Ireland · Materials & DIY
U-Value Calculator
Enter each layer's thickness and thermal conductivity. The calculator sums the resistances and returns the U-value, with every step shown so a BER assessor or building control officer can follow it.
How is a U-value calculated?
A U-value is the reciprocal of the total thermal resistance of a build-up. Add the inside surface resistance, the resistance of every material layer (thickness in metres divided by its thermal conductivity), any air cavity, and the outside surface resistance. Divide one by that total. A lower U-value means less heat lost per square metre.
How to use this calculator
- Pick the elementWall, roof or floor. This sets the inside and outside surface resistances, which differ because heat moves differently up, down and sideways.
- Enter each layer outside-inThickness in millimetres and the thermal conductivity, lambda, in watts per metre kelvin. Use the manufacturer's declared lambda where you have it — product figures vary more than generic tables suggest.
- Add the cavity if there is oneAn unventilated air gap of 25 mm or more carries a standard resistance. A ventilated or partially filled cavity does not behave the same way, so leave it out and treat the insulation layer on its own.
- Set a target if you are checking complianceType in the U-value your project has to meet and the result tells you whether the build-up clears it and by how much.
How the calculation works
Thermal resistance is additive. Each material layer resists heat flow in proportion to how thick it is and inversely to how well it conducts, so the resistance of a layer is its thickness in metres divided by its lambda value. A 100 mm layer of insulation at 0.022 W/m·K contributes 0.1 ÷ 0.022, or about 4.55 m²K/W. The same thickness of dense concrete block at 1.13 contributes 0.089 — roughly fifty times less.
Two further resistances sit at either face. The inside surface resistance, Rsi, accounts for the still air film against the internal finish. The outside surface resistance, Rse, does the same on the weather side. Standard values are 0.13 and 0.04 for walls, 0.10 and 0.04 for roofs, and 0.17 and 0.04 for floors, following BS EN ISO 6946.
Add everything, then take the reciprocal. That final step is why the arithmetic feels counter-intuitive: doubling your insulation does not halve the U-value, because the other layers and the surface films are still in the total.
U = 1 ÷ (Rsi + Σ(d ÷ λ) + Rcav + Rse)Worked example: a filled cavity wall
Take a wall of 100 mm dense concrete block outer leaf at λ 1.13, 100 mm of rigid PIR insulation at λ 0.022, a 100 mm block inner leaf at λ 1.13, and 13 mm of plasterboard at λ 0.21.
The layer resistances are 0.088, 4.545, 0.088 and 0.062 m²K/W. Adding the wall surface resistances of 0.13 and 0.04 gives a total R of 4.953. One divided by 4.953 is a U-value of 0.20 W/m²K.
Now swap the insulation for 150 mm of the same board. The insulation layer becomes 6.818, the total becomes 7.226, and the U-value falls to 0.14. Fifty per cent more insulation bought a thirty per cent improvement — the diminishing return that catches people out when they specify by thickness rather than by result.
Typical lambda values
Use these as a starting point when you are testing an idea. For anything that will be submitted, use the declared lambda from the product's own technical data sheet, because the spread within a material class is wide.
| Material | Typical λ (W/m·K) |
|---|---|
| Phenolic insulation board | 0.020 – 0.022 |
| PIR / PUR insulation board | 0.022 – 0.028 |
| Expanded polystyrene (EPS) | 0.032 – 0.038 |
| Mineral wool quilt | 0.038 – 0.044 |
| Aircrete block | 0.11 – 0.18 |
| Softwood timber | 0.13 |
| Plasterboard | 0.21 – 0.25 |
| Medium-density concrete block | 0.44 – 0.60 |
| Gypsum plaster | 0.57 |
| Brick, outer leaf | 0.77 |
| Dense concrete block | 1.13 – 1.75 |
| Sand and cement screed | 1.15 |
What the result actually means
A U-value is watts lost per square metre of element for every degree of temperature difference. A wall at 0.20 W/m²K with 20°C inside and 5°C outside loses about three watts per square metre. Across eighty square metres of external wall, that is a quarter of a kilowatt — before you count the roof, the floor, the glazing and the air leakage.
