Water Vapor Transmission DUK159

With the advent of modern, airtight, When a vapor-pressure differential The amount and rate of water vapor highly insulated buildings, it has exists, water vapor will move toward transmitted through building become increasingly important to the lower pressure independently of materials is dependent on: (1) the consider vapor flow characteristics air. For instance, with a winter gradient (differential) in vapor when designing exterior walls. condition of 0 °F and 75% RH, an pressure from one side of the Condensation was rarely a problem outside vapor pressure of 0.027 in. material to the other, (2) the area of in older buildings because they had Hg would exist, while inside a the material and (3) its permeance such high air-exchange rates just building heated to 70 °F and with (ability to permeate vapor passage). from air leakage that water vapor 35% relative humidity, vapor essentially traveled freely through pressure would equal 0.259 in. Hg. A good rule of thumb to insure safe the walls without ever reaching The vapor pressure inside would be outward diffusion of vapor is that the pressures that would result in nearly 10 times as high as outside. combination of exterior materials condensation. Like other gases, water vapor moves should have a total installed from an area of high pressure to an permeance of at least five times the The need for vapor retarders and area of low pressure until equilibrium total permeance of the interior vapor their proper location within a wall is established. During cold weather, barrier plus interior finish materials. assembly is influenced by the interior the difference in pressure between Water vapor permeance is a and exterior environmental inside and outside causes vapor to conditions as well as the wall’s measure of water vapor flow through move out through every available a material of specific thickness, or an thermal and vapor flow crack and directly through many characteristics. assembly of several materials. The materials that are permeable to reciprocal of permeance is called Generally speaking, buildings in cold water vapor. When vapor passes vapor resistance. northern climates will benefit from through pores of homogeneous having a vapor retarder installed walls, which are warm on one side Water vapor permeability is the near the interior side of the wall while and cold on the other, it may reach permeance of a 1 inch thickness of a its dew point and condense into homogeneous substance. buildings in warm, humid climates generally need to avoid low water within the wall. In warm, humid perm: permeance materials at those climates, the flow of water vapor will The unit of permeance which locations. be reversed and flow from the states the amount of vapor flow outside to the inside. Under such in grains per hour, per square It is important to note that each circumstances, it is necessary to building is fairly unique in terms of foot of surface, per 1 in. Hg. avoid use of interior wall coverings, vapor pressure gradient. wall construction, interior usage and i.e., vinyl wallpaper, which have low environmental conditions and should permeance to water vapor. perm inches: be evaluated individually by the Designation for permeability building designer. Dryvit Systems, Inc. offers a Water stating the amount of vapor flow The measure of the amount of water Vapor Transmission (WVT) analysis through 1 inch of material, in vapor in the air is known as relative at no cost to the customer to grains per hour, per square foot humidity (RH) and is stated as a determine the potential for of surface, per 1 in. Hg. vapor percentage. At 100% RH, air condensation in a specific wall. A pressure gradient contains the maximum amount of Water Vapor Transmission Request vapor it can hold, and such air is said can be submitted to Dryvit with Materials which have a permeance to be saturated. The temperature at appropriate information. The WVT (perm rating) of 1 perm or less are which saturation occurs is called the analysis will be returned using the referred to as vapor barriers. dew point temperature. format on the enclosed sample. One of the reasons for using 1 pcf density expanded polystyrene is that The concentration of water vapor The actual vapor pressure is it has the highest permeance of any may also be stated by giving its compared to the saturation vapor of the cellular plastics. It does not pressure, commonly expressed in pressure at each point in the wall. If create a vapor barrier on the exterior inches of mercury (in. Hg). actual vapor pressure is less than of the wall. The requirements for the saturation vapor pressure, no Water vapor establishes a pressure vapor barriers with Dryvit Outsulation condensation occurs. Conversely, if are not unusual. It is good practice to proportional to the amount of water actual vapor pressure is more than vapor present within the air mix. Air be aware of unusual design the saturation pressure, the potential conditions associated with particular with more vapor has a higher vapor for condensation exists. It still must pressure. At a particular temperature building functions or climatic regions be decided whether the and do a water vapor transmission and at saturation, the water vapor condensation rate is serious, since it exerts a definite pressure (Vapor analysis if there is a legitimate might readily be evaporated during question. Pressure at Saturation). the condensation period without excessive wetting. Water Vapor Transmission DUK159

Sample Water Vapor Transmission Analysis Project: ABC Building Interior Temp. (F): 72 Location: City State Interior R.H. (%): 35 Condition: Winter Exterior Temp. (F): 5 Date: 5/7/97 Exterior R.H. (%): 67

Material Thickness Therm Res Moist Res Temp Saturation Actual Cond (in.) (h ft^2 f/Btu) (inHg ft^2 (F) Vapor Vapor Rate h/gr) Pressure Pressure (gr/h ft^2) (inHg) (inHg) Interior 72.0 0.8020 0.2807 Inside Air Film Non-Ref 0.01 0.68 0.001 69.8 0.7442 0.2806 (Still Air) Latex Paint 2 Coats 0.002 0 0.3 69.8 0.7442 0.2443 Latex Primer 0.0012 0 0.16 69.8 0.7442 0.2250 Gypsum Wall Board - 2 0.5 0.45 0.027 68.3 0.7069 0.2217 Kraft Paper Facer 0.01 0 1 68.3 0.7069 0.1008 Fiberglass Batts - unfaced - 2 3.5 11 0.028 32.5 0.1864 0.0974 Dens Glass Gold - 1 0.5 0.56 0.029 30.7 0.1718 0.0939 Polystyrene, Expanded 2 7.7 0.4 05.6 0.0508 0.0455 (1.0PCF) Primus Base Coat 0.063 0 0.038 05.6 0.0508 0.0409 Sandpebble- 0.06 0 0.065 05.6 0.0508 0.0331 DPR Finish Outside Air Film Winter 0.01 0.17 0.001 05.0 0.0492 0.0330 (15MPH) Exterior 5.0 0.0492 0.0330

Total: 6.6562 20.56 2.049

2 Water Vapor Transmission DUK159

WATER VAPOR TRANSMISSION REQUEST FORM

Please type necessary information in the table below, print and fax to IJ Valainis, Engineering Department, (401) 823-8820. Project Name Project Location Firm Name Contact Person Address Zip/Postal Code Country Phone No. FAX E-mail Distributor/Sales Rep Date Needed By Notes Interior Design Temp. °F _____Summer _____Winter Interior Relative Humidity % _____Summer _____Winter Exterior Design Temp. °F _____Summer _____Winter Exterior Relative Humidity % _____Summer _____Winter List all materials, including paints, from the interior surface to the exterior Wall Construction: surface. Layer Thickness (Inches) Material A. 0.100² Inside Air Film B. C. D. E. F. G. H. I . J. K. L. M. N. O. 0.100" Outside Air Film Building Interior Use Is The Interior Relative Humidity

Controlled? If Yes, Describe This Information Provide By Date

Dryvit UK Ltd Unit 4 Wren Park Hitchin Road Shefford, Bedfordshire SG17 5JD 01462 819555 www.dryvit.co.uk