R-Value Calculator: Working Out Insulation Thermal Resistance

Summary

R-value and U-value are two sides of the same calculation, but they answer different questions. U-value (covered in detail in u value calculator) tells you whether a finished wall, roof, or floor build-up meets a Building Regulations Part L target. R-value tells you how much resistance to heat flow a single material or layer provides — which is the number on the back of a roll of mineral wool, the spec sheet for a PIR board, or the figure you need when working out "how much more insulation do I need to add" or "can I swap this product for that one and get the same performance."

For a tradesperson on site, R-value calculations come up more often than full U-value calculations: topping up loft insulation, deciding whether a thinner PIR board can replace a thicker mineral wool layer in a stud cavity, working out how many layers of a given product are needed to hit a target, or explaining to a customer why "thicker" isn't always "better value" once you compare pound-per-R-value across products.

This article covers the R-value formula, how R-values combine across layers (the part most people get wrong), how to convert between R and U, and three fully worked examples using real UK insulation products and real numbers. For full wall/roof/floor build-ups against Part L targets, see u value calculator; for loft-specific depth tables, see loft insulation depth.

Key Facts

• R-value formula — R = d ÷ λ, where d = thickness in metres, λ = thermal conductivity in W/mK. Units: m²K/W.
• Higher R = better insulator — R-value is a resistance; you want more of it. This is the opposite sense to U-value, where lower is better.
• λ (lambda) is a material property — fixed per material/product, independent of thickness; always use the manufacturer's declared or BBA-certified λ, never a generic textbook figure
• R-values sum in series — for layers stacked in the direction of heat flow (which is how almost all insulation build-ups work), R_total = R1 + R2 + R3 + ... This is the calculation that makes "topping up" and "layer swapping" straightforward.
• U-value is the reciprocal of total R — U = 1 ÷ R_total (including surface resistances Rsi and Rse)
• Surface resistances — internal (Rsi) 0.13 m²K/W, external (Rse) 0.04 m²K/W per BS EN ISO 6946:2017 — always include both when converting a build-up's R-value to a compliance U-value
• Typical λ values — PIR board: 0.022–0.023 W/mK; mineral wool (glass/rock): 0.034–0.044 W/mK; EPS: 0.030–0.038 W/mK; XPS: 0.033–0.038 W/mK; woodfibre: 0.038–0.050 W/mK; sheep's wool: 0.038–0.040 W/mK; blown cellulose: 0.038 W/mK
• Common retail packaging figures — many loft roll and slab products are labelled with a nominal R-value for a given thickness (e.g. "100mm roll, R 2.65") rather than requiring you to calculate from λ yourself — but always check this matches the declared λ, as packaging figures can round generously
• Air gaps have an R-value too — an unventilated cavity of 25mm or more contributes approximately 0.18 m²K/W; a ventilated cavity behind cladding is treated as part of the external environment, not counted as R
• R-value is additive but not simply proportional across products — doubling the thickness of the same product doubles its R-value (R = d/λ is linear in d), but you cannot assume the same relationship holds when comparing two different materials — always calculate each material's R separately
• Compression reduces effective R — mineral wool and other fibrous insulation compressed under boarding or foot traffic has reduced effective thickness and therefore reduced R-value, even though the labelled thickness is unchanged
• Moisture reduces R-value — wet insulation conducts heat far better than dry insulation; a damp loft or a cavity wall with failed insulation performs significantly worse than its rated R-value
• BR 443 (BRE) — the UK reference document for U-value calculation conventions, including how to handle bridged and inhomogeneous layers where simple R-addition doesn't apply directly
• Cost-per-R-value comparison — when comparing products, divide the cost per m² by the R-value achieved to get a genuine like-for-like cost comparison, not cost per mm or cost per roll

Quick Reference Table — R-Value per 100mm for Common UK Insulation Materials

Detailed Guidance

The core calculation — R = d ÷ λ

Take the thickness in metres and divide by the material's thermal conductivity:

R = d (m) ÷ λ (W/mK)

Example: 120mm PIR board with λ = 0.022 W/mK:

• d = 0.120m
• R = 0.120 ÷ 0.022 = 5.45 m²K/W

The same calculation for 200mm of standard mineral wool at λ = 0.040 W/mK:

• d = 0.200m
• R = 0.200 ÷ 0.040 = 5.00 m²K/W

This immediately shows why PIR is specified where depth is limited (loft hatches, floor build-ups over doorways, dormer cheeks) — it delivers a similar R-value in roughly 40% less thickness, at a materially higher cost per m².

