Is it appropriate to add attic-floor insulation after converting the attic with open-cell spray foam?

A homeowner in Northern Virginia ran into trouble after a whole-attic upgrade meant to bring the attic space inside the home's thermal envelope. The work included spraying open-cell foam insulation on the roofline and attic walls, sealing the ductwork (a process called Aeroseal), and installing a new 3-ton variable-capacity heat pump system. The idea was that foaming the roof and attic walls would make the attic part of the conditioned house, since the air handler and ductwork for the top floor sit up there.

After the work was done, the system couldn't keep the house cool on hot days. With outdoor temperatures above 92°F, the indoor temperature stalled around 79-80°F instead of reaching the 75°F setpoint, even with the equipment running at full capacity. A tightness test confirmed the house was sealed much better than before, but no one could pinpoint an obvious flaw in the foam or building envelope. The contractor's proposed fix is to add 6 to 10 inches of blown-in insulation back onto the attic floor, in the same spot the original insulation was removed from, betting that this will cut the cooling load enough for the existing equipment to keep up. The alternative on the table is upsizing to a 4-ton unit.

The homeowner is raising a legitimate concern for anyone who has spray-foamed an attic roofline: adding floor insulation back in creates two competing thermal boundaries, which can leave the attic in an unclear in-between state and raise questions about moisture, humidity, and how the ducts and equipment up there will perform long-term.