Insulation R-Value & U-Value Calculator
Build a wall, roof or floor layer by layer — R, U and the insulation it needs.
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- Free preview: the U-value or R-value of the element in full, with the first rows of the layer table (up to 3) and a watermarked drawing; every other figure is hidden.
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Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Insulation R-Value & U-Value Calculator
Put together a wall, roof or floor from its layers — brick, block or concrete, plaster, boards, an air gap, the insulation — and see its thermal resistance (R-value) and thermal transmittance (U-value) the way ISO 6946 works them out: each layer’s thickness divided by its conductivity, plus the inside and outside surface resistances for the direction the heat flows. Conductivities come from the table in India’s Eco-Niwas Samhita energy code, or enter a product’s own λ or the R-value printed on its label. Studs or joists through the insulation are included with ISO 6946’s combined method.
The result is in metric (m²K/W, W/m²K) and US units (ft²·°F·h/Btu). Compare it with the ENERGY STAR R-values for your US climate zone or Eco-Niwas Samhita’s U-value limits, and let the calculator find the insulation thickness that reaches a target. An estimate to compare options — not a professional design.
Without a pass, the free preview shows the U-value or R-value of the element in full (the R-value in US units), with the first rows of the layer table and the drawing marked as a preview; the complete result, Copy result and the CSV come with a Pro pass.
How to use it
- Choose what you are building — a wall, a roof keeping heat in, a roof in a hot climate, or a floor — or set the surface resistances yourself.
- Start from a common construction, or add layers from outside to inside: pick each material and type its thickness. Change the conductivity if the product’s datasheet gives another λ, or pick “known R-value” for a product sold by its R-value.
- If studs or joists run through the insulation, tick “Studs” on those layers and enter the share of the area they take (stud width ÷ spacing).
- Pick a target — an ENERGY STAR R-value, an Eco-Niwas Samhita limit or your own U or R — and mark the insulation layer to see the thickness it needs.
- Read R and U, each layer’s share, and the drawing; Copy result or Download CSV — with a pass, or after unlocking this result.
Examples
R = 0.13 + 0.012/0.721 + 0.23/0.85 + 0.012/0.721 + 0.04 = 0.474 m²K/W → U = 2.11 W/m²K (R-2.7 in US units).
R = 0.17 + 0.15/1.58 + 0.012/0.721 + 0.04 = 0.322 m²K/W → U = 3.11 W/m²K, well above Eco-Niwas Samhita’s 1.2 limit.
The EPS adds 0.05/0.035 = 1.43 m²K/W: R = 1.81 m²K/W, U = 0.55 W/m²K. 16 mm of that EPS would just reach U = 1.2.
Between studs R = 2.55, through a stud 0.88 m²K/W; ISO 6946 upper limit 2.15, lower 2.07 → R = 2.11 m²K/W (R-12.0), U = 0.47 W/m²K.
Common uses
- Check whether a roof meets Eco-Niwas Samhita’s U-value limit before you build, and how much insulation it would take.
- Compare an AAC block wall with a brick wall, or a filled cavity with an empty one.
- See how much timber studs take off a batt’s label R-value.
- Get U-values for a heat-loss or cooling-load calculation.
How R and U are worked out (ISO 6946)
- Layer: R = thickness ÷ conductivity (m ÷ W/m·K = m²K/W). A product sold by its R-value is entered as that R.
- Total: R_T = R_si + R₁ + R₂ + … + R_se. The surface resistances: inside 0.13 m²K/W for walls (heat flows sideways), 0.10 when heat flows up through a roof or ceiling, 0.17 when it flows down (a roof in hot weather, a floor); outside 0.04 — the values Eco-Niwas Samhita Table 45 uses.
- U-value = 1 ÷ R_T, in W/m²K: the watts that pass through each square metre for each degree between inside and outside.
- US units: 1 m²K/W = 5.678 ft²·°F·h/Btu, so R 2.5 m²K/W is R-14.2. US product labels give the R of the insulation alone; the assembly R here also counts the other layers and both air films.
- Air layers: an unventilated gap has its own resistance, 0.10–0.21 m²K/W depending on its thickness and the direction of heat flow (Eco-Niwas Samhita Table 47); a ventilated gap does not count.
