Heating pipes are freezing - how do I insulate them?

The simplest way is to wrap the pipes with foam or expanded polyethylene. Ready-made pipe shells for different diameters are sold - you just slip them on and secure them with tape or wire. If you're on a tight budget, regular fabric in several layers works fine - old rags, blankets, newspapers. The key is to wrap it tightly with no gaps where cold air can get through. For damp places (basement, unheated attic), it's better to use foam or mineral wool - they don't rot and keep the insulation working longer.

If the pipes are in cracks or openings, seal them with expanding foam before insulating. That'll block the drafts and help a lot. In really critical spots - like where cold air flows directly - you can also wrap a self-regulating heating cable around them, but that gets pricey and is more of a hassle.

Also check if maybe you just didn't insulate it properly or if there's air in the system. Sometimes it helps to just open the bleeder valve and let the air out of the radiator if it's not heating right.