Wall assembly

What it does

Keep rain and air under control while limiting heat flow through a framed wall.

Follow water, air, thermal and vapour control layers through the wall and its junctions.

When it fits

Studying layer continuity at windows, rims and the foundation.

The trade-offs

The temperature and dew-point illustration is not a transient moisture-transport analysis.

Context. An educational model. Geometry, material assumptions and the reference building are described in the illustrated study.

An exterior wall has more jobs than any other part of a house. It holds up the floor and roof, keeps the rain out, stops the wind blowing through, slows heat on its way out, and keeps damp air from soaking into the wood. No single material does all of that well, so the wall is built as a sandwich in which each layer does one job. The figure takes a 12 ft (3.7 m) piece of the reference house’s wall, with the foundation and floor below it, a window in it and the next floor above, and lets you take it apart a layer at a time.

The layers are only half the story. Most walls that fail do so at a joint: where the wall meets the foundation, where a floor interrupts it, or where a window punches through it. A layer that does its job across the middle of the wall but stops at a joint has a hole in it. That is why the figure shows the wall with its neighbours, and why you can trace each control layer through them.

Four control layers

Building scientists list the jobs in order of how much damage a failure does. Each one belongs to a particular layer of this wall.

Water: The housewrap, a spun plastic sheet stapled over the sheathing. The siding sheds most rain, but some always gets behind it; the wrap is the surface that water runs down.
Air: The sheathing, with its joints taped. Wind and the pressure of warm air rising inside push air through every gap. That air carries heat and, far more than diffusion does, moisture.
Vapour: A retarder on the side that is warm and damp for most of the year, or none at all where the wall needs to dry that way. Its position depends on the climate.
Heat: The batts between the studs, and the rigid foam outside them when the wall has any.

A designer checks a section by drawing each of these layers as a line, from the roof down to the footing, without lifting the pen. Choose a layer under Trace and the figure turns to face its cut end and draws that line. Rings mark the joints the line has to cross. The air line is the one that most often breaks. Here it runs down the taped sheathing, straight across the rim joist of the floor above, and down to a bead of sealant where the sheathing laps the foundation. A foam gasket closes the joint under the sill, and foam in the shim space joins it to the window frame.

Gypsum board on the inside could serve as the air barrier too, but every floor, partition and outlet box breaks it. On the outside nothing interrupts the sheathing except the windows, and those are taped to it.

Every lap faces down

Water runs downhill and is pulled sideways into narrow gaps. A water control layer therefore never relies on a seal alone. Each piece is lapped over the one below it, like shingles, so that water moving down a face always crosses a lap from the outside. The housewrap goes on from the bottom up, each course 6 in (150) over the last. At its foot it hangs down past the sheathing and over the top of the foundation, outside it, so water drips clear of the sill rather than running behind the concrete’s dampproofing.

A window is the hardest place to keep the laps right, which is why the figure installs it in order. Step back through the layers to watch it:

The wrap is cut in a modified I. Its sides are folded into the opening, and a flap above the head is cut at 45° and taped up out of the way.
A flexible sill pan goes across the sill, 6 in up each jamb and out over the wrap below.
The window is set into sealant at its head and jambs but not its sill, so anything that gets past it can drain out across the pan.
Tape goes over each side flange, lapping over the pan’s wings, then over the head flange, lapping over the side tapes.
The flap is folded down over the head tape and its cuts are taped. Water on the wall above now meets only laps that face down.

Housewrap is one of four ways to make the water layer. They differ in how they handle laps, air and vapour, and in how much they forgive a careless crew:

What keeps water and air out behind the siding?

Decided by rain exposure · the cladding over it · whether it must also be the air barrier · the crew's skill with laps and tapes

HousewrapThis house
A spun-bonded polyolefin sheet fastened over the sheathing, lapped like shingles and taped.
Choose it when most wood-frame walls; vapour must pass so the wall dries outward; taped and sealed, it is the air barrier too.
The catch tears in wind before the siding is on; every lap at every window must face down; under stucco it needs a second layer or a drainage wrap.
Wrap on the house No. 03 →
Felt or building paper
Asphalt-saturated felt or kraft paper lapped in courses.
Choose it when under stucco and stone, where two layers make a drainage plane; a forgiving, traditional layer.
The catch tears easily; no air barrier; more courses and more labour.
Fluid-applied membrane
A liquid sprayed or rolled onto the sheathing that cures into one seamless sheet.
Choose it when complex walls with many penetrations; a seamless air and water barrier; over concrete or masonry.
The catch costs more; its thickness depends on the applicator; weather limits when it can go on.
Taped sheathing
OSB with a factory-bonded water-resistive face; the joints are taped instead of wrapping the house.
Choose it when one trade does bracing, water and air barrier at once; a quick, tear-free dry-in.
The catch the tape is the system: every seam must be right; one maker's products for repairs; less open to vapour than housewrap.

