Comparative Evaluation of the VTT Mold Index Model and the DR SIM Model for Moisture Risk Assessment

Abstract

Hygrothermal simulations are useful in forensic investigations of moisture problems and in design analysis for identifying and mitigating potential risks both in new construction and retrofit of existing buildings. Damage models, including mold growth models, are often used as a means of assessing risk; such models use simulation outputs such as temperature and relative humidity or moisture content. The Finnish mold growth model (or VTT mold index model), a pioneering model used widely by researchers and practitioners, was developed based on laboratory data for a wide range of materials, and several studies have shown qualitative agreement between field observations and model predictions. However, a round-robin study, which evaluated six different models, found that the VTT model was highly sensitive to user input selections and assumptions. Furthermore, under field conditions with considerable variation in temperature and humidity, the VTT model significantly underestimated the extent of mold growth compared with observations using microscopy. To provide more realistic predictions of mold growth over the range of hygrothermal conditions experienced in-situ, the Dose-Response Simple Isopleth for Mold (DR SIM) model was developed. This paper compares the DR SIM model with the VTT model in terms of model structure, statistical goodness of fit to laboratory mold growth datasets from the literature, and accuracy of pass/fail predictions based on mold index thresholds. We find that the DR SIM model is more accurate than the VTT model overall, but the possibility of overfitting needs further investigation. We highlight the advantages and limitations of each model and emphasize the need for further laboratory and field mold growth data on North American materials.

Citation

Glass, Samuel V.; Lepage, Robert; Farkas, Natalia; Zelinka, Samuel L. 2025. Comparative Evaluation of the VTT Mold Index Model and the DR SIM Model for Moisture Risk Assessment. In: Proceedings Buildings XVI Conference, Thermal Performance of the Exterior Envelopes of Whole Buildings. Clearwater Beach, FL. Dec. 8-11 2025. Peachtree Corners, GA: ASHRAE: 1-10.