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There are a few reasons for the exceptional performance of thermal-mass buildings despite the low per-inch R-value of the wall components. First (and most easily documented) is the heat capacity of the mass itself, known as the K-value. Rather than resisting the flow of energy from outside to inside, as an insulated wall does, a mass wall stores heat energy for return to the living spaces as it is needed.

The second is due to the effect that heat transfer through radiation has on our bodies. When we stand, sit, or walk near a mass wall, the energy in the wall will cause direct sensations of warmth or coolness, regardless of the temperature of the air in the room. Direct radiant energy provides a soothing, subtle, and effective alternative to forced-air heat and mechanical air conditioning.

In very cold regions, an exposed, uninsulated wall will become cold even with a supply of warm air on the interior surface. The cold walls will "wick" heat out of the living spaces, adding to the fuel bills as energy is expended in an attempt to offset the heat loss. Despite warm air temperatures, the walls will also create the sensation of cold through a constant radiant heat loss between the occupants and the wall mass. In these situations, insulation, either embedded within the wall or applied to the outside surface of the walls and protected with stucco or paneling, is economically justifiable.

At the lower left is a rambling, large-scale project.

In mild climate zones, but where some winter protection is helpful, it might make sense to explore alternatives to insulation, especially if an uninsulated wall can work to your advantage in maintaining cool temperatures during a long, hot summer. Some of these alternatives might involve altering your design to provide greater solar access if winter heat is your predominant re quirement, or adding a wide porch if summer cooling is the higher priority.

If your winters are cold but short and sunny, perhaps the most feasible alternative would be the addition of solar panels to heat water for circulation through the floor slab, thereby offsetting the heat loss through the walls with inexpensive heat gain. If the summers are hot and dry, consider installing a nighttime evaporative cooler to force-ventilate the house and chill down the walls.

If you live on rural property, try stacking bales of hay or straw against the north side of the house during the cold winter months, then using the straw for summer mulch on the garden. Whether you rely on an attached greenhouse or an annual straw-bale blanket, there are a wide variety of alternatives worth evaluating before buying into synthetic insulation and stucco protection. Even in the coldest of climates, you might decide to build your house without exterior insulation and test how well it performs for a year or two. You can always add the insulation and stucco later if the heating bills are too high.

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