Damn, I kinda feel like I already asked this question and got the same response like 8 month earlier, if you feel that too - sorry for wasting ur time. I promice I will improove 
I've checked what non-manifold means and I am very shocked really, it kinda feels like there is absolutely no way to create something like a shirt mesh without running into this problem. How does one avoids creating non-manifold meshes? Especially given your requirenment of mesh being one-sided?
No really I am stuck: I cannot imagine how can I create anything that would be non-maniford cloth-wise. I am just stuck.
I am pretty sure the "Flag" mesh you use in your project is non-manifold. How is that working? Seriously I am VERY confused.
I've checked what non-manifold means and I am very shocked really, it kinda feels like there is absolutely no way to create something like a shirt mesh without running into this problem. How does one avoids creating non-manifold meshes? Especially given your requirenment of mesh being one-sided?
No really I am stuck: I cannot imagine how can I create anything that would be non-maniford cloth-wise. I am just stuck.
I am pretty sure the "Flag" mesh you use in your project is non-manifold. How is that working? Seriously I am VERY confused.