{"id":39374,"date":"2017-07-18T10:00:18","date_gmt":"2017-07-18T14:00:18","guid":{"rendered":"https:\/\/www.constructionspecifier.com\/?p=39374"},"modified":"2017-09-11T15:52:27","modified_gmt":"2017-09-11T19:52:27","slug":"illustrating-the-dew-point-an-effective-way-to-determine-its-value","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/illustrating-the-dew-point-an-effective-way-to-determine-its-value\/","title":{"rendered":"Illustrating the Dew Point: An effective way to determine its value"},"content":{"rendered":"
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Photo \u00a9 BigstockPhoto<\/span><\/em><\/span><\/figcaption><\/figure>\n

by Anthony Katona, CDT
\n<\/strong>Although many roof designers and construction professionals understand the basic meaning of the Dew point temperature, there remains a real need for greater awareness about how to accurately perform and demonstrate the necessary calculations. This author\u2019s preferred method is to graphically illustrate the process. This allows one to accurately determine the value in relation to above-deck insulated, low-slope built-up roofing (BUR) assemblies.<\/p>\n

Relied on by architects, engineers, HVAC designers, and roof consultants alike, the dew point is defined by the National Roofing Contractors Association (NRCA) as \u201cthe temperature at which air becomes saturated with water vapor; the temperature at which air has a relative humidity (RH) of 100 percent.\u201d In other words, it is the point when water vapor condenses and changes from a vapor to a liquid.<\/p>\n

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Figure 1<\/strong>: The National Roof Contractors Association (NRCA) map. <\/span>
Image courtesy Professional Roofing<\/span><\/em><\/span><\/figcaption><\/figure>\n

According to NRCA, dew point calculations are necessary whenever the outside average January temperature is below 4 C (40 F), and when the expected interior winter RH is 45 percent or greater. Average exterior temperature can be determined from historical climatic data compiled by either the National Weather Service or local private weather services. When local climatic data is unavailable, the map in Figure 1 can be used to determine the general regions with outside January temperature below 4 C.<\/p>\n

NRCA considers the roof system designer to be responsible for determining whether a vapor retarder is necessary. When dew point calculations have not been correctly performed before construction, this often results in a liability. Additionally, costs can be significant when the calculations must be conducted in the field after the fact. Therefore, understanding how to calculate dew point is critical.<\/p>\n

For this article, the step-by-step procedures presented use hypothetical constant temperature values. However, actual dew point temperatures and related RH values constantly change in typical building environments. Therefore, this author recommends any findings be confirmed using methods advocated by NRCA. In the event a vapor barrier is required, further support can be obtained from the U.S. Army Corps of Engineers\u2019 (USACE\u2019s) Cold Regions Research and Engineering Laboratory (CRREL) and the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE). (Vapor barriers and vapor retarders are not synonymous. The former is a layer of material[s] used to prevent the transfer of moisture, primarily in the form of vapor, from permeating into adjacent materials within a building environment. A vapor retarder is a layer of material[s] used to prevent the transfer of water vapor, primarily in the form of liquid, from permeating into adjacent materials or anywhere within a building structure.)<\/p>\n

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Figure 2<\/strong>: Comparison of old R-values and new (after long-term thermal resistance [LTTR]), for different thicknesses of polyisocyanurate (polyiso) insulation.<\/span><\/figcaption><\/figure>The basics
\n<\/strong>Roofing assemblies create varying degrees of barriers between areas of differing vapor pressures. Therefore, if a roof is insufficiently insulated, warm, moist air will rise into the assembly above, where it can cool to its dew point temperature, creating the necessary conditions for condensation to occur. The rate at which the air cools depends on the R-value of the material it is passing through, with the choice and thickness of insulation having the greatest impact.<\/p>\n

Vapor drive from interior to exterior is likely to be strongest when the exterior temperature and RH are low and the interior temperature and RH are high. These conditions occur most often during winter months in cooler regions, as warmer interior air exerts a higher vapor pressure than cooler air outside.<\/p>\n

The primary goal of the method\u00a0 explored in this article is to establish the conditions at the warm side of the vapor retarder (i.e.<\/em> the underside), where it must remain warmer than the dew point temperature of the entire roof assembly. This article uses as its central example an assembly including an above-deck rigid polyisocyanurate (polyiso) insulation board\u2014a material chosen because of its high thermal resistance or R-value (per inch of thickness).<\/p>\n

Additionally, for this article, this author calculates the dew point temperatures using the recently updated long-term thermal resistance values (LTTR). These R-values, which are based on ASTM C1289-11A, Standard <\/em>Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board,<\/em> became effective on January 1, 2014. LTTR values predict product R-value after five years, which is equivalent to a time-weighted thermal design R-value for 15 years (Figure 2).<\/p>\n

A practical example
\n<\/strong>The first step in this process is to create a list of the components being evaluated. In this article\u2019s example, there are eight individual components comprising the roof assembly necessary to develop an accurate R-value profile:<\/p>\n