Substack diary no.128: Sunday, 19th. April 2026

Clear blue skies, sunshine, a negligible breeze and 10 degrees celsius out there, so a fairly typical spring morning. In the sitting room the temperature has just this minute risen from 15 to 16 degrees and yet I do not ‘feel’ as cold as I did four weeks ago. I was reminded of studies in environmental psychology for my post-graduate degree course in 2004.

I was already aware of metabolic gain when modelling heat loads and demand for mechanical heating in buildings, especially houses. It is overlooked by most people, but not by energy professionals and, I would like to think, by architects and house builders. Metabolic gain is taken into account in the SAP model – the Standard Assessment Procedure – for housing energy surveys in the UK. I am now sitting in an armchair and writing this and in so doing I am emitting approximately 60 watts or so of energy in the form of heat. If there was another person in the room with me we would be generating at least 120W of heat; simply standing up would increase the heat gain and moving around would increase it further! That is why crowded rooms always feel warm.

It is not metabolic gain that is interesting me at the moment, but solar gain. This is free energy from the sun falling on the external fabric of the home and gradually warming it up, and is most noticeable if you sit by a window upon which the sun is shining. Most of us have warm and cold spots in the home, and these often vary during the day, depending on the orientation of the building. This reminded me of the older generation in my family, and in particular of some who were lucky enough to have a fairly large house with two ‘reception’ rooms, one of which they sometimes referred to as the ‘morning’ room and one the ‘afternoon’ room. They would literally sit in one in the morning and move to the other in the afternoon! Quaint but demonstrating an innate common sense that seemed to die with that generation.

There is no doubt that I feel warmer this morning than I did on an equivalent morning a month ago, despite the ambient temperature being exactly the same. Solar gain is not the only source of my feeling of comfort – psychology is in there somewhere too – but it is an important contributor.