"G&M" wrote in message
...

"Andy Hall" wrote in message
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What I meant was that if you treated the outside wall as not existing
as you were for insulation calculation, you have outside temperature.
If you add the wall back in, you have not only its U value

but not much - even 2 feet of stone has a very poor insulation value.

but more importantly a huge thermal mass.

This is more important though. However one has to be careful not to end
up
with a thermal lag so bad that south facing living rooms stay cold all day
and need more heat than expected to keep them warm until evening when they
suddenly warm up dramatically.

If south facing you enlarge windows and use passive solar gain.