S. M. Cornick

A Moisture Index to Characterize Climates for Building Envelope Design

Premature failures of building envelopes in the 1990s, notably in coastal areas of North America, point to problems with Moisture management by Exterior Wall Systems (MEWS)1. The MEWS Consortium sought to combine field experience with lab testing and hygrothermal modeling to understand and deal with these problems. The method proposed in this paper was used in MEWS to (1) characterize climate with respect to the risk of related building envelope problems, (2) select locations of interest for a detailed hygrothermal parametric study, and (3) to select moisture reference years for the parametric study (not covered in this paper). This paper describes a method proposed for mapping North American climatic regions according to moisture loading on one hand, and the potential for drying on the other. The composite Moisture Index can be used either with hourly records or summary data and shows promise for application to specific problems, such as decay or corrosion, depending on the nature and mechanisms of the problem being investigated.

Selecting moisture reference years using a Moisture Index approach

Abstract Recent history has documented the premature failures of building envelopes in various regions—in North America most notably on the West Coast and the East Coast. The MEWS Consortium, a project undertaken by IRC and its partners, has addressed this issue in detail. The strategy for answering these questions was based on predicting the moisture management performance of wall systems as a function of climate, wall construction, and material properties through mathematical modeling. A key task was to determine what years to use as input for the simulations. Moisture Reference Years were selected using a Moisture Index approach developed for MEWS. This paper will develop the approach and compare it with other methods of selecting moisture reference years for hygrothermal simulations.

A Comparison of Empirical Indoor Relative Humidity Models with Measured Data

The focus of this study is to examine the reliability of models that are available in the open literature for simulating the interior moisture conditions, comparing the predicted interior relative humidity (RH) to measured data. Four models, for predicting the indoor RH in houses are tested against measured RH data for 25 houses. The models considered are primarily developed as design tools. The models tested are the European Indoor Class Model, the BRE model, and the ASHRAE 160P simple and intermediate models. The RH in each house is measured in two different locations producing 50 data sets. The ASHRAE intermediate model seemed to be the most robust exhibiting lower errors when compared to measured data. The European Indoor Class also performed well and can be used when data regarding moisture generation and/or air change rates is not available. As a design tool, however, it is not universally conservative in estimating the indoor RH. The BRE is problematic and generally exhibits large positive errors for most of the houses surveyed. It is found to be not reliable for the North American houses investigated in the comparisons. The ASHRAE simple model also exhibited large positive errors and does not trend well with the measured conditions. Models that greatly overestimate the design loads should be used with caution as they may lead to complicated inefficient designs.

Verification and validation

.......................................................................................................................................................... 4 INTRODUCTION.................................................................................................................................................. 4 VERIFICATION .................................................................................................................................................... 5 An Analytical Exercise ...................................................................................................................................... 5 Time step dependency .................................................................................................................................... 5 Grid dependency ............................................................................................................................................ 6 Error Analysis ................................................................................................................................................ 6 A Common Exercise .......................................................................................................................................... 8 Error Analysis ................................................................................................................................................ 8 Grid dependency ............................................................................................................................................ 8 Time step dependency .................................................................................................................................... 9 VALIDATION ....................................................................................................................................................... 9 Laboratory Testing ........................................................................................................................................... 12 Field validation ................................................................................................................................................. 13 DISCUSSION ...................................................................................................................................................... 16 CONCLUSIONS .................................................................................................................................................. 18 ACKNOWLEDGEMENTS ................................................................................................................................ 18 REFERENCES ..................................................................................................................................................... 19