The most conductive material in your house is the foundation walls as it gets as cold as the frozen ground outside. A major source of air leaks in from the rim joists. Spray foam serves excellently as a thermal barrier and air barrier. The less heat your basement walls lose the warmer the floors. Warm floors make the space above feel more comfortable. Crawlspaces are very similar, but it is also important to block the humidity coming out of the ground, but still allow the free geothermal heat to warm the area.