Masonry Magazine August 1988 Page. 17
BIA Technical Notes
on Brick Construction
Brick Institute of America 11490 Commerce Park Drive, Reston, Virginia 22091
7D
REVISED
May
1988
MOISTURE RESISTANCE OF BRICK MASONRY WALLS
CONDENSATION ANALYSIS
Abstract: Moisture, formed by the condensation of water vapor, can cause many problems in brick masonry walls. Among these are: efflorescence, spalling, corrosion and interior finish damage.
This Technical Notes outlines a method used by designers to assess the possibility of condensation occurring in a given wall section; and, describes how to alleviate condensation problems through the use of vapor barriers and/or ventilation.
Key Words: brick, condensation, dew point, humidity, permeance, relative humidity, saturated vapor pressure, saturation, vapor barrier, vapor pressure, vapor resistance, walls.
Brick Institute of America
May 1988
4d
BRICK
MASONRY
INTRODUCTION
When a vapor pressure differential exists, water vapor will move independently of air. The vapor movement through common building materials is at a relatively high rate for common pressure differentials. When vapor passes through pores of homogenous walls, which are warm on one side and cold on the other, it may reach its dew point and condense into water within the wall; but, if the flow of vapor is impeded by a vapor-resistant material in the wall, the vapor may not reach that point in the wall at which the temperature is low enough to cause condensation.
Condensation problems are most frequent during the heating season when buildings of tight, highly insulated construction have occupancies and/or heating systems which produce humidity. This gain in moisture content of the interior air increases the interior vapor pressure substantially above that existing in the outdoor atmosphere. This tends to drive vapor outward from the building through any vapor-porous materials that comprise the wall assembly. This may be controlled either by the use of a properly placed vapor barrier or by decreasing the vapor pressure differential across the wall section through the use of ventilation.
Technical Notes 7C contains a discussion of the principles of condensation of water vapor, both on the wall surface and within the wall system. This Technical Notes is devoted to the analysis of wall systems to determine at what point or points in the wall assembly condensation might be expected to occur.
EFFECTS OF CONDENSATION
Many building materials are affected by water. For example, wood expands with increasing moisture content. If conditions of varying humidity occur in different parts of the cross-section of a single wood framing member, there will be a tendency to warp. High humidity can also cause the decay of wood. Water promotes the corrosion of metal, and many insulating materials show permanent change over the course of time when in contact with water. The insulating value of most materials is greatly reduced by the presence of free water. Volumetric changes in fired clay masonry units due to gains in moisture content are to be expected and should be given consideration in the design process.
Currently, a coefficient of moisture expansion of 0.0005 in./in. is suggested for design to accommodate brick masonry movement. Alternate freezing and thawing of clay products when saturated may lead to eventual deterioration, such as cracking and spalling. If soluble salts are present in or in contact with brick masonry, moisture caused by condensation may contribute to efflorescence.
CONDENSATION ANALYSIS
This Technical Notes describes the method used to determine the temperature and vapor pressure gradients of a wall when the exterior and interior design temperatures and relative humidities are known. Accompanying this method is an example of determining the points in a wall system where condensation may be expected to occur.
Method
The step-by-step method outlined assumes a steady-state heat loss procedure. The wall section and the interior and exterior temperatures and relative humidities are therefore held constant. This procedure is easily adapted to the cooling season by keeping in mind that the temperature and vapor pressure gradients are always plotted across the wall section from the warm side to the cool side.