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Surface Temperature Factor (fRsi) Calculator (BS EN ISO 13788, BRE IP 1/06)

The surface temperature factor fRsi, from a U-value or a surface temperature, beside the critical factor BRE IP 1/06 sets for the building type.

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A flat wall or roof away from junctions follows from its U-value; a junction needs a surface temperature from a model or a measurement.

For a plane element the factor is 1 − U × Rsi exactly. At a junction, a lintel or a window reveal the heat flow is two- or three-dimensional, and the factor comes from a model to BS EN ISO 10211 — which reports a surface temperature, or the factor itself — or from a surface temperature read on site.

The element's thermal transmittance, surface resistances included.

A wall or roof calculated to BS EN ISO 6946, or a window's declared or NFRC-rated figure. The box follows the unit switch.

The surface film the factor is worked with — larger than the one used for heat loss, on purpose.

BS EN ISO 13788 takes 0.25 m²K/W (1.42 hr·ft²·°F/BTU) on opaque surfaces for mould and condensation, to allow for corners, furniture and curtains; for windows and doors it keeps 0.13 (0.74) with horizontal heat flow. A film coefficient quoted as 8 W/m²K is a resistance of 1 ÷ 8 = 0.125 m²K/W.

The room's temperature — strictly the operative temperature, the mean of the air and the surrounding surfaces.

Twenty degrees Celsius (68 °F) is the usual design figure for a heated room; it only sets the surface temperature the page reports, not the factor itself.

The outdoor air temperature for the day or month being assessed.

For a design check, the local design external temperature; for a diagnosis, the temperature when the surface was read.

BRE IP 1/06 sets a critical factor by the humidity the building's use generates.

The wetter the use, the warmer a surface has to stay to keep its surface humidity below the level at which mould can grow, so the critical factor rises from storage buildings to swimming pools.

Temperature factor fRsi

0.925 ratio

High confidence

For a plane element the factor is 1 − U × Rsi, whatever the weather: the temperatures only place the surface temperature on the day. The factor is at or above the 0.75 BRE IP 1/06 gives for dwellings, residential buildings and schools.

Surface temperature at these air temperatures
65.3 °F
Critical factor for this building type (BRE IP 1/06)
0.75
Surface temperature the critical factor corresponds to
59 °F
Then change the inputs to see how far the answer moves.

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How this was calculated

Formula source(s)

  • BS EN ISO 13788:2012, 3.1.2: fRsi = (θsi − θe) ÷ (θi − θe), calculated with an internal surface resistance Rsi; for a plane element fRsi = 1 − U × Rsi
  • BS EN ISO 13788:2012, 4.4.1: Rsi = 0.25 m²K/W for condensation or mould growth on opaque surfaces, representing corners, furniture, curtains or suspended ceilings; for windows and doors the values of Table 2 (0.10 up, 0.13 horizontal, 0.17 down)
  • BRE Information Paper IP 1/06, Assessing the effects of thermal bridging at junctions and around openings — critical temperature factors: storage 0.30; offices and retail 0.50; dwellings, residential buildings and schools 0.75; sports halls, kitchens and canteens 0.80; swimming pools, laundries and breweries 0.90

Inputs used

What You Know About the Surface
A plane element's U-value
U-Value of the Element
0.05 BTU/(hr·ft²·°F)
Internal Surface Resistance Rsi
1.42 hr·ft²·°F/BTU
Internal Temperature
68 °F
External Temperature
32 °F
Internal Surface Temperature
59 °F
Building Type (for the Critical Factor)
Dwellings, residential buildings and schools

Intermediate steps

Surface temperature at these air temperatures
65.3 °F
Critical factor for this building type (BRE IP 1/06)
0.75
Surface temperature the critical factor corresponds to
59 °F
Final result0.93 ratio

Confidence note: For a plane element the factor is 1 − U × Rsi, whatever the weather: the temperatures only place the surface temperature on the day. The factor is at or above the 0.75 BRE IP 1/06 gives for dwellings, residential buildings and schools.

