You definitely don't want to oversize the current equipment to compensate for a loft space, which by its nature will be hard to condition relative to the entire upstairs space. An oversized unit would make matters worse by running shorter cycles (less dehumidification and more importantly, less conditioning of the space that has a higher heat gain relative to the rest of the upstairs).

First: Has a load calculation been done on the upstairs to determine the capacity needed for each room (including the loft)? Airflow balancing is likely in order to improve airflow to the room, which may require larger supply ducts. But you still may not be comfortable all the time. I'm assuming the thermostat is in the main hallway or master bedroom, nowhere near the loft. Zoning is not really an efficient or feasible option with that small of a space and that large of a single-stage system. You could try running the fan in the "on" position for air circulation. You could also install a thermostat with averaging capabilities to average the temp in various upstairs rooms, including the loft.

I really like the mini-split option. Mitsubishi mini-splits are VERY nice, and I think this is a great application for one. The inverter systems are variable capacity, so the output will match the demand of the loft.

Thanks for this. I have not done a load calculation, we bought the home after it had been extensively remodeled and expanded. I assume that a load calculation was done when they built the loft, but of course, that's only an assumption. The thermostat, is just like you say, in the master bedroom, nowhere near the loft and considerably cooler in the summer.

With larger ducts to the loft, there's a chance it would help, but no guarantee - what would replacing ducts cost? something like 20 feet worth?

Thanks, everyone seems to like ductless splits, except for a few folks! I'd be fine with it, if it was about 1/2 the cost.

I am a fan of mini-splits, but it appears that the current central system is very close so ducting revisions would be simple and inexpensive. It is well worth investigating that.

Evaluating the whole house with special emphasis on the relationship between that room and the attic would be a good idea. Insulation and air sealing are the most economical things that you can do. If you can over-insulate that room, it might be cool enough relative to the rest of the house.

If the house is new to you, you might want to get a non partial energy rater to help you identify the low-hangning fruit for efficiency improvements and advice on fixing the hot room. Start at Resnet.

Thanks, I had an energy audit done early and the attic was an area of concern so insulation was added to the walls adjacent to this loft space and elsewhere, although further spray foam sealing is probably necessary. One large area that is being fixed now was the door which is 3X4 and just plywood. My wife wants the temperature fixed in the loft, so I don't think I can convince her to insulate more, fix the duct work, and test this summer, but I think I can convince her to insulate more and fix the duct work if she wants to get the split, so that seems to be the direction we are headed!

This situation is tailor made for a ductless split. That way you can control the zone that you need cooling (lofts and attics tend to be hotter as heat rises) without making the rest of the home uncomfortable from over cooling. The other advantage is you can leave the system off until you use the room as you can cool much quicker with a mini split then a large central system saving you $ from cooling a space that's un occupied.

Here is a link that might be useful: Mini splits