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

I am in Northern Virginia and recently completed an integrated attic-insulation and HVAC project on my townhouse. I would appreciate input from people with building-science and residential HVAC-design experience.

The original project included:

• Removal of the existing attic-floor insulation;

• Installation of open-cell spray foam at the roofline and other exterior attic boundaries;

• Aeroseal duct sealing; and

• Replacement of the HVAC system with a 3-ton variable-capacity hybrid system.

My understanding was that insulating and air-sealing the roofline would bring the attic within, or substantially within, the home’s thermal envelope. The air handler and ductwork serving the top floor are located in the attic.

Immediately after the new HVAC system was installed, it was unable to cool the house adequately during hot weather. On several days when the outdoor temperature exceeded 92°F, the system operated at full capacity for extended periods, but the indoor temperature remained around 79–80°F and could not reach a 75°F setpoint.

The contractor’s HVAC technician observed the problem during installation and returned twice afterward. The technician confirmed that the equipment was operating at full or nearly full cooling capacity, but it still could not bring the house down to the thermostat setting.

A post-project blower-door test later showed that the house was substantially tighter than it had been before the work. The test-out technician did not identify an obvious building-envelope or spray-foam defect that would explain the cooling problem.

The contractor initially discussed replacing the 3-ton outdoor unit and indoor coil with a 4-ton system. They are now recommending that I first add approximately 6 to 10 inches of blown-in insulation on the attic floor. This insulation would be installed in the same general location from which the original attic-floor insulation was removed.

The contractor believes this may reduce the cooling load enough for the existing 3-ton system and has described the work as reversible if it does not solve the problem.

My questions are:

1. Is it considered a sound building-science approach to install substantial insulation at both the roofline and the attic floor?

2. Would this create two competing thermal boundaries, or is that concern overstated?

3. After adding attic-floor insulation, would the attic still reasonably be considered conditioned or semi-conditioned space?

4. Could this configuration create humidity, condensation, mold, or other moisture risks, particularly because the attic has no dedicated dehumidifier or intentional ventilation?

5. How would the added attic-floor insulation affect the air handler, supply and return ducts, and bathroom exhaust ducts located in the attic?

6. Would the attic need dedicated supply air, a return-air pathway, transfer grilles, mechanical ventilation, dehumidification, or another form of temperature and moisture control?

7. Could this approach reduce or negate some of the intended benefit of moving the thermal and air-control layers to the roofline?

8. Before accepting this proposal, what calculations, measurements, or design information should the contractor provide?

I would also appreciate recommendations for objective performance criteria. For example, should the contractor be expected to demonstrate that the system can maintain a specified indoor temperature and relative humidity at an agreed outdoor design condition?

I am not opposed to a technically sound corrective measure, but I am concerned about materially changing the attic assembly without understanding the long-term moisture, durability, and HVAC-performance consequences.

Thank you for any guidance!