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Creating High-Performance Walls

In the first installment of a four-part blog series, Hammer & Hand details the exterior walls of Karuna House

Posted on Sep 29 2015 by Zack Semke

Our Evolution of Enclosure exhibit at AIA Portland (which ran through September 10) examined the role that buildings — especially building enclosures — can play in helping to diffuse climate change. As examples, the exhibit drew on four projects built by Hammer & Hand: Karuna House designed by Holst Architecture; Pumpkin Ridge Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. and Glasswood Commercial Retrofit, both designed by Scott | Edwards Architecture; and Madrona Passsive House, designed by SHED Architecture & Design.

The exhibit itself included a set of four beautiful full-scale cross sections, built by Hammer & Hand’s Jason Woods, of the high-performance wall assemblies that these four projects used to achieve Passive House performance.

Each of these wall cross sections is accompanied by a series of illustrations created for us by Ryan Sullivan of Paste in Place. The illustrations explain how each wall manages heat, air, and moisture. In a series of four posts, we'll share these illustrations and the analysis embedded within in them.

This week: Karuna House.

Heat management

At Karuna House, the client and designers determined the form of the building first and the strategy to bring that design to high performance levels was overlayed second. So the wall assembly had to be especially thermally resistant to allow for the form’s articulations and increased surface area. The key challenge was to do this while not making the wall extremely thick.

The assembly’s high R-57 insulative value is provided by (A) three 2-inch layers of foil-faced polyisocyanurate insulation (R-36) with seams offset by the Z-girts that hold the layers in place and (B) 5 1/2 inches of high-density cellulose (R-21).

Water management

The first line of defense for bulk water management is Karuna House’s siding (A) and the ventilated rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. cavity (D) behind that siding that allows water to drain harmlessly away. The second barrier is the foil face of the polyiso insulation (B). And if any water were to penetrate beyond that, the final barrier is the fluid applied, vapor-open air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. (C).

Vapor management

Starting just inboard of the innermost layer of foil faced polyiso (the foil is a vapor barrier), the assembly is vapor-open to the inside (B) but not the outside. This strategy works because the polyiso keeps the structural sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. warm and protected, as shown in the THERM diagram (see Image #2 at the bottom of the page). Any water that finds it way between the polyiso and the fluid-applied air barrier gets warm, becomes vapor, and then is transferred harmlessly inside through the vapor-open side of the assembly.

Air management

The air barrier for the Karuna House wall assembly is a fluid-applied, vapor-open membrane manufactured by Prosoco.

To read the next installment of this series, click this link: High-Performance Walls, Part 2.

Zack Semke is the director of business development at Hammer & Hand a company specializing in high-performance building with offices in Washington and Oregon.

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  1. Hammer & Hand

Sep 29, 2015 8:04 AM ET

Weird R-ithmetic
by D Dorsett

"The assembly’s high R-56 insulative value is provided by (A) three 2-inch layers of foil-faced polyisocyanurate insulation (R-21) with seams offset by the Z-girts that hold the layers in place and (B) 5 1/2 inches of high-density cellulose (R-36)."

Somebody needs an editor.

A: Three 2" layers of polyiso is more like R36, not R21.

B: 5.5" of 3.5lb cellulose is more like R21, not R36.

R21 + R36 = R57 (and not R56 thus.)

Sep 30, 2015 1:23 PM ET

Hi Dana - you're right. Typos
by Zachary Semke

Hi Dana - you're right. Typos happen...thanks for catching it.

Sep 30, 2015 1:37 PM ET

by Scott Gibson

Thanks for spotting that, Dana. The text has been corrected.

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