Attic insulation, radiant barrier, and ductwork
Background: The house is located in Atlanta Georgia and is a 3000 square feet two story with a high roof attic and the second floor HVAC and ductwork present in the attic. The house was built in 1984. Cooling bills avg about $55-60 per month this Summer. I am after better comfort. Attic Temperature highs around 103.
Question 1 – I have an area with a house fan on the second floor hallway ceiling that has wood decking over the old insulation in the attic. I want to keep the decking to get to the furnace. The area in the hallway has an HVAC return and the thermostat is located there. I want to increase the insulation without raising the floor. I would like to remove the old settled blown in insulation about 6″ or so and replace in the cavity with several cut pieces of XPS foam board 3 to 4 pieces stacked in the cavity and then reinstall the plywood foor. I will also remove the house fan. My concern is moisture build up between the ceiling dry wall and the foam board. This would not be the whole attic floor by about 10% of it max.
Question 2 – In the same attic I have installed radiant barrier under the roof rafters. I did not want to use spray foam due to outgassing and cost. My thought was to attach 2 inch XPS or polyiso to the rafters to improve the attic insulation. However, I am not sure if this can cover the underside of the radiant barrier directly. In addition, I have R 3.8 duct insulation on the ductwork which is raised above the floor. If I add additional insulation will this help much. I have already sealed the duct leaks with mastic.
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