Check that your windows aren't the main issue (sunrooms have lots of 'em) - infiltration will trump insulation by a freaking mile. A tiny gap letting cold air in will mitigate all the insulating work pretty quickly. And if the windows are low-E, they may be radiating cold in (technically leaching heat out), so storm windows/etc. may be a more useful option. If you're sure the windows are tight and don't feel like touching a freezer wall when it's cold out, I imagine you'll get the latest scoop on insulation as the comments pile up! And consider the floor as well, it may be a big cold-sink depending on its construction.