Retrofit Assessment Best Practice – Top 5

What is the Thermal Envelope?

It is essential to identify, record and present (in an easily understandable format) the thermal envelope of the home. Typical challenges include communal corridors outside flats, integrated garages, porches and conservatories. Assessments should identify whether these areas are heated and by what means. Sometimes this may include secondary heating. This will help ensure that the Retrofit Coordinator and Designer can effectively produce the energy model and identify the most effective EEMs.

Measure, measure, measure

A tape measure is critical here. Once the thermal envelope has been identified, the assessment should measure each of the thermal elements so the overall heat loss of the property can be easily calculated. This includes measuring each of the windows, external doors, floor areas, floor perimeters and so on. A common mistake is to use the RdSAP default figures of “A Typical Amount” for windows. Each home is unique, so it should be measured rather than relying on an algorithm. 


Homes will have varying existing mechanical ventilation from none at all (typically in older dwellings) to a range of ventilation systems. This could include IEV, MEV, PIV, to MVHR. The most common will be a form of IEV installation. Whilst you will often find fans in people’s homes which may appear to make them compliant, once you test these, you often find that these fans are performing way below the requirements to make them functional (we often see these operating at 10-20% of there designed performance). It is a requirement in PAS2035 to check the flow rates of these fans. 

And more ventilation

Mechanical ventilation is only one part of each home’s ventilation strategy. Background Ventilation (Trickle Vents / Wall vents), ventilation between rooms (door undercuts or transfer grilles) and Purge Ventilation is also critical to the overall ventilation performance of the home. Trickle vents should be recorded for their size and location (not all trickle vents are the same), doors should be closed, and the undercut should be measured to the nearest mm. Doors are of various widths, so a 10mm gap to a door may be too much or too little. A pen or a pencil isn’t 10mm (about 5-6mm typically), and the actual requirement is 7600 mm2 which aligns with building regulations. Somebody should check that windows are open, and photos should be taken with the windows open to their maximum amount. 

And even more ventilation

Ventilation also includes the existing ventilation of the fabric measures already installed within the home. BS5250 is an important document to read and understand as this identifies the requirements for each fabric measure. When looking at the roof, is there existing insulation? Is there insulation at the eaves? Is there existing eaves ventilation (soffit vents/vent trays etc.)? Is the roof membrane breathable or non-breathable? Does the mechanical ventilation vent through the roof? Are there ridge vents or other tile vents already installed? Are there gaps in the ceiling / around the loft hatch? All of these will influence the suitability and design of any future measures. What might look like a simple loft top-up could be significantly more complicated. 

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