Here Cheap, open to vapour so the wall dries outward, and taped at its seams it is the house's air barrier as well. The windows are flashed into it in a set order, so every lap faces down.

A gap that drains

The siding does not sit on the wrap. It is nailed to furring strips ¾ in (19) thick over the studs, leaving a rainscreen gap behind it. Water that gets past the siding can no longer be held against the wrap by the siding pressing on it. Instead it runs down the gap and out at the bottom, past an insect screen. Air moving through the same gap dries whatever got wet. Turn on Rain to see the three paths water takes: down the face of the siding, down the glass and over the sill, and down the wrap in the gap where the siding is cut away.

The siding here is fibre cement. It is the first line against rain and what the street sees, and the choice of it changes the wall behind: brick needs a ledge and ties, stucco a drainage gap, vinyl a better water layer.

What is the house clad in?

Decided by rain and sun · fire exposure · upkeep · first cost against life · the look of the street

Fibre-cement lapThis house
Boards of cement, sand and cellulose fibre, primed in the factory and nailed 7 in to the weather.
Choose it when the look of painted wood without rot, insects or fire: it does not burn; paint that lasts 10 to 15 years.
The catch heavy and brittle: two people to carry a board; silica dust when cut, so it is sheared or cut wet; kept clear of roofs and soil.
Siding on the house No. 03 →
Vinyl
Extruded PVC panels hung loosely on nails so they can slide as they warm and cool.
Choose it when the lowest first cost; no painting, ever.
The catch burns and melts in fire; brittle in cold, warps near heat; hangs loose, so the water barrier behind does more of the work.
Wood
Cedar or pine clapboards or shingles.
Choose it when the traditional look; easy to repair and renew.
The catch paint or stain every 5 to 8 years; rots if it stays wet; burns.
Brick veneer
One wythe of brick an inch off the sheathing, tied to the frame and standing on the foundation.
Choose it when a century without painting; fire resistance; the regional look.
The catch a foundation ledge to stand on; weeps and flashing at every base and opening; the frame shrinks and the brick does not.
Ties that hold a veneer No. 13 →
Stucco
Portland cement plaster in three coats on metal lath.
Choose it when dry, sunny climates; a seamless, hard skin.
The catch over wood in a wet climate it traps water unless a drainage gap is behind it; it cracks; skilled plasterers.
Metal panels
Steel or aluminium in lap, board-and-batten or flat profiles.
Choose it when wildfire country; a long life with little upkeep; a modern look.
The catch dents; moves with heat; costs more.

Here It looks like the painted clapboard on the rest of the street, will not rot or burn, and over the house's life costs less than wood once repainting is counted.

Heat flows through layers in series

Each layer resists heat flow, and resistances in a row simply add. The total sets how much heat leaves, and each layer takes a share of the temperature drop in proportion to its own resistance:

ΔT_layer = ΔT_total × R_layer ÷ R_totalR in ft²·°F·h/Btu (RSI = R × 0.176) · ΔT across the whole wall, room air to outdoor air

For the cold-climate wall, with 2 in (51) of foam outside, a 70°F (21°C) room and a January average of 15°F (−9°C) outside:

Layer	R	Drop
Inside air film	0.68	1.1°F
½ in gypsum	0.45	0.8°F
5½ in batts	21	34.9°F
7⁄16 in OSB	0.62	1.0°F
2 in foam	10	16.6°F
Siding and outside air film	0.32	0.5°F
Total	33.1	55°F

That is the resistance between the studs. Through a stud the batts are replaced by 5½ in of wood at about R-1.25 per inch, so that path adds up to only R-19. In a typical wall about a quarter of the area is framing (studs, plates, headers and sills), and the two paths carry heat side by side. Their conductances, not their resistances, add:

1 ÷ R_wall = 0.25 ÷ R_stud + 0.75 ÷ R_cavityframing factor 25%

Without foam, R-21 batts make a wall of only R-16.5 (RSI 2.9) overall: the studs are thermal bridges. Foam outside covers the studs, plates and rim joists alike. With 1 in the whole wall rises to R-22.4, with 2 in to R-27.9 and with 4 in to R-38.4. Trace Heat without foam and the insulation line breaks at every plate and rim: foam inside the rim joists and on the basement wall carries it past the floors, with wood bridging between. With foam the line runs unbroken down the outside.

