You need to look at doing 100% foam insulation at the roof level first. That will reduce the effective available height for the whole space. You will have a moisture and heat problem otherwise that would prohibit any type of storage. And you will create a home ventilation air exchange issue doing that as well, so you have to look at the multiple systems involved here. What is the actual ceiling height at the ridge, and what type of insulation exists in it now?

Hi Sophie -

Thanks for the response. There's a dropped ceiling in the bedroom there now. The ceiling height is 7' which I believe is the minimum allowed in Massachusetts.

From what I can tell by looking behind the knee wall space, there is standard fiberglass insulation. I have had no moisture issues in the 10 years I've been in the home so assume there is insulation in the ceiling space today.

I plan on upgrading the insulation - likely with spray foam and installing a mini-split for heat and cooling. Currently there is forced hot water baseboard heating.

You need to research unvented attic assemblies. You are taking space that is not currently conditioned, and wanting to move it within the conditioned envelope. That means a completely different, and expensive, approach to the insulation and ventilation of the home, as a whole system.