Wood Shear Walls with Continuous Insulation

Continuous insulation code requirements, and its elected use, are ever increasing. Continuous insulation advantages are proven – it significantly reduces thermal bridging, particularly with repetitive framing like roof rafters and wall studs. It increases construction costs, but it results in reduced operating costs by reducing heating and cooling demand.

But there are structural implications. In light-framed buildings, wood and cold-formed steel alike, the exterior walls often act as both thermal barriers and shear walls. The in-plane (parallel to the wall) strength necessary to resist lateral wind and earthquake loads is highly dependent on the direct interaction between the sheathing, nails, and studs:

Add a layer of rigid insulation, which has very low shear strength, between the studs and sheathing, and the wall shear capacity is greatly reduced. A 2014 APA study (hyperlink: https://www.apawood.org/Data/Sites/1/documents/technicalresearch/wood-structural-panel-and-foam-insulation-systems.pdf) found through testing that the shear capacity of a 2×4 wood stud wall with a 1-1/2” layer of EPS insulation is only 63% of the same assembly without the insulation. Huber published data for their modular ZIP R-Sheathing, and we found the strength reduction to be similar to the APA study.

 

In many of our recent designs, we’ve found that the wall assembly with a layer of continuous insulation between the stud and sheathing does not meet structural load demands. If we are unable to increase the lateral force resisting system capacity by increasing the total wall length (or reducing the window/door openings), what other strategies can we employ?

1. Insulation outside the sheathing.

The easiest (at least structurally) and most obvious approach is to maintain the strength of the wall with sheathing directly against the stud, then insulation outside of that:

The primary drawback to this approach: siding often relies on direct attachment to a firm substrate like sheathing (vinyl) and sometimes the fasteners must also engage the stud (cedar clapboard). So, onto #2.

2. Add another layer of sheathing.

A second layer of sheathing, or strapping if OK with the siding manufacturer, now provides a fastening substrate for the siding and maintains the shear strength of the traditional wood shear wall. But this is material and labor-expensive, and the assembly is starting to become quite wide.

3. Let-in bracing

Let-in bracing is still prescriptively permitted by both the IRC and IBC. For wood walls, a diagonal 1x is let-into the studs. Or, as is the case with cold-formed steel walls, a thin metal tension strap is installed outside (or inside) the studs.  Structural sheathing is not required when let-in bracing is used, allowing insulation to be installed directly against the studs. The shear capacity per foot of wall length is significantly lower than sheathed walls, so this option requires more wall length, and/or fewer door and window openings. Another drawback: this approach is not permitted for buildings in Seismic Design Category C or higher, and there are building story and height limitations.

4. Other creative approaches may be viable:

• Sheathing on the wall interior
• Staggered studs
• Continuous insulation inside the stud

 

5. More testing and studies.

There is limited data available, and some scenarios are untested, as continuous insulation is an emerging approach. We would benefit from more studies that test assemblies to provide data on common and creative assemblies alike. For example, what is the shear strength of a wall that combines sheathing and let-in bracing with continuous insulation?

We are committed to developing a practical structural solution (or solutions) that meet load demands, and all other envelope requirements. Stay tuned, as we’re constantly encountering this situation and will provide updates on our progress.

 

Dan Martel, P.E.

Senior Structural Engineer