I like to monitor the temperatures in my shop and I receive the occasional question about how I gather these measurements. Well, in stark contrast to expensive commercial monitoring systems, using commonly available parts one can construct a reliable building monitoring system for the cost of an old PC and about $150 worth of sensors. Here is an overview of the components used to build this system.
Sensors are from iButtonLink and are based upon the 1-Wire network which uses simple twisted pair wiring to connect sensors in a variety of topologies. I used an old Amphenol 6 port RJ45 harmonica and wired the ports into a passive hub configuration to create a star topology. The PC connects to the iButtonLink LinkUSB USB master which is in turn plugged into the hub, as are the sensors. All sensors are iButtonLink T-Probes except for the iButtonLink MS-TH which provides both temperature and humidity readings. Note that the DS2438 temperature sensor used in the MS-TH is not as accurate as the DS18B20 used in the T-Probes—a minor nit.
The PC runs Linux (I like CentOS) with OWFS 1-Wire software to read from the sensor network and RRDtool recording/graphing software. A simple shell script called from cron on 5 minute intervals provides for regular data collection.