Metal are the worst thing you can think of for thermal mass. They are the best conductor of heat, and therefore the worst insulation.
Better to go with concrete, rock or bricks to do a thermal mass and use cooper to move the heat from one side to the others. If you've got enough temperature difference between your cold and hot side you can even make water circulate in a pipe and build a turbine, it has been done before but isn't really cost effective.
You can dig a hole in the basement and fill it with concrete with a few cooper rod in it, lets the cooper rod go to the first floor or second floor if you have one with a heat sink. The copper rod will transfer heat from the concrete mass to the house, hot in winter, cold in summer, relatively speaking. They will always have the same temperature and will try to bring the house at the same temperature as it is underground. Assuming your underground is at zero degree Celsius, it will try to pump the heat in the house down to 0 in summer effectively cooling the house, and in winter it will heat the house from (example) -20 up to 0 leaving your central heating job only from 0 to +20 and not from -20 to +20 saving you cost in heating.
A better way to do this would be to build a whole house underground except 1 side facing the sun, like Earthship house :
http://en.wikipedia.org/wiki/EarthshipMaking the whole thing integrated into the house by design, way more effective. As far as i know there is nothing more cost-effective and efficients than Earthship house, the whole house is the thermal mass. It require almost no heating or cooling.
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