Page 469 - Forensic Structural Engineering Handbook

MASONRY STRUCTURES 13.15

2
where k = thermal conductivity, Btu/(h·ft ·°F·in)
b
W = density of brick masonry, lb/ft 3
ω= moisture content, percent
For interior walls ω≈ . 1 percent; for exterior walls in winter ω≈ . 7 percent, and in summer
ω≈ . 2 percent or after a few rainy days ω ≈ . 12 to 20 percent.
Concrete Masonry. Thermal conductivity of solid concrete masonry k may be esti-
c
mated as k ≈ . 0.082w 3.84, where w is the concrete density in pounds per cubic foot (see
c
Table 13.2).
Air and Vapor Transmission
Air and vapor transmission is important to structural engineers, because condensation can
affect serviceability of structural materials, principally steel corrosion and masonry durabil-
ity. Mismatch in vapor transmission properties of masonry repair and replacement materials
can cause deterioration at interfaces between original and repair materials. Porous stone and
brick pointed with portland cement–lime mortar are susceptible to this type of deterioration.
Examples of building types in which the need for vapor retarders in masonry wall assem-
blies should be investigated include laundries, indoor swimming pools, hockey rinks, and cold
storage facilities. When required, a vapor retarder should be placed on that side of the insula-
tion which is warmer for most of the year. Vapor permeance of a nominal 4-in (100-mm) wythe
2
2
of brick masonry is about 0.8 perm [gr/(h·ft ·inHg)] [46 ng/(Pa·s·m )]. Vapor permeance of a
nominal 8-in (200-mm) wythe of hollow CMU (lime-stone aggregate) is about 2.4 perm.
Air leakage is usually more important to condensation control than vapor retardation.
When an air pressure differential occurs across a wall, air may pass through or around the
wall. Wind, fan pressure, or stack effect may cause a pressure differential. In winter when
warm air rises in a tall building, a positive pressure is created in upper stories and a nega-
tive pressure in lower stories. Air supply by mechanical ventilation may increase air pres-
sure. Exhaust fans may reduce air pressure. A windward facade experiences pressure, and
a leeward encounters suction. These phenomena may combine to create a significant pres-
sure differential across a wall.
In winter, if interior pressure is greater, then warm, moist interior air may exfiltrate to a
cooler part of the wall where condensate can form. In hot, humid climates during summer,
the warm, moist exterior air may infiltrate and cause interstitial condensation. If interior
surface material becomes moist, mold may flourish. The smell and other effects may make
human occupancy intolerable, causing evacuation of rental property. See Case History for
“A Low-Rise Hotel.”
An air barrier on the cavity side of the interior wythe would limit air leakage and thus
mitigate the problem. Air barriers must be continuous, durable, impermeable, and struc-
turally capable of sustaining the air pressure differential. A perfect air barrier is not attain-
able in conventional buildings. Complete control of air and vapor flow is not feasible. For
that reason and because of permeance of masonry to wind-driven rain, corrosion protection
for metal connectors in masonry is an important design consideration. 47
For additional information on air and vapor retarders see Ref. 1.

Codes, Standards, and Reference Works
This section provides information on national standards specifications for masonry prod-
ucts and their assembly, on methods for sampling and testing, and on methods for structural
design of masonry.