some dear friends I'll start construction next week.