Construction · Lesson 6 of 13Intermediate10 min read

The Building Envelope

The envelope is everything that separates inside from outside: the foundation, walls, windows, doors and roof. A good one controls four things: liquid water, air, water vapor and heat. This lesson takes them one at a time.

One wall, four jobs

Moisture moves into a wall four ways: as bulk water (rain, snowmelt, groundwater), by capillary wicking through porous materials, carried along by air that leaks through, and by vapor slowly diffusing through materials. Heat moves too, by conduction, convection and radiation. Each needs its own control layer, and the layers have to work together. Try each one here.

Pick a tab. Water: remove the barrier to see what happens. Air: leave a gap at the outlet. Vapor and heat: change the outdoor temperature and add foam sheathing. The heat numbers use OpenStax’s R-11 for 3½-inch batts and about R-1.2 per inch for wood; drywall, paint and air films are left out.

Water: the drainage plane

Siding sheds most rain, but some always gets behind it. So behind the siding there’s a weather-resistive barrier (WRB): house wrap, building paper, taped foam sheathing or a coating that’s sprayed or rolled on. Together with the flashing and the roofing, it forms the drainage plane. University of Minnesota Extension puts the rule simply: “Every element of the drainage plane must be installed so it will direct water out and over the layer below it.” Layers are lapped like shingles, and tape is no substitute for lapping them the right way.

Every hole in a wall is a chance for a leak, and a house typically has 20 or more of them on its outside walls: pipes, vents, wires, lights and more. Each one has to be flashed into the drainage plane. Windows and doors are the biggest interruptions, and leaks around them often aren’t found until damage has already happened inside the wall.

Every layer laps over the one below it, so gravity carries water out. Don’t seal the bottom flange: water that gets in must be able to drain out. Sources: University of Minnesota Extension; DOE Building Science Education Solution Center.

Flashing a window

Window flashing goes on in shingle order: sill flashing first, then the sides lapped over it, then the head flashing, and finally the house wrap above folds down over the head flashing. The sides and top of the window’s flange get sealed, but the outside of the bottom flange should not be caulked or taped: water that gets into the window opening has to be able to drain back out.

Air: seal the leaks

Air leaks make a house drafty, put extra strain on heating and cooling equipment, and let in moisture that can damage wood and encourage mold. The Department of Energy compares the two jobs this way: insulation slows heat flow, but “air sealing acts like a windbreaker.” Air leaks are among the greatest sources of energy loss in a home, and DOE says sealing and weatherizing can save 10 to 20 percent on heating and cooling bills.

The usual leaks are around windows and doors, between the attic and the rooms below, where siding meets the foundation, and wherever pipes, vents, wires, ducts and fans pass through outside walls. Walls and rim joists make up more than 40 percent of a home’s envelope area, so they matter too. Caulk seals cracks and gaps; spray foam fills bigger ones; weatherstripping seals doors and the moving parts of windows. Gaps near chimneys, flues and furnace vents need special high-temperature materials, a job DOE suggests leaving to a pro.

Vapor: put the retarder on the warm side

A vapor retarder slows how fast water vapor moves through a material. They come in three classes by permeance: Class I at 0.1 perm or less, Class II between 0.1 and 1.0 perm, and Class III at 1.0 perm or more. Vapor retarders belong closest to the warm side of a wall: the inside in cold climates, the outside in hot, humid ones. No vapor barrier is required in mild or hot climates; the building code requires them in colder climates (zones 5 through 8), where a Class II or Class III retarder such as kraft-faced insulation or latex paint is often enough.

A vapor retarder doesn’t stop air. Moisture carried by leaking air needs an air barrier, which is why air sealing and vapor control go together. Lesson 10 shows how vapor becomes liquid water at a cold surface.

Heat: insulation and R-value

Heat moves three ways: by conduction through solid materials, by convection as air or another fluid moves, and by radiation. Most insulation slows heat flow by conduction and convection: its fibers or foam cells hold air still, and still air conducts heat poorly (OpenStax lists air at 0.023 watts per meter-kelvin). The rate of heat flow through a layer depends on its area, the temperature difference across it, its thickness, and its thermal conductivity. Builders combine thickness and conductivity into one number, the R-value: the larger the R, the better it insulates. R-values of layers in a row add up.

heat flow per square foot = (T_inside − T_outside) ÷ R (Q/t = kA(T₂ − T₁)/d, with R = d/k)

Kinds of insulation

Batts and rolls (blankets) of mineral fibers such as fiberglass, sized to fit between standard stud and joist spacing. OpenStax gives about R-11 for 3½-inch fiberglass batts, used in walls, and R-19 for 6½-inch batts, used in ceilings.
Loose-fill fiberglass, rock wool or cellulose, blown in with special equipment; good for attics and odd-shaped spaces with wires and pipes in the way.
Foam-in-place (spray foam), which insulates and also reduces air leakage through holes and cracks.
Rigid foam board, used as exterior wall sheathing and on basement walls.

