Ray writes from Aston, Pennsylvania: "What is the proper way to insulate a crawl space with a cement floor and one vent. The size is 12 x 20".

If the crawl space is dry I would seal off the vent and insulate the walls of the crawl space. One vent doesn't really give you enough air movement to fully vent the space so it is typically better to seal it off. Basically, I would treat the space like a mini basement. If you have a sump pump in the crawl and the space seems damp, it might also be a good idea to run a dehumidifier. Your other option is to insulate the floor of the house, and leave the crawlspace as is.