Roof U-Value
Calculator
Get the U-value of a roof assembly from its insulation and the other layers. Roofs use the upward heat-flow interior surface resistance (Rsi = 0.10), lower than walls.
Who it's for: architects and energy designers checking a flat or pitched roof build-up against thermal targets — enter insulation thickness and conductivity plus the combined resistance of the other roof layers at design development.
How to use this calculator
The formula
R_insulation = d (m) ÷ λ
U = 1 ÷ (Rsi + R_insulation + R_other + Rse)
d — insulation thickness in metres (enter in mm; the tool divides by 1 000)
λ — thermal conductivity of the insulation (W/m·K), from the product datasheet
Rsi — interior surface resistance: 0.10 m²K/W (ISO 6946, upward heat flow)
Rse — exterior surface resistance: 0.04 m²K/W (ISO 6946)
R_other — combined resistance of deck, membrane, ceiling lining, and any air gap
Roofs use Rsi = 0.10 (not the 0.13 used for walls) because heat flowing upward creates a thinner, less insulating air film at the interior face.
Worked example
When this estimate will be off
- Uses a simple series build-up. Thermal bridges through structural upstands, steel fixings, or penetrations are not modelled — the real whole-roof U-value will be higher than the result shown.
- Surface resistances are fixed at ISO 6946 upward-flow values (Rsi 0.10, Rse 0.04). Floor and ceiling assemblies under downward heat flow use different surface resistances and need a separate calculation.
- The "other layers R" field combines all non-insulation layers into one figure. If those layers are uncertain, the accuracy of the result is limited by that estimate.
- Assumes uniform insulation depth. Tapered insulation designed for drainage varies in thickness — use the area-weighted average depth, not the minimum, for a representative result.
Frequently asked questions
A common mistake: Uses a simple series build-up. Thermal bridges through structural upstands, steel fixings, or penetrations are not modelled — the real whole-roof U-value will be higher than the result shown.
The interior surface resistance depends on heat-flow direction. ISO 6946 specifies Rsi = 0.10 m²K/W for upward flow (roofs), 0.13 m²K/W for horizontal flow (walls), and 0.17 m²K/W for downward flow (floors). Heat rising toward a roof surface creates a thinner, less insulating air film, so Rsi is lower — making the total R slightly smaller and the U-value slightly higher than a wall with the same layers.
UK Building Regulations Part L 2021 targets new-build flat roofs at 0.15 W/m²K; many EU energy codes sit in the 0.15–0.20 W/m²K range. The Passive House standard requires 0.15 W/m²K or lower in most Central European climates. Retrofit targets are typically relaxed to 0.18–0.25 W/m²K where full replacement is not feasible.
Add the R-value of every non-insulation layer: concrete or metal deck, waterproof membrane, ceiling lining, and any enclosed air cavity. A metal deck + single-ply membrane + plasterboard ceiling typically totals 0.25–0.40 m²K/W. ISO 6946 Annex B gives fixed R values for enclosed and strongly ventilated air layers. For a quick check, 0.30 m²K/W is a reasonable default for a typical flat-roof make-up.
Not for this calculator — it sums resistances in series regardless of layer order. In a warm roof the insulation sits above the deck; in a cold roof it sits below. What changes is the risk of interstitial condensation, not the U-value arithmetic. Enter all layer resistances in the relevant fields; position within the build-up does not affect the total R or the resulting U-value.
Sources
- ISO 6946:2017 — Building components and building elements — Thermal resistance and thermal transmittance — upward heat-flow surface resistance values (Rsi 0.10, Rse 0.04) and series calculation method used here
Sources and review record
Sources and method basis
- ISO 6946:2017 homogeneous-layer series resistance method
- https://www.iso.org/standards.html
Limitations
- One homogeneous insulation layer plus user-supplied other resistance only; tapered/repeating construction, bridges, cavities, corrections, penetrations, and compliance are excluded.
This free Roof U-Value Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.