Eversprings work fine for me (ST814) and the batteries seem to last about a year. I used to have six or so of these deployed around the place, they've been no trouble at all. However, as you say, if you don't need the display then what else is there? In Europe I would suggest a one-wire system connected to a Fibaro universal sensor, or one each to a door sensor. I've recently swapped out most of my Eversprings for Netatmo units, which don't need Vera at all (although there is an excellent plugin!), and don't have a display, but are more expensive. Perhaps you could use their CO2 sensor to detect if anything was living in the crawl space? It certainly detects human occupancy of a room quite quickly.