Many older buildings were built at a time when energy was inexpensive. The hot-water piping may not have been insulated at all, or it could be wrapped to a less than optimum thickness. Pipe routed through unconditioned spaces such as the basement or attic should never be without insulation, even in moderate climate zones. Insulation protects against freezing, but it also prevents condensation ~ even a small drip, drip of water can over time cause rotting wood and framing, mold inside the walls, and even structural damage.
Insulating piping is an easy DIY project. The foam insulation is already molded to fit the outside diameter of the pipe. You just slip it on and tape the seams. Some insulation brands even have adhesive on the edges: fold the sides together, and the foam insulation sticks in place.
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Excerpted from Going Green, available as an eBook at this list of eBook retailers online.