my heroine with a full head of hair still shows a bald scalp through the heir in this render!

isn't there someway to cover that scalp?

drawn

Without seeing what the scalp cap looks like in the Material Room, there may be several issues happening.  This is assuming that the hair piece was designed for the model.  If it is a conforming hair piece, it does not fit well on the model and shows poke throughs.  The scalp may either be tinted or transparent.  But my guess is that it is not fitting right.  We need workflow information.

As hborre says,please provide following for your query:

1. What figure are you using?

2. The hair-- is it a figure or a prop? 

3. What are your steps/ workflow? Load figure, add hair... are you conforming? Are you applying a fit pose? Are you dialing it? Are yoy adding a material to the hair?

All of those are dependent on #1 and #2, but all are crucial for us to understand how to help you.

4. What render engine, what lights, what settings. Those also may impact  how your render looks.