Systems and specification
A spray foam U-value estimator for an existing commercial roof, wall or soffit. Choose the construction and the work; it estimates the U-value with closed-cell foam, or the depth that reaches the target, and sets it against Approved Document L. The method and every assumption are shown beside the result.
Free, no sign-upBS EN ISO 6946 methodApproved Document L comparison
01The calculator
Say what you want to find out, then choose the construction closest to yours and change its layers if yours differ: the U-value at a depth of closed-cell foam, or the depth that reaches a target. The comparison with Approved Document L, the caveat and the assumptions sit beside every result. No sign-up and no personal details.
For guidance only. This calculator gives an idea of the U-value and depth for a typical build-up. It is not a design calculation, and it may not be the depth your building needs. The right depth depends on the construction and its condition, how the building is used, the temperature and humidity inside, the local climate and the condensation risk. We calculate your project with recognised U-value and condensation risk software, to BS EN ISO 6946 and BS EN ISO 13788, with WUFI where the risk needs a closer look, and confirm the depth before we spray.
Choose the construction closest to yours. It fills in the layers for you; change them if yours differ.
Outside at the top, inside at the bottom.
Only if metal fixings pass through the insulation. Leave empty if not.
Enter a number between 0 and 100 per m².
Enter a diameter between 0.1 and 50 mm.
Enter an area between 1 and 100,000 m².
Only used to show heat loss in W/K, before and after.
No climate and no product picker. Section 03 says what the estimate leaves out and why.
02The report
An example of the calculation we run before we spray, based on a real calculation by our team with the project details removed.
This example is a seamed steel roof over a timber deck, with 50 mm of closed-cell spray foam under the deck and mineral wool quilt below it. On your roof the depth is typically 25 mm or 50 mm closed-cell, depending on the roof make-up, confirmed before we spray.
Every layer from the steel sheet down to the paint is drawn and named, so the calculation matches what is actually on the roof.
| Layer, outside to inside | Thickness mm | Conductivity W/mK | Resistance m²K/W |
|---|---|---|---|
| Outside surface resistance Rse | 0.0400 | ||
| 1Seamed steel roof sheet | 2 | 50.000 | 0.0000 |
| 2Slightly ventilated air layer (upward heat flow) | 10 | 0.133 | 0.0752 |
| 3Air and vapour control membrane (outer) | 0.1 | 200.000 | 0.0000 |
| 4Oriented strand board (OSB) deck | 18 | 0.130 | 0.1385 |
| 5Closed-cell spray foam 95% / softwood joists 5% | 50 | 0.027 foam, 0.130 timber (combined 0.032) | 1.5552 |
| 6Mineral wool quilt 95% / softwood joists 5% | 250 | 0.042 wool, 0.130 timber (combined 0.046) | 5.3879 |
| 7Fully bonded vapour barrier | 1.2 | 0.230 | 0.0052 |
| 8Gypsum plasterboard | 12.5 | 0.250 | 0.0500 |
| 9Gypsum plaster skim | 3 | 0.570 | 0.0053 |
| 10Emulsion paint | 0.1 | 0.200 | 0.0005 |
| Inside surface resistance Rsi | 0.1000 |
Total thickness 346.9 mm. Adding the listed resistances gives the lower limit of the combined method in panel 3, 7.35 m²K/W, to the report's rounding.
Each layer's thickness and conductivity give its thermal resistance. Where foam or wool sits between timber joists, both the insulation and the timber are counted.
0.14W/m²K, the roof as built
Averaging the two calculation limits gives a total resistance of 7.41 m²K/W. A small correction for air gaps is added, giving a U-value of 0.14 W/m²K.
In every month the inside surface stays well clear of mould risk: 0.970 against the 0.754 needed in the worst month. Inside the roof, any moisture that forms in winter dries out in summer.
At one interface, the underside of the steel sheet, a trace of condensate forms in winter, 0 g/m² to the nearest gram, and all of it is predicted to evaporate during the summer months. Outside conditions are a 20-year monthly weather record with the BS 5250 cold-year allowance (1 °C colder, 4% more humid); inside 20 °C at 57 to 65% relative humidity.
In January the dew point only meets the temperature at the underside of the steel sheet. Everywhere else the roof stays warmer than its dew point, and the monthly check in panel 4 shows that point dries out each year.
03The method
Approved Document L Volume 2 points to BR 443 for U-value conventions (paragraph 4.1). The calculator adds the resistance of each layer in your list, the air spaces and the surface resistances, takes the reciprocal and adds the corrections. Where timber bridges a layer, it uses the combined method.
| Element | Rsi | Rse |
|---|---|---|
| Roof, heat flow upward | 0.10 | 0.04 |
| Wall, heat flow horizontal | 0.13 | 0.04 |
| Floor over an open space, heat flow downward | 0.17 | 0.04 |
| Material | λ | Published on |
|---|---|---|
| Closed-cell spray foam | 0.027, declared | Closed-cell system data |
| Mineral wool, existing | 0.042 | U-values and Approved Document L |
| Softwood timber | 0.13 | U-values and Approved Document L |
Every other layer in the library carries its own conductivity and source, shown under every result and listed in the printout: steel sheet 50 W/mK, reinforced concrete 2.3 W/mK and OSB deck 0.13 W/mK from BS EN ISO 10456, fibre-cement sheet 0.48 W/mK from the manufacturer's declared value, and so on. Air spaces take the BS EN ISO 6946 Table 8 value for their thickness and direction of heat flow; a slightly ventilated space counts half, and a well ventilated space takes the layers outside it out of the sum, with Rse set equal to Rsi.
