U-value calculator
Build the wall, roof or floor layer by layer and watch the section draw itself — then set a target and get the missing insulation in millimetres, in the material you'd actually use.
In the editor, save a build-up as an assembly: assign it to walls and the heat-loss sheet (Pro) sums U × A over the whole drawn building — every wall, roof, floor and window.
The physics
Each layer resists heat flow by R = thickness ÷ λ (metres over W/m·K).
Resistances add in series, plus fixed surface films on each face — for a wall,
Rsi 0.13 inside and Rse 0.04 outside (EN ISO 6946). Then
U = 1 ÷ ΣR. The default build-up: 20 mm render (R 0.02) + 150 mm mineral
wool (R 4.05) + 200 mm aerated block (R 1.25) + 13 mm plasterboard (R 0.06) + surfaces
0.17 = 5.56 m²K/W, so
U = 0.18 W/m²K.
The insulation-to-target answer inverts the same equation: the resistance still missing is
1 ÷ Utarget − ΣR, and millimetres = missing R × λ × 1000, rounded
up to the next 5 mm.
The section is drawn to scale and hatched by material family the way a construction detail is — diagonals for masonry, cross-hatch for insulation, grain for timber, an empty cavity left empty. That is not decoration: seeing 13 mm of plasterboard beside 150 mm of wool is the fastest way to understand why one layer carries the U-value and the other barely registers.
Common questions
Related: the editor's assemblies & heat-loss sheet apply these same values to a whole drawn building; the paint calculator for the cheaper layer of the same wall.
What is a U-value?
The rate a building element loses heat: watts through one square metre for each degree of temperature difference, W/m²K. Lower is better — a solid brick wall sits around 2.0, a modern insulated wall at 0.15–0.20. Multiply U by area and by the temperature difference and you have the heat flow in watts, which is exactly how a whole-building heat-loss sheet is assembled.
What U-value should I aim for?
The target list runs from 0.30 (an older renovation standard) down to 0.10 (passive house), with 0.20 as a common new-build wall — but those are typical European figures, not any one country's regulation. Codes are national, sometimes regional, and they change, so look yours up and enter it under "Custom" if it is not on the list. The physics (EN ISO 6946 form) is the same everywhere; only the bar moves.
How much insulation do I need to reach a target?
Set the target U and the calculator answers directly: it computes the missing thermal resistance and converts it to millimetres of the insulation you pick. The differences between materials are real — reaching the same target takes roughly 60% more mineral wool than PIR, because λ 0.037 against 0.023 — but any of them gets there with enough thickness.
Can I use a material that isn't in the list?
Yes — pick "Custom material…" at the bottom of any layer's list and type a name and its λ. Use the manufacturer's declared λ from the product datasheet, which is the number that actually applies to what you are buying; the built-in library is deliberately small and generic. A custom layer counts exactly like a library one, is drawn in the section, and travels in the shared link, so you can send someone a build-up using a material we have never heard of.
Are these values certified?
No — they are indicative design values (EN 12524 / ISO 10456 territory) computed in the EN ISO 6946 form with standard surface resistances, good for comparing build-ups and sizing insulation. A certified assessment uses the manufacturer's declared λ for the exact product, moisture corrections and thermal bridging. The same library and the same honesty note apply inside the Gridsnap editor's assemblies.
What about thermal bridges and windows?
This calculates the plain element between bridges — studs, junctions and fixings raise the real average, which is why bridged layers like a timber frame are entered as a composite with an effective λ. Windows and doors have their own certified U-values from the maker (typically 0.8–1.4 for modern units). Gridsnap's heat-loss sheet (Pro) sums U × A over every element and opening of the drawn building.
Flooring & Tile Calculator · Paint Calculator · Stair Calculator · Floor Plan Scale Calculator
One wall is a calculation. A house is a drawing.
Draw the building, assign assemblies, and the whole-building heat loss — every element, every window, per room — assembles itself.