When I think sunroom, I imagine lots of windows. Triple paned, argon filled, coated windows are still only ~R5. Older double-paned windows, are probably closer to R2-R3. (Typical 2x4 fiberglass insulation is like R12-15). Closed cell spray foam is ~R7/in, You don't typically fill the wall space 100%, so Maybe R18.

If the windows make up, say 50% of the wall, your whole wall will only be ~R7. Don't forget all the wall framing surface area is counted at R1 and probably makes up about 15% of your wall space.

Depending on how much glass you have, More insulating is probably not going to help much. Newer windows can help a lot if you have older windows. Filling drafts will make a huge difference. New window installers will usually use spray foam in the gaps, so that also helps.