Between rafters or joists, heat takes two paths: through the foam and through the timber. The upper bound R′T combines the two paths in parallel; the lower bound R″T uses an equivalent conductivity in each bridged layer. RT is the mean of the two. The timber fraction is the timber width divided by the centres, so 47 mm at 600 mm centres is 7.8% of the area; noggings are not counted. Foam deeper than the timbers, or a layer that runs behind them, is counted as continuous. If R′T ÷ R″T is above 1.5, the combined method does not apply and the calculator says the construction needs a full calculation instead of giving a figure.
The U-value is shown to two decimal places, and the comparison is made on that displayed figure. The depth search compares the unrounded value, so a depth it returns always reaches the target: where a depth gives 0.1815, which would display as 0.18, the calculator returns the next 5 mm step.
Net, the estimate can still read a little better than a full calculation. It is not a project calculation and should not be read as one: the figure for a building comes from our calculation, with the layers and corrections listed. The full calculation, step by step.
04The comparison values
The 2021 edition incorporating 2023 amendments, checked against the published document. A new or replacement element is compared with Table 4.1; a renovated element with Table 4.2, which sets a threshold and an improved value.
| Element | Limiting value |
|---|---|
| Flat roof | 0.18 |
| Pitched roof | 0.16 |
| Wall | 0.26 |
| Floor | 0.18 |
| Element | Threshold | Improved |
|---|---|---|
| Pitched roof at ceiling level | 0.35 | 0.16 |
| Pitched roof at rafter level | 0.35 | 0.18 |
| Flat roof or roof with integral insulation | 0.35 | 0.18 |
| Wall with internal or external insulation | 0.70 | 0.30 |
| Floor | 0.70 | 0.25 |
Sheet roofs and concrete decks are renovated as a flat roof or roof with integral insulation. For new work, a pitched sheet roof is compared with the pitched roof value and a flat one with the flat roof value. A timber pitched roof uses the row for insulation at rafter level.
A result is above the value of its row; at or better than the threshold and above the improved value, for a renovation; or at or below the value of its row. It is an estimate and a comparison, not a compliance position: that belongs to the project's designer and building control.
Paragraph 4.8: if the improved value is not technically, functionally or economically feasible (15-year simple payback), the lowest feasible value applies, generally no worse than 0.7; your designer documents it.
The 2026 edition lists one roof value, 0.18 W/m²K, and applies from 24 March 2027 for most work, subject to transitional rules. This tool uses the current edition.
For a building in Wales, Approved Document L (Wales) applies and its values differ; this tool uses the England values.
Building regulations U-values for commercial buildings sets out the tables in full.
05Examples
Figures from HOLVEM project calculations. A project figure comes from the full calculation of that building; the calculator gives an estimate.
Each figure was calculated for the building, with its own layers and corrections, and is published on its project record. Where the calculator estimate differs, the card says by how much and why.
Project record
about0.53W/m²K
50 mm closed-cell from below. Treated-roof U-value about 0.53 W/m²K, calculated for that build-up.
For the same inputs the calculator estimates 0.50 W/m²K: the full calculation counts the fixings and laps this estimate leaves out unless you enter them.
Project record
135mm closed-cell
Concrete deck on profiled steel decking, sprayed to a 0.18 W/m²K target set at the start.
For the same target the calculator returns about 145 mm: the full calculation counts the fixings and the other layers this estimate leaves out unless you add them.
Project record
0.18W/m²K
Concrete soffits, 120 mm closed-cell. Specified 0.18 W/m²K, achieved.
For the same inputs the calculator estimates 0.21 W/m²K: the full calculation counts the other layers and corrections this estimate leaves out unless you add them.
Published calculation
0.14W/m²K
Metal standing-seam roof: 50 mm closed-cell between the timbers, with 250 mm of mineral wool. U-value 0.14 W/m²K.
With 47 mm timbers at 600 mm centres, the deck, the foam and the wool, the calculator estimates 0.15 W/m²K: the full calculation used the roof's own timber fraction and every layer, membranes and lining included.
Where this page hands off
The method in full, the declared figures for the foam, and how the objective sets the depth before any target does.
Your next step
If you already know the thickness or U-value you need, we price to it and confirm it is right; if not, we work it out with you. Tell us the objective and what you know about the roof; the depth is set by the objective and the calculation, and someone who does this every day comes back to you.
The U-value calculation report
Start a project brief and we will run a full U-value calculation for your building, as a written report. It counts the layers this estimate leaves out, with the bridge included. We confirm the figure by calculation before we spray.