Building insulation and air sealing: the envelope field guide

Building insulation and air sealing, and why both jobs matter

Building insulation and air sealing are the two jobs that make a building envelope work. The envelope is the shell that separates conditioned space from the weather: the walls, the roof, the floor, the windows and doors, and every joint between them. Insulation slows the heat that conducts through that shell, and it is rated in R-value, the resistance to conductive heat flow. Air sealing stops the air that leaks through the gaps, and that leaking air carries far more energy and moisture than most people credit. The Building Science Corporation puts uncontrolled air leakage at roughly a quarter to forty percent of a home's energy loss, which is why a thick blanket of insulation full of holes underperforms a thinner assembly that is actually sealed.

The hook for the whole subject is this: air sealing matters as much as the R-value, and on a leaky building it matters more. R-value only describes conduction. It says nothing about the air pouring through a gap around a recessed light or up an open stud bay, and that air drags heat and water vapor with it.

The work is three things detailed together. Get the air barrier continuous, get the insulation right for the assembly, and get the vapor control on the correct side for the climate. Pull them apart and the wall rots or the building runs cold. The roof side of this lives in two sibling guides, one on roof insulation and cover board and one on the roof vapor retarder and air barrier. This guide is the whole-building version.

Why air seal before you insulate?

Air seal first, then insulate. That sequence is the building-science order, and getting it backward is the most common way a retrofit underperforms. Two reasons drive it. The first is access. The best places to seal, the top plates in the attic, the rim joist, the penetrations through the framing, are open and reachable before the insulation goes in. Blow cellulose into the attic first and you have buried the top-plate gaps under a foot of loose fill you now have to dig back out to seal.

The second reason is physics. Insulation slows conduction, but it does not stop moving air. Air pushes straight through fiberglass like it is not there, carrying heat and moisture with it, so a leaky assembly never delivers the R-value on the label no matter how much you pack in.

Think of it as air barrier, then thermal barrier. Find and close the holes, confirm the barrier is continuous, and only then add the insulation that the sealed assembly can actually use. On a tight budget, the dollar spent sealing the big leaks usually buys more comfort and lower bills than the dollar spent on extra R. Seal first. Insulate second.

What is the difference between R-value and air leakage?

R-value is resistance to conductive heat flow, the heat that moves through a solid material from the warm side to the cold side. Air leakage is convective: bulk air moving through gaps, and it carries heat and water vapor along with it. Different mechanisms, measured differently, fixed differently. R-value is a property of the insulation. Air leakage is a property of the assembly and how it was detailed.

Both matter, and they matter because they fail in different places. A wall can hit its rated R in the cavity and still bleed energy through an unsealed top plate, because the calculated R never accounted for air movement. In real-world terms the leakage is usually the bigger loss and the bigger problem, because moving air also carries moisture to cold surfaces where it condenses. R-value alone does not move water. Air leakage does, which is why it shows up as both an energy loss and a rot risk.

The practical line: rate the insulation by R-value, but judge the building by how tight it is. A blower-door number tells you about the air leakage that the R-value cannot. The two together describe the envelope. Either one alone misses half of it.

Insulation types and where each fits

Six families cover almost all building insulation, and they trade R per inch against cost, moisture tolerance, air-sealing ability, and fire performance. No single product wins everywhere. The right call is the one that fits the assembly, the climate, and the moisture conditions, sized to the energy code after any real-world de-rate.

The R per inch figures below are rated values at standard temperature. Treat them as a starting point. Polyiso loses R in the cold, cellulose settles, and any batt loses R if it is compressed or gapped, so the installed number runs below the label. Confirm the rated values against the manufacturer's published data for the specific product before you size to them.

Fiberglass and mineral wool batt

Batt insulation is the cheap, common cavity fill, and its weakness is not the material. It is the install. A fiberglass or mineral wool batt only delivers its rated R when it fills the cavity completely, with no gaps, no compression, and no voids behind wires and boxes. Compress an R-19 batt into a too-shallow cavity and you do not get R-19, you get less, because R-value rides on the trapped air the loft holds. Leave a gap at the top of the bay, or tuck the batt behind a wire instead of splitting it, and you have left a channel for air and a spot of near-zero R.

