Masonry Magazine August 2016 Page. 44
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By Ted Winslow
WHILE THE QUEST FOR INCREASED A-VALUES HAS LED TO DRAMATIC REDUCTIONS IN AIR MOVEMENT THROUGH BUILDING ASSEMBLIES, it has also reduced their ability to dry when they get wet. In�wall systems allow this moisture to dissipate while maintaining the desired degree of airtightness. One way to create moisture escape routes is through construction techniques. Another is through the use of products that employ advanced air/moisture barrier technology.
The "Moisture Sandwich"
THE INTENDED CONSEQUENCE of the tight-envelope construction techniques mandated by various building codes is significantly less air leakage. The unintended consequence is that moisture, which finds its way inside the wall cavity -and moisture will always find a way into wall cavities -can get trapped there, creating a "moisture sandwich."
There are four primary physical mechanisms by which moisture infiltration takes place: water flow, airborne mois�ture flow, vapor diffusion and capillary suction.
Water flow is the most basic of these mechanisms. As rain or snow falls, gravity pulls it down a building's ex�terior, where improperly installed flashing can divert it into the building. Wind-driven rain can penetrate open�ings in the exterior cladding at imperfect mortar joints, laps, utility and electrical cutouts, etc., and gain access to a wall's interior.
Vapor diffusion occurs when water vapor flows through building materials due to dif�ferences in air pressure on either side of the wall. The flow direction is from the high-pres�sure side to the low-pressure side. The di�rection and intensity of this flow vary with season and geographic location. Higher tem�peratures inside a building increase vapor dif�fusion to the outside in cold climates, while the direction is reversed when outside tem�peratures are warm/humid and the inside of a building is cool and dry (due to air condi�tioning).
Whichever the direction ofthe flow, mois�if there's no mechanism for it to dry quickly, problems like wood rot and mold can occur.
Capillary suction is a result ofsurface tension and adhe�sive forces between water and the vertical plane of the wall. Moisture held there can be driven through tiny pores in the weather-resistant barrier and permeable wall sheathing and into the wall's interior by the heat ofthe sun -a phenome�non known as solar or vapor drive.
Masonry products like brick and cultured stone are sometimes referred to as "reservoir" cladding because they absorb and store moisture. As soon as this moisture is driven inside the wall and encounters a cold surface, it con�denses, and the potential for mold growth and damage to wood framing increases the longer the area stays damp.
Helping Moisture Escape
EXTERIOR WALL AND ROOF SYSTEMS are the first line of defense against rain, snow and subtler problems like
.____Drywall (depending on paint) Unfaced batts with poly
._______ OSBsheathlng
,7
______:Hou~sewr~ap(water-resistive barrier)
Continuous extruded
polystyrene Insulation (.XPS)
r ------Moisture-retentivedadding
(fiber cement/brick/stucco) 3-35perms so.1 perm Approximately 2 perms
5-SOperms
s.1 perm
Vapor permeable/ moisture retentive
Moisture will get trapped in the wall between the continuous extruded polystyrene
ture can accumulate when this vapor con�
insulation (XPS) foam board (on the exterior) and poly (on the interior), with no ability to
denses on cold surfaces inside the wall, and
dry.This can create a "moisture sandwich."
42 I MASONRY � August 2016 � www.masoncontractors.org The Voice of the Masonry Industry