These openings create a path for moisture laden air to travel through the wall assembly until it finds a cold surface on which to condense, typically the back of the precast or within the steel stud assembly. During the winter months, this moisture can freeze and turn to ice. As the weather warms in the spring, the ice melts, turning into water which then makes its way out of the wall assembly, leaving the components of the assembly damp or moist. The end result is wet, sagging batt insulation, and the strong possibility of corroding steel studs.

Assuming that air laden with moisture is likely to enter the wall assembly, additional measures must be provided to minimize any detrimental effects. One means of doing this is by providing an air/moisture resistant Insulation on the interior surface of the architectural precast concrete panel. Recent publications like Canada Mortgage and Housing Corporation’s (CMHC) Best Practice Guides for Steel Stud Construction and Architectural Precast Concrete Walls discuss the use of a “cavity" insulation, which can be placed on the outside of the studs to move the dew point outside the wall assembly. The guide suggests that where fibrous batt insulation is used between the studs, a minimum of one inch of semi-rigid or rigid insulation is needed on the outside to prevent condensation developing within the steel stud wall assembly. The addition of a rigid insulation between the architectural precast concrete panel and the steel studs also eliminates the potential for thermal bridging. Blanketing the studs keeps the studs warm within the wall assembly.

ASHRAE 90.1 • 1989 recognizes thermal bridging in steel stud wall assemblies. It provides a Table of Parallel Path Correction Factors used to determine effective thermal values of steel stud walls which do not incorporate an insulated sheathing product. The accompanying Table 1 is taken from ASHRAE 90.1-1989 and is modified to show effective R-value using the correction factors given. For example, a 2x6 steel stud wall 24 inches on centre, insulated with a H-20 ban insulation results in an effective R-value of 9 (R20 multiplied by correction factor of 0.45).

Table 1

Note: Correction factors in this table may be used with metal studs of 16 Ca. or lighter.

Table 2
Effective R Values When an Insulated sheathing is incorporated

Effective R-values when an insulated sheathing is incorporated on National Energy Code Calculations
                                                                                      
Table 2 shows the effect of adding one or 1.5 inch of insulated sheathing. By changing the wall assembly to 2x4 steel studs, 16 inches o.c. with a one-inch rigid extruded polystyrene insulated sheathing on the exterior, the effective thermal value will be increased by as much as 25 per cent, without increasing the thickness of the wall. Ship lapped edges on the insulation will act as a secondary barrier against any air/moisture that makes its way past the exterior cladding.

In conclusion, architects and builders who seek the aesthetic appeal of architectural precast concrete, now have an effective way to control moisture penetration in the building envelope. Proper caulking of the joints and the placement of rigid Insulation between the steel stud back up and the architectural precast cladding provides an economical way to enhance the integrity of the building envelope and reduce the potential for problems.

The Canada Mortgage and Housing Corporation Best Practice Guides are available at www.cmhc-schl.gc.ca/ or www.cpci.ca .

The article was provided by Canadian Precast/Prestressed Concrete Institute (CPCI), an Ottawa, ON-based non-profit organization aimed at advancing the use of structural precast prestressed concrete, architectural precast, concrete and post-tensioned concrete in Canada. 1-877-937-2724; www.cpci.ca.