Mineral wool batt runs a little higher per inch than fiberglass, around R-4 against fiberglass near R-3.1 to R-3.4, holds its shape, resists fire, and does not slump when it gets damp. Fiberglass is cheaper and lighter. Either one is friction-fit, cut slightly oversize so it springs into the bay and holds without sagging.

What separates a good batt job from a bad one is the energy rater's grade. A Grade I install fills the cavity with no defects. A Grade III can lose a quarter of the rated R to gaps and compression. The batt does not air seal anything, so it rides on top of a separate air barrier, and it does not forgive a sloppy hand. Cut it to fit the bay, around the box, around the wire, every time.

Blown cellulose and fiberglass

Blown insulation is the default for attics and the way to fill an existing closed wall. Loose fill blows in fast and covers the irregular attic floor, getting into the gaps a batt bridges over. Cellulose, ground-up recycled paper treated with borate for fire and pests, runs about R-3.2 to R-3.8 per inch and packs around obstructions well. Blown fiberglass is lighter and faster but lower per inch, around R-2.2 to R-2.7 loose in an attic.

Two field facts decide whether blown insulation performs. The first is coverage depth. An attic is specified by installed inches and by a settled R-value, and the bag has a coverage chart that tells you how many bags hit the target depth over the square footage. Skimp on bags to save money and the attic is short on R from day one. The second is settling. Cellulose settles roughly a fifth over time as it compacts, so it has to be installed deeper than the final target, or dense-packed in a cavity where it cannot settle.

Dense-pack is the wall version: cellulose blown to a high density into a closed cavity, which both insulates and slows air movement through the bay. It is the common way to insulate an existing wall without opening it. Dense-pack cuts air leakage, but it is not a substitute for a real air barrier at the top and bottom plates.

Rigid foam board: XPS, EPS, and polyiso

Rigid foam board is where continuous exterior insulation comes from, the layer that goes outside the sheathing to wrap the framing and stop thermal bridging. Three foams cover the work. EPS, expanded polystyrene, is the cheapest per R at about R-4.2 per inch and stable across temperature. XPS, extruded polystyrene, runs about R-5 per inch fresh and settles toward R-4.5 as its blowing agent leaves over years, and it resists water well. Polyiso gives the most R per inch on the label, around R-5.6 to R-6.5, with a good fire rating.

The polyiso catch is the cold de-rate. Polyiso loses R as it gets cold, the opposite of what you want in a heating climate, so a board rated near R-6 at room temperature can perform closer to R-4.5 on a cold winter day. The roof insulation and cover board sibling guide covers this in depth for above-deck roofs, and on walls the same lesson holds. In a cold climate, do not size polyiso continuous insulation on the warm label. Design to the cold value, or pick a foam that holds its R in the cold.

Where the board goes drives the choice. EPS and XPS for below-grade and foundation work where water is present, polyiso for above-grade walls where its higher R and fire rating pay off, and the seams taped so the board doubles as part of the air barrier.

Open-cell vs closed-cell spray foam: which one?

Spray foam does two jobs in one pass, air sealing and insulating, which is its real value. It expands into every gap and seals as it fills, so it is both the thermal barrier and the air barrier where it is applied. The choice is open-cell against closed-cell, and they are different materials for different problems.

Open-cell foam is the lighter, cheaper one, around R-3.5 to R-3.8 per inch. It is vapor-open, so the assembly can dry through it, and it acts as an air barrier at a few inches. It expands a lot, fills irregular cavities, and is the common pick for interior walls, the underside of a roof deck, and sound control. Closed-cell foam is denser and higher R, around R-6 to R-7 per inch, which makes it the choice where space is tight. It is a vapor retarder at roughly two inches and up, it adds real rigidity to a wall, and it resists water, which is why it is the foam for rim joists, below-grade, and flood-prone assemblies.

Two warnings ride with foam. It needs a fire-protection layer, covered below, and a closed-cell layer in the wrong spot can trap moisture by stopping the assembly from drying. Cost is the other factor. Foam costs more per R than batt or blown, so it earns its place where the air seal and the high R per inch are worth paying for, not as a default for the whole house.

The continuous air barrier

The air barrier is the system that stops air from leaking through the envelope, and the word that makes or breaks it is continuous. It is not one product. It is a connected plane of materials and sealed joints that wraps the whole conditioned space with no holes. The field of a wall is the easy part. The failures live at the transitions: the top and bottom plates, the rim joist, the window and door openings, the penetrations for plumbing and wiring, and the joint where the wall meets the roof and the foundation.

