House Insulation Rating Comparison
Compare two insulation materials by thermal resistance, estimated heat transfer and matching thickness, with clear assumptions for your installer discussion.
Comparison Inputs
Breakdown
| Step | Material A | Material B |
|---|---|---|
| R-value = thickness ÷ λ | - | - |
| Resulting U-value | - | - |
Scenario Analysis
How the two materials compare when installed at the same thickness (like-for-like), rather than the thicknesses you entered above.
| Scenario | Material A Estimated U / Layer Proxy | Material B Estimated U / Layer Proxy |
|---|---|---|
| Your entered thicknesses | - | - |
| Both at Material A's thickness | - | - |
| Both at Material B's thickness | - | - |
Insulation Upgrade Timeline
Check your starting pointFind your existing U-value from an EPC report, or use one of this tool's rough starting presets.
Compare materials hereWeigh R-value per millimetre against cost, availability and how the material fits your joists, cavity or lining.
Check suitabilityConfirm ventilation, moisture and fire requirements for the material and location — a good U-value on paper is not enough if the build-up traps moisture.
Get quotes or buy materialsSee Home Insulation Comparison: Cost & Payback for typical installed costs and payback.
Verify after installationFor Building Regs or an EPC uplift, a qualified assessor confirms the as-built U-value — this tool's figures are a planning estimate, not a certificate.
What Is R-Value and U-Value?
R-value (m²K/W) measures how strongly a single layer resists heat flow — the higher the better. U-value (W/m²K) measures how much heat passes through a whole element (a wall, roof or floor) for every degree of temperature difference across it — the lower the better. The basic layer model uses U = 1 ÷ total R. A full construction calculation needs surface resistances and corrections as well. Read this explanation of the thermal resistance equation; this tool only provides a simplified comparison.
How Insulation Materials Compare
Illustrative conductivity (λ) defaults are around 0.040 W/mK for mineral wool, 0.036 W/mK for EPS board, 0.035 W/mK for XPS board, and as low as 0.023 W/mK for PIR/PUR rigid foam board — meaning PIR needs under 60% of the thickness of mineral wool for the same R-value. Sheep's wool (around 0.038 W/mK), wood fibre board (around 0.038 W/mK) and blown cellulose (around 0.039 W/mK) sit closer to mineral wool. Exact figures vary by specific product, which is why every value in this calculator can be overridden with your own product's datasheet number (checked 2026-09-25).
Loft Insulation Targets
Energy Saving Trust describes 270mm of mineral wool as a recommended depth. Using this tool’s illustrative λ of 0.040 W/mK gives R = 0.270 ÷ 0.040 = 6.75 m²K/W. The intermediate R = 5.0 band is CoryVu’s comparison convention, not an official “good” rating. Matching either resistance does not show that another material is suitable for a loft. Check fire, moisture, installation and ventilation requirements separately.
Floor Insulation and Building Regs
A floor with an assumed starting U-value of 0.7 W/m²K and 180mm of insulation at λ = 0.040 gives 1 ÷ (1/0.7 + 4.5) = 0.169 W/m²K in this simplified model. This is not a compliance result. Ground-floor calculations need geometry and ground conditions; suspended floors also need bridging and ventilation details. Rules differ between UK nations and projects. Ask a competent designer or building control which calculation and standard applies.
Why Wall Results Don't Claim Building Regs Compliance
Walls and floors are rated only on the modelled reduction from the entered starting U-value: Strong at 70% or more, Moderate from 40% to below 70%, Minimal below 40%. These are CoryVu comparison bands, not regulatory thresholds. The 1.5 W/m²K cavity-wall, 2.0 solid-wall, 0.7 timber-floor and 1.0 concrete-floor presets are illustrative starting assumptions; replace them with a construction-specific assessment. An EPC may not give a measured U-value.
Method and Limits
This calculator uses the additive-resistance method from BS EN ISO 6946 (R = thickness ÷ λ; U = 1 ÷ ΣR), a simplified layer comparison; it is not a full standards-compliant U-value assessment.
- Assumes the loft's ceiling and roof void add little resistance. For loft results, U is approximated as 1 ÷ the insulation layer's own R-value, ignoring the ceiling and ventilated roof space above it — a standard simplification, not a full roof calculation.
- Does not derive a full ground-floor U-value. Real ground floors follow BS EN ISO 13370, which accounts for exposed perimeter and ground conditions. This tool instead adds one insulation layer to a floor U-value you already know or estimate from a preset.
- Does not model thermal bridging, moisture, fire or ventilation. Junctions, timber studs, wall ties, condensation risk and ventilation requirements can matter as much as the raw U-value and are not modelled here.
- Does not price materials or installation. See Home Insulation Comparison: Cost & Payback for cost and payback figures.
- Input limits. Conductivity must be 0.015–0.060 W/mK, thickness 0–600mm and existing U 0.05–5 W/m²K. Values outside this tool’s scope need a specialist calculation. Loft thickness must exceed zero.
Figures checked .
Frequently Asked Questions
What's the difference between R-value and U-value?
R-value (m²K/W) measures how much a layer resists heat flow — higher is better. U-value (W/m²K) measures how much heat passes through a whole element per degree of temperature difference — lower is better. A complete U-value also needs surface resistances and construction corrections; the loft output here is only 1 ÷ insulation-layer R.
Is a thinner, lower-conductivity material always the better choice?
Not necessarily. PIR/PUR board typically has a lower λ (around 0.023 W/mK) than mineral wool (around 0.040 W/mK), so it needs less thickness for the same R-value — useful where space is tight, such as internal wall lining. But mineral wool is usually cheaper per m² and easier to fit around irregular joist spacing, so the "better" material depends on the job, not just the number. See the insulation cost comparison for price and payback.
Can I mix two different insulation materials in the same location?
Thermally, yes — resistances of layers in series simply add together, so an existing layer of one material plus a new layer of another gives a combined R-value equal to the sum. This calculator compares two materials against each other rather than modelling them stacked together, but the Breakdown section shows the arithmetic you'd use to add a second, stacked layer's R-value yourself.
Why doesn't this tool say whether my wall insulation meets Building Regs?
Neither wall nor floor results prove compliance: this model omits bridging, ground geometry and construction details. Its 40% and 70% improvement bands are CoryVu conventions. Loft results compare resistance with a 270mm mineral-wool reference, not a whole-roof calculation. Ask a qualified designer or building control about your project.
Sources
- Kingspan — thermal resistance calculation (thickness in metres divided by conductivity)
- gov.uk — Approved Document L: Conservation of fuel and power, Volume 1: Dwellings
- CoryVu — Diall XPS Underlay U-Value Calculator
- CoryVu — Home Insulation Comparison: Cost & Payback
Last updated: 2026-09-25. This page gives an estimate only and is not legal, tax, financial, or building-control advice.
About this tool
Created and maintained by CoryVu. Read how we check our tools and consult the sources and assumptions on this page.
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