Thermal Index Sheets

Thermal Index values for the quantitative analysis of thermal imaging building surveys.

Thermal Index Sheet: 0.75

A simple Thermal Index sheet for quickly determining minimum internal temperatures.

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Thermal Index Calculator

Download our Excel spreadsheet for modifying the Thermal Index value.

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What is a thermal index?

The Thermal Index (TI) is a fixed value used for determining the risk of surface condensation or mould growth within an area. As an industry standard, a Thermal Index value of 0.75 is recommended to provide a consistent guideline for temperature assessment.

Thermal Indexing is used to define a quantitative pass/fail criteria that can be used to determine whether a thermal anomaly within a building fails to meet its design intent for thermal performance.

Thermal Index Calculation Example

The Thermal Index (TI) value is used when calculating the Minimum Internal Surface Temperature using the formulae below, and using it as a guideline for temperature measurement.

Minimal Internal Surface Temperature = (Thermal Index x (Internal Temperature – External Temperature)) + External Temperature

In practice, we know that the Thermal Index value will be 0.75, and the internal and external temperatures will be recorded during the survey.

For example, if during a building survey the internal temperature was 20ºC and the external temperature was 10ºC, the formulae would work as follows:

Minimal Internal Surface Temperature = ( 0.75 x ( 20 – 10 ) ) + 10

Minimal Internal Surface Temperature = 17.5 (ºC)

In this example, this would mean that if any area within the building measured less than 17.5ºC with a thermal imaging camera, it would “fail” according to this criteria.


Thermal Indexing is an advanced method of thermal analysis primarily used by Category 2 Thermographers when undertaking a survey for BREEAM Credit.

For basic data collection by a Category 1 Thermographer, Thermal Indexing is not a requirement.