A hole defeats the barrier for the area it serves, and the cost of a hole is not small. The roof vapor retarder and air barrier sibling guide carries the number worth memorizing: a one-inch square hole can pass on the order of a hundred times more water by air leakage than a whole sheet passes by diffusion over a heating season. The air barrier can be the drywall sealed at its edges, the exterior sheathing with taped seams, a house wrap detailed as a barrier, the foam itself, or a dedicated membrane. Whatever it is, it has to connect to itself at every change of plane and material.

This is the number-one air-seal detail. Detail the transitions like they are the job, because for the air barrier they are. A perfect field of wall with an open rim joist is a leaky building.

Where the air leaks actually are

The leaks are not spread evenly, and chasing the small ones first wastes the budget. The big losses cluster at the top and bottom of the building, driven by the stack effect, plus the penetrations through the envelope. Go after the big ones first.

At the top, the attic is the worst offender. Warm air rises and pushes out through every gap in the ceiling plane: the top plates of interior and exterior walls, the dropped soffits over cabinets, the attic hatch, recessed lights, bath fans, and the holes drilled through top plates for wiring. At the bottom, the rim joist, where the floor framing sits on the foundation, and the sill plate are the classic make-up-air entry, pulling cold air in to replace what left the top. In between sit the penetrations: plumbing stacks, dryer vents, electrical and cable entries, and the gaps around windows and doors.

The field method is to seal the biggest holes first, then work down to the small ones. A foot-square open chase to the attic moves more air than every outlet gasket in the house combined. Find the big openings, the chases, the dropped ceilings, the attic top plates and penetrations, and close them before you fuss with the trim.

The stack effect and the pressure it creates

The stack effect is the pressure that drives most of the air leakage in a heating climate. Warm interior air is lighter than the cold air outside, so it rises and pushes out through any opening high in the building. As it leaves the top, it creates negative pressure low in the building that pulls cold outside air in through the bottom. The building behaves like a chimney, and the taller it is and the colder the day, the harder it pulls.

This is why the leaks at the top and bottom matter more than the ones in the middle. There is a neutral pressure plane around mid-height where the pressure difference is near zero. Above it, air is pushing out. Below it, air is pulling in. Seal only the bottom and the stack effect still drives air out the top and finds new make-up paths. Seal only the top and you cut the engine that drives the whole loop.

The rule is to seal the top and the bottom together. Close the ceiling plane to stop the air from leaving, close the rim joist and sill to stop the cold from coming in, and the stack effect loses its driving force. In a cooling climate the effect runs weaker and can reverse, so the same top-and-bottom sealing pays off for different reasons. Confirm the dominant direction for the climate.

Vapor control and the climate-zone rule

Vapor control is where the climate zone decides the detail, and copying a detail from the wrong climate is how walls rot. A vapor retarder slows water vapor from diffusing through the assembly, and the rule for where it goes is the warm-in-winter side. In a cold climate that is the interior side, so you stop interior moisture before it drives out into the cold part of the wall and condenses. The IRC ties this to climate zone: it generally calls for a Class I or Class II vapor retarder on the interior side of frame walls in climate zones 5, 6, 7, 8, and Marine 4, and it does not require an interior vapor retarder in zones 1, 2, and 3.

In a hot-humid climate the rule flips. The vapor drive runs inward most of the year toward the cool, dehumidified interior, so an interior vapor retarder is on the wrong side and traps that inward moisture. The detail that is right in Minnesota is backward in Houston. This is the part to hedge hard. The required vapor retarder class and side come from the adopted code edition and the climate zone, and a dew-point analysis settles the assembly. The roof side of vapor control lives in the roof vapor retarder and air barrier sibling guide. The wall and floor side follows the same physics.

Variable-perm retarders, sometimes called smart retarders, exist for the assemblies that have to dry both ways. They tighten in winter and open in summer to let trapped moisture out. Where an assembly needs to dry to the inside part of the year, that product can be the right answer. Confirm the requirement against the code, the climate, and the manufacturer.

Do not trap moisture in the assembly

The number-one envelope failure is trapping moisture, and it almost always comes from a vapor barrier on the wrong side, or two vapor barriers with the wall sealed between them. Every assembly takes on some moisture. The question is whether it can dry. Put a low-perm layer on the cold side, or sandwich the insulation between two low-perm layers, and the water that gets in has nowhere to go. It accumulates, the framing stays wet, and the wall rots from the inside while the surfaces look fine.