That is why fabric numbers dominate a BER result and why the difference between 0.20 and 0.14 is worth arguing about on a drawing. It is also why chasing the wall alone rarely pays: once one element is good, the heat finds the next weakest path, and thermal bridges at junctions start to matter more than another 50 mm of board.
This calculator handles homogeneous layers. Where a layer is bridged — timber studs through insulation, joists through a ceiling, mortar joints through lightweight blockwork — the real U-value is worse than this simplified sum suggests, sometimes substantially. For a bridged build-up you need the full combined method with an upper and lower resistance limit.
Assumptions and limitations
- Layers are treated as homogeneous and continuous — no correction is applied for timber studs, joists or mortar bridging
- Standard surface resistances from BS EN ISO 6946 are used, not project-specific values for sheltered or exposed sites
- Ground floor U-values here ignore the perimeter-to-area ratio, which materially affects a real floor calculation
- Air cavities are assumed unventilated; a ventilated cavity contributes little and a partially filled one behaves differently again
- No correction is applied for mechanical fixings penetrating the insulation, or for air gaps behind poorly fitted boards
- The result is a design figure. Workmanship on site is the single biggest reason a finished wall underperforms its calculation
Questions people ask about this in Ireland
What is a U-value and how is it calculated for Irish building regulations?
A U-value measures how much heat passes through a square metre of a building element for each degree of temperature difference, in W/m²K. It is calculated as one divided by the total thermal resistance of the build-up, adding the inside surface resistance, each layer's thickness divided by its thermal conductivity, any unventilated cavity, and the outside surface resistance.
Irish projects are assessed against the Technical Guidance Documents to Part L, which set maximum U-values by element. Those figures change between regulation cycles, so enter the target that applies to your project rather than relying on a number quoted in an article.
Is a higher or lower U-value better?
Lower. The U-value is a rate of heat loss, so a smaller number means less heat escaping. This trips people up because most building metrics improve as they rise. If you want a figure that goes the right way, use the R-value — the total resistance — which the calculator also shows.
Why does my calculated U-value differ from the one on the product data sheet?
Because the data sheet quotes a lambda for the insulation on its own, not a U-value for your wall. The finished element also contains blockwork, plaster, cavities and surface films, all of which are in the total. A board with a superb lambda still sits inside a build-up that dilutes it.
Does this calculator handle timber-frame walls?
Only approximately. Timber studs conduct far better than the insulation between them, so a real timber-frame U-value has to be worked out with the full combined method, averaging an upper and lower resistance limit weighted by the fraction of area each path covers. Treating a studded layer as homogeneous will flatter the result, typically by 10 to 20 per cent.
What surface resistances does this use?
The standard values from BS EN ISO 6946: Rsi 0.13 and Rse 0.04 for walls, 0.10 and 0.04 for roofs, and 0.17 and 0.04 for floors. The internal figure differs by element because convection carries heat away from a surface differently depending on whether the flow is horizontal, upward or downward.
Can I use this for a BER assessment?
Use it to test build-ups while you are designing. A BER assessment is produced by a registered assessor using the DEAP methodology, which takes far more than element U-values into account — thermal bridging, air permeability, ventilation strategy, systems and fuel. This tool tells you whether a wall is worth building; it does not produce a rating.
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Method, source and review date
- How it is worked out
- BS EN ISO 6946 combined method, simplified for homogeneous layers
- Formula used
U = 1 ÷ (Rsi + Σ(thickness ÷ λ) + Rcavity + Rse)- Checked against
- BS EN ISO 6946 elemental method
- Last reviewed
- Confidence
- High
Spotted a figure that has moved, or a formula you can show is wrong? Tell us — send a correction. Fixes to rates and formulas are treated as urgent.
Disclaimer
This is a planning estimate, not a substitute for a design signed off by a qualified engineer. Anything structural, electrical or gas-related must be checked and certified by a competent person before it is built or connected.