Performance of Compact, Closed Cavity, Double-skin Curtain Wall

Thermal and Energy Simulations to Assess the Risk of Condensation

Overview

Authors

Photo of Julien Schwartz, M. Eng.

Julien Schwartz, M. Eng.

Building Energy Consultant

Morrison Hershfield

jschwartz@morrisonhershfield.com

Photo of Katie Hay, P. Eng.

Katie Hay, P. Eng.

Building Science Consultant

Morrison Hershfield

khay@morrisonhershfield.com

Photo of Stéphane Hoffman, M. Arch., M. Eng, PE

Stéphane Hoffman, M. Arch., M. Eng, PE

Vice President, Building Science Analytics

Morrison Hershfield

shoffman@morrisonhershfield.com


Keywords


Abstract

A proposed design for a compact, closed cavity, double-skin curtain wall system in the Marine climate of the West Coast raises questions of potential for interstitial condensation. The design calls for a closed cavity enclosed with insulated glazing units on the exterior side of an aluminum frame and a single glazed light on the interior. The potential for air infiltration into the closed cavity creates the risk for condensation within the cavity during extended periods of cold weather. Condensation is considered problematic since the sealed nature of the system would not allow for access to clean the glass. In a first step a three-dimensional analysis of the proposed curtain wall design is undertaken to identify surface temperatures. This allows accurate assessment of the potential for condensation. The analysis confirms that condensation is likely assuming the curtain wall framing experiences air infiltration. Strategies for mitigating the effects of condensation were examined to better understand their benefits and shortcomings. An estimate of the volume of desiccant required to mitigate the potential for condensation is calculated. The volume of desiccant proves to be excessive and the option of pressurizing the interstitial cavity to minimize infiltration is examined. The option of using the building’s mechanical system to pressurize the cavity is analyzed. Energy modeling software is used to simulate the air exchange and evaluate the impact of pressurizing the cavity using the mechanical system. The results of this analysis confirm that a dedicated ventilation system will be required to substantially mitigate the risk of condensation.

Introduction

The design for a new institutional building in the downtown core of a West Coast city in the United States called for a compact, closed cavity, double skin curtain wall

Members Only

Method

Thermal Modeling of Surface Temperatures

The surface temperatures for this analysis were determined using 3D heat transfer software from Siemens called Nx*1. The models used published material properties and information provided

Members Only

Results

Thermal Analysis

The results of the thermal analysis are shown for the intermediate horizontal mullion detail. The critical locations are highlighted on the assembly in the associated thermal profile (Figure 2)

Members Only

Explanation

Thermal Analysis

From Table 1 it can be seen that for the typical exterior design temperature of 55°F and assumed interior of 68°F, condensation could occur when the humidity spikes above

Members Only

Conclusion

The combination of thermal simulations and energy simulations provided insight on the performance of the proposed design. The thermal analysis accurately determined the surface temperature indices that allowed an assessment

Members Only

Rights and Permissions

ASHRAE Handbook of Fundamentals, ASHRAE, 2017
ASHRAE Research Report 1365, Thermal performance of Envelope Details for Mid & High-Rise Buildings, ASHRAE, 2011
BC Hydro. “Building Thermal Bridging Guide.” http://www.bchydro.com/thermal... (accessed 2018)
OneBuilding. “Links: Building Simulation Resources –Climate.OneBuilding.Org Weather Files” http://climate.onebuilding.org... (accessed 2018)
Sorbent Systems. “Products” https://www.sorbentsystems.com... (accessed 2018)