{"id":4527,"date":"2026-04-27T14:56:09","date_gmt":"2026-04-27T14:56:09","guid":{"rendered":"https:\/\/sleepy-hofstadter.94-76-228-222.plesk.page\/nota_tecnica\/water-vapour-permeability\/"},"modified":"2026-09-01T14:29:54","modified_gmt":"2026-09-01T14:29:54","slug":"water-vapour-permeability","status":"publish","type":"nota_tecnica","link":"https:\/\/ediltec.com\/en\/technical-notes\/water-vapour-permeability\/","title":{"rendered":"WATER VAPOUR PERMEABILITY"},"content":{"rendered":"
\n
\n


\nMoisture and insulation performance
\n<\/span><\/p>\n

Water vapour behaviour in building structures<\/h2>\n

Unlike liquid water, water vapour can diffuse more easily through materials when a difference in temperature and vapour pressure exists between the two sides of a building structure.<\/p>\n

If not properly managed, moisture can affect the performance of insulation, increase thermal conductivity and promote condensation within the building assembly.<\/p>\n<\/div>\n

\n

Water has a thermal conductivity of approximately
\n0.56 W\/mK<\/strong>, significantly higher than that of the main
\ninsulation materials. Even a limited amount of moisture can therefore
\nnoticeably reduce the insulating performance of a material.<\/p>\n<\/div>\n

\n

\n\u03b4p
\n<\/span><\/p>\n

Water vapour permeability<\/h3>\n

Water vapour permeability indicates the amount of water vapour that can
\npass through a material when there is a difference in vapour pressure
\nbetween its surfaces.<\/p>\n<\/article>\n


\n\u03bc
\n<\/span><\/p>\n

Vapour diffusion resistance<\/h3>\n

The dimensionless factor \u03bc<\/strong> indicates how many times
\na material resists water vapour diffusion more than a layer of air
\nof the same thickness.<\/p>\n<\/article>\n


\n\u03bb
\n<\/span><\/p>\n

Thermal performance<\/h3>\n

Moisture within a material can increase its thermal conductivity.
\nTo maintain high performance, insulation should remain as dry as possible
\nthroughout its service life.<\/p>\n<\/article>\n<\/div>\n

\n

How the \u03bc factor is calculated<\/h3>\n

The water vapour diffusion resistance factor is the ratio between
\nthe water vapour permeability of air and that of the material being considered.<\/p>\n

\n


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

\u03bc = \u03b4 air \/ \u03b4 material<\/div>\n

For air, the reference value is \u03bc = 1.<\/p>\n<\/div>\n<\/div>\n

\n

Open-cell and closed-cell materials<\/h3>\n

Most building materials are permeable to water vapour to varying degrees.
\nThe internal structure of a material has a significant influence on the rate
\nat which water vapour can pass through it.<\/p>\n

\n

\nMore open structure
\n<\/span><\/p>\n

Lower vapour resistance<\/h4>\n

A material with a more open structure and a lower \u03bc factor allows
\nwater vapour to diffuse more rapidly. If the building assembly is not
\nproperly designed, this may increase the risk of moisture accumulation.<\/p>\n<\/article>\n


\nClosed-cell structure
\n<\/span><\/p>\n

Higher vapour resistance<\/h4>\n

Closed-cell materials generally provide greater resistance to water
\nvapour diffusion and lower sensitivity to moisture absorption.<\/p>\n<\/article>\n<\/div>\n<\/div>\n

\n

The risk of interstitial condensation<\/h3>\n

The main risk occurs when water vapour passes through the building assembly,
\nreaches a sufficiently cold area and condenses within the construction layers.<\/p>\n

The presence of water can alter the properties of the insulation, reduce
\nits efficiency, contribute to deterioration and compromise the durability
\nof building components.<\/p>\n<\/div>\n

