{"id":3966,"date":"2026-02-12T15:18:05","date_gmt":"2026-02-12T15:18:05","guid":{"rendered":"https:\/\/ikoknowledgecenter.com\/docs\/uncategorized\/is-there-a-significant-difference-between-a-v3-and-v4-bituminous-vapour-barrier\/"},"modified":"2026-04-23T11:57:13","modified_gmt":"2026-04-23T11:57:13","password":"","slug":"is-there-a-significant-difference-between-a-v3-and-v4-bituminous-vapour-barrier","status":"publish","type":"docs","link":"https:\/\/ikoknowledgecenter.com\/en\/docs\/flat-roof-systems\/is-there-a-significant-difference-between-a-v3-and-v4-bituminous-vapour-barrier\/","title":{"rendered":"Is there a significant difference between a V3 and V4 bituminous vapour barrier?"},"content":{"rendered":"\n

A frequently asked question when selecting a bituminous vapour barrier is whether there is a major difference between V3<\/strong> and V4.<\/strong> The short answer: no, functionally they are very similar, but there are some technical nuances that can be decisive in certain situations.<\/p>\n\n

According to Buildwise TV 280 (table 13), both V3 and V4 bituminous vapour barriers belong to the same E3 class<\/strong>. This means that, from a normative and performance perspective, they are equivalent in their application as vapour barriers.<\/p>\n\n

<\/div>\n\n

So where is the difference?<\/strong><\/span><\/h3>\n\n

The differences are mainly found in three aspects:<\/p>\n\n

1. Thickness<\/span><\/h5>\n\n
    \n
  • V3<\/strong>: 3 mm<\/strong><\/li>\n\n\n\n
  • V4<\/strong>: 4 mm<\/strong><\/li>\n<\/ul>\n\n

    The greater thickness of a V4 results in a more robust product, which is particularly relevant for less even substrates.<\/p>\n\n

    2. Vapour diffusion resistance<\/span><\/h5>\n\n

    For IKO Base products (V3\/V4 T\/F 10 m), we see:<\/p>\n\n

      \n
    • V3<\/strong>: \u00b5 \u2248 150<\/strong><\/li>\n\n\n\n
    • V4<\/strong>: \u00b5 \u2248 200<\/strong><\/li>\n<\/ul>\n\n

      The V4 therefore has a slightly higher vapour diffusion<\/strong> resistance, but this difference is generally not decisive<\/strong> for the overall roof performance.<\/p>\n\n

      3. Substrate and roughness<\/span><\/h5>\n\n

      In practice, the choice between V3 and V4 is often determined by the roughness of the substrate<\/strong>. TV 280 (table 9, page 43) provides clear guidelines:<\/p>\n\n

        \n
      • b \u2264 2 mm<\/strong> \u2192 use of\u00a0V3<\/strong><\/li>\n\n\n\n
      • b \u2264 3 mm<\/strong> \u2192 use of V4<\/strong><\/li>\n\n\n\n
      • b \u2265 4 mm<\/strong> \u2192 application with hot bitumen<\/strong> or cold bituminous adhesives<\/strong><\/li>\n<\/ul>\n\n

        b = flatness deviation<\/p>\n\n

        In other words: the rougher the substrate, the thicker the vapour barrier required<\/strong> to ensure proper and durable adhesion.<\/p>\n\n

        For more information, contact your IKO advisor.<\/p>\n","protected":false},"excerpt":{"rendered":"

        A frequently asked question when selecting a bituminous vapour barrier is whether there is a major difference between V3 and V4. The short answer: no, functionally they are very similar, but there are some technical nuances that can be decisive in certain situations. According to Buildwise TV 280 (table 13), both V3 and V4 bituminous […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","template":"","meta":{"footnotes":"","_members_access_role":[],"_members_access_error":""},"doc_category":[63],"glossaries":[],"doc_tag":[65,952,307,897,858],"class_list":["post-3966","docs","type-docs","status-publish","hentry","doc_category-flat-roof-systems","doc_tag-flat-roof","doc_tag-foundation-en","doc_tag-roofing-en","doc_tag-vapor-barrier","doc_tag-vapor-pressure-distribution"],"year_month":"2026-09","word_count":230,"total_views":"1508","reactions":{"happy":"0","normal":"0","sad":"0"},"author_info":{"name":"Luna Degraeve","author_nicename":"luna","author_url":"https:\/\/ikoknowledgecenter.com\/en\/author\/luna\/"},"doc_category_info":[{"term_name":"Flat roof systems","term_url":"https:\/\/ikoknowledgecenter.com\/en\/docs-category\/flat-roof-systems\/"}],"doc_tag_info":[{"term_name":"Flat roof","term_url":"https:\/\/ikoknowledgecenter.com\/en\/docs-tag\/flat-roof\/"},{"term_name":"foundation","term_url":"https:\/\/ikoknowledgecenter.com\/en\/docs-tag\/foundation-en\/"},{"term_name":"roofing","term_url":"https:\/\/ikoknowledgecenter.com\/en\/docs-tag\/roofing-en\/"},{"term_name":"vapor barrier","term_url":"https:\/\/ikoknowledgecenter.com\/en\/docs-tag\/vapor-barrier\/"},{"term_name":"vapor pressure distribution","term_url":"https:\/\/ikoknowledgecenter.com\/en\/docs-tag\/vapor-pressure-distribution\/"}],"knowledge_base_info":[],"knowledge_base_slug":[],"_links":{"self":[{"href":"https:\/\/ikoknowledgecenter.com\/en\/wp-json\/wp\/v2\/docs\/3966","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ikoknowledgecenter.com\/en\/wp-json\/wp\/v2\/docs"}],"about":[{"href":"https:\/\/ikoknowledgecenter.com\/en\/wp-json\/wp\/v2\/types\/docs"}],"author":[{"embeddable":true,"href":"https:\/\/ikoknowledgecenter.com\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/ikoknowledgecenter.com\/en\/wp-json\/wp\/v2\/comments?post=3966"}],"version-history":[{"count":3,"href":"https:\/\/ikoknowledgecenter.com\/en\/wp-json\/wp\/v2\/docs\/3966\/revisions"}],"predecessor-version":[{"id":3969,"href":"https:\/\/ikoknowledgecenter.com\/en\/wp-json\/wp\/v2\/docs\/3966\/revisions\/3969"}],"wp:attachment":[{"href":"https:\/\/ikoknowledgecenter.com\/en\/wp-json\/wp\/v2\/media?parent=3966"}],"wp:term":[{"taxonomy":"doc_category","embeddable":true,"href":"https:\/\/ikoknowledgecenter.com\/en\/wp-json\/wp\/v2\/doc_category?post=3966"},{"taxonomy":"glossaries","embeddable":true,"href":"https:\/\/ikoknowledgecenter.com\/en\/wp-json\/wp\/v2\/glossaries?post=3966"},{"taxonomy":"doc_tag","embeddable":true,"href":"https:\/\/ikoknowledgecenter.com\/en\/wp-json\/wp\/v2\/doc_tag?post=3966"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}