Ambient vs. actual fermentation temp

Does anybody know how much warmer a beer ferments at as compared to the ambient temperature? I'm sure there are many factors that affect this including type of vessel, yeast type, maybe even gravity of wort? I'm thinking about fermenting in a space where the ambient temp is 14-15C, thinking that the fermenting temp of the beer would be about 18-19C, so about 4-5 degrees different. I'm just guessing though. any thoughts?
I usually ferment in a glass carboy and don't want to leave a thermometer inside while it ferments. For my next batch I'm thinking about fermenting in a plastic bucket with a floating thermometer to monitor the difference. just wondering if anyone has tried this and has some insight. thanks in advance!

 

You can get a good idea using a simple stick on thermometer(fermometer) on the outside of your fermentor. I use a temperature controlled fermentation chamber, and I've at times had to set the air temp as much as 7-8*F below what I want the fermentation temp to be.