\n


\nIndicative comparison
\n<\/span><\/p>\n

Thermal conductivity and vapour resistance of insulation materials<\/h3>\n
\n\n\n\n\n\n\n
Property<\/th>\nXPS<\/th>\nEPS<\/th>\nPIR\/PUR with permeable facings<\/th>\nPIR\/PUR with impermeable facings<\/th>\nMineral wool<\/th>\nWood fibre<\/th>\n<\/tr>\n<\/thead>\n
Thermal conductivity W\/mK<\/strong><\/td>\n0.032\u20130.038<\/td>\n0.033\u20130.050<\/td>\n0.025\u20130.028<\/td>\n0.022<\/td>\n0.034\u20130.044<\/td>\n0.038\u20130.050<\/td>\n<\/tr>\n
Indicative \u03bc factor<\/strong><\/td>\n80\u2013150<\/strong><\/td>\n20\u201360<\/strong><\/td>\n30\u201350<\/strong><\/td>\nVery high<\/strong><\/td>\n1\u20132<\/strong><\/td>\n4\u20138<\/strong><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

The values are indicative and may vary depending on the product,
\ndensity, thickness, facing and the relevant technical declaration.<\/p>\n<\/div>\n

\n


\nEdiltec solutions
\n<\/span><\/p>\n

Products with different levels of vapour resistance<\/h3>\n

Ediltec offers extruded polystyrene
\nX-FOAM\u00ae<\/strong> products and
\nPOLIISO\u00ae<\/strong> polyurethane panels with different facings
\nto meet a wide range of design requirements.<\/p>\n

The choice between a more vapour-permeable product and a product with
\nhigh vapour-resistance facings should be based on the building assembly,
\napplication, environmental conditions and expected hygrothermal behaviour.<\/p>\n<\/div>\n

\n

Indicative characteristics of Ediltec product ranges<\/h3>\n
\n\n\n\n\n\n\n
Property<\/th>\nX-FOAM\u00ae with extrusion skin<\/th>\nX-FOAM\u00ae without extrusion skin<\/th>\nPOLIISO\u00ae AD \/ EXTRA<\/th>\nPOLIISO\u00ae PLUS<\/th>\nPOLIISO\u00ae ED \/ SB \/ VV<\/th>\n<\/tr>\n<\/thead>\n
Thermal conductivity W\/mK<\/strong><\/td>\n0.032\u20130.036<\/td>\n0.032\u20130.036<\/td>\n0.022<\/td>\n0.022<\/td>\n0.025\u20130.028<\/td>\n<\/tr>\n
Indicative \u03bc factor<\/strong><\/td>\n100\u2013150<\/td>\n80<\/td>\nVery high<\/td>\n125<\/td>\n30\u201350<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n
\n

Vapour control layer or vapour barrier?<\/h3>\n

Depending on the construction type, internal relative humidity,
\nintended use and temperature differences, it may be necessary to combine
\nthe insulation with a specific vapour control layer.<\/p>\n

\n

\nVapour control layer
\n<\/span><\/p>\n

Limits and regulates the passage of water vapour without completely
\npreventing it, helping to control the moisture balance within the
\nbuilding assembly.<\/p>\n<\/article>\n


\nVapour barrier
\n<\/span><\/p>\n

Provides very high resistance to vapour diffusion and is used when
\nit is necessary to strongly limit the entry of water vapour into the
\nbuilding assembly.<\/p>\n<\/article>\n<\/div>\n<\/div>\n

\n

Hygrothermal assessment of the building assembly<\/h3>\n

The choice of insulation material should not be based exclusively on its
\nthermal conductivity. Resistance to water vapour diffusion and the behaviour
\nof the entire building assembly must also be considered.<\/p>\n

A hygrothermal assessment, carried out according to the applicable methods
\nand standards, makes it possible to identify the risk of surface or
\ninterstitial condensation and to correctly design the position of any
\nvapour control layers or vapour barriers.<\/p>\n<\/div>\n<\/section>\n","protected":false},"author":1,"featured_media":0,"template":"","meta":{"_yoast_wpseo_focuskw":"water vapour permeability","_yoast_wpseo_title":"WATER VAPOUR PERMEABILITY","_yoast_wpseo_metadesc":"Contrary to what happens with liquid water, the water vapor spreads more easily inside the structures, when a gradient of pressure 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