{"id":5164,"date":"2026-01-29T09:36:39","date_gmt":"2026-01-29T09:36:39","guid":{"rendered":"https:\/\/idelco.eu\/insulation-values-explained-what-do-%ce%bb-value-and-rd-value-mean\/"},"modified":"2026-01-29T09:36:39","modified_gmt":"2026-01-29T09:36:39","slug":"insulation-values-explained-what-do-%ce%bb-value-and-rd-value-mean","status":"publish","type":"post","link":"https:\/\/idelco.eu\/en\/insulation-values-explained-what-does-the-lambda-value-and-rd-value-mean\/","title":{"rendered":"Insulation values explained: what do \u03bb\u2011value and Rd\u2011value mean?"},"content":{"rendered":"
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\u00a0Anyone who looks into insulation will inevitably come across them: the \u03bb\u2011value and the Rd\u2011value. They determine how well a material resists heat transfer, but what do they actually mean and what is the difference?<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/section>\n
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In this blog, you\u2019ll find a clear and simple explanation of what these insulation values are, how to calculate them, and which values are considered good for your roof, wall, or floor.<\/p>\n<\/div>\n

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Overview: insulation values at a glance<\/strong><\/h2>\n<\/div>\n
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Term<\/span><\/th>\nMeaning<\/span><\/th>\nUnit<\/span><\/th>\nGood to know<\/span><\/th>\n<\/tr>\n<\/thead>\n
\u03bb-Value<\/span><\/td>\n\n
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Thermal conductivity of a material (how well a material conducts heat)<\/p>\n<\/div>\n<\/td>\n

W\/m\u00b7K<\/span><\/td>\n\n
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The lower, the better. PIR \u2248 0.022 W\/mK is excellent.<\/p>\n<\/div>\n<\/td>\n<\/tr>\n

Rd-Value<\/span><\/td>\n\n
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Thermal resistance of a single insulation layer<\/p>\n<\/div>\n<\/td>\n

m\u00b2K\/W<\/span><\/td>\n\n
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Determined by thickness and \u03bb\u2011value: Rd = d \/ \u03bb<\/strong><\/p>\n<\/div>\n<\/td>\n<\/tr>\n

U-Value<\/span><\/td>\n\n
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Thermal transmittance of the entire construction (all layers combined)<\/p>\n<\/div>\n<\/td>\n

W\/m\u00b2K<\/span><\/td>\n\n
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The lower, the better. Used in EPB regulations.<\/p>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n

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What is the \u03bb\u2011value (lambda value)?<\/strong><\/h2>\n<\/div>\n
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The \u03bb\u2011value, or lambda value, indicates how well a material conducts heat. The lower the \u03bb\u2011value, the better the material insulates. The unit is W\/m\u00b7K (watts per meter per kelvin).<\/p>\n

Examples:<\/strong><\/p>\n

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  • PIR: \u03bb \u2248 0.022 W\/mK (excellent)<\/li>\n
  • Glass wool: \u03bb \u2248 0.035 W\/mK<\/li>\n
  • Concrete: \u03bb \u2248 1.7 W\/mK (poor insulation)<\/li>\n<\/ul>\n

    The \u03bb\u2011value is a material property, meaning it does not change with the thickness of the material.<\/p>\n

    Tip:<\/strong> the lower the \u03bb\u2011value, the less thickness you need to achieve good insulation. That\u2019s why PIR performs so well where space is limited.<\/p>\n<\/div>\n

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    What is the Rd\u2011value?<\/strong><\/h2>\n<\/div>\n
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    The Rd\u2011value indicates how much thermal resistance an insulation product provides. It is therefore a product property, dependent on both the thickness and the \u03bb\u2011value.<\/p>\n

    Formula:<\/strong> Rd = d \/ \u03bb (d = thickness in metres, \u03bb = thermal conductivity)<\/p>\n

    Example:<\/strong>
    \n10 cm (0.10 m) PIR with \u03bb = 0.022 W\/mK \u2192 Rd = 0.10 \/ 0.022 \u2248 4.55 m\u00b2K\/W<\/p>\n

    The higher the Rd\u2011value, the better the product insulates.<\/p>\n<\/div>\n

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    What is the U\u2011value and how does it differ from the Rd\u2011value?<\/strong><\/h2>\n

    The U\u2011value<\/strong> represents the insulation performance of the entire construction<\/strong> (insulation + structural elements + finishes). It is calculated from the total thermal resistance (Rtot<\/strong>):<\/p>\n

    U = 1 \/ Rtot<\/strong><\/p>\n

    Difference:<\/strong><\/p>\n

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    • Rd<\/strong> = thermal resistance of a single insulation layer (e.g. an insulation board).<\/li>\n
    • U<\/strong> = overall thermal performance of the complete roof, wall, or floor.<\/li>\n<\/ul>\n

