Warm, Dry, and Built to Last: Wall and Roof Assemblies for the Classical Wood-Framed House

A classical house does not have to be a cold one. A cornice, a pitched roof, and deep-set windows are compatible with some of the highest insulation levels we know how to build. In many cases they help. Here is how we approach it, using a residence now in design at Aureum.

FOUR JOBS, IN ORDER

Every wall and roof has four jobs: control rain, control air, control vapor, and control heat. They rank in that order of importance, and the best practice is to put these layers on the outside of the structure, where they protect the wood from water and temperature swings.1 Our assemblies follow that principle.

THE WALL

From the inside out:

• Painted gypsum board, a vapor-open finish
• 2x6 advanced framing at 24 inches on center, with 5½ inches of mineral wool batt
• Taped structural sheathing, which serves as the air and water barrier
• Four inches of continuous mineral wool board, in two staggered two-inch layers, over the sheathing
• A three-quarter-inch rainscreen gap, formed by furring, for drainage and drying
• Fiber-cement siding

The continuous layer matters because wood framing carries heat at about R-1 per inch. In a simple parallel-path estimate, with framing at 25 percent of the wall, four inches of exterior mineral wool lift the effective R-value of this wall from about R-18 to about R-37.²

THE ROOF

Classical roofs have hips, valleys, dormers, and returns, and these are difficult to ventilate. We move the air and thermal control to the roofline instead. The assembly combines closed-cell and open-cell spray foam below the structural sheathing with two staggered layers of mineral wool above it, and the roofing sits on its own deck, lifted on spacers. The cornice framing is carried outside the insulation, so a cornice with real projection and profile does not puncture the air barrier.

MASONRY OVER STEEL

The same logic holds for a heavier wall, with one reversal. The commercial wall leaves a steel stud cavity uninsulated and puts all of the insulation outside, because steel carries heat straight through cavity insulation.³ Our example reaches just over R-40 with these layers, inside to outside:

• Painted gypsum board, about R-1
• 16-gauge steel stud cavity, uninsulated, no R-value counted
• Exterior gypsum sheathing, about R-0.6
• A continuous air, water, and vapor control membrane
• Six inches of polyisocyanurate in staggered layers, about R-39 at a labeled R-6.5 per inch
• A drained and ventilated air space
• Stone and masonry veneer

The veneer is tied back across seven inches or more of cavity, which is beyond the masonry code's prescriptive limit. The anchors, their capacity, spacing, and fastening must be specified and calculated by the project's engineer.⁴

THE NUMBERS

The goals come from a proportional rule for cold climates: R-20 at the basement wall, R-40 at the wall, and R-60 at the roof, with resistance scaled to the temperature difference each assembly faces.⁵ The roof clears its goal by about a point and the wood wall by about three, with a little over two-fifths of the wall's insulation outboard of the framing. The steel-stud wall sits at its goal.

WHY CLASSICAL DETAILS HELP



• The eave. Best practice puts the overhang run at twice the drop from the eave to the window head. A generous classical eave meets that standard.⁶

• The inset window. A window set in the plane of the sheathing has a deep reveal, which is good proportion, and it is also the simplest way to line up the water control layers.⁷

• The rainscreen. A gap behind the cladding drains and dries the wall.

• Tight, then ventilated. Old houses leak, which is different from breathing. Air leakage can account for 25 to 40 percent of a building's heat loss or gain.⁸ We make the shell tight and ventilate on purpose.

BUILT FOR GENERATIONS

The most durable wall we know how to build is called the 500-year wall, the kind used for museums, courthouses, and libraries.¹ A house meant to be handed down deserves walls built the same way.

— Kenneth Shane, Principal

Aureum Architects, LLC

A note on use. The assemblies and numbers in this piece show how we think about wall and roof design. They are not construction documents and not a basis for design. Every wall and roof must be designed for its own climate, loads, and code, with the products, anchors, and fasteners specified and the calculations performed by the project's design professionals. R-values are nominal, and effective values will be lower.

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NOTES

1.     Building Science Corporation, "Insight BSI-001: The Perfect Wall," rev. July 2010.

2.    Aureum illustrative calculation from the project's wall sheet: nominal R-22 cavity, R-16.8 exterior (two 2-inch layers), about R-4 for sheathing, air films, and finish; framing at about R-1 per inch and a 25 percent framing fraction. Not a whole-wall test result.

3.    Building Science Corporation, "Insight BSI-001: The Perfect Wall," rev. July 2010, Figure 8.

4.    Brick Industry Association, Technical Note 3C (2022 TMS 402/602, anchored veneer). Nominal values: ⅝-inch gypsum board about R-0.56, interior air film about R-0.68, polyisocyanurate at a labeled R-6.5 per inch; at R-6.0 per inch the same wall would total about R-38.

5.    Building Science Corporation, "Insight BSI-081: Zeroing In," October 2014; The Build Show, Build Science 201 (2025), pp. 14–16. The numbers are set for Climate Zones 5 and 6.

6.    The Build Show, Build Science 301 (2025), p. 12.

7.    Building Science Corporation, "Insight BSI-085: Windows Can Be a Pain," April 2015.

8.    Build Science 201, p. 8.