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Be Careful When Adding Air Conditioning to a Spray Foamed Attic
Conditioned attics are a smart move. You bring the ductwork inside the building envelope, insulate and air seal the roof deck all at once, and the space performs better. It's a solid strategy — especially in the South, where heat and humidity are always working against you.
But here's where it gets tricky: in hot, humid climates, moisture-laden air rises. If it gets trapped in an encapsulated attic, you've got a problem.
Building code recognized this. Back around 2016, amendments were proposed to require a minimum amount of conditioned air be supplied to these spaces — enough to keep moisture in check without overcooling the area. The mechanical system is there to control humidity and semi-condition the space. That's it.
Which brings us to the real issue. Because when it comes to air conditioning, more isn't always better.
What Went Wrong in Nashville
Our friends at PurEstate Home Performance and Koala Insulation in Nashville ran into exactly this situation. The project was straightforward on paper: spray foam the roof deck, add a bedroom to the newly conditioned attic. Classic encapsulated attic conversion.
Not long after the new bedroom was occupied, mold started showing up. Most of it was coming from a knee wall area where the ductwork and mechanical equipment were located. More appeared on both sides of the roof peak.
Koala called in PurEstate's home performance team to investigate. Infrared cameras, thermometers, moisture meters — they had the tools, and the problem became obvious fast.
What They Found
The mechanical contractor had added two supply runs to the attic space. One open duct was placed near the roof peak. The other dumped cold air directly near the HVAC unit itself. No grilles. No restrictive control. Just butterfly dampers running near full capacity with no monitoring of the space.
The numbers told the story:
· Near the HVAC unit in the knee wall: 52°F, 83% RH
· At the peak duct against the vaulted ceiling drywall: nearly identical
· With no return air in the space, the trunk lines and ducts were saturated — and dripping
Inside the bedroom, things weren't much better. The ceiling on the mechanical side was around 67.5°F. On the opposite side — no mechanical — it was in the low 80s. The attic without mechanical ranged from 69–75°F at around 80% RH, while the bedroom was being kept at 67°F.
Here's the thing: the side with no mechanical had high humidity too. But it didn't have a mold problem. The side with the open ducts — making the space too cold, too fast, with no return — condensed immediately.
The Fix
The open duct runs were the core problem. Making an attic space colder than the conditioned room below draws moisture straight to it. There's no good reason to overcool these spaces.
The right approach: eliminate the open ducts entirely, or baffle them down to a true minimum. Better yet, install a dehumidifier in both attic spaces. Control the moisture directly instead of chasing it with overcooled supply air.
