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Drainage, ventilation drying and enclosure performance

Straube, J. F. and Burnett, E. F. P.
1998
Thermal Performance of the Exterior Envelopes of Buildings VII, Clear water Beach, Florida, pp. 189
cladding system, cavity wall

Straube, J. F. and Burnett, E. F. P., (1998), "Drainage, ventilation drying and enclosure performance", Thermal Performance of the Exterior Envelopes of Buildings VII, Clear water Beach, Florida, pp. 189.
Abstract:
This paper explores the influence and role of both drainage and ventilation drying on the ability of enclosure assemblies to control moisture. Drainage is often the most direct method of removing water from within a wall (i.e., from exfiltration condensation or rain penetration), but it is often not sufficient to provide moisture contro.l Design approaches that rely solely on drainage to remove moisture from behind the outer layers or cladding ignore the significant quantities of moisture that can be stored in the outer layers of most enclosure walls.

Most cladding systems have relatively low vapor permeability and therefore tend to restrict diffusive drying. Moisture trapped in or behind the cladding can be transported into the enclosure by solar-driven diffusior4 especially in air-conditioned buildings. Rather than control vapor diffusion, a 6 mille vapor retarder close to the interior may, in many instances, exacerbate wetting and greatly retard drying.

The role of ventilation within walls especially for North American conditions, has not been well researched and there does not appear to be any consensus with respect to the effect of ventilation on drying. We have found that airflow behind the cladding (ventilation) can be an important means of removing moisture stored within and behind vapor impermeable cladding. Calcu-lations, lab experiments3field monitoring, and anecdotal evidence all show that ventilation cannot only improve the drying capac-ity of wall assemblies, it is sometimes necessary for proper performance.

Several years of temperature, humidity, and moisture data collected from full-scale wall assemblies installed in a natural exposure and test facility are used to demonstrate these points.
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Author Information and Other Publications Notes
Straube, J. F.
Department of Civil Engineering, University of Waterloo, Waterloo, Ontario N2L 3T2 Canada
  1. A review of rain control and design strategies
  2. Driving rain and building facades
  3. Field testing of filled-cavity wall systems
  4. Indoor air quality, healthy buildings, and breathing walls
  5. Methodology and design of field experiments for monitoring the hygrothermal performance of wood frame enclosures
  6. Moisture control in enclosure wall systems
  7. Moisture fundamentals and mould
  8. Moisture in buildings
  9. Moisture movement in building enclosure wall systems
  10. Overview of hygrothermal (HAM) analysis methods
  11. Pressure moderation and rain penetration control
  12. Rain control and screened wall systems
  13. Simplified prediction of driving rain deposition
  14. The influence of low-permeance vapor barriers on roof and wall performance
  15. The role of hygrothermal modeling in practical building design: case studies
  16. Vents, ventilation drying, and pressure moderation  
Burnett, E. F. P.
  1. A review of rain control and design strategies
  2. Development of design strategies for rainscreen sheathing membrane performance in wood frame walls
  3. Driving rain and building facades
  4. Field testing of filled-cavity wall systems
  5. Moisture movement in building enclosure wall systems
  6. Overview of hygrothermal (HAM) analysis methods
  7. Rain control and screened wall systems
  8. Simplified prediction of driving rain deposition
  9. The Ontario wall drying project
  10. Uncertainty analysis for vapor pressure measurement
  11. Ventilation drying in enclosure wall systems
  12. Vents, ventilation drying, and pressure moderation  


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