Arkansas HVACR NewsMagazine March 2026

HVACR NewsMagazine March 2026

Tech News

onto the material. That can lead to its own set of problems, like roof rot.

What will happen to the warm air at the top when it gets close to the cold wall? It’ll sink. This process will keep happening as more warm air moves toward the cool wall to replace the air that just sank, forming a convective loop. By extension, ventilation strategies that rely on natural convection won’t work the same way to prevent sweating when you have a cool knee wall on one side. In those cases, ducts and air handlers should be as far away from the knee wall as possible for the best chance at reducing sweating. Everything in this tech tip so far has been about vented, unconditioned attics. So, what about sealed attics? It turns out that heat and moisture behave the same way in sealed, unvented, conditioned attics. The “ping pong” effect still happens, and moisture still accumulates at the ridge. Ridge vents defeat the purpose of a sealed attic by giving air a path, so we need to rely on something else to allow moisture to escape. Ideally, we’d need a membrane that is air closed (to prevent air from entering and bringing water with it) but still vapor-open (to let water vapor diffuse through it and get out), like a passive pressure-relief valve. That way, we keep our sealed attic and still give moisture an escape path. Well, we’re in luck. Those membranes are a real thing, and they’re called vapor diffusion ports . Vapor diffusion ports are air-closed, vapor open building materials, meaning they allow water vapor to pass through (and exit the structure) but block air movement (and the negative effects of air entering the structure). In other words, they are air barriers but NOT Moisture Management in Sealed Attics

The Real Effects of Poorly Designed Ventilation

In very humid climates, like Florida, it’s not unrealistic to introduce a gallon of moisture every hour for every 110 CFM that comes in through those vents and is allowed to move across the attic. You can imagine how that moisture math quickly gets out of hand over time with inadequate exhaust ventilation. One gallon per 110 CFM per hour is 24 gallons per day per CFM, etc. The gallon per 110 CFM moisture rate is a Florida-based rule of thumb and isn ’t as extreme in other climates, but it’s a reality for many homes in Florida and other coastal areas of the Southeast. Of course, there are many more reasons why ducts and air handlers sweat in the attic, but improper vent location is a big reason.

A Word of Caution for Knee Walls

The idea that air comes in through soffit vents, rises along the outer area of the attic, and exits through the ridge vents only works if convection is allowed to work uninhibited. Now, what might hinder natural convection?

Enter knee walls.

Knee walls separate a conditioned room from an unconditioned attic. In a hot climate, there will be a clear temperature difference between a conditioned room and an unconditioned attic; the room will be much cooler. Heat naturally wants to move from hot to cold (high to low all over again), so there will be heat losses across the wall via conduction as heat in the attic moves toward the cooler space.