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Indeed. I’m not claiming to be an expert on this but have done a fair bit of reading on the subject. Whilst there is definitely benefit to having some thermal mass a huge amount is not always beneficial.
“Thermal mass is most appropriate in climates with a large diurnal temperature range. As a rule of thumb, diurnal ranges of less than 6°C are insufficient; 7°C to 10°C can be useful depending on climate; where they exceed 10°C, high mass construction is desirable”
I think the UK range is usually quite small – most parts don’t get extremes of temperature, and when we do the daily range is fairly small (nothings far from the sea.)
A huge amount of thermal mass wouldn’t have been much benefit during either the cold extremes of this last winter (when it stayed cold day and night) nor in the recent heatwave (when night time temperatures were still high).