Voom function from limma package and Normalization on counts data
I'd like to know answer for this myself. It seems the normalize.method argument is set to none by default. You can edit by looking at the methods in normalizeBetweenArrays. 
But how do I know if and when I have to use normalize.method in my analysis?
The output from voom is log2CPM of count values (adding 0.5 to them).