How R-values combine — the calculation most people get wrong

R-values in a build-up add in series, layer by layer, in the direction heat is flowing through them:

R_total = R_layer1 + R_layer2 + R_layer3 + ... (+ Rsi + Rse if converting to U-value)

This is different from λ, which does NOT average or combine directly across different materials — you must calculate each layer's individual R first, then add the R-values together. A common site error is trying to average two λ values before calculating R; this gives the wrong answer unless the two layers happen to be the same thickness.

Worked example — topping up existing loft insulation

A loft currently has 100mm of old mineral wool (λ = 0.044 W/mK, a lower-grade older product). The homeowner wants to know how much additional insulation, laid crosswise over the joists, is needed to bring the loft up to the Part L1B renovation target of 0.16 W/m²K (see loft insulation depth for the full target table).

Step 1 — calculate existing R: R_existing = 0.100 ÷ 0.044 = 2.27 m²K/W

Step 2 — calculate the R needed to hit the target U-value: Target U = 0.16 W/m²K → R_total required = 1 ÷ 0.16 = 6.25 m²K/W Subtract surface resistances and the plasterboard ceiling (12.5mm, λ = 0.21, R = 0.06): R_insulation_required = 6.25 − 0.13 (Rsi) − 0.04 (Rse) − 0.06 (plasterboard) = 6.02 m²K/W

Step 3 — subtract the existing insulation's R: R_still_needed = 6.02 − 2.27 = 3.75 m²K/W

Step 4 — convert to a thickness of the new product being used to top up. If topping up with a premium glass wool roll at λ = 0.035 W/mK: d = R × λ = 3.75 × 0.035 = 0.131m = 131mm, round up to the next standard roll thickness — 150mm, which is standard stock for a cross-laid top-up roll.

Result: lay 150mm of premium glass wool crosswise over the existing 100mm of old mineral wool (crosswise both to cover the joists themselves, which are otherwise a thermal bridge, and to avoid compressing the existing layer). This combined build-up comfortably exceeds the 0.16 W/m²K target.

Worked example — matching R-value when substituting a different product

A stud wall cavity is designed for 100mm of standard mineral wool (λ = 0.040 W/mK, R = 2.50 m²K/W), but the merchant is out of stock and the only mineral wool available is a slightly different grade at λ = 0.044 W/mK. The customer doesn't want to lose thermal performance. What thickness of the substitute product is needed to match the original R-value?

Step 1 — target R to match: 2.50 m²K/W (from the original spec)

Step 2 — rearrange R = d ÷ λ to solve for d: d = R × λ = 2.50 × 0.044 = 0.110m (110mm)

Since the stud cavity is 100mm, the substitute product at a slightly worse λ cannot quite match the original R-value within the same 100mm cavity (it would only achieve R = 0.100 ÷ 0.044 = 2.27 m²K/W, roughly 9% less than the original spec). In practice, either accept the small shortfall, use a batt product designed to be compressed slightly into a service void, or switch to a higher-performance product (e.g. a premium glass wool at λ = 0.032, which gives R = 0.100 ÷ 0.032 = 3.13 m²K/W — comfortably exceeding the original spec in the same 100mm).

Worked example — cost-per-R-value comparison for a customer quote

A customer is choosing between two loft insulation options and asks which is "better value." Real indicative UK prices (check current supplier pricing before quoting):

Both options deliver a near-identical R-value (and therefore near-identical thermal performance and running-cost saving), but the standard product is cheaper per unit of thermal resistance achieved — the premium product's advantage is the reduced thickness (200mm vs 270mm), which matters where loft headroom, joist depth, or hatch clearance is restricted, not where performance-per-pound is the only criterion. Present both figures to the customer rather than defaulting to "premium is better."

