The moisture content of building materials that comprise the building assemblies all remained below the equilibrium moisture content of the materials as specified in ASHRAE 160 P under this evaluation approach. The author and the publisher of this article have used their best efforts to provide accurate and authoritative information in regard to the subject matter covered. This wall assembly will dry from the vapor barrier inwards and will dry from the vapor barrier outwards.WUFI: PC Program for Calculating the Coupled Heat and Moisture Transfer in Building ComponentsThe Difference Between a Vapor Barrier and an Air BarrierMoisture Control in the Modern Building Envelope: The History of the Vapor Barrier in the US – 1923 to 1952
In this wall assembly the vapor barrier is also the drainage plane and air barrier.This assembly has all of the thermal insulation installed on the interior of the concrete block construction but differs from * In very cold and subarctic/arctic regions vapor impermeable foam sheathings are recommended* In very cold and sub arctic/arctic regions vapor impermeable foam sheathings are recommended – additionally the thickness of the foam sheathing should be determined by hygro-thermal analysis so that the interior surface of the foam sheathing remains above the dew point temperature of the interior air (see Applicability – Limited to mixed-humid, hot-humid, mixed-dry, hot-dry and marine regions – can be used with hygro-thermal analysis in some areas in cold regions (Zone 5, but not Zone 6; see This wall is a flow through assembly – it can dry to both the exterior and the interior. Therefore this wall assembly should not be used in cold regions or colder. This is easy to understand, logical, with a long historical basis.The fundamental principle of control of water in the solid form is to not let it get solid and if it does – give it space or if it is solid not let it get liquid and if it does drain it away before it can get solid again. The function of a vapor barrier is to retard the migration of water vapor. Latex painted gypsum board (one coat of latex paint) is a Class III vapor retarder.Plywood sheathing and oriented strand board (OSB) have perm values of greater than 1 perm when using the wet cup test. This is a known weak spot for conventional cold climate construction. The spray foam insulation installed on the interior of the precast concrete is non-moisture sensitive and allows the wall to dry inwards. The drainage plane in this assembly is the latex painted precast concrete. However, it is virtually impossible to create a perfect vapor barrier without any holes or leaks that can fully prevent air and moisture from moving through the wall assembly. This is simple to understand, except we have trouble deciding what side of a wall is the cold or warm side. (not the actual service conditions for typical residential occupancy – but the design conditions for the simple steady state design procedure being used)(not the actual service conditions for typical residential occupancy – but the design conditions for the simple steady state design procedure being used)Polyethylene is a Class I vapor retarder. When this air moves from location to location due to an air pressure difference, the vapor moves with it. The wall assembly does contain water sensitive cavity insulation (except where spray foam is used) and it is important that this assembly can dry inwards – therefore vapor semi impermeable interior finishes such as vinyl wall coverings should be avoided. It is also a durable assembly due to the block construction and the associated moisture storage (hygric buffer) capacity. It is also a flow through assembly similar to Applicability – Limited to mixed-humid, hot-humid, mixed-dry, hot-dry and marine regions – should not be used in cold, very cold, and subarctic/arctic regionsThe vapor barrier in this assembly is the precast concrete itself. Note that adsorbed water is different than absorbed water (see The fundamental principle of control of water in the liquid form is to drain it out if it gets in – and let us make it perfectly clear – it will get in if you build where it rains or if you put your building in the ground where there is water in the ground. The location of the barriers in the wall assembly is dependent on the exterior climate and interior use. * taken somewhat from ASHRAE Fundamentals 2001, Chapter 23.The unit of measurement typically used in characterizing the water vapor permeance of materials is the “perm.” It is further proposed here that there should be several classes of vapor retarders (this is nothing new – it is an extension and modification of the Canadian General Standards Board approach that specifies Type I and Type II vapor retarders – the numbers here are a little different however):ASTM E-96 Test Method A (the desiccant method or dry cup method)The current International Building Code (and its derivative codes) defines a vapor retarder as 1.0 perm or less (using the same test procedure). If professional advice or other expert assistance is required, the services of a competent professional shall be sought. Of particular concern is the fact that the thermal insulation is air permeable (except where spray foam is used). As a result, in mixed climates, the wall should be able to dry in both directions, changing with the season. It has a Class III vapor retarder on the interior of the assembly (the latex paint on the gypsum board).
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