My last question was about IR or infrared light's ability to go through glass, but…?

…I’m still confused. I was reading about PIR, or passive infrared motion sensors on www.howstuffworks.com and how they cannot detect anything behind a window. The article states that glass is transparent to visible light, but opaque to infrared. If motion detectors are supposed to detect IR energy (heat) and IR energy can pass through glass as some people state, then how come motion detectors don’t get tripped?

Another thing I read was about greenhouses, and how they work by trapping in long wave IR energy on the inside. In other words, the IR energy isn’t able to go through glass.

Err, could someone please clarify this for me? Thanks!