Should I insulate air conditioner coolant pipes?
The large, cold, low pressure line, carrying the evaporated refrigerant from the house, should be insulated to prevent condensation. The small, warm, high pressure line, carrying condensed refrigerant into the house, should not be insulated. I am not an air conditioning professional. However, en.allexperts.com, www.bobvila.com, and www.familyhandyman.com, all agree with me. See references below.
Explanation: The unit outside the house compresses the refrigerant from the large line. This makes the refrigerant very hot. Then the refrigerant is run through the condenser coils and the fan blows outside air over them to remove the heat and condense the refrigerant back to liquid. The small line now carries the warm liquid refrigerant back into the house. No insulation is needed on this line because it is still warmer than the outside air. If the air cools it some more, that only helps. In the house, the pressurized liquid refrigerant is allowed to rapidly decompress into the evaporator coil. The instant drop in pressure causes an instant drop in temperature, making the evaporator coil very cold. Interior air is blown over this coil, cooling the air and warming the coil and refrigerant. The evaporated refrigerant, which is still rather cold, now flows out of the house in the large pipe. It does not help us for this refrigerant to get warmer after it leaves the evaporator coils. It just makes it harder to compress and cool in the condenser coils. So we want to prevent condensation on this pipe. That is the reason for the insulation.

(source: reimanpub.com)
From this site:

&quot;the skinny tube doesn't need insulation&quot;

From another source:

&quot;As you have seen, the large refrigerant line carries cold vapor while air conditioning, and on a humid day it sweats. The main reason we insulate the pipe is to prevent humid air from coming into contact with it.&quot; &quot;For the small copper tube you do not need insulation. Just make sure that it does not contact masonry or other metal.&quot;

The following site provides good step by step maintenance instructions.