Use Spray Foam to Stop Register Grilles From Sweating
Our resident energy nerd suggests that a lack of air-sealing and insulation may be the cause of condensation around air ducts in a cathedral ceiling.
I have a client here in Maryland who is complaining about wet air grilles during air-conditioning season. The grilles get rusty, and there may be mold. It appears limited to the master bathroom on the upper floor, where the vents are in the cathedral ceiling. The air handler for these vents is located in an attic space above the adjacent hallway, where the ceiling is flat. The fiberglass-batt insulation in this attic space is under the roof, not on the ceiling below. The air ducts, which are insulated as well, appear to run between the insulation and the roof sheathing. The client had an HVAC contractor take a look at it, and he said that the duct insulation looks good. I have two questions. First, is the duct location causing the problem? Second, is there a reasonably easy fix, since relocating the ducts would be an expensive proposition given the cathedral ceilings?
—DAVID, via email
Martin Holladay, editor of Green Building Advisor, responds: In order to get this type of condensation, you need two elements: humid air and a cold surface. To eliminate the condensation, you need to cover the cold surfaces with vapor-impermeable insulation or seal up the leaks that allow the humid exterior air to come into contact with the cold surfaces.
It’s certainly true that the ducts in this house aren’t located properly. Ducts should never be installed in a cathedral ceiling. The best solution is to relocate the forced-air register and the duct serving the register— for example, by building an airtight interior soffit to hide the ductwork.
My guess is that the rafter bay where this duct is located is leaky, allowing hot, humid exterior air to come into contact with the cold register. As a first step, assuming that the register boot is uninsulated, you could try insulating the metal register boot with canned spray foam. If this isn’t sufficient, it’s time to open up the ceiling so that you can either seal the cracks that allow exterior air to reach the register boot, or move the duct to a better location.
This article appeared in Fine Homebuilding Magazine issue 281 titled “Sweaty Register Grilles”