This U-value calculator is free to use, with no account and no password. It runs entirely in your browser; enter your name and email to reveal your result, and we can send you a copy.
The tool builds a construction one layer at a time, from the inside face outwards, and returns the thermal transmittance of the whole element in W/m²K. It covers external walls, party walls, pitched roofs insulated at ceiling or at rafter level, warm and inverted flat roofs, solid and suspended ground floors, exposed soffit floors, and floors over unheated space. Each element type sets its own heat-flow direction and its own surface resistances, because a U-value is not a property of the materials alone.
It is built for the stage where the build-up is being decided rather than checked. A material library with editable conductivity values, a bridging picker, and a solver that works backwards from a target U-value are all there so that the specification can move while the number stays honest.
Every field starts blank. Nothing is assumed, and no result appears until the checklist is complete and you press Generate result. There is a step by step walkthrough of every field in the U-value calculator guide.
The calculation follows BS EN ISO 6946. Heat passing through a construction meets a series of resistances: the internal surface resistance, then each material layer in turn, then the external surface resistance. Add them up and you get the total thermal resistance. The U-value is its reciprocal.
A material layer contributes its thickness divided by its thermal conductivity. An unventilated air cavity contributes a tabulated resistance that depends on the cavity thickness, the direction of heat flow, and whether the cavity faces are low emissivity. A slightly ventilated cavity contributes half that value, capped at 0.15 m²K/W. A well ventilated cavity contributes nothing at all: the standard excludes it, excludes every layer outboard of it, and sets the external surface resistance equal to the internal one. The tool applies all three rules and tells you when it has done so.
Real constructions are rarely uniform. A timber stud, a joist, a batten or a mortar joint bridges the insulation and carries heat faster than the material around it. ISO 6946 handles this with the combined method: calculate an upper limit of resistance by treating each thermal path in parallel, calculate a lower limit by combining the bridged layers in series, and take the average of the two. The tool enumerates every path through the build-up rather than approximating, and it reports both limits alongside the answer.
The combined method has a validity boundary. When the ratio of the upper limit to the lower limit exceeds 1.5, the averaging is no longer reliable and ISO 10211 numerical modelling is required instead. The calculator computes that ratio, shows it, and warns you when you cross the line rather than quietly returning a number that cannot be submitted.
Three corrections from Annex F are then added. Air voids in the insulation layer add a penalty scaled by the square of the insulation resistance over the total. Mechanical fasteners penetrating the insulation add a term built from the fastener conductivity, cross-sectional area and count. An inverted roof adds a rainwater cooling term. Each is reported separately so you can see which one is costing you.
Ground floors are different again, and the tool switches to ISO 13370 for them. There the U-value depends on the characteristic dimension of the floor, the ratio of its area to half its exposed perimeter, because heat leaves a slab sideways through the ground as well as downwards. A solid floor gets an equivalent thickness built from the wall thickness, the soil conductivity and the floor construction resistance. A suspended floor adds the ventilated underfloor void in series, with its own term for the void wall area and the ventilation opening ratio.
The element type is the first and most consequential choice. It fixes the direction of heat flow, which changes the tabulated cavity resistances, and it fixes the surface resistances: 0.13 m²K/W internal for a wall, 0.10 for upward flow through a roof, 0.17 for downward flow through a floor, and 0.04 external in each case. A party wall and a floor over unheated space both get an external resistance equal to the internal one, because neither face sees outdoor air.
Each layer takes a material and a thickness. Conductivity comes from the library but is editable per layer, which is the point: certified product data beats a library default every time, and a submission needs the certified figure. Air cavities take a ventilation condition instead of a conductivity. Insulation products that ship a fixed declared resistance rather than a conductivity are handled as such.
Bridging is set per layer. The picker carries the common repeats, timber studs at 400 and 600 centres, joists and rafters, battens, mortar joints, thin-joint mortar, and steel framing, each with a default area fraction that you can override. Steel framing produces a warning rather than a silent answer: the combined method is not valid for metal bridging, and the standard sends you to Annex D or to tested data.
The corrections panel controls the air-void installation level, the fastener parameters, and the ground geometry. The reference set fixes the target the verdict is measured against. Choosing a target also unlocks the solver, which searches for the insulation thickness that just meets it and tells you plainly when no thickness up to 500 mm will do.
Take a masonry cavity wall: plasterboard on the inside face, a blockwork inner leaf bridged by mortar joints, a partial-fill rigid insulation board bridged by nothing, a residual unventilated cavity, and a brick outer leaf. Load the worked example for an external wall and the tool fills exactly that build-up.
Generate the result and the panel returns the corrected U-value, the uncorrected value before the Annex F terms, the total resistance, the upper and lower resistance limits with the validity ratio between them, each correction term on its own line, and the total build-up thickness. If a target is set, the verdict pill says whether the element meets it and by how much.
The value of seeing all of that at once is that it shows you where to push. If the validity ratio is close to 1.5 the bridging is doing more damage than the insulation thickness is fixing. If the fastener term dominates, a different fixing pattern is worth more than another 20 mm of board.
The reference sets cover the ones that come up most often in this region and in the standards Indonesian practice borrows from. SNI 6389:2020 is included as an indicative envelope figure with an explicit caveat: for a commercial envelope in Indonesia the governing metric is the overall thermal transfer value, not the element U-value, and that is what an authority will ask for. Use the OTTV calculator for the governing check and this tool to build the wall that feeds it.
Permen PUPR 21/2021 covers the green building route. ASHRAE 90.1 climate zone 1A is there because it is the closest mainstream reference for a hot and humid climate. Two UK Approved Document L1 2021 sets are included, limiting and notional, because they are widely used as a stricter benchmark on internationally funded work.
A custom target is available for a project-specific brief. Whichever you choose, the verdict compares the corrected U-value, not the uncorrected one, which is the figure a reviewer will check.
A U-value describes one-dimensional steady-state heat flow through one element. It says nothing about the junctions between elements, and on a well insulated building the junctions are often where most of the heat leaves. Linear thermal transmittance at a corner, a jamb, a sill or a slab edge needs ISO 10211 modelling, and so does any build-up where this tool tells you the validity ratio has been exceeded.
The calculation is also dry and steady. It does not model moisture, and interstitial condensation risk is a separate assessment that can change a build-up completely. It does not model thermal mass, which matters for comfort and for cooling load in a tropical climate even when the U-value is unremarkable. And it does not model air leakage, which is measured, not calculated.
Treat the output as an early-design figure that is defensible in a design review, and as a specification target rather than a submission document. Certified product data and, where the validity ratio is exceeded, numerical modelling are what a submission needs.
This calculator sits alongside the other envelope and comfort tools in the design calculators set. The OTTV calculator takes the element U-values you build here and checks the whole facade against the regulated limit. The natural ventilation calculator covers the other half of a passive strategy, sizing the openings that reduce the cooling load in the first place.
For the reference documents behind the numbers, see the standards library, and for the vocabulary, the terminology reference.
Envelope performance work is part of ALTA Integra's passive design and sustainability practice. If you need the full assessment rather than an element check, thermal bridging modelling, condensation risk, or a Greenship or SNI submission package, talk to the team.
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