Stack Effect and the Neutral Pressure Plane

Why this matters

The same house can frost its attic sheathing over the upstairs bathroom, pull cold air through the basement outlets, and run the upstairs bedrooms noticeably warmer than the living room, all on the same January night, with every fan in the house switched off. None of that is a broken part. It is stack effect, the one pressure driver that works every hour of the heating season without anyone turning it on. A tech who can picture where the house is pushing out and where it is pulling in knows which leaks matter, which way the moisture is going, and why sealing the wrong end of the house can make a problem worse.

Why warm air pushes

Air gets lighter as it warms. A column of 70 F indoor air weighs less than a column of 10 F outdoor air the same height. At the bottom of the house, the heavy outdoor column presses harder than the light indoor one, so outdoor air is pushed in through any low hole. At the top, the indoor air, having lost less pressure with height than the denser outdoor air beside it, is pushed out through any high hole.

So in a heated house in winter:

• The bottom runs negative to outdoors. Air leaks in at the basement rim joist, slab cracks, low wall penetrations, the bottom of the front door.
• The top runs positive to outdoors. Air leaks out at attic bypasses, top plates, recessed lights, the attic hatch, upper-floor window heads.
• Somewhere between is the neutral pressure plane (NPP), the height at which indoor and outdoor pressure are equal and air at a hole there goes nowhere.

The total stack pressure, bottom leak to top leak, grows in a straight line with height, and grows with the indoor-outdoor temperature difference. That makes it the steadiest of the three drivers: wind comes and goes, fans cycle, but the stack keeps pushing as long as the house is warmer than outdoors. See related: The House as One Pressure System, for how stack adds to wind and fans.

Worked example: how big the stack is

A two-story house over a basement. The lowest significant leaks are at the basement rim joist, the highest at attic bypasses above the second-floor ceiling, about 20 feet apart. Outdoors 10 F, indoors 70 F.

The relationship. In the form the ASHRAE Handbook, Fundamentals gives it, the stack pressure per unit height is gravity times the difference in density between the outdoor and indoor air. At sea-level pressure, that works out to about 3,460 times the difference between the reciprocals of the two absolute temperatures, in pascals per meter.

The numbers. 10 F is about 260.9 K and 70 F is about 294.3 K. One over 260.9 is 0.003833; one over 294.3 is 0.003398. The difference is 0.000435. Times 3,460 gives about 1.51 Pa per meter. Twenty feet is about 6.1 meters, so the full stack is 1.51 times 6.1, about 9.2 Pa from the bottom leaks to the top leaks.

Check it another way. The densities directly: air at 10 F and sea-level pressure is about 1.353 kg per cubic meter, at 70 F about 1.200. The difference, 0.153, times gravity, 9.81, is about 1.50 Pa per meter. The two methods agree.

Split by the neutral plane. If the leaks are spread evenly top and bottom, the NPP sits near mid-height, so the basement rim runs about 4.6 Pa negative and the attic bypasses about 4.6 Pa positive. Those are the pressures pushing air in and out through each hole.

Taller house. The same temperatures on a three-story house with 30 feet between leaks: 1.51 times 9.1 meters is about 13.8 Pa, half again as much. Stack scales with height, so tall houses, houses on walkout basements and houses with a tall stairwell or open foyer connecting all floors feel it hardest.

           attic: air pushed OUT
           +4.6 Pa  ^      ^
   +-------------------------------+
   | 2nd floor       out ->        |   20 ft
   |                               |   between
   |- - - - neutral pressure plane |   lowest
   |        (0 Pa, mid-height)     |   and
   | 1st floor       <- in         |   highest
   |                               |   leaks
   | basement   <- in at rim joist |
   +-------------------------------+
           -4.6 Pa  outdoor air pulled IN
   winter: 70 F inside, 10 F outside

What moves the neutral plane

The NPP sits wherever the air coming in balances the air going out. Add holes high and more air can leave up there, so the house drops lower in pressure overall until the low holes bring in enough to match. The plane rises. Add holes low and the plane falls.

Worked shift. Take the same 9.2 Pa house and give it twice as much leakage area at the top as at the bottom, which is common in an older house with an open plumbing chase, unsealed recessed lights and a leaky attic hatch. Treating each hole like a simple orifice, where flow goes with the square root of pressure, the plane rises until the pressure across the low holes is four times the pressure across the high ones, because twice the area at the top needs only a quarter of the pressure to pass the same air. That puts the NPP at four fifths of the height. The basement now runs about 0.8 times 9.2, or 7.4 Pa negative, and the top only 1.8 Pa positive. Real house leaks behave a little differently from orifices, and with a typical leak exponent the plane lands nearer three quarters of the height, about 6.8 Pa negative at the bottom. Either way, the lower floors work much harder.

