{"id":1471,"date":"2026-10-07T07:04:05","date_gmt":"2026-10-07T07:04:05","guid":{"rendered":"https:\/\/mywarmhome.co.uk\/stage\/?p=1471"},"modified":"2026-10-11T14:52:07","modified_gmt":"2026-10-11T14:52:07","slug":"thermal-values-explained","status":"publish","type":"post","link":"https:\/\/mywarmhome.co.uk\/stage\/2026\/10\/07\/thermal-values-explained\/","title":{"rendered":"Thermal Values explained."},"content":{"rendered":"\n
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Insulation \u2022 Thermal performance<\/p>\n\n\n\n

Sometimes in life,
it\u2019s better to lower your values.<\/span><\/h2>\n\n\n\n

U-values, R-values and thermal conductivity can look technical. In reality, they are simply different ways of describing how easily heat moves through materials and building elements.<\/p>\n<\/div>\n\n\n\n

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01<\/p>\n\n\n\n

R-value<\/h3>\n\n\n\n

How strongly a material resists heat flow. Higher is better.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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02<\/p>\n\n\n\n

U-value<\/h3>\n\n\n\n

How much heat passes through a building element. Lower is better.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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03<\/p>\n\n\n\n

Lambda \u03bb<\/h3>\n\n\n\n

How readily an individual material conducts heat. Lower is better.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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04<\/p>\n\n\n\n

Thickness<\/h3>\n\n\n\n

The same material performs differently depending on how much of it is installed.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n\n\n\n

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In short<\/p>\n\n\n\n

Thermal values tell us how easily heat escapes.<\/h2>\n\n\n\n

When insulating a home, the material, thickness, building construction, moisture balance and the way different layers work together all affect the finished thermal performance.<\/p>\n\n\n\n

R-values help describe resistance to heat flow. U-values describe the overall thermal transmittance of a building element. Lambda values describe the conductivity of an individual material.<\/p>\n\n\n\n

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The simple rule<\/p>\n\n\n\n

R-value:<\/strong> higher is better. U-value:<\/strong> lower is better. Lambda:<\/strong> lower is better.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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Typical U-value targets you may see<\/h2>\n\n\n\n

These are illustrative figures retained from the original article, not universal requirements for every project. Applicable limits depend on the work, building element and current regulations.<\/p>\n\n\n\n

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Roofs<\/p>\n\n\n\n

0.16<\/p>\n\n\n\n

W\/m\u00b2K<\/p>\n<\/div>\n<\/div>\n\n\n\n

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Walls<\/p>\n\n\n\n

0.30<\/p>\n\n\n\n

W\/m\u00b2K<\/p>\n<\/div>\n<\/div>\n\n\n\n

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Floors<\/p>\n\n\n\n

0.25<\/p>\n\n\n\n

W\/m\u00b2K<\/p>\n<\/div>\n<\/div>\n<\/div>\n\n\n\n

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Replacement windows<\/h3>\n\n\n\n

1.4 W\/m\u00b2K or a B Window Energy Rating.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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Doors over 60% glazed<\/h3>\n\n\n\n

1.2 W\/m\u00b2K.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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Other doors<\/h3>\n\n\n\n

1.0 W\/m\u00b2K, with a limiting value of 1.6 W\/m\u00b2K.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n\n\n\n

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Insulation basics<\/h2>\n\n\n\n

Effective insulation can reduce heat loss and improve comfort, but choosing the right product involves more than simply picking a material from a shelf.<\/p>\n\n\n\n

Thermal performance, moisture control, wall or floor construction, property age and the characteristics of the insulation all need to be considered together.<\/p>\n\n\n\n

A fabric-first approach means reducing unwanted heat loss before upgrading or replacing the heating system. Less heat escaping from the building means the heating system has less work to do.<\/p>\n\n\n\n

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Explore insulation<\/a><\/div>\n<\/div>\n<\/div>\n\n\n\n
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Three values. Three different jobs.<\/h2>\n\n\n\n
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R-value<\/p>\n\n\n\n

Resistance<\/h3>\n\n\n\n

The R-value measures the thermal resistance of a material. The higher the R-value, the greater its resistance to heat transfer.<\/p>\n\n\n\n