      In Flanders<\/strong>, EPB regulations work with minimum Rd\u2011values<\/strong> for insulation products and maximum U\u2011values<\/strong> for the full construction. Rc\u2011values<\/strong>, as used in the Netherlands, do not apply here.<\/p>\n<\/div>\n

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      How do you calculate the insulation value yourself?<\/strong><\/h2>\n<\/div>\n
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      Do you want to know whether your insulation meets the required standards? Then use the formula Rd = d \/ \u03bb<\/strong>.<\/p>\n

      Assume:<\/strong><\/p>\n

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      • PIR board of 80 mm (0.08 m)<\/li>\n
      • \u03bb = 0.022 W\/mK<\/li>\n<\/ul>\n

        Rd = 0.08 \/ 0.022 \u2248 3.64 m\u00b2K\/W<\/strong><\/p>\n

        This allows you to check whether you meet the minimum Rd\u2011value and whether the construction as a whole achieves the required U\u2011value.<\/p>\n

         <\/p>\n<\/div>\n

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        Which insulation values apply in Flanders in 2025?<\/strong><\/h2>\n

        Minimum Rd\u2011values (insulation material):<\/strong><\/p>\n

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        • Roof \u2265 4.5 m\u00b2K\/W<\/li>\n
        • Fa\u00e7ade \u2265 3.0 m\u00b2K\/W<\/li>\n
        • Floor \u2265 2.0 m\u00b2K\/W<\/li>\n<\/ul>\n

          Maximum U\u2011values (construction):<\/strong><\/p>\n

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          • Roof \u2264 0.24 W\/m\u00b2K<\/li>\n
          • Fa\u00e7ade \u2264 0.24 W\/m\u00b2K<\/li>\n
          • Floor \u2264 0.30 W\/m\u00b2K<\/li>\n<\/ul>\n

            Anyone aiming for an energy\u2011efficient new build or renovation should target higher Rd\u2011values (and therefore lower U\u2011values).<\/p>\n<\/div>\n

             <\/p>\n

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            Frequently asked questions<\/strong><\/h2>\n

            1. What is the difference between Rd and the U\u2011value?<\/strong>
            \nRd is the thermal resistance of a single insulation layer. U is the insulation performance of the entire construction.<\/p>\n

            2. What is a good Rd\u2011value?<\/strong>
            \nThat depends on the application. For a roof, it\u2019s best to aim for Rd \u2265 4.5 m\u00b2K\/W. The higher, the better.<\/p>\n

            3. Which \u03bb\u2011value is considered good?<\/strong>
            \nThe lower the \u03bb\u2011value, the better. Materials with a \u03bb around 0.022 (such as PIR) perform exceptionally well.<\/p>\n

            4. How do I know if my insulation meets the standards?<\/strong>
            \nUse the formula Rd = d \/ \u03bb<\/strong> and compare the result with the required Rd\u2011value for your roof, fa\u00e7ade, or floor.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/section>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/section>\n<\/section>\n<\/div>\n

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            Conclusion: what does this mean for your project?<\/strong><\/p>\n

            If you choose an insulation material with a low \u03bb\u2011value, you need less thickness to achieve a high Rd\u2011value. PIR insulation from Idelco Insulation has a \u03bb\u2011value of 0.022 W\/mK and therefore offers excellent performance for roof, wall, and floor applications.<\/p>\n

            Want to know which board is best suited for your project?
            \nVisit our product page or request expert advice with no obligation.<\/p>\n<\/div>\n

            \"Isolatiewaarden<\/p>\n

             <\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/section>\n<\/section>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

            \u00a0Anyone who looks into insulation will inevitably come across them: the \u03bb\u2011value and the Rd\u2011value. They determine how well a material resists heat transfer, but what do they actually mean and what is the difference? In this blog, you\u2019ll find a clear and simple explanation of what these insulation values are, how to calculate them,<\/p>\n","protected":false},"author":3,"featured_media":4539,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":""},"categories":[115],"tags":[],"class_list":["post-5164","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-not-part-of-any-category"],"acf":[],"_links":{"self":[{"href":"https:\/\/idelco.eu\/en\/wp-json\/wp\/v2\/posts\/5164","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/idelco.eu\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/idelco.eu\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/idelco.eu\/en\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/idelco.eu\/en\/wp-json\/wp\/v2\/comments?post=5164"}],"version-history":[{"count":0,"href":"https:\/\/idelco.eu\/en\/wp-json\/wp\/v2\/posts\/5164\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/idelco.eu\/en\/wp-json\/wp\/v2\/media\/4539"}],"wp:attachment":[{"href":"https:\/\/idelco.eu\/en\/wp-json\/wp\/v2\/media?parent=5164"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/idelco.eu\/en\/wp-json\/wp\/v2\/categories?post=5164"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/idelco.eu\/en\/wp-json\/wp\/v2\/tags?post=5164"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}