I'm looking to build-my-own storage shed. I'll be storing quite a few things in here, from a riding mower, to various tools, tillers, etc, as most people do.

I'll also be using it to store some liquids and possibly even plant materials, i.e., things I don't want to freeze.

Therefore I want to keep it heated (probably just to 32 - 34 degrees F), and insulated to minimize wasted heat and money.

I plan to use 2 X 4 construction and fiberglass insulation on the roof and walls, and cover the walls with something easier/cheaper than drywall.

I don't think I'll have any windows, since it will have power and a light inside. I will, however, have a door, probably something like a single-car garage size so I can drive the mower out.

Is there some way to figure out the r-value of the entire structure, more or less, by calculating the area of each type of material that touches the outside, so I know how much BTU power I need to heat it to 32 on the coldest nights? We occasionally get as cold as about 10 below here, but last winter we were even colder. We've been as cold as -20 in the past so that's the bottom line.

The shed plans are for 12' X 15' dimensions. In my head if I use R-13 insulation a 1500 watt heater should do the trick, but I can't figure out how much loss I'm adding by having a garage door or something similar added to the equation.

As a side note, if insulation is important, what is the best type of door to use that would be more or less the width of a single-car garage door or perhaps somewhat smaller, but big enough for a large lawn tractor/riding mower?