Comments made here are my own and, according to my children, do not reflect the opinions of any other person... anywhere, anytime.
Charlie, I can certainly relate to your disclaimer, but mine would read 'kids and grandkids'.
Hi Bob
In a former career I made a lot of heating load calculations for various buildings. The loads from biggest to smallest are usually;
1. Air (makeup, leakage etc)
2. Window transmission (well it was a long time ago and there were a lot of single paned windows)
3. Roof transmission
4. Wall transmission
5. Other (in your case the doors could be pretty bad)
So wall insulation is not so important. I think you will be surprised at the effectiveness of your plastic cocoon.
But you will want to insulate. It just doesn't have to be ultra insulation in the walls. Your roof vent item implies a need for air flow from the eaves to the ridge above the insulation. This may need more thought.
Spray foam insulation is really effective and quick, but really ugly. Look before you leap.
Bob,
I'm sorry I didn't see your message until just now. For the walls I used 1" foam board insulation. I had to cut it down to fit between the studs. I have the foil side facing towards the inside of the building. I did not cover up the walls. Here in eastern Washington our coldest temperatures will be as low as minus ten but our normal winter days will be in the teens. Though this board does not provide a large R value I find it very adequate.
For the ceiling I used this Rockwood insulation. I chose this because its easy to work with and doesn't itch. You'll want to wear a dust mask as its quite dusty to work with. Once in place its fine. I may one day cover up the ceiling with hardiboard.
With just the two above I kept my thermostat set at 68 (plus or minus) and it was fine. But the floor was always cold.
So this fall I insulated the floor with 1" insulation boards, the 1" version of this board. Lowe's doesn't stock the 1" but they ordered it and had it the next day. It has good R value and a high compression rating. I then used this tongue and groove subfloor right over the foam boards. I used vinyl flooring on top of that.
Whatever I set my thermostat to, say 68 degrees, the walls, floor and ceiling are all 68 degrees. My electric bill floats between $35 and $50 a month when leaving the heater on 24/7. The one thing I probably could do is use a bit of foam insulation around the two window frames.
Thank you Tom. I appreciate it.