ZigBee-Wireless Mesh Networks (ZigBee-WMNs) are recognized as a cost-effective and flexible solution for building automation and control systems (BACs). They have the potential to unify the methods of data communication for sensors, actuators, appliances, and asset-tracking devices in BACs. They offer a means to build a reliable but affordable network backbone to facilitate building control. ZigBee operates primarily in the license-free 2.4 GHz ISM (Industrial, Scientific and Medical) band. This feature makes the technology easily applicable, but also potentially vulnerable to interference by other electronic systems working in the same band such as WLAN-systems of IEEE 802.11 and microwave ovens. This paper reports the experimental results to quantify the impact of different interferers on ZigBee-WMNs. They show that, among different interferers, interference from microwave oven is indeed a major problem. In particular, it results in bigger drop rate and retrial rate compared to the impact of WiFi on WMN. It is expected that the results can provide a quantitative basis for designing reliable ZigBee-WMNs.