{"id":6914,"date":"2013-02-01T00:00:00","date_gmt":"2013-02-01T00:00:00","guid":{"rendered":"https:\/\/insulation.org\/io\/articles\/perspectives-on-water-vapor-permeance-of-mechanical-insulation-systems\/"},"modified":"2017-06-09T20:25:21","modified_gmt":"2017-06-09T20:25:21","slug":"perspectives-on-water-vapor-permeance-of-mechanical-insulation-systems","status":"publish","type":"articles","link":"https:\/\/insulation.org\/io\/articles\/perspectives-on-water-vapor-permeance-of-mechanical-insulation-systems\/","title":{"rendered":"Perspectives on Water Vapor Permeance of Mechanical Insulation Systems"},"content":{"rendered":"

Insulation
\nmaterial comparison often involves contrasting the physical properties of the
\nmaterials as represented on the products’ data sheets. Going through this exercise,
\nit is important to be sure that the physical properties being compared are
\ntested to the same test method and procedure, and the values are expressed in
\nthe same units. If not, one is comparing apples to oranges, resulting in an
\ninaccurate analysis of materials. It also is essential to understand the effect
\nthat the physical property will have on the insulation performance in relation
\nto the units of measure.<\/span><\/p>\n

A good example of the importance of understanding physical
\nproperty terms is in defining a material’s ability to resist penetration of
\nmoisture from the air. Water vapor permeability and permeance are both measures
\nof a material’s ability to resist penetration of moisture from the air. The
\nterms are defined in ASTM Standard C168 ? 10, “Standard Terminology Relating to
\nThermal Insulation” as follows:<\/span><\/p>\n

<\/span><\/p>\n

Water
\nvapor permeability<\/span><\/b> – the
\ntime rate of water vapor transmission through unit area of flat material of
\nunit thickness induced by unit vapor pressure difference between two specific
\nsurfaces, under specified temperature and humidity conditions.<\/span><\/p>\n

<\/span><\/p>\n

Water
\nvapor permeance<\/span><\/b> – the
\ntime rate of water vapor transmission through unit area of flat material or construction
\ninduced by unit vapor pressure difference between two specific surfaces, under
\nspecified temperature and humidity conditions.<\/span><\/p>\n

<\/span><\/p>\n

Permeability is measured in units of perms-inch and is used
\nto compare materials that are typically used in a variety of thicknesses (\u00bc” or
\ngreater). Permeance is measured in units of perms and is used to describe
\nthinner materials (e.g., jacketing products) that are used in the field in the
\nexact thickness at which the material is tested.<\/span><\/p>\n

A similar relationship exists between the terms used to
\ndefine thermal conductivity: k-factor and R-value. The unit to describe k-factor
\nis defined at a standard thickness of per 1″. This allows an end user to
\ncompare materials on an equal basis, regardless of thickness. In contrast,
\nR-value is a measure of the material in the thickness it is used in the field,
\nand varies depending on the referenced thickness. In regards to units used to
\nmeasure moisture penetration resistance, permeability is similar to the
\nk-factor, and permeance is similar to the R-value. A material with a
\npermeability listed as 1.0 perm-inch would have a permeance of 1.33 at \u00be”
\nthickness (1.0\/.75 = 1.33). For moisture penetration resistance values, the
\nsmaller the number, the better the value.<\/span><\/p>\n

Derived moisture penetration resistance results can be
\nconverted from one term to the other using appropriate conversion factors
\n(referenced in the table on page 29).<\/span><\/p>\n

A common test method to measure this property is ASTM E96,
\n“Standard Test Methods for Water Vapor Transmission of Materials.” Two test
\nprocedures (see images above) are called out in the Standard: wet cup
\n(Procedure A) and dry cup (Procedure B). Both procedures start by conditioning,
\nmeasuring thickness, and weighing the sample. For the wet cup method, the
\nsample is placed over a pan with water in it and the edges sealed.\u00a0 For the dry
\ncup method, the sample is placed over a pan with a desiccant in it and the
\nedges sealed. The pans are then placed in an environmental chamber at a
\nspecified temperature and humidity, and weighed daily, until the weight gain or
\nloss reaches equilibrium (shown in the chart on page 29). At this time, the
\npermeability or permeance of the material can be established. For some
\ninsulation or jacketing materials, the procedure used can make a slight
\ndifference in the stated value. Hence, to be sure one is comparing apples to
\napples, one should use values obtained from the same procedure (which should be
\nstated on the product’s technical data sheet).<\/span><\/p>\n

