The Stack Effect in New Orleans Homes: How Humid Air Gets Pulled Into Your House

Almost every explanation of the stack effect you will find online was written for a house in a cold climate. Warm air rises, escapes through the attic, and cold air gets pulled in at the bottom to replace it. That description is correct, and for six months of the year in Louisiana it is the wrong way round.

Getting this right matters, because the stack effect is the argument for air sealing. If you only know the cold-climate version, you will aim the work at the wrong surface, and you will miss the part that actually costs you money here, which is moisture.

What the stack effect actually is

Warm air is less dense than cool air. When the air inside a building is a different temperature from the air outside, the two columns of air weigh different amounts. That difference creates a pressure difference across the envelope that varies with height.

The result is a neutral pressure plane somewhere in the middle of the house, with the envelope pushed one way above it and the other way below it. Air leaves through the leaks on the high-pressure side and enters through the leaks on the low-pressure side.

Nothing about that mechanism requires cold weather. What the weather decides is the direction.

Winter here: the version you have read about

On a cold January night in New Orleans, the inside of the house is warmer than the outside. The indoor air column is lighter, the top of the house is pushed outward, the bottom is pulled inward, and the house behaves like a chimney. Air leaves through the attic and is replaced by air drawn in at the floor, which in a raised pier-and-beam house means outdoor air from the crawl space.

The Department of Energy describes the same mechanism from the practical end: sealing holes in the attic ceiling reduces the house’s suction, or stack effect, so fewer contaminants such as radon and other soil gases are drawn up into the house from the ground.

Summer here: the direction flips, and the pressure is smaller

Run the air conditioning in August and the inside of the house becomes the cool side. The indoor air column is now the denser one, so the pressure pattern reverses. The upper part of the envelope is pulled inward and the lower part is pushed outward.

The second thing that changes is scale. The stack pressure depends on how big the temperature difference is. A northern house in January can face a difference of fifty or sixty degrees. A New Orleans house in August, at 95 outside and 75 inside, is working with about twenty. The summer stack effect here is real and it is weaker than the winter version most articles describe.

That is why a purely buoyancy-based story does not explain what a blower door finds in a local house in summer. Two other drivers often matter more, and which of them dominates in your house is something a test can separate rather than something an article can tell you.

The drivers that often matter more in a Gulf Coast summer

Duct leakage. If the air handler and ducts sit in an unconditioned attic, every leak on the return side pulls superheated attic air into the system, and every leak on the supply side dumps conditioned air where it does no good and puts the house under negative pressure. That negative pressure has to be relieved by air pulled in through the envelope. A leaky duct system is effectively a fan that imports outdoor air all day. Louisiana’s code has a related local rule: in climate zone 2A, supply and return ductwork must not be buried in insulation.

Exhaust appliances. Range hoods, bathroom fans, and dryers all remove air from the house. Whatever they remove has to be replaced, and in a house with no designed makeup air, it is replaced through the leaks.

Wind. Windward walls get pushed; leeward walls get pulled. On a breezy day, this can swamp the stack pressure entirely.

The part that costs you money: latent load

Here is why any of this matters more in New Orleans than in most of the country.

In a cold climate, infiltration is mostly a heating penalty. Air comes in cold and has to be warmed. In a hot humid climate, infiltration is mostly a moisture penalty. Every cubic foot of outdoor air that gets in during summer arrives carrying water vapor, and your air conditioner has to condense that moisture out before the house feels comfortable. That is latent load, and it is the reason a house can hit its thermostat setpoint and still feel clammy.

The direction of the vapor drive is well documented locally. A year-long study of twelve Louisiana homes, eight of them in New Orleans, run jointly by the USDA Forest Products Laboratory and the LSU AgCenter, found that the predominant vapor drive was inward from May through October, the months when air conditioning is running. During the rest of the year, the difference between indoor and outdoor water vapor pressure was small.

The same study found that in open pier and beam crawl spaces, the water vapor pressure was essentially the same as outdoors and the air temperature tracked outdoor temperature closely. In other words, the space under a raised New Orleans house is outside, whatever it feels like when you crawl into it.

If your house feels sticky at 74 degrees, indoor humidity rather than air temperature is usually what you are reacting to.

What this changes about how you seal a house

If the winter version were the whole story, you would seal the ceiling plane and stop. Because both directions happen and because ducts and exhaust fans dominate in summer, the work has to be more complete than that.

1. Seal the ceiling plane. Attic hatches, dropped soffits, chases, recessed lights, and top plates. This is the highest-value surface in the heating season, and it is cheap to reach.
2. Seal the floor plane. Band joists and penetrations through the subfloor. In a raised house, this is the boundary against outdoor air, and it is the one most often ignored.
3. Seal the vertical connections. Plumbing and flue chases and balloon-framed wall cavities join the two planes and bypass everything you did to either of them.
4. Deal with the ducts. In a hot attic, duct leakage can move more air than the envelope does. Sealing the house without sealing the ducts leaves the biggest fan in the building still running.
5. Then insulate. Fibrous insulation slows heat and does not stop air. The full list of local leak paths is in our guide to the leak points behind a failed blower door test.

Moving the insulation and air barrier from the attic floor to the roof deck brings the ducts and air handler inside the conditioned envelope, which removes the duct problem rather than patching it. That is the reasoning behind an unvented attic, and it is a design decision rather than an upgrade you bolt on later.

Tighten the house, and you change its ventilation

A tighter envelope means less uncontrolled air exchange, which is the point. It also means the fresh air that used to arrive by accident has to be designed in.

Louisiana wrote the trigger into the code. Where a blower door test is performed and the air infiltration rate comes in under 3 air changes per hour at 50 pascals, the dwelling must be provided with whole-house mechanical ventilation in accordance with Section M1507.3. The Department of Energy makes the same point in plainer terms for anyone air sealing an existing home: work out how much fresh air the house needs, and how combustion appliances will get their air, before you start closing holes.

Spray foam can take a house past 3 ACH50 without much difficulty. Have the ventilation conversation first. Our guide to Louisiana’s blower door code requirements sets out the rule and who it applies to.

Seeing it for yourself

A blower door test makes all of this visible in an afternoon. The fan puts the house under a steady pressure difference, and a technician walks it with a smoke pencil or an infrared camera while the pressure is on, so the air paths show themselves instead of being argued about.

If that is what you want before spending money on the envelope, that is what blower door testing and a full energy audit deliver.

Frequently asked questions

Find out what your house is actually doing

Star Spray Foam is based in New Orleans at the address below. Ask whether your address is inside the working area when you call. If you want the air paths mapped before anyone quotes you for insulation, get in touch and ask about a diagnostic blower door test.

Star Spray Foam
5826 Louis XIV St, New Orleans, LA 70124
504-229-6500

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