"Yes, that is the idea, but my thinking is that the thermal mass rarely gets hot enough and is often acting as a heat sink, meaning you need extra heating energy (this is for the UK climate, not a mile high at 40°N)"

What I am noticing here in Tenerife in an end flat with East, South and West elevations is how the building temperature affects our comfort zones. We go down to around 15º C during the winter which starts around December but we only need heat from late January on. This is only an impression at the moment as I haven't monitored temperatures but it appears as if the building was slowly releasing the heat it gained during the Summer and what is more pertinent to this discussion, it still feels cool inside today although internal air temps are around 22º C. Wall temps are obviously affecting our physiological comfort level and I must obviously get down to collecting some data for you!

Marktime, yes, it's a lot more complicated than just what the outside air temperature is.

Does it get cloudier as you go into January, perhaps?

As you say, the temperature of the building will lag the outside temperature a bit though I'd be surprised if is was by more than a week or so for a conventional build.

Also as you say, the wall temperatures will have an effect on the perceived comfort level. Within reasonable ranges, the walls being 1 °C warmer will compensate for the air being 1 °C cooler and vice-versa. I use a Lidl IR thermometer to measure temperatures here (caravan in NE Scotland) pointing it at internal walls, windows, floor and ceiling to get an idea of the radiant temperatures and at a bit of paper hanging over the edge of my table to for an air temperature proxy. (Current air temp just under 19 °C but feeling a lot chillier with bit singe glazed windows a 11 °C, floor away from the oil-filled heater about 14 °C and walls just under 18 °C).

There's also the physiological adaptation effect - you adjust to the general temperatures around you over the last few days or longer though this would tend to have the opposite effect to what you describe.

Hi Ebeneezer, welcome to the forum.

Posted By: EbeneezerSpecifically the "house" with thermal mass retains heat into the evening (18:00-00:00) when people are back from work and want to use it.

I wonder what proportion of homes are actually used that way, though, with people working at home, retired, with very young children, sick, unemployed and so on. Also, with a lot of non-9-5 type work many homes will have at least one person in during what's a working day for others (e.g, one person working M-F and another working weekends with days off in the week).

Even in houses occupied solely by people who spend most working days outside there are still weekends and holidays.

Overall, I'd guess that on average more home·days are occupied than not.

The more you insulate a house the less benefit Thermal Mass is because the temperature rarely fluctuates. In our timber frame Solar Enhanced Passive Houses the thermal mass of the floor slab heated to 24 degrees is sufficient to heat the house for 4 days.

The ground floor slab in a 160m2 two storey house is about 10m2 so about 25 tonnes! When you add in the solar gain the heat-loss for the Solar Passive House is about 2kWh/day in December.