What this calculation does not cover

  • From a U-value the factor holds for a plane element only. Junctions, lintels, window reveals and corners conduct in two or three dimensions and need a model to BS EN ISO 10211, or a measured surface temperature.
  • The critical factors are BRE's, derived for the internal humidity each building type is expected to generate; a building used more humidly than its type needs its own assessment to BS EN ISO 13788.
  • The factor is about the surface: it says nothing about condensation between the layers of the construction, which BS EN ISO 13788's interstitial method assesses separately.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-22 · v1.0.0

Regulatory standards & verification citations3
  1. BS EN ISO 13788:2012, 3.1.2: fRsi = (θsi − θe) ÷ (θi − θe), calculated with an internal surface resistance Rsi; for a plane element fRsi = 1 − U × Rsi
  2. BS EN ISO 13788:2012, 4.4.1: Rsi = 0.25 m²K/W for condensation or mould growth on opaque surfaces, representing corners, furniture, curtains or suspended ceilings; for windows and doors the values of Table 2 (0.10 up, 0.13 horizontal, 0.17 down)
  3. BRE Information Paper IP 1/06, Assessing the effects of thermal bridging at junctions and around openings — critical temperature factors: storage 0.30; offices and retail 0.50; dwellings, residential buildings and schools 0.75; sports halls, kitchens and canteens 0.80; swimming pools, laundries and breweries 0.90

Which documents these citations point at

Standards referenced: ISO 13788 (International Organization for Standardization, International).

Cite this page

Your workspace

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Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

How to calculate surface temperature factor (fRsi) (BS EN ISO 13788, BRE IP 1/06) in 8 steps

  1. What You Know About the SurfaceA flat wall or roof away from junctions follows from its U-value; a junction needs a surface temperature from a model or a measurement.
  2. U-Value of the ElementThe element's thermal transmittance, surface resistances included.
  3. Internal Surface Resistance RsiThe surface film the factor is worked with — larger than the one used for heat loss, on purpose.
  4. Internal TemperatureThe room's temperature — strictly the operative temperature, the mean of the air and the surrounding surfaces.
  5. External TemperatureThe outdoor air temperature for the day or month being assessed.
  6. Internal Surface TemperatureThe coldest point of the surface, read on site or taken from a thermal model.
  7. Building Type (for the Critical Factor)BRE IP 1/06 sets a critical factor by the humidity the building's use generates.
  8. Temperature factor fRsiThe tool computes the temperature factor fRsi from those figures and shows the formula, its sources, and a confidence rating alongside it.

Temperature factor fRsi by U-value of the element

Page defaults, not your figures above.

U-Value of the ElementTemperature factor fRsi (ratio)
0.04 BTU/(hr·ft²·°F)0.943
0.06 BTU/(hr·ft²·°F)0.915
0.08 BTU/(hr·ft²·°F)0.886
0.1 BTU/(hr·ft²·°F)0.858

Frequently asked questions

Why use a factor rather than the surface temperature itself?
Because a surface temperature is only true for the pair of air temperatures it was worked out at, while the factor is a property of the construction. A detail with a factor of 0.75 sits three quarters of the way from the outdoor temperature to the indoor one whatever the weather, so one number describes it on every day of the winter.
Why is the internal surface resistance 0.25 and not 0.13?
Because BS EN ISO 13788 assesses mould and condensation on opaque surfaces with a larger film to represent the still air behind furniture and curtains and in corners, where surfaces are coldest. For windows and doors it keeps the ordinary values — 0.13 m²K/W (0.74 hr·ft²·°F/BTU) with horizontal heat flow.
What does the critical factor mean?
It is the lowest factor at which the surface stays warm enough to keep its humidity below the level mould needs, for the internal humidity BRE assumed for that type of building. BRE IP 1/06 gives 0.75 for dwellings, residential buildings and schools, less for drier buildings and more for wetter ones.
Can I find the factor from a thermal camera?
Yes, with care: read the coldest point of the surface and the room and outdoor temperatures at the same time, on a cold day with a steady difference across the wall. The factor from those three figures describes the detail as it is built, which is what a diagnosis of damp needs.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.