Key risks of Retrofitting

There can be a number of detrimental risks occurring from poorly detailed and/or inappropriate energy efficiency measures. This can range from affecting the occupant’s health and deterioration of the building fabric. They can be interrelated and hence exacerbated.  

Moisture and damp

Traditional building materials allow the movement of moisture and have hydrothermal properties.  If there are not allowances for this when making energy efficiency measures, there is the risk of a build-up of condensation and trapping moisture in the existing historic fabric, which can lead to damp and deterioration. In extreme cases, this can impact the structural integrity of the building.

Natural air leakage in buildings and the properties of the traditional construction materials manage the moisture movement through the building. When considering draught-proofing or installing insulation there must be an understanding on the probable effect of this so there is not a build-up of moisture. 

Relative humidity can influence indoor air quality and is subject to moisture levels and air temperature. Relative humidity can affect human health, thermal comfort and building health, particularly as there is an increased potential for pests and dust mites.

Airtightness and Ventilation

Many energy efficiency measures can have impacts on the ventilation of a building. Ventilation in traditional buildings can happen in a number of ways that manages moisture levels, including controlled ventilation, features such as chimneys, as well as infiltration and air flow through vapour permeable building fabric. Natural ventilation is affected by wind speed and the building’s location and orientation. As noted above, a combination of poor ventilation and moisture can result in detrimental risks to a building and its occupants. This includes the deterioration of the fabric causing mould, thereby harming the occupants’ health through poor indoor air quality.

Building Regulations Approved Document F paragraph 3.2 sets out ventilation requirements and considerations when carrying out works in existing dwellings:

  1. replacing a window or door
  2. doing energy efficiency work the ventilation of the dwelling should either:
  3. meet the standards in the relevant approved document
  4. not be less satisfactory than before the work was carried out.

NOTE: Ventilation through infiltration should be considered to be part of the ventilation provision of a dwelling. Reducing infiltration might reduce the indoor air quality of the dwelling below the standards given in Appendix B.

Balancing ventilation is important and can be complex, particularly if the building performance is not understood. Un-controlled ventilation can impact on the energy efficiency of the building and thermal comfort of the occupants, whereas too little ventilation can result in damp and overheating.   

Rebound and performance gap

Understanding the objective of the project should determine the level of retrofit. Not using the Whole House Approach and considering the occupant’s comfort can result in deep retrofit and rebound effects. Rebound effects are largely where, post energy efficiency measures, the occupants want a higher level of comfort, therefore the energy usage and bills can increase. In these instances, if the occupants’ requirements had been understood first, some works to the building may not have been required, and others may have been more appropriately undertaken.       

When predicted energy savings are set out before the works, for instance achieving space heating targets or U-values, and energy efficiency measures are installed, which do not then achieve the predicted energy savings, this is known as the performance gap. There are a number of reasons why this can happen, including poor understanding of the building’s performance pre-works, lack of understanding of the occupant’s requirements, and poor design and/or installation of the energy efficiency measures.                  

Further information on risks

Kate de Selincourt, The risks of retrofit

IHBC, How to deal with retrofit risks

Iain McCaig, Retrofit in Heritage Buildings: Understanding the Risks

Approved Document L, Conservation of fuel and power, Volume 1: Dwellings



Neil May and Chris Sanders, Moisture in buildings: an integrated approach to risk assessment and guidance

STBA, Interstitial/surface Condensation

STBA, Trapped/accumulated moisture

STBA, Thermal Bridges


Airtightness and Ventilation

Ventilation: Approved Document F

CAT, Airtightness and Ventilation

STBA, Sufficient ventilation?

STBA, Overheating

Retrofit Academy, Ventilation guide



Overheating: Approved Document O

Good Homes Alliance, Overheating in Retrofit and Existing Homes – Tool and Guidance

Historic England, Overheating and Historic Buildings

CAT, Keeping your home cool


Performance gap

BRE, Providing better estimates of solid wall insulation savings

Designing Buildings, Performance gap between building design and operation

Page updated: 26/05/2023

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