The recent report to Government before COP26 from the Climate Change Committee highlights the problem of overheating in new homes as one of 8 crucial measures.

The conclusion is that we need to adopt passive measures, specifically shading with external blinds and shutters, to prevent heat gain from reaching the windows.

Failure do so now, will, if left for a further five years, lock-in an expensive retro-fit for as many as 1.5m new homes and renovations. That will be a cost of as much as 4 times the amount to the consumer compared to including in the original build. That is also likely to be the outcome of the latest revision of the Building Regs. which will be that passive measures must be considered before mechanical cooling. Or we make our windows smaller which is not the passive house concept.

The report refers to a study, by London South Bank University and BBSA, of an apartment building in Camden London which looked at methods for reducing overheating and found that when no shading was present operative temperatures in September reached highs of 47.5°C. Shading with Hallmark Trojan external venetian blinds reduced that by up to 18°.


As from a previous CCC report 'Shading – shutters or awnings – is not costly or difficult to install, it is just that we are not doing it'.

Typically, we are still not doing it, and yet it is a proven, passive, common sense solution.

A leading architects practice for a development in East London has recently announced that to meet the proposed Part L changes the windows in their Passive House design will have to be smaller.

WRONG. The proposal requires an assessment of passive measures before considering cooling or reducing window size. That is external blinds that are a passive measure that can save throughout their lifecycle up to 60 times the energy used to create them with minimal operating costs.

Except that many of the leading modelling tools do not correctly calculate dynamic shading and their effect on overheating. That is the main reason why over 20% of new buildings overheat, not because their designers wish to cook the occupants. In our London case study the operative temperatures in the modelling predicted 28° as a maximum, even in September the temperature was 47° in the unshaded room, the room with a blind was 29°

Modelling in Energy Plus or EQUA is not 'black box' and calculates to ISO standards so check your modelling in the free to use EQUA Early Stage Building Optimisation programme for a realistic comparison of outcomes.

Then you will realise that stopping over 90% of the heat gain before it gets to the glass just makes sense. External blinds are the facilitator of larger areas of glass which the Passive House concept needs to maximise on free energy from the sun in the heating season whilst maintain a comfortable temperature in the cooling season. Less glass is not the answer.

Look for Trojan and Hemera external blinds and links to ESBO at www.hallmarkblinds .co.uk