A SIMULATION INVESTIGATION OF STUCCO CLADDING WALL SYSTEM
VAPOR TRANSPORT PERFORMANCE IN A COLD CLIMATE:
PHASE III: HYGROTHERMAL RESPONSE TO INTERIOR WATER SOURCES AND
THE DRYING POTENTIAL OF DRAINAGE PLANES

FINAL REPORT

Date: February, 2007
Revision: B

ACKNOWLEDGEMENT AND CERTIFICATION
The research described herein has been performed with funding provided by the Minnesota Lath and Plaster Bureau. While this support is gratefully acknowledged, the Principal Investigator assumes complete responsibility for the contents herein.

EXECUTIVE SUMMARY

This report is a sequel to the report describing phases I and II of the “Vapor Transport Performance in a Cold Climate” research project. As such, it needs to be read in conjunction with the previous report as descriptions of the methodologies used, boundary conditions and other background information are not repeated herein.

This report investigates the following two aspects of stucco cladding system performance:

• The hygrothermal impact on the wall system of an internal moisture source adjacent to the exterior surface of the water resistive barrier (WRB).

• The potential of using an air gap between the WRB and the stucco as a means of drying internally wetted stucco and, in so doing, decoupling the stucco moisture source from the framing system.

The first aspect was studied using the WUFI-2D (version 2.1) simulation program with all its attendant limitations as described in the phase I and II report. Given these limitations, the internal moisture loading was accomplished by setting the initial moisture content of the stucco adjacent to the WRB at a saturated state at the beginning of the simulation year on July 1. The moisture performance was inferred from the resulting relative humidity and moisture content transient temporal profiles as well as from the annual performance summary data used in Phases 1 and II.

The second aspect was studied using the computational fluid dynamics module of the ALGOR finite element program. The velocity profiles within the air gap were simulated as a function of gap width, top and bottom slot height as well as gap configuration. Three gap configurations were investigated, namely a uniform thickness extending over the full wall height; an extruded polystyrene layer at the bottom of the wall on the interior side of the air gap; and, the same configuration except with the insulation layer on the exterior side of the gap.

The simulations of a stucco wall system subject to a transient interior wetting event show that this moisture loading yields a phenomenology that is different from that which occurs with exterior only wetting events that were addressed in phases I and II of this research. While the performance of the base stucco system nominally is adequate under single event transient internal moisture loading, the data indicate a cause for concern as to what could happen with repeated transient internal moisture loading events. Decreasing the vapor coupling between the wet stucco moisture source and the framing cavity (such as replacing 2 layers of grade D building paper with one layer of no. 15 felt) while not changing the rest of the system seems to improve the performance. Increasing the coupling between the framing cavity (such as replacing a warm-side polyethylene vapor retarder with a PA-6 membrane) improves the hygrothermal performance in some respects while exacerbating it in others. Hence implementing both of these modifications together offers the potential of effectively managing internally wetted stucco systems. Taken as a whole, the results do offer some clues as to possible mechanisms for field-observed failures that often defy conventional forensic analysis (such as a patch of sheathing mold in the center of a windowless wall).

Using a vented drainage gap of adequate width offers an alternative approach to effectively decoupling the wetted stucco vapor source from the wall cavity system that could be used with a warm side polyethylene vapor retarder. In particular, a drainage gap with 12 in. of extruded polystyrene insulation installed on its bottom interior face shows promise of yielding flow rates adequate to achieve sufficiently robust drying of internally wetted stucco when driven by horizontal temperature gradients alone. However, implementing such a system is likely to engender strong resistance from the stucco community owing to the complexities of achieving a structurally robust stucco system with a relatively large (0.5- 1”) gap between the WRB and the stucco. Conversely, however, current practice with drainage air gaps in the 1/8-1/4” range with bottom slots (or weep screeds) only are not effective as ventilation planes and it is erroneous to claim otherwise.
 

Download the full report:  Final3-B.pdf

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