by Caoimhan » Tue 19 Jul 2005, 12:46:21
$this->bbcode_second_pass_quote('JoeW', '')$this->bbcode_second_pass_quote('Caoimhan', ' ')Thermal masses are not insulators. They are usually stuccos, masonry, specialized cement, or masses of water.
I slightly disagree. All of the materials you suggested as "thermal masses" are categorized as ceramics, which (in general) are decent insulators, when compared to metals. That's why coffee mugs are often ceramic.
Modern insulation for homes is styrofoam (polystyrene), a plastic, which is a better (cheaper!) insulator. I have about 14" of blown insulation in my attic. Works great.
I believe insulation is the biggest money-saver in home energy heating and cooling efficiency.
JW
It's not the plastic that is the insulator, but the air bubbles trapped in the plastic. Air is a poor conductor of heat, unless it's moving (convection). Most insulation is a medium that traps tiny pockets of air. The best insulator known to man, aerogel, is 99.9999% air.
When speaking of insulators and thermal masses... we're actually talking about two different qualities, though there is some correllation between the two.
Insulators are materials that transmit heat slowly.
Thermal masses are materials that have a large heat capacity (the ability to store thermal energy).
The stucco, masonry, and cement I refer to are moderate insulators, but we don't insulate our homes with them, do we? If they were good insulators, we would. But they're actually not so good insulators. Sure, metals transmit heat faster, but masonry is much worse than fiberglass matt at insulating.
Intelligently designed homes will contain a lot of thermal masses INSIDE the external insulation envelope. The external insulation envelope reduces the transfer of heat into the home during hot seasons, and out of the home during cold seasons. During cold seasons, passive solar gain is a great thing... but only works during the daytime, unless you have some means of storing the thermal energy gain. It does you no good to have an 80 degree home at 3 PM, if it drops to 55 degrees at 3 AM. The air in your home is really bad at storing that solar energy you got during the daytime, especially since you have to recirculate your air, exhausting stale, moist air out of your home, and bringing in fresh air.
This is where thermal masses come in. Using thick plaster, stucco, masonry, and cement inside your home, where the solar gain can be stored up is intelligent design. All that thermal mass will be absorbing solar during the daytime, keeping your 3 PM temp to a comfy 70 degrees (instead of 80), and then re-radiate the thermal energy into your home at night, keeping it 65 degrees (instead of 55).
The fact that these materials are good insulators means that they will re-radiate that heat slowly... all night long... which is a good thing.
Edit: Oh, and the reason your coffee mug is ceramic is because it's a thermal mass. It will keep your drink warmer longer than a metal mug. Most people brew teas and coffees at around 140 degrees F. When you pour your drink into your coffee mug, the mug will absorb a lot of heat from the liquid, bringing it down to drinking temp (about 110 degrees) in a reasonable time. But once the temp of the mug and the liquid are equalized (around 120 degrees), the liquid has a tendency to cool faster than the mug. The mug will then slow the cooling of the liquid inside it by re-radiating heat into the liquid.