Studs and joists (bridged layers)
Where timber studs or joists cut through the insulation, heat takes the easier path through the timber. ISO 6946’s combined method works out an upper limit — the wall as two side-by-side walls, one all insulation and one all stud, weighted by area — and a lower limit — each bridged layer averaged first — and takes their mean, with a maximum error of half their difference. The calculator shows all three. The method is not meant for insulation bridged by metal (steel studs, fixings); those need a numerical calculation (ISO 10211).
The stud share is the stud width divided by the spacing — 38 mm studs at 400 mm centres are 9.5% — plus extra for plates, noggings and openings, which make up a larger share in a real wall.
Targets: ENERGY STAR and Eco-Niwas Samhita
ENERGY STAR (for retrofitting existing wood-framed homes, from the 2021 IECC Table R402.1.3) recommends adding attic insulation of R30 in zone 1, R49 in zones 2–3 and R60 in zones 4–8 to an uninsulated attic, or R25 / R38 / R49 over 3–4 inches already there; floors R13 (zones 1–2), R19 (3, 4A and 4B), R30 (4C, 5 and 6) and R38 (7–8). These are R-values of the insulation itself, so the calculator compares them with the layer you mark as the insulation.
Eco-Niwas Samhita (BEE’s energy code for homes in India) limits a roof’s U-value to 1.2 W/m²K in every climate, and in the cold climate the U-value of the walls and the rest of the envelope to 1.8 W/m²K; it rounds the U-value to one decimal place before comparing. Walls in the other four climates are checked with RETV, a heat-gain measure this calculator does not compute.
Sources
- BEE Eco-Niwas Samhita 2024 (Energy Conservation Building Code for residential buildings) — U-value method, Tables 45–47 and the roof and cold-climate limits
- ENERGY STAR — recommended home insulation R-values
- ISO 6946, Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods
Limitations
- An estimate to compare constructions — not a professional design. Real walls lose more at thermal bridges (lintels, slab edges, fixings) than a one-dimensional calculation shows.
- Conductivities are typical table values; a product’s declared λ from its datasheet is better and can be typed in. Wet materials conduct more heat.
- Ground floors and basement walls lose heat through the ground; their U-value depends on the floor’s size and perimeter (ISO 13370), which this layer calculation does not include.
- It does not check condensation inside the wall; a vapour-control layer may be needed in cold climates.
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Frequently asked questions
What do I get without a pass?
Without a pass, Insulation R-Value & U-Value Calculator shows the U-value or R-value of the element in full, with the first rows of the layer table (up to 3) and a watermarked drawing; every other figure is hidden. Until you unlock it, the result can’t be downloaded or copied. A Pro, Premium or Ultimate pass, a one-time payment unless you choose automatic renewal, unlocks the full result. The pricing page lists the passes and their prices.
What is the difference between R-value and U-value?
R-value is thermal resistance — higher is better insulation. U-value is its inverse for the whole element including the surface resistances — lower is better. A wall with R 2.5 m²K/W has U = 0.4 W/m²K: 0.4 watts pass through each square metre for each degree of temperature difference.
How do I convert R-value to US units?
Multiply m²K/W by 5.678 to get ft²·°F·h/Btu: R 1 m²K/W is R-5.7, and R-13 is 2.29 m²K/W. U-values convert the other way: 1 W/m²K is 0.176 Btu/h·ft²·°F.
What is the U-value of a 230 mm brick wall?
With 12 mm of cement plaster on both sides and Eco-Niwas Samhita’s conductivities (brick 0.85, plaster 0.721 W/m·K), R = 0.474 m²K/W and U = 2.11 W/m²K. A 200 mm AAC block wall plastered the same way is about 0.78.
How thick should roof insulation be?
Enough to reach your target. For a 150 mm RCC roof with a 40 mm screed in a hot climate, 16 mm of EPS (λ 0.035) just meets Eco-Niwas Samhita’s U ≤ 1.2, and 50 mm gives U = 0.55. Mark the insulation layer and pick the target — the calculator finds the thickness.
Why is my wall’s R lower than the insulation’s label?
Studs, joists and other bridges let heat bypass the insulation. R-13 batts between 10% timber studs give about R-12 for the whole wall with plywood and plaster; more framing, steel studs or gaps make it worse.
Which surface resistance should I use?
ISO 6946: 0.13 m²K/W inside a wall, 0.10 when heat flows upward (a ceiling in winter), 0.17 when it flows downward (a floor, or a roof under the summer sun), and 0.04 outside. Choose the element and the calculator sets them.