Can someone help me with one-sided material models?

This may be more on the blender side but maybe someone can point in the right direction.

I often find myself sending models into unreal engine 4 (4.4.0 if it matters), and they are only one sided…which is fine in many cases. Although some occurrences the only fix I’ve found is to make whatever material I was using in unreal on said model a two-sided material.
My question is really…how Do I set my models to be either two sided in blender or how do I decide which side is being used?? Like a building for example where you’d like to be INSIDE only and not outside.

I know this is a blender based question but could someone tell me What it is I’m looking for?? the terminology or what not? I’d be more than happy to Google for help but I’m not even good at explaining what I need here.

The models collision on one sided models is only on one side…and I don’t know who to tell blender (or unreal) how to set the collision to the side I wish.

Any help would honestly be a life saver. This has been one of my biggest problems sense I started playing with all of this stuff 5 months ago.

thanks everyone!!!

You’re looking for “reverse normals”.

Excellent. THank you