Thermal mass is an advantage masonry has over almost every other material.
Mass inside the house complements the way fireplaces heat - radiantly. The masonry fireplace and chimney wall gets warm. The fireplace also heats up the other surfaces in the room - the walls, ceiling and floor - bringing up the "mean radiant temperature" so that people are more comfortable at cooler air temperatures and with good ventilation.
Mean radiant temperature - the average temperature of all the surfaces in a room - is roughly inversely proportional to air temperature. If the walls, windows, etc. average 60 degrees F and the air temperature has to be 72 degrees F to feel comfortable, you can reverse that and feel comfortable at 60 degrees F if you can bring up the mean radiant temperature to 72 degrees F.
When the air in the room isn't so warm and there is less differential between indoor and outside air temperature, the insulation works better and good ventilation doesn't cost so much.
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Buckley Rumford Fireplaces
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