If you've ever used luminosity masks, you know how perfect they can be for creating specific looks, effects and styles, and are also hugely purposeful for specific utility processes in your workflow. There are many ways to go about creating luminosity masks, but have you considered simply using the Gradient Map adjustment layer for this?
If you're not familiar with luminosity masks, they are simply detailed masks, created from the luminance of your image, which allow you to mask out specific areas of your shot for an unlimited amount of uses. Mostly, these areas are some assortment of highlights, midtones and shadows.
With many of the Photoshop Actions out there to create luminosity masks (including my own), your Channels palette can become quite full with several or even dozens of different luminosity masks for you to pick from. Apart from that causing a bit of a mess, it also increases your .psd file size radically if you opt to keep the channels when you save your work. This method is useful, pretty flexible, and common, but if massive files and unnecessary extra channels don't excite you, you're not alone, especially when you simply needed that one, simply mask to select your shadows and that's it.
With some other panels for luminosity masks, multiple layers are created when you go to generate said masks. Like the channels, this can start to get messy, despite the control and flexibility afforded to you with this approach.
With my own panel, Lumizone, we've opted to not create channels, not create layers, and keep things streamlined by letting you create luminosity masks quickly, easily, and then adjust them to any range with a click.
That said, there are times when a quick, efficient, manual method just gets the job done for what you need. And to that end, I've been espousing the use of Gradient Maps (specifically black and white ones) combined with luminosity selecting of your RGB channel, to make a fast mask in whatever range you need, all with granular control.
The video above reviews how I go about doing it, so check it out and let me know if it works for you!