A full-scale experimental programme to study the ad freeze/heaving problem of an insulated basement is currently underway at the National Research Council of Canada. Apart from heaving results this experiment has also yielded interesting findings related to the natural convection in concrete-block wall cavities. Both the field data and a simplified finite element model calculation show that natural convection in a block basement wall can be a significant factor contributing to heat losses. Insulating only the inside upper portion of the wall, as is current practice, may not be effective in reducing heat losses with block walls, while full length insulation may lead to significant lowering of ground temperatures near footing levels.