We have a small (1200 sq. ft.) Civil War cabin. It has two stone rooms (one story) with a two story wooden addition (circa 1880). Picture attached.

It is in Kansas, Climate Zone 4A.

We’ve insulated the wooden addition as best as possible as well as the roof of the stone rooms. Most of the heating when we’re there is by a wood-burning stove with windfall as fuel. We also have a forced air heat pump system.

We are going to add a bedroom – a one-story wooden addition on the north side of the 1880’s two story wooden addition. Keeping the design in accord with the existing structure is extremely important. Therefore we have only 4”- 4.5” of roof thickness (bottom of sheathing above the rafters to top of shingles) to work with. The rafters beneath will be exposed, so we cannot insulate below the plywood sheathing.

Thus we cannot achieve the desired R value for the roof. We’re prepared to live with that fact. (We’re not subject to any code.)

However, the literature indicates that we must have enough insulation above the plywood sheathing to keep the plywood warm enough to prevent sorption.

It also appears that Polyiso loses effectiveness below freezing, unless topped with a different type of insulation.

My thought is to create a sandwich with plywood sitting on the rafters, then a vapor barrier of some type, then 3 – 3.5” of Polyiso, and then roofing Zip on the top (with shingles nailed onto the Zip).

I realize it would not achieve the desired R value, but with the rest of the construction it would probably be the best insulated room in the house.

My concerns include:
1. the two vapor barrier system;
2. the sorption possibility for the plywood sheathing; and
3., I’m missing something.

Any thoughts will be greatly appreciated.

Thanks, Bruce WaughLittle house spring.jpeg