"Historiecally, moisture control recomendations have focused on control of diffusion from the inside by vapor retarders and limitting water entry from the outside, but more recently air movement has become an important consideration."

"The effectiveness of the polyethylene vapor retarder almost ceretainly depended on its function as an air barrier, a function it was not intended to perform.

ABSTRACT

Wind pressures can play an important role in the wetting of exterior walls (driving rain). In response, the rain screen concept, including compartmentalization and air spaces, has been developed to provide pressure equalization and limit water entry into the wall. However, conventional construction such as wood lap siding has not been evaluated as to its ability to function as a rain screen. As part of a two-year project assessing the performance of hard board lap siding, we measured air pressure differences across the siding over extended periods of time in two single-story wood-frame buildings, specially constructed for this study in southern Florida. Three different wall constructions were included in the study. We found that the conventionally installed lap siding provided substantial air pressure equalization. Inward air pressure differences across the siding did not appear strong enough, or long enough in duration, to raise concern about significant water penetration through the siding overlapse even during windy days.

Air leakage has been recognized as an important mode of water vapor transport, and airtight construction is therefore recognized as an important ingredient of designing for high moisture tolerance. As part of the same study, we monitored air pressures across the siding, sheathing, and gypsum board in two of the walls. We found that wind-induced air pressures across the exterior walls were predominantly exfiltrative, even on the windward side of the building. Infiltrative pressures only occurred near windward comers of the building during short periods of time. We also found significant air leakage past the top plate into the wall cavity. This paper also presents a method for coordinating the design for airtightness (airflow retarders) with the design for control of vapor diffusion (vapor retarders). The calculations show that a vapor retarder of] perm (57ng/Pa-s-m2) should be complemented 10^-5 M^2/M^2)

with an air barrier system (ABS) with an ELA of 0.003 in.21ft2 (2x or less. Specifying lower-per, vapor retarders

(eg., 0. 1 perm, or 5. 7 ngIPa.s.m2) would require specifying an extraordinary level of-airtightness.

• air barrier : vapor retarder is unintended air barrier
• rainscreen, pressure equalized rainscreen principle : rain screen concept provides pressure equalization and limit water entry into wall
• vapor barrier, vapor retarder : vapor retarder is unintended air barrier
• wind driven rain : Wind pressures important in wetting of exterior walls (driving rain)
• wind induced pressure on building : wind-induced pressures across exterior walls were predominantly exfiltrative
• wood frame houses and walls, wood structure : pressure differentials are monitored across/within wood frame wall

1. A mathematical model for indoor humidity in homes during winter
2. Accuracy of wood resistance sensors for measurement of humidity
3. Effect of cavity ventilation on moisture in walls and roofs
4. Interior moisture design loads for residences
5. Issues related to venting of attics and cathedral ceilings
6. Manufactured housing walls that provide satisfactory moisture performance in all climates
7. Moisture management in buildings
8. Moisture transfer through materials and systems in buildings
9. Performance of back-primed and factory-finished hardboard lap siding in southern florida
10. Ventilation, humidity, and condensation in manufactured houses during winter
11. Weather Data  

1. Accuracy of wood resistance sensors for measurement of humidity
2. Accuracy of wood resistance sensors for measurement of humidity
3. Decay of wood and wood-based products above ground in buildings, Mechanism of Brown-Rot Decay: Paradigm or Paradox
4. Effect of cavity ventilation on moisture in walls and roofs
5. Performance of back-primed and factory-finished hardboard lap siding in southern florida
6. Rainwater intrusion in light-frame building walls  

1. Performance of back-primed and factory-finished hardboard lap siding in southern florida