Hint: There should be no insulation between an enclosed pipe (including a drain pipe) and the interior wallboard/flooring/ceiling paneling. Hint: The only way the unheated garage will ever get above outside temperature in winter is due to heat leakage or seepage from heated interior space above or to the side. If you are not going to use the garage space as living space (or for storing materials that can't stand the cold), it is better to concentrate on providing adequate heat in the living space and not worry about heating the garage. Hint: If there are pipes in an exterior wall and an access panel on the inside, it is a good idea to have the panel open all winter. Also, cabinets along outside walls with plumbing inside or in the wall behind should have the doors left open in winter.