For receiving, an outside antenna is preferably to an inside/attic antenna for a few reasons:
First, an inside antenna is more likely to pick up interference from electronic devices in the house. Computers, TVs, appliances, switching power supplies on virtually everything today, etc. This isn't a problem for transmitting, of course.
Second, the received signals can be attenuated somewhat by the house itself, especially if there's a lot of metal. If you've got aluminum siding, you're might essentially be inside a Faraday Cage.
Third, with an outside antenna, you probably have a better chance of getting it higher in the air, if you have a lot of tall trees in your yard.
Fourth, you can usually have a larger antenna antenna outside.
This is not to say that an antenna in the attic can't work, just that you'd probably get better results from one outside. What are the dimensions of your lot, and where you do have trees?