Installed insulation can perform below its rated R-value if it has gaps, voids or compressed spots, or if air moves through it. And wood conducts heat two to four times better than glass wool (0.08 to 0.16 versus 0.042 watts per meter-kelvin), so every stud is a small thermal bridge through the insulation.

ENERGY STAR’s recommended R-values for wood-framed houses (from the 2021 IECC). Your local code sets the legal minimum.
Climate zone	Attic, if uninsulated	Attic, if it has 3–4 inches now	Floor
1	R-30	R-25	R-13
2	R-49	R-38	R-13
3	R-49	R-38	R-19
4A, 4B	R-60	R-49	R-19
4C, 5, 6	R-60	R-49	R-30
7, 8	R-60	R-49	R-38

Windows and doors

Windows are rated by the National Fenestration Rating Council (NFRC), and its label lets you compare products. Two numbers matter most:

U-factor is the rate at which a window, door or skylight lets non-solar heat flow through. The lower the U-factor, the more energy-efficient it is.
Solar heat gain coefficient (SHGC) is the fraction of the sun’s radiation that gets through. The lower the SHGC, the less solar heat comes in. ENERGY STAR suggests a low SHGC, or shading, for windows that face east and west.

ENERGY STAR sets limits for each climate region. It says certified windows lower household energy bills by an average of up to 13 percent nationwide when they replace single-pane windows.

ENERGY STAR Version 7 criteria for residential windows (effective October 23, 2023)
Climate region	U-factor	SHGC
Northern	≤ 0.22	≥ 0.17
North-Central	≤ 0.25	≤ 0.40
South-Central	≤ 0.28	≤ 0.23
Southern	≤ 0.32	≤ 0.23

You canYou can

Find drafts: on a windy day, DOE suggests carefully holding a lit incense stick or a smoke pen next to windows, doors and outlets and watching where the smoke blows.
Caulk gaps around trim and weatherstrip doors and windows. DOE says these often pay for themselves in a year or less.
Check your attic insulation depth against the table above.

StopBefore you tighten up a house

DOE says the first step in air sealing is working out how much fresh air the home needs. Without proper ventilation, moisture and harmful combustion gases can build up, and fuel-burning appliances that don’t get enough air may release carbon monoxide into the home. A professional energy auditor or certified HVAC contractor can test the house.

In an attic, watch for vermiculite: loose, pebbly insulation. EPA lists attic and wall insulation containing vermiculite among the products that may contain asbestos. Leave it alone, and if you plan work that could disturb it, have it checked by a trained, accredited asbestos professional first.

Check yourself

Tap an answer to check it, or open “Show the answer.”

Which layer carries away the water that gets past the siding?
Show the answer
The weather-resistive barrier, part of the drainage plane. The WRB, lapped like shingles with the flashing, drains water down and out.
Why shouldn’t you caulk the outside of a window’s bottom flange?
Show the answer
It traps any water that gets into the opening. Water that gets in must be able to drain out at the bottom.
A lower U-factor means a window…
Show the answer
lets less heat through. U-factor is the rate of non-solar heat flow. Lower is more efficient.
In a cold climate, the vapor retarder goes on which side of the insulation?
Show the answer
The warm inside. Vapor retarders go closest to the warm side: the inside in cold climates.
According to OpenStax, about what R-value do 3½-inch fiberglass batts have?
Show the answer
R-11. About R-11. Thicker 6½-inch batts are about R-19.

Go deeper

U.S. Department of Energy: Consumer Guide to Air Sealing the Home DOE’s consumer guide to air sealing, with a diagram of common leaks.
ENERGY STAR (EPA and DOE): Recommended Home Insulation R-Values ENERGY STAR’s R-value table and wall and basement guidance.
U.S. Department of Energy, Energy Saver: Energy Performance Ratings for Windows, Doors, and Skylights DOE on U-factor, SHGC, visible transmittance and air leakage ratings.
OpenStax, College Physics 2e, 14.5 Conduction (R factors) OpenStax on conduction and R factors, with worked examples.
Lesson 10: Moisture, Mold and Building Science Lesson 10: condensation, dew point and mold.

Sources

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A free course from Teddy Rook Book. It explains how houses are built in general and follows U.S. sources; the local building code, the building department and the manufacturer’s instructions are the final word.