Scott, Right on target. The thermal mass creates a phase lag. ICF operate a little differently since they are insulated on both the inside and outside. The external insulation slows the heat flow from outside going into the wall and the inside insulation does the same for heat in the wall going into the house. The outside insulaltion reduces the amplitude of the temp fluctuations in the concrete core as well as delays the time when the core reaches its max temp. Then the inside insulaltion slows the flow of heat from the core into the house. This thermal system can be "tuned" for optimum effect, sort of...

There have been attempts to optimize the insulation versus thermal mass to derive max benefit from controling the captured heat. I built my mom a sunporch with a slab floor and a southern exposure window wall. In the winter in the early morning before sunrise the temp in the (unconditioned) sun room averages about 15-20 degrees higher than outside due to storing solar heat in the slab. My mom went out and bought a carpet for the sun porch. [img]/forums/images/icons/confused.gif[/img]

I told her it would cause the room to overheat in the middle of the day and be a lot colder at night and in the morning but... Oh well she wants the floor more comfortable.

The carpet will warm up quickly, converting sunlight to heat and overheat the room around noon and early afternoon. The carpet will insulalte the slab from the solar heat so there will be no storage to draw upon overnight so plants will freeze etc. (Never got below freezing so far with bare slab. The beneficial phase shift of the slab's storage will be lost. This ranks right up there with the in-floor hydronic heat that works really well until someone decides to put a nice wool oriental rug down and doesn't grasp why a nice warm rug would make the room so cold.

Thermal masses can be too large and not let the temp rise enough to be useful. Another not to well known concept which flies inthe face of the general belief that if a little bit is good then a whole lot more should be a whole lot better.

Gee Scott, I never stayed at the Holiday Inn much...

Pat