Which insulation, and where, is a choice of its own. The reference house, in milder climate zone 5, stops at batts in the cavities; the figure’s colder walls add the foam outside, and the margin says why when you move Outside insulation.

What insulates the walls?

Decided by the R-value the climate asks for · the depth of the cavity · air sealing · where the wall must dry · cost and installer skill

Fibreglass battsThis house
Blankets of glass fibre cut to fit between the studs.
Choose it when open cavities of standard spacing; the lowest cost; a crew that cuts neatly around every box and pipe.
The catch gaps and squashed batts lose a third of their value; no air barrier on their own.
Insulation in the house No. 03 →
Cellulose
Shredded recycled paper treated with borates, blown loose into attics or packed dense into walls behind netting.
Choose it when attics, where loose fill is deep and cheap; filling odd-shaped or existing cavities completely; quieter walls.
The catch dense-pack needs a skilled installer; it soaks up water, though it dries again; loose fill settles.
Open-cell spray foam
Polyurethane sprayed into the cavity, expanding about a hundredfold; soft and open to vapour.
Choose it when sealing air and insulating in one step; irregular spaces and roofs insulated at the rafters.
The catch only about R-3.6 per inch; two to three times the cost of batts; needs a vapour retarder in cold climates.
Closed-cell spray foam
Dense polyurethane, R-6 to R-7 per inch, an air barrier and vapour retarder at once.
Choose it when rim joists, crawl spaces, basements; thin spaces needing a high R-value.
The catch the most expensive; blowing agents with a climate cost, though newer ones are far lower; hard to remodel through.
Mineral wool
Stone wool spun into dense batts or boards that cut cleanly and keep their shape.
Choose it when fire separations and sound control; places that may get damp; boards outside the sheathing.
The catch costs more than glass fibre; heavier to handle.
Continuous exterior foam
Rigid boards over the sheathing, unbroken by the studs.
Choose it when cold climates and the latest energy codes, which add R-5 to R-10 outside; warming the sheathing so it cannot condense; stopping heat through the studs.
The catch thicker walls: windows and trim are detailed again; siding fastened through it on long screws or furring; the water barrier must be detailed with the foam.

Here R-21 batts fill the 2×6 cavities, rigid foam is cut in against the rim joists and sealed, and 14 in of loose fill is blown over the ceiling: the cheapest way to meet climate zone 5, provided the air sealing comes first and the batts are cut neatly.

Where water would condense

Room air in winter carries moisture. At 70°F and 35% relative humidity its dew point is 41°F (5°C), and any surface colder than that which the air reaches gets wet. In a heating climate the first such surface is the back of the sheathing, the cold side of the batts. The figure draws the temperature through the wall on its cut end, with the dew point dashed across it, and rings the place to watch.

Outside insulation	Back of sheathing, cold (15°F out)	Back of sheathing, mixed (36°F out)
None	17°F (−8°C)	37°F (3°C)
1 in, R-5	27°F (−3°C)	43°F (6°C)
2 in, R-10	33°F (1°C)	47°F (9°C)
4 in, R-20	42°F (5°C)	53°F (11°C)

Two strategies follow, and the figure’s climates use both. One is to keep room air and vapour away from a cold sheathing with a vapour retarder on the warm side. The other is to put enough foam outside that the sheathing never gets cold. In Minneapolis the sheathing stays below the dew point unless the foam is close to 4 in thick, so this wall uses 2 in of foam and a polyethylene sheet. In Washington 1 in is enough, and a smart membrane can replace the sheet. It is tight in dry winter air and opens in humid summer, so the wall can dry either way.

Climate moves the vapour control

In a hot, humid place the problem turns round. On a July afternoon in Houston the air outside is at 94°F (34°C) with a dew point near 75°F (24°C), and the air conditioning keeps the room at 72°F (22°C). Vapour is driven inward, and the coldest surface it reaches is the back of the gypsum, at about 73°F. A polyethylene sheet there would be below the outdoor air’s dew point and would run with water, out of sight behind the wall. So a hot-humid wall has no vapour retarder inside: latex paint on the gypsum lets it dry toward the cooled room. Switch Climate and watch the sheet come and go and the temperature line tip the other way.

The same wall stands on its footing in the spread footing figure and is framed stick by stick in platform framing. As a braced panel resisting wind it appears in lateral stability.