The classic version is polyethylene sheet, a Class I vapor barrier, installed on the interior of a wall in a climate where the wall needs to dry inward, or used together with closed-cell foam or foil-faced board on the exterior. Now the assembly is closed on both sides. The code warns about this directly: a Class I interior vapor retarder combined with a Class I exterior layer needs an approved design, because the default is a wall that cannot dry.

The rule is to give the assembly a drying path in at least one direction. Control vapor on the warm-in-winter side, keep the other side more open so trapped moisture can escape, and design it for the specific climate rather than copying a detail. Hedge hardest here. The safe assembly is the one a building-science analysis signed off on for that climate zone, the adopted code, and the materials actually used. Two vapor barriers and the wall is a rot you built in.

Thermal bridging and why the wall is less than the cavity R

Thermal bridging is the heat that conducts around the insulation through the framing, and it is why the real assembly R is always less than the cavity R on the label. Wood and steel conduct heat far better than insulation, so every stud, plate, and header is a short circuit straight through the wall. The framing typically makes up around a quarter of a wood-frame wall's area, more at corners, headers, and intersections, and steel framing is far worse, often half the area or more.

The number that surprises people: a wall with R-20 in the cavity and a 25 percent framing factor performs closer to R-15 on a whole-wall basis, because the framing drags the average down. Cavity insulation alone commonly loses on the order of 20 to 30 percent of its label R to thermal bridging. The R-value on the batt is not the R-value of the wall.

The catch follows directly. You cannot solve thermal bridging from inside the cavity, because the studs are in the cavity. The fix is a layer that covers the studs too, which is continuous exterior insulation. Until you break the bridge, adding more cavity R gives diminishing returns, because the heat just takes the framing path around it.

Continuous exterior insulation

Continuous exterior insulation is a layer of rigid board, or mineral wool board, installed outside the sheathing so it wraps the whole wall, framing included. It does two jobs at once. It breaks the thermal bridge, because the board runs unbroken across the studs the cavity insulation cannot cover. And it moves the dew point outward, keeping the sheathing and the framing warmer in winter so interior moisture is less likely to condense inside the wall.

This is the direction the energy code has been moving. The IECC now writes many wall requirements as a cavity R plus a continuous R, with prescriptive paths such as R-20 cavity plus R-5 continuous, or R-13 cavity plus R-10 continuous, in the colder zones. The continuous layer carries a second rule in cold climates: there has to be enough of it to keep the sheathing above the interior dew point, which the IRC sets by climate zone, or the exterior foam becomes the cold condensing surface it was supposed to prevent. Too little exterior foam can be worse than none.

The field reality is that continuous exterior insulation changes the wall. The window and door details get deeper, the cladding attachment has to reach through the foam to the structure, and the foam seams become part of the air barrier. The values and the required continuous thickness shift by climate zone and code edition, so pull them from the adopted code, not from the last job.

The assemblies: attic, wall, rim joist, and foundation

Each part of the envelope wants a slightly different mix of air sealing, insulation, and vapor control, because each one faces a different load. The principle is the same everywhere, air seal first, insulate for the assembly, control vapor for the climate, but the execution changes.

The attic is mostly an air-sealing job before it is an insulation job, because the stack effect makes the ceiling plane the biggest leak. The rim joist is the spot closed-cell foam earns its keep, sealing and insulating an awkward, leak-prone joint in one pass. The foundation and slab edge are about moisture-tolerant insulation and a capillary break. The table below maps the common approach. The project spec and the climate zone control the actual R.

What is a blower door test?

A blower door test measures how leaky a building is. A calibrated fan mounted in an exterior doorway pulls air out of the building to hold a standard pressure difference of 50 pascals between inside and outside, and the fan reports how much air it has to move to keep that pressure. That airflow is the leakage rate. Read in cubic feet per minute it is CFM50. Divided by the building's volume to give air changes per hour, it is ACH50, the number most codes and programs use.

The test does two things. It puts a single number on the air leakage, so you can verify the air sealing actually worked instead of assuming it did, and run with a smoke pencil or an infrared camera it shows you where the air is moving so you can find and close the leaks. A foreman who has sealed a house and then watched the blower-door number stay high has learned where the hidden leaks were.

