Absolutely. The Entry Sensors purchased with your do-it-yourself security system can be used to protect doors or windows. The Motion Sensor can be used to fill in coverage in a room with lots of windows. Plus, SimpliSafe was designed to be expandable - you can easily order extra Entry Sensors along with your system if there are additional windows that you’d like to protect. According to the National Burglar and Fire Alarm Association, almost 80% of all home invasions occur through the front door, back door or a first floor window.