Building Technology: How a Wall Keeps the Weather Out

It is January and the window in the bedroom is wet on the inside. A week later a brown stain appears on the ceiling under the roof, but only after wind with the rain, never after plain rain. The corner of the living room is always cold, and there is a draught at the skirting board that has nothing to do with any window. Nothing is broken. Each symptom is a building doing exactly what its details told it to do.

A wall is not "a thing that keeps the weather out". It is a stack of thin layers, each given a job: shed rain, stop wind, hold back vapour, slow heat, resist fire. A building technologist learns to see the layers, the order they go in, and the joints between them, because that is where buildings fail. Walls rarely fail in the middle. They fail at the edges.

Rain: shed it, then drain what gets in

People think of waterproofing as sealing. Old builders knew better: they shed water. Slate, tile, shingle and clapboard all work by overlap and slope, so gravity does the work and nothing needs to be sealed. The overlap must be long enough that water cannot be pushed or drawn up and over it. Two forces try to do that.

The first is wind pressure: a dynamic pressure of ½ρv² piles up on the windward face and pushes water in through any gap. The second is capillary action: water is drawn up a narrow gap, higher the narrower it is, with a rise of about 15 mm for a 1 mm gap. A designer's answers are an overlap long enough, a slope steep enough, and a gap wide enough to break the capillary pull. And behind the outer layer, a drainage plane and a cavity so that water that does get in runs back out. This is the rainscreen: the outer skin is not meant to be watertight, only to take most of the rain.

At 20 m/s (72 km/h) a 100 mm lap at 30° beats the push by 16 mm. At 35 m/s (126 km/h) the push is 75 mm of water and the same lap loses. That is the wind-with-rain stain in the hook. Steepen the roof to 60° and the same lap wins again; widen the gap to 6 mm and the capillary part nearly vanishes. Real roofs add a second line of defence, an underlay, for the storm that beats the first.

Heat: a wall is parallel paths, not one layer

In Heat Transfer (34/16) a wall is a series of resistors, and the biggest wins. That is the first step. The second is that a real wall is not uniform: wherever something conductive passes through the insulation (a timber stud, a steel stud, a concrete floor slab poking through the facade, a window frame), heat takes the easy path. That is a thermal bridge. The wall is then a set of parallel paths, and the average is dominated by the leaky one.

The second surprise is how much of a house's heat loss is not the wall. Windows have U-values of roughly 5.7 W/m²·K for single glass down to about 0.8 for triple low-e glass, and the cracks around everything add air leakage. Try them.

With the default settings, the 14 cm of insulation looks like R 3.9, but the studs cut it to an effective R 3.2, about a fifth less. The total loss is about 2.3 kW, and walls are barely a fifth of it. Windows are 24%, and air leakage 36%. Change to single glass and 1.5 air changes and the total is about 9 kW. This is why high-performance houses chase the details: continuous insulation outside the studs, and thermally broken window frames. The voluntary Passivhaus standard, for example, asks for a heating demand of 15 kWh/m² per year (or a peak load of 10 W/m²), and an airtightness of no more than 0.6 air changes per hour at 50 Pa. Passivhaus is one standard among many and numbers differ by country; the point is that envelope detail makes the number.

Air: the leak you can't see

Air moves through a building for three reasons: wind, fans, and stack effect. In winter the air inside is warm and lighter than the air outside. In a tall building, the warm air wants to rise and escape through leaks high up, and cold air is sucked in low down. The pressure that drives it grows with height and with the temperature difference.

At 100 m and −10 °C outside the stack effect makes a pressure difference of about 139 Pa between bottom and top, and about 69 Pa at each end. That is a push of about 130 newtons on a lobby door, enough to make it hard to open and elevator doors whistle. The same pressure carries smoke up stairwells and elevator shafts in a fire, which is why fire designers treat shafts as dangerous. For houses the effect is weak but constant: it is the draught at the skirting board. The cure is an air barrier, a continuous airtight layer (a membrane, taped sheathing, or plaster) that is checked with a fan-pressurised blower-door test. Airtightness is the cheapest improvement a building can make, and the most often skipped.

Vapour: where the wall's water goes

Warm indoor air carries moisture, from cooking, breathing and showers. Vapour moves through materials from the high-pressure side to the low, and a wall is a gradient of temperature. If, somewhere inside the wall, the temperature drops below the dew point of the air that has got there, the vapour condenses to liquid water. The moisture then sits in a place that cannot dry, and timber rots.

The classic check is the Glaser method (a steady-state calculation named for Helmut Glaser, who published it in the late 1950s