The energy code increasingly requires it. Recent IECC editions set a maximum ACH50 verified by a blower-door test, commonly around 5 ACH50 in the warmest zones and 3 ACH50 in zones 3 through 8, with stricter programs going lower. Passive House targets 0.6 ACH50. The exact limit and whether testing is mandatory depend on the adopted code edition and the AHJ, so confirm the number for the jurisdiction.

Build tight, ventilate right

A tight building needs mechanical ventilation, and skipping it is how a well-sealed house gets bad air. The old leaky house ventilated itself by accident through all its gaps. Seal those gaps and you stop the accidental air exchange that was carrying away moisture, cooking byproducts, radon, and indoor pollutants. The answer is not to leave the building leaky. It is to build tight and then ventilate on purpose, with a fan sized to bring in the fresh air the occupants need.

ASHRAE 62.2 sets the residential ventilation rate, and the common form adds a fraction of the floor area to a per-bedroom allowance, often written as 0.03 cubic feet per minute per square foot plus 7.5 cfm for each bedroom plus one. Commercial work runs to ASHRAE 62.1. The point is that the ventilation is designed and controlled, through a continuous exhaust, a supply system, or a balanced heat- or energy-recovery ventilator that brings in fresh air while reclaiming heat from the exhaust.

Do not just seal it up. A tight envelope without planned ventilation traps moisture inside, and that moisture finds cold surfaces and grows mold. Tight and ventilated is a healthy, efficient building. Tight and unventilated is a moisture problem waiting to happen. The two halves are one design.

What R-value does the code require?

The required R-value is set by the energy code for the building's climate zone, and on most work that means the IECC or ASHRAE 90.1, whichever the jurisdiction has adopted. Both split the country into climate zones numbered roughly 1 through 8, hot to cold, and the colder the zone the more R the envelope has to carry. The requirement is written by assembly, with separate numbers for the ceiling, the walls, the floor, and the foundation, and walls increasingly carry a cavity-plus-continuous format.

As a rough map only, residential ceilings commonly run from around R-30 in the hot zones up to R-60 in the cold ones, and walls from around R-13 in the cavity in the warm zones up to a cavity-plus-continuous combination such as R-20 plus R-5 in the cold zones. These numbers move every code cycle, differ between the IECC and 90.1, and have generally tightened. Do not carry a number from the last job.

Hedge hard here. The exact required R-values, the U-factor alternatives, the air-leakage limit, and whether a blower-door test is mandatory all come from the adopted code edition, the climate zone, and any local amendments, and the AHJ enforces them. Pull the requirement from the code in force for the actual address, confirm whether the project is taking the prescriptive R path or a U-factor or performance path, and verify it at plan check rather than from memory.

Fire code and foam safety

Foam plastic insulation has to be separated from the interior of the building by a fire-protection layer, and this is not optional. Rigid foam board and spray foam both burn and both give off smoke, so the IRC requires a thermal barrier between the foam and the living space, generally a 15-minute thermal barrier. The recognized default is half-inch gypsum board, which covers the great majority of jobs. Leave foam exposed to an occupied space and you have an inspection failure and a real fire-spread hazard.

There is a lighter requirement for spaces nobody occupies. In an attic or crawlspace with limited access, the code allows an ignition barrier instead of a full thermal barrier, a thinner layer such as mineral fiber, a wood structural panel, or a coating rated for the purpose. Some foams carry a tested coating that qualifies them to be left exposed in those spaces. That approval is product-specific and has to be confirmed against the listing.

Two more notes. Spray foam off-gases during and shortly after application, so the space has to be vented and unoccupied during the cure window the manufacturer specifies. And the fire rating is tested as a whole assembly, so the thermal barrier, the foam, and the substrate have to match a tested or listed combination. Confirm the fire-protection requirement against the adopted code and the manufacturer's listing for the specific foam.

Common mistakes

• Insulating without air sealing first, so the new insulation buries the leaks it was supposed to stop.
• Compressing or gapping batts, leaving voids behind wires and boxes, and losing a quarter of the rated R.
• Treating the field of an assembly as the air barrier while the rim joist, top plates, and penetrations stay open.
• Putting the vapor retarder on the cold side, or sandwiching the wall between two low-perm layers, so it cannot dry and rots.
• Ignoring thermal bridging and adding cavity R while the framing carries the heat straight around it.
• Sizing cold-climate polyiso or exterior foam on the warm rated R instead of the cold design value.
• Sealing the building tight with no mechanical ventilation, trapping moisture and indoor pollutants inside.

