ASHRAE 160 And Cold Sheathing: Implications For TJI And Truss Wall Construction

Energy wall design has seen a resurgence in TJI and truss wall construction – sometimes employing exterior insulation in conjunction with their deep 12” to 16” insulation-filled cavities.   The downside of this approach includes a high potential for cold sheathing and subsequent moisture accumulation during the winter months.

The Risk
Wood sheathing will accumulate moisture when the rate of moisture sorption exceeds the drying rate.   The risk for moisture accumulation can be greater during the winter months due to moisture exfiltration and the combined effect of moisture penetration during transitional months in late fall and early spring.   As part of the ASHRAE 160 design criteria, the 1% wind-driven rain requirement provides a basis for accommodating realistic moisture burdens against weather-resistive barriers.  As air temperatures decline, the air’s capacity for moisture storage also declines.  This results in higher potentials for moisture sorption and accumulation.  Combining the 1% rain penetration fraction with thicker, insulated cavities means that sheathing remains colder and wetter for longer periods of time.

Often the phenomenon of ‘cold sheathing’ is unfairly attributed to OSB due to its lower vapor permeance at increasing relative humidity.  While it is true that wetted OSB exhibits lower drying rates than dimensional lumber and plywood, the potential ill effects of cold sheathing are pertinent to any sheathing material.

Hygrothermal Performance
To illustrate the effects of moisture penetration combined with cold sheathing effects, we have modeled a 14” fiberglass batt-filled wall assembly sheathed with OSB and clad with metal panels.  Our assembly includes a 3/8” rainscreen ventilated with an assumed constant of 40 ACH.   Our modeled climate is Madison, Wisconsin.  Further details are outlined in Hygrothermal Snapshot No. 11.

The figure below illustrates highly favorable conditions when the assembly lacks a 1% wind-driven rain fraction:

Hygrothermal Modeling of Energy WallsAdding the 1% fraction significantly reduces wall performance, resulting in borderline or elevated conditions throughout most of the year.  Particularly noteworthy are the transitional periods of fall and spring:

Hygrothermal Modeling of Energy WallsLastly, we illustrate the effects of further reducing sheathing temperatures by filling the cavity with cellulose fiber rather than fiberglass batts, which increases the assembly’s R-value from 48 to 57.  While the results are similar, we see the expected conditions of slightly higher humidity:

Hygrothermal Modeling of Energy Walls

Conclusions
The cold sheathing condition, when combined with the requirements of ASHRAE 160, can be a daunting challenge.  Switching to plywood will not necessarily provide the desired effect.  Likewise, increasing rainscreen depth/ventilation also has limited benefits.  Furthermore, care must be taken when adding exterior insulation as ever-smaller amounts of incidental moisture between the sheathing and insulation will cause even greater damage. The preferred solution is one that accommodates rapid drying, employs low nutritive materials, and provides redundant safeguards for safe moisture storage and release.  These goals can still be achieved by incorporating exterior insulation; however, performance assumptions must accommodate the realistic expectation that a breach will occur and that some amount of moisture will enter behind the outbound insulation.  After all, the unexpected always happens.