A guide to PAS 2035 retrofit evaluation

A guide to PAS 2035 retrofit evaluation

PAS 2035 is the over-arching document in the retrofit standards framework which was introduced following the recommendations of the Each Home Counts review. Part of the PAS 2035 process requires that the monitoring and evaluation of the retrofit is carried out in order to claim compliance. Three different ‘levels’ of retrofit evaluation are required under PAS 2035, namely basic, intermediate or advanced, depending on the performance of the individual retrofit project.

Basic monitoring and evaluation

Under PAS 2035, basic monitoring includes the use of measures-specific questionnaire distributed to the Client and the occupant(s) (if different) of the dwellings that have been retrofitted to establish areas of interest. The aim of the questionnaire is to establish:

Basic monitoring and evaluation shall be completed and reported within three months after handover of the retrofit project. This is the least invasive and relies upon the opinions of the occupants to pick up on any underlying issues with the retrofit. This is carried out for every project wishing to claim compliance with PAS 2035. It does not involve the use of any monitoring equipment to provide data to back up claims.


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basic monitoring

Intermediate monitoring and evaluation

Intermediate monitoring and evaluation is the next stage, used where basic monitoring and evaluation indicates that the outcomes are significantly different from those originally agreed, or there are unintended consequences of the retrofit work.

More thorough investigative measures are utilised at the intermediate level, ranging from inspections of the Energy Efficiency Measures (EEMs) that have been installed to use of verification equipment. Notably post-installation air-tightness testing is to be done to confirm that the property is properly sealed. Recording the internal temperatures and relative humidity through a monitoring period can pick up on any potential issues for occupant comfort, as well as indicating if there is a risk of mould. Identifying any occupancy factors that might be contributing to poor outcomes (such as incorrect operation of heating systems, window-opening patterns etc.) is more difficult, but has a marked effect on the energy performance of a home if not properly considered by the occupants.  

Intermediate monitoring and evaluation shall be completed and reported within six months after basic monitoring and evaluation have been completed, or as soon as seasonal weather conditions allow.

intermediate monitoring

Advanced monitoring and evaluation

Advanced monitoring and evaluation is the final, most thorough level, and includes all the elements of intermediate monitoring as well as further investigative measures. Thermographic surveys will identify areas of significant heat loss from the dwelling (e.g. from poorly sealed jambs), and sub-metering of energy use by new building service systems will identify any potential reasons as to why energy use has changed. A post-construction review will be carried out to confirm exactly what was installed in the property, and to determine if the installation is consistent with retrofit design. Investigative monitoring of internal conditions (including temperature, relative humidity and CO2 concentration) for at least one year, combined with monitoring of moisture levels within the building fabric and/or at the interfaces between insulation and masonry will be used to investigate occupant comfort, and identify where (if any) mould issues are arising from. Investigation of any defects identified by monitoring must be done to ensure they are properly addressed. A post-occupancy evaluation, consisting of two detailed questionnaire-based interviews with occupant(s) are carried out at one year intervals, to ensure the results for the occupants are achieved at a longer timescale.

Advanced monitoring and evaluation shall be completed within two years after basic monitoring and evaluation has been completed.

advanced monitoring


The evaluation of retrofit projects required by PAS 2035 is carried out to determine whether the intended outcomes of the retrofit project have been realised, and to identify and learn from any issues raised. Three levels of monitoring and evaluation are specified, with each level being more intrusive investigation than the last and only being triggered if issues have been identified. Evaluating the performance of retrofit projects and, importantly, learning from them is a crucial step in addressing the energy performance of our existing housing stock. Only with effective retrofit of our existing buildings can the ambitious UK target of net zero by 2050 even be considered.

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Article published 16/11/21

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