When opening the cold side of sink, I get warm water for a minute or two before going cold

This is water that is sitting in pipe in a heated area. The heat is transferred to water through pipe. Insulating pipe will reduce the transferred a bit but not totally.

This can also be caused by cold water pipe to close to a heat sauce. It is common in most cold weather areas.

Chuck

Hi dave, This also could mean that you have hot water bleeding to cold through the mixer. What type of
Faucet setup do you have? This problem is more common in single handle shower sets . I did however
See it happen in a 3 handle kitchen faucet when one of the check valves failed.

Sorry dave I answered the wrong question,

Install nipples that allow water flow in one direction only. You can buy them in Home Depot. Install them on top of the heater. Make sure you follow the Arrows. One nipple is for Hot water and the other for Cold. See the pic... Let us know if the problem continues. Back to you. Milo