11/10/2026
Two quotes. Two thicknesses. Two very different numbers. Millimetres alone tell you very little.
A product’s declared thermal conductivity (λ) and installed thickness determine its declared thermal resistance (R). The target for a roof or loft is usually expressed as a U-value for the complete build-up, not just one layer. Different products and thicknesses may be specified toward a similar target, so two quotes saying “100mm” may describe very different things.
Declared figures aren’t the whole installed result. Coverage, gaps, compression, continuity over joists and around edges or penetrations, clear services, ventilation and inspection routes, and correct vapour-control-layer positioning all matter.
Reflective multi-layer insulation and breathable sheep wool aren’t comparable by millimetres alone; they’re specified differently. Wool may suit some older buildings, while multi-layer products may suit some newer builds. Suitability depends on the survey, existing roof build-up and ventilation, manufacturer guidance and current official guidance.
Ask about any quote:
1. Which product?
2. What declared performance?
3. What installed thickness, and exactly where?
4. What does the specification assume about the rest of the build-up?
Product suitability, moisture and condensation behaviour, and thermal performance depend on the property and installation. No specific savings, rating, comfort or compliance outcome is promised. This is an illustrative project example, not a diagnosis of your property.
Midlands homeowner or landlord? Arrange a free, no-obligation roof and loft survey and tailored written quotation. We’re TrustMark registered, rated 5★ on Google and Bark, and serve the East and West Midlands. Call 01332 480429 or visit nextgenerationinsulation.co.uk.