Balancing the Control of Heat, Air, Moisture, and Competing Interests
The design of a building for either new construction or rehabilitation must consider the control of heat, air, and moisture (HAM) flows. A large number of buildings in the Lower Mainland of B.C. have been rehabilitated because of extensive deterioration of the building envelope as a result of rainwater penetration. Rehabilitation designs have focused on eliminating rainwater penetration as a damage mechanism, which has typically included an increased control of air flow through the envelope. Because these designs have resulted in buildings that are more air tight than before rehabilitation, there is an increased need for mechanical ventilation to maintain good building performance. Without adequate ventilation the balance humidity levels in the suites increase and, depending on the wall design, this can result in condensation on the windows or interior drywall, or within the wall assembly itself.
This paper draws on one rehabilitated wall assembly with high humidity levels to explore the assumptions and decisions faced by the design team when completing a rehabilitation project with respect to controlling heat, air, moisture and the resulting affect on building performance. Decisions and assumptions discussed include:
- Indoor environment: acceptable and assumed ranges of conditions for temperature, ventilation, moisture generation, and the resulting balance relative humidity.
- Cost and existing construction: constraints of the existing construction including wall dimensions and the cost of rehabilitation.
- Current norms for building design and construction: current accepted norms for the control of heat, air and moisture transport and the importance of balancing these controls.
This paper presents a combination of calculated values for the indoor environment using a moisture balance between the exterior and indoor environment, HAM computer modeling, and field measurements of a rehabilitated project to support this discussion. Conclusions and recommendations are drawn from the lessons learned from completing this work.
Authors: Mark Lawton, P.Eng., Patrick Roppel, M.A.Sc, P.Eng., Brian Hubbs, P. Eng.
Date Published: 2007