When simple R-addition breaks down

The R = d/λ and R_total = ΣR method assumes homogeneous layers with heat flow perpendicular to the layer (the "combined method" in BS EN ISO 6946). It does not directly apply where:

• A layer is bridged by a different material — e.g. timber studs (λ ≈ 0.13 W/mK) running through mineral wool insulation (λ ≈ 0.035–0.044 W/mK) in the same plane. The bridged sections have much lower R than the insulation-only sections, and the wall's effective R/U must be calculated using the upper/lower resistance limit method in BR 443, not a simple weighted average.
• The insulation is compressed — a batt rated for 100mm compressed to 70mm under boarding does not deliver the 100mm R-value; use the compressed thickness in the calculation, not the nominal thickness.
• Ground floors — heat loss through the ground follows BS EN ISO 13370, not the simple series-resistance method, because of the perimeter-to-area effect; see u value calculator for the simplified approach and when to use full software.

Frequently Asked Questions

What's the difference between R-value and U-value?

R-value (thermal resistance) measures how much a layer or build-up resists heat flow — higher is better, units m²K/W. U-value (thermal transmittance) is the reciprocal of the total R-value including surface resistances — lower is better, units W/m²K. UK Building Regulations Part L set targets in U-value; insulation products are more often compared and specified by R-value because it's the additive, layer-by-layer figure.

If a roll of insulation says "R 2.65" on the packaging, is that the whole story?

It's the R-value of that product at that specific thickness, tested to a declared λ. It does not account for surface resistances, other layers in the build-up, compression once installed, or thermal bridging from structural elements. Use the packaging R-value as one input into the full calculation, not as the final answer for whether a wall or roof complies with Part L.

Can I just add up the R-values on the packaging of two different products to get my total?

Yes — this is exactly how R-values are meant to be combined, provided both products are correctly installed at their stated thickness (no compression) and are genuinely in series in the direction of heat flow. This is the calculation used in the loft top-up worked example above.

Why does PIR cost more than mineral wool if they can achieve the same R-value?

PIR's lower λ means it achieves a given R-value in roughly 40-45% less thickness than standard mineral wool, which matters where space is constrained (flat roof upstands, floor build-ups, loft hatches, dormer cheeks). Per unit of R-value achieved, PIR is usually more expensive than mineral wool — the premium buys thinner construction, not necessarily better ultimate performance per pound spent.

Does R-value account for air leakage (draughts)?

No. R-value and U-value are both steady-state conductive heat loss calculations — they say nothing about air leakage through gaps, poorly sealed loft hatches, or uncontrolled ventilation. A well-insulated but draughty building can still lose significant heat that no R-value calculation captures; airtightness is assessed separately (air permeability testing, Part L requirement for new dwellings).

Regulations & Standards

• BS EN ISO 6946:2017 — Building components and building elements — thermal resistance and thermal transmittance — calculation methods; defines the R = d/λ combined method and standard surface resistances

• BR 443 (BRE, 2019) — Conventions for U-value calculations in the UK; the authoritative reference for bridged layers and non-standard build-ups

• BS EN ISO 13370:2017 — Heat transfer via the ground; required for ground floor calculations where simple R-addition does not apply

• Building Regulations Approved Document L (2021 edition) — sets U-value targets that the R-value calculation ultimately feeds into

• BBA Agrément Certificates — the certified source for declared λ-values of proprietary insulation products; always use the declared value over a generic table figure for a formal calculation

• BR 443 — BRE Group — Conventions for U-value calculations

• Building Regulations Approved Document L (2021) — GOV.UK official publication

• Kingspan Insulation — technical λ values and R-value data — manufacturer BBA-certified thermal data

• Knauf Insulation — technical data sheets — manufacturer λ and R-value data for mineral/glass wool products

• u value calculator — full multi-layer U-value calculation method and Part L target values, worked wall/roof/floor examples

• loft insulation depth — loft-specific depth tables by material and target U-value

• cavity wall insulation types — mineral wool vs EPS bead vs phenolic full-fill comparison for cavity walls

• natural insulation materials — sheep's wool, hemp fibre, wood fibre, cellulose and cork λ-value comparison