What to document

The assembly you built is what the energy compliance, the warranty, and the next renovation are all measured against, so the record has to name each layer, how it was sealed, and the climate-driven decisions behind it. The question it answers later is whether the wall that is growing mold, or the building that is running cold, was built the way it was specified. Without the record, that argument is unwinnable.

Capture the insulation type and design R per inch and total R for each assembly, the air barrier material and where it runs continuous, the air-sealing details at the top, the bottom, and the penetrations, the vapor retarder class and side, the continuous exterior insulation R and thickness, the blower-door result, the ventilation system and rate, and the climate zone and code edition the design was built to. If a dew-point analysis drove the vapor placement, keep it.

Field checklist

Standards and references

The energy code sets the R-values and the air-leakage limit. On most work that is the IECC or ASHRAE 90.1, whichever the jurisdiction adopted, with the required R-values, U-factor alternatives, and maximum ACH50 set by climate zone. Both have moved toward continuous exterior insulation and mandatory blower-door verification in recent cycles. The figures shift every edition and have generally tightened, so confirm them against the adopted edition and local amendments before citing them.

The building code, the IRC for residential, carries the vapor retarder and fire rules. R702.7 ties the vapor retarder class and side to the climate zone, using the perm classes: Class I at 0.1 perm or less, Class II from 0.1 to 1.0, Class III from 1.0 to 10. R316 requires a thermal barrier, generally half-inch gypsum, between foam plastic and the interior, with an ignition-barrier allowance in limited-access attics and crawlspaces. Ventilation for a tight building runs to ASHRAE 62.2 for low-rise residential and 62.1 for commercial.

For practice and design judgment, building-science sources lead: the Building Science Corporation for assembly detailing and the air-leakage-versus-diffusion math, ASHRAE for ventilation and energy, and the insulation manufacturer for the rated R, the cold de-rate, and the listed fire assembly. Material standards include ASTM C1289 for polyiso board, ASTM C578 for the polystyrenes, ASTM E2178 and E2357 for air-barrier permeance, and ASTM E96 for vapor perm. Above all of it, the adopted code, the project specification, the manufacturer's listing, and the AHJ control. The roof side is detailed in the roof insulation and the roof vapor retarder and air barrier sibling guides. Air seal first then insulate, make the air barrier continuous and control vapor for the climate, and break the thermal bridging and ventilate a tight building.

Units, terms, and conversions

The envelope carries a handful of terms that get used loosely, and mixing them up is how the wrong layer ends up in the wrong place. Keep the two heat paths separate: R-value is conduction resistance, air leakage is convective and carries moisture. Keep the two moisture controls separate: the air barrier stops bulk air, the vapor retarder slows diffusion.

R-value is thermal resistance in hr-ft2-degF/Btu, and its inverse at the assembly level is the U-factor the code may check instead. R per inch times thickness gives the total R of a layer, before the framing and de-rate corrections. Air leakage is reported as ACH50, air changes per hour at 50 pascals, or CFM50, the airflow at that pressure. Vapor permeance is in perms, with the classes drawn at 0.1, 1.0, and 10. Continuous insulation, written c.i., runs unbroken across the framing.

Building envelope

The shell separating conditioned space from the weather: walls, roof, floors, windows, and the joints between them

R-value vs air leakage

R-value is resistance to conductive heat flow; air leakage is bulk air through gaps, carrying both heat and moisture

Air barrier

The continuous system that stops air leakage through the envelope, rated by air permeance and required to be continuous

Vapor retarder

A layer that slows vapor diffusion, rated by perm; Class I at 0.1 perm or less is the tightest, often called a vapor barrier

Thermal bridging

Heat conducting around the insulation through framing, so the whole-assembly R falls below the cavity R

Continuous exterior insulation (c.i.)

Rigid board outside the sheathing that wraps the framing, breaks the thermal bridge, and warms the sheathing

Stack effect

Warm air rising and leaking out the top of a building, pulling cold air in at the bottom

Blower-door test / ACH50

A calibrated fan test of air leakage at 50 pascals; ACH50 is air changes per hour at that pressure

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