Unit:<\/strong> m\u00b2K\/W<\/p>\n<\/div>\n<\/div>\n\n\n\n

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U-value<\/p>\n\n\n\n

Transmittance<\/h3>\n\n\n\n

The U-value describes heat transfer through a complete building element or combination of materials. Lower U-values indicate better thermal performance.<\/p>\n\n\n\n

Unit:<\/strong> W\/m\u00b2K<\/p>\n<\/div>\n<\/div>\n\n\n\n

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Lambda \u03bb<\/p>\n\n\n\n

Conductivity<\/h3>\n\n\n\n

Lambda describes the thermal conductivity of an individual material. Lower conductivity means the material transfers heat less readily.<\/p>\n\n\n\n

Unit:<\/strong> W\/mK<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n\n\n\n

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Why R-value matters<\/h2>\n\n\n\n

Higher thermal resistance means the insulation is better at slowing heat transfer. That can help a home retain heat in winter and reduce unwanted heat gain in summer.<\/p>\n\n\n\n

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Comparing two 100mm insulation products<\/h3>\n\n\n\n
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  • 100mm mineral loft roll:<\/strong> R-value around 2.50 in the example used on the original page.<\/li>\n\n\n\n
  • 100mm PIR rigid board:<\/strong> R-value around 4.30 in the example used on the original page.<\/li>\n<\/ul>\n<\/div>\n\n\n\n

    This is why a higher-performance insulation product may achieve a similar thermal result at a smaller thickness. Cost, moisture behaviour, detailing and suitability still need to be considered alongside the thermal number.<\/p>\n<\/div>\n\n\n\n

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    Why U-value matters<\/h2>\n\n\n\n

    The U-value is used to describe the overall rate of heat transfer through a building element. A wall, roof or floor is normally made from several layers, so each layer contributes to the total thermal resistance.<\/p>\n\n\n\n

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    Basic relationship<\/p>\n\n\n\n

    U = 1 \u00f7 total R<\/p>\n\n\n\n

    The original page uses the reciprocal relationship as a simple way to explain why increasing total resistance lowers the U-value.<\/p>\n<\/div>\n\n\n\n

    Real building-element calculations can involve several materials, surface resistances, repeating thermal bridges, air spaces and product-specific data, so manufacturer or professional calculation tools are normally used for final design and compliance checks.<\/p>\n<\/div>\n\n\n\n

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    Choosing insulation<\/h2>\n\n\n\n

    Space often determines what can realistically be installed. A suspended timber floor with limited joist depth may need a product that achieves a higher thermal resistance at a smaller thickness. Where more depth is available, a different material may be practical and more economical.<\/p>\n\n\n\n

    Window and door reveals can also be difficult because there may be very little room for insulation. Thinner specialist products may be considered where conventional insulation cannot physically fit.<\/p>\n\n\n\n

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    Do not choose on thermal performance alone<\/h3>\n\n\n\n

    Cost, available space, moisture behaviour, fire considerations, installation method and the construction of the existing building can all influence the final choice.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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    Practical example<\/h2>\n\n\n\n

    The original MyWarmHome example considers insulation beneath a suspended floor with a target U-value of 0.25 W\/m\u00b2K.<\/p>\n\n\n\n

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    100mm loft roll<\/p>\n\n\n\n

    Example U-value: 0.44 W\/m\u00b2K<\/strong>
    Does not meet the example target.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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    150mm loft roll<\/p>\n\n\n\n

    Example U-value: 0.29 W\/m\u00b2K<\/strong>
    Does not meet the example target.<\/p>\n<\/div>\n<\/div>\n<\/div>\n\n\n\n

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    200mm loft roll<\/p>\n\n\n\n

    Example U-value: 0.22 W\/m\u00b2K<\/strong>
    Meets the example target.<\/p>\n<\/div>\n<\/div>\n\n\n\n

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    100mm PIR<\/p>\n\n\n\n

    Example U-value: 0.22 W\/m\u00b2K<\/strong>
    Meets the example target.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n\n\n\n

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    Try it yourself<\/p>\n\n\n\n

    Simple U-value calculator<\/h2>\n\n\n\n

    Add material layers and thicknesses to get a simple indicative result. For final specification or Building Regulations compliance, use the relevant manufacturer or professional calculation method.<\/p>\n\n\n\n