It also is important to understand the
\nsignificance of both the physical property itself and the value stated. One of
\nthe key determining factors to consider when evaluating the significance of a
\ntest value is the precision of the test. A result that goes beyond what the
\ntest can accurately measure does not provide added value. Also, the definition
\nof what constitutes a good value, “low perm” in this case, often changes over
\ntime. Further, within the permeability or permeance value, one should consider
\nthe value of the entire system?specifically, how much weight should be placed
\non the permeability of the material itself versus the permeability of the seams.
\nFor example, aluminum jacketing has a very low permeance, but if the seams are
\nnot properly sealed, the system loses much of its integrity so that no matter
\nhow good the jacketing is, the overall insulation system may not perform as
\nexpected. In most cases, slight differences in permeability would require many
\nyears to make a difference in the performance of the system, even if the
\npercentage change may seem large. Other factors probably would play a much
\ngreater role, such as damage to the jacket or insulation.<\/span><\/p>\n

Another key point to understand is what
\neffect the property will have on the performance of the insulation.
\nPermeability is primarily an issue with insulation on low-temperature lines
\nwhere humidity is high over long periods of time. On systems that do not have
\nthese conditions, permeability will not be a key consideration. The lower the
\nline temperature and the higher the humidity, the greater the importance will
\nbe. Hence, in Florida and the Gulf States, it can be a key factor. Over time,
\nwith highly permeable materials, air with moisture will come in contact with
\nthe cold pipe and form condensation between the pipe and the insulation. The
\nmoisture creates wet insulation, which increases the chances for pipe
\ncorrosion, mold growth, and degradation of the insulation’s thermal
\nconductivity, resulting in insulation and system failure. This situation is
\nsevere and would warrant removal of the insulation. However, this is a long,
\nslow process taking years to develop. In contrast, if there is evidence of
\nmoisture between the insulation and the pipe in a short period of time, the
\ncause is almost always an open seam or termination point where moisture-laden
\nair can travel unabated to the cold pipe and form condensation. <\/span><\/p>\n

In summary, there are typically several ways to describe a
\nmaterial’s performance, including its ability to resist penetration of moisture
\nfrom the air. When comparing products or determining if a material meets a
\nspecification requirement, one must pay careful attention to the units and identified
\ntest method. There are many reasons why it is important to understand the
\nphysical property terms used to describe or compare materials and to be sure
\none is comparing like terms expressed in the same units. Further, it is vital
\nto understand the effect that a particular property has on the performance of
\nthe insulation system.<\/span><\/p>\n

\n
<\/a>Figure 1<\/b><\/div>\n
<\/a>Figure 2<\/b><\/div>\n
<\/a>Figure 3<\/b><\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

Insulation material comparison often involves contrasting the physical properties of the materials as represented on the products’ data sheets. Going through this exercise, it is important to be sure that the physical properties being compared are tested to the same test method and procedure, and the values are expressed in the same units. If not,<\/p>\n","protected":false},"author":[],"featured_media":0,"template":"","categories":[23,301],"class_list":["post-6914","articles","type-articles","status-publish","hentry","category-condensation-control","category-design"],"acf":[],"yoast_head":"\nPerspectives on Water Vapor Permeance of Mechanical Insulation Systems - Insulation Outlook Magazine<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/insulation.org\/io\/articles\/perspectives-on-water-vapor-permeance-of-mechanical-insulation-systems\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Perspectives on Water Vapor Permeance of Mechanical Insulation Systems\" \/>\n<meta property=\"og:description\" content=\"Insulation material comparison often involves contrasting the physical properties of the materials as represented on the products’ data sheets. 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