Existing situation –

We have a cabin and outbuildings built by my wife’s great grandfather in 1930. We like the character exactly as it is. This is a vacation home, used a few months in the summer, a few weeks in the winter. The building is not heated when it is vacant. We want to make the 20’x18’ garage space into two bedroom spaces with a high ceiling, and insulate the building. On the garage there is 1×5 bead board ponderosa pine siding nailed horizontally directly to studs for the walls. So the boards are weathered to great looking brown colors. There are no interior walls or insulation. The lower 6” to 12” of some bead board walls and some wall 2×4 have some rot, and the rotted portions will need to be replaced. We really do not want to remove the exterior cladding, as we are afraid the boards might split or crumble. We would only add a toilet and sink. No shower, fan, nothing creating lots of moisture. We would not add an air conditioner, but would add electric wall heaters for the winter. Located in eastern Oregon, climate zone 5, in the summer it averages a high 85, low 45. In the winter high 40, low 25.

 Reading the “Insulating walls in an old house with no sheathing” article by Martin Holladay, February 12, 2016,

https://www.greenbuildingadvisor.com/article/insulating-walls-in-an-old-house-with-no-sheathing gave me hope of keeping our 90 year old siding on our garage as is, and still adding effective insulation. However, it does not speak to thermal bridging.

 So my proposed WALL –

1.       Leave the 1×5 wood siding (R-1.25) nailed to the 2×4 studs, which are 24” on centers.

a.       We likely need to add new 2×6 next to the original 2×4 since the bottoms of the original 2×4 studs need to be cut off because of some rot.  But this adds to thermal bridging.

b.      Add 2×2 boards to the end of original 2×4 studs to extend depth of the wall to make it 2×6.

c.       Nail 3/4” x 3/4” wood spacers to the studs to create an air gap between the wood siding and new insulation.

d.      Insert 1-1/2” EPS solid insulation (R-5.78) onto the 3/4″ x 3/4″ boards; add tape to the studs to create a vapor barrier.  In TechNote TN06 “Vapor Retarders: Reducing Moisture Risk in Frame Walls” https://www.iccsafe.org/wp-content/uploads/proclamations/TN06-Vapor-Retarders_pdf.pdf  it says that in climate zone 5 the building should have a Class 2 vapor retarder, like XPS foam sheathing. Is my proposed design with the EPS solid insulation in the correct location?

2.       Install   3-1/2 inch deep fiberglass batt insulation (R-15) between the 2×6 studs.

3.       Nail ½”plywood (R-0.63) or OSB to the 2×6 studs to add racking bracing strength for the structure. The walls don’t have this now, only the 1×5 siding is providing racking support.

4.       I want to minimize the thermal bridging through the original siding and on through the original 2×4 and new 2×6 studs, so I thought I could nail sheets of 1” of EPS solid insulation (R-4.0) to the 2×6 studs to diminish the thermal bridging between the inside and outside of building. (Although this would give two surfaces for condensation as the wall breathed? Or should I make the 1-1/2” EPS above thicker to 2-1/2” EPS and not add it here to minimize the thermal bridging?  For the walls code requires R-15, is R-27.91 overkill for a building only used a few months of the year?)

5.       Nail 1”x 9” vertical ponderosa pine paneling (R-1.25) vertically onto the EPS insulation and into the studs.

6.       Total R value through the insulation is 1.25 + air space + 5.78 + 15 + 0.63 + 4 + 1.25 = 27.91.

a.        But the total R value through the studs and the insulation is 1.25 + 4.38 (2×4 endwise) + 1.88 (2×2 added) + .63 (1/2 inch plywood) +4 (1” EPS) +1.25 (1” ponderosa pine paneling) = 12.38.

 Condensation, air barriers, vapor barriers versus vapor retarders. I want to get this right for the wall. This is a remodel of an existing building and only used a few weeks in the winter so the insulation minimum is R-15, so getting R-27.91 should be great I would think.

 In the primary use period in the summer, the electric heater might be turned on a bit in the morning when there is a chill, but by 10, everything is warmed up outside. Should there be much concern about condensation on any of the interior wall material surfaces?

 By the fall, the water and heat is turned off, and the buildings are locked up for the winter.

 The garage will now be used for sleeping too when we have larger groups. The building will now be insulated in the walls, ceiling, and foundation. Sitting empty and unheated, it should get to be as cold as the surrounding in the winter, maybe 10 to 40 degrees. So when we arrive in the winter, we would go inside to heat up the space with electric wall heaters. That should warm it up and reduce the humidity, but that will push any moisture in the inside into the walls? We want the moisture pushed to the outside so it doesn’t condense on a surface on the interior of the wall? Then the space is heated for a week or two. At the end of this use, the heat is turned off, the door is shut, and the garage will sit unheated for a few months until spring or summer use again. We want the building so it can more easily be kept warm in the winter, and also be kept cool in the summer. And also don’t want condensation on the interior of the walls. How can we successfully do that?

 The more I read the more I realize that I do not know. I would appreciate your comments.