We’re doing a down-to-the-studs-and-slab “rebuild” of a small 2×4 wood frame construction cottage. All heating and appliances are electric,

This is in Eugene, Oregon, which is climate zone 4C, a mild climate with moderately high humidity in the winter and low humidity in the summer. Interior vapor production in the winter would arise from one or two occupants and steam from cooking (but no natural gas use). We’ll use and HRV for ventilation.

Our design approach for the walls is
* Vapor barrier on the interior
* Rock wool batts for the stud cavities (R-15)
* Walls that “breathe to the outside”

We’re limited to not increasing the cavities to the inside because of: the cost, the reduction in room size, and that adding more insulation inside the sheathing isn’t the best choice for vapor management.

So, we’re looking at adding insulation on the outside of the sheathing.

We removed all the old 1×8 board sheathing and replaced it with 3/4″ CDX (moderate permeability).

The basic plan is to add SIGA Majrex 200 as the WRB (vapor-permeable) + 1/2″ PT furring strips + primed and painted Cedar beveled lap siding.

We’re limited in how much depth we can add to the outside of the wall assembly because the rafter tails are very short outside the sheathing and the wall height is under 8′; thus, extending the rafter tails would require reworking the roof and would bring the edge of the roof too low in any case.

We could probably fit 0.59-inch Neopor on the wall (under the PT furring strips), which would add R-3.

See Foam Board Insulation .59 in x 4 ft x 8 ft R-3 GPS ProBoard (10 PSI) – Insulation Supply

Please share your comments and advice.

Thank you,

Paul Conte
Earth Advantage Accredited
* Sustainable Homes Professional
* Accessory Dwelling Unit (ADU) Specialist