Free tool
Roof simulator
Build up your roof layer by layer. The simulator works out its U-value, checks the grant threshold and spots condensation risks, using the methods of the European standards.
Start from an example
Winter conditions for condensation 20 °C, 50 % / -5 °C, 80 %
Build-up, layer by layer
From the room to the outside: pick each material and its thickness, then set the order with the arrows.
Heated room
-
Softwood (boarding, sarking boards) 20 mm
-
Vapour barrier sd 18.00 m
-
PIR polyurethane (Utherm Sarking K) 140 mm
-
Breathable roofing underlay (HPV) sd 0.10 m
Ventilated air gap, then the roof covering
U-value of the roof 0.147W/m²·KRoof section
The temperature through the roof, and the dew point of the water vapour passing through it: as long as the dew point stays below the temperature, the vapour does not condense.
- Temperature
- Dew point
- Condensation
Layer details
| No. | Layer | Thickness (mm) | λ (W/m·K) | R (m²·K/W) | sd (m) | Temperature on the outer side (°C) |
|---|---|---|---|---|---|---|
| Surface exchange, inner side | 0.10 | 19.6 | ||||
| 1 | Softwood (boarding, sarking boards) | 20 | 0.130 | 0.15 | 1.00 | 19.1 |
| 2 | Vapour barrier | 18.00 | 19.1 | |||
| 3 | PIR polyurethane (Utherm Sarking K) | 140 | 0.022 | 6.45 | 8.40 | -4.6 |
| 4 | Breathable roofing underlay (HPV) | 0.10 | -4.6 | |||
| Surface exchange, outer side | 0.10 | -5.0 | ||||
| Total | 160 | 6.80 | 27.50 |
How it is worked out
- U-value following EN ISO 6946: surface resistances Rsi 0.10 and Rse 0.04 m²·K/W (heat rising towards the roof). Under a ventilated roof covering, anything beyond the air gap does not count and Rse is 0.10.
- Rafters or joists within a layer: combined method of the standard (average of the upper and lower limits), softwood with λ 0.13 W/m·K.
- Condensation: Glaser method (EN ISO 13788), between the rafters, in a steady winter state. It shows where vapour may condense; it does not work out drying over the year.
- Sarking boards: resistance declared by the manufacturer for each thickness made, as in the simulator on the Services page.
- Not counted: thermal bridges, fixings, workmanship defects, unventilated air gaps, summer comfort.
Where the values come from
- French Th-Bât rules, Materials booklet (housing ministry, 2017), the French version of the values of the European standard EN ISO 10456: plaster, timber, OSB, plywood, μ factor of mineral wool and polyurethane.
- Unilin datasheets (Utherm Sarking K and Utherm Sarking L Comfort) and STEICO (STEICOtherm dry, ACERMI certificate 13/134/897): thicknesses made, declared resistances, μ factor of the wood fibre.
- ACERMI certificates: Isover GR 32 (02/018/100, λ 0.032) and Rockwool Rockmur kraft (02/015/025, λ 0.035).
- French roofing and insulation codes (NF DTU 40.29, 45.10 and 31.2): vapour barrier Sd ≥ 18 m; breathable underlay (HPV) Sd ≤ 0.10 m (class Sd1). Change the sd value to match the datasheet of the product fitted.
- Grant thresholds: French energy-savings sheet BAR-EN-101 (R ≥ 6 for pitched roofs, R ≥ 7 for cold lofts), along with the other grant conditions (RGE contractor, household income).
Indicative results: they replace neither a thermal study nor a roofer’s on-site advice. The exact build-up of your roof is set during the free visit.
Contact
A roofing project? Let’s talk.
Free quote, no obligation. We come out, assess the work and get back to you within 48 hours.
First contact by email through the form on this site.