That has three consequences a tradesperson should know:

• Leaky tops pull basements negative. The house with big attic bypasses has the most negative basement, which is where the natural-draft water heater, the sump and the radon live. See related: Radon and Soil Gas Entry Paths.
• Exhaust fans raise the plane too. Running exhaust drops the whole house lower in pressure, so more of the house sits below the plane and pulls in. Supply-only ventilation pushes the plane down. See related: Counting Every Exhaust Device in the House.
• An open fireplace damper is a big high hole. A masonry flue with its damper open moves the plane up and depressurizes the basement, and in winter the cold chimney itself often runs the other way. See How Draft Actually Works.

Seal the top first, and why it matters which end

Because stack flow in winter goes in low and out high, sealing only the low end of the house has a side effect: with less area at the bottom, the plane rises, so the basement and lower floors run more negative, which is where the natural-draft water heater and the soil-gas paths are. Meanwhile every attic bypass is still open, so warm, moist indoor air keeps leaving through the ceiling, meeting cold sheathing and condensing, frosting and later dripping. That is why a basement rim-joist foam job alone does not cure attic frost when the attic was never touched.

Sealing the top first does the reverse. It closes the exits where moist air does its damage. The plane drops back toward the middle, so the pressure across the few high holes that remain rises a little, but with most of them gone far less air leaves through the ceiling. By the same arithmetic the basement runs less negative. The standard advice to air seal the attic floor before anything else comes from this. See How to Air Seal Penetrations Into the Attic for the method.

The caution runs the other way for combustion. Any sealing tightens the house overall, and a tighter house is depressurized more by the same exhaust fans. The trade that seals should ask whether a natural-draft appliance remains. See related: Which Trade Jobs Change House Pressure and Moisture.

Summer stack is weaker and backwards

In an air-conditioned house in summer, indoors is cooler and denser than outdoors, so the stack runs the other way: the top of the house is pulled in and the bottom pushes out. It is also much weaker, because the temperature difference is smaller.

The same 20-foot house in July. Indoors 75 F, outdoors 90 F. That is 297.0 K and 305.4 K, reciprocals 0.003367 and 0.003274, difference 0.0000926. Times 3,460 is about 0.32 Pa per meter, times 6.1 meters is about 2 Pa across the whole height. Against the winter 9.2 Pa, that is about a fifth.

Two summer effects still matter:

• Hot attic air comes in at the top. With the upper floor slightly negative, attic air, often well above 120 F on a sunny afternoon, and dusty, is drawn in through the same ceiling bypasses that leaked out all winter. Insulation dust tracking at a recessed light or an attic hatch is the visible trace.
• Cool indoor air leaves the bottom. A cool basement pushing out is not a moisture problem by itself; the problem is when a basement is depressurized by fans and pulls humid air in through the rim. See related: Relative Humidity Versus the Moisture Actually in the Air.

On a mild day, with the house near outdoor temperature, stack is close to zero, which is why spring and fall service visits see a house behaving nothing like it does in January.

Field cues that tell you the plane is working

• Frost or staining on attic sheathing directly above a bath, a chase or the attic hatch, worse near the ridge: air leaving at the top.
• Cold drafts at first-floor and basement outlets and baseboards in winter, gone in mild weather: air entering low.
• Upstairs rooms warmer than downstairs in winter beyond what the duct design explains.
• Interior doors to an attic or bonus room that push open or whistle: pressure across them.
• A smoke pencil at a basement rim joist crack drawing in, and at an upstairs ceiling light pushing out, on the same day. See How to Use a Smoke Pencil to Read Air Direction.

Each one is a reason to think about stack before blaming the duct system, the insulation or the window that happens to be nearest the symptom.

References

• See related: The House as One Pressure System; Wind Pressure on a House and What It Moves
• See related: How Draft Actually Works (stack inside a vent, the same physics in a smaller column)
• See related: How to Air Seal Penetrations Into the Attic; How Moisture Moves Through a Building
• ASHRAE Handbook, Fundamentals, chapter on ventilation and infiltration (stack pressure and neutral pressure level)