The greater the temperature difference, the higher will be the rate of heat flow and the more obvious will be those areas of lower resistance. Temperature difference must therefore be taken into account in the interpretation of the thermographic data. Warm air flowing upward from radiators or hot air registers, or cold air washing downward from windows can cause serious distortion of surface temperatures. Stack pressure encourages infiltration on the lower floors of a building and exfiltration on the upper floors. This results in cooling of the walls at the lower levels and warming at the upper levels.
Wind can affect surface temperatures in the same manner as stack pressure.
That is, pressurization will exaggerate defects when they are viewed from the exterior and make them less apparent when viewed from the interior. The opposite occurs when a building is depressurized. Measuring the pressure difference across the enclosure is therefore important. In some cases, buildings may be pressurized or depressurized purposely to exaggerate deficiencies and make them easier to detect. Conduction The rate of conductive heat flow across a building envelope depends on the temperature difference across the envelope and the thermal resistance of the system.