In many southern states, it’s common for an HVAC contractor to install an air handler and ductwork in a vented unconditioned attic. In July, the attic temperature rises to 120°F. Perhaps the homeowner, irritated by his hot bedroom and concerned about his high electricity bills, sticks his head through the attic access hatch, and nearly faints. It’s a sauna up there.
One solution to this problem is to convert the vented unconditioned attic to an unvented conditioned attic by applying spray foam to the underside of the roof sheathing. That’s a good solution, because it brings the attic air handler and attic ductwork into the home’s conditioned space, but the price tag is very high.
So the homeowner thinks of a different solution, and posts a question on GBA: Can I build a mechanical room with insulated walls in my attic, so that the air handler is (more or less) in conditioned space?
I usually advise, “That’s a bad idea.” Let’s investigate why.
The many features of a mechanical room
First of all, let’s describe the features of an adequate mechanical room. It needs to have an insulated floor, walls, and ceiling, and these assemblies need to be relatively airtight. The walls should be finished with 5/8-inch drywall. The room needs to be big enough to allow maintenance workers to access the equipment comfortably, and to swap failed equipment with new equipment when necessary.
Can you create this type of mechanical room in an attic? Sometimes, but not always. What are the hurdles?