I get an insufficient storage space error when trying to download an app and I have several gigabytes of free space I have my phone set to use my SD card as default storage. When I attempts to download the Facebook for Hulu app from the Google store it always gives me this error message. I checked both apps both take around 40 some odd megabytes. I have around 4 gigabytes of free space on my SD card. I have around two gigabytes of free space on my internal storage. Can you tell me what's wrong?
Apps don't necessarily install on an external SD card. Normally, they would install on the system memory and then, if capable, the app can be "moved" to the SD card. Moving still leaves part of the app on the phone memory and usually increases the overall size of the app.

Long story short- make sure you have room on your device's internal memory to install apps.