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Builders searching house plans for someplace to put a bit more insulation often find help under the floor. It costs less to add insulation beneath the floor than to build thicker walls or to add foam sheathing. Even the typical attic seldom has room for more than 18 inches of insulation (R-50 to R-60) without the added expense of raised heel trusses. Floor insulation is especially effective over an unconditioned basement or crawlspace. The myth that heat rises was used for many years to justify not insulating floors at all, which led to some very cold feet. The truth is that heat doesn't rise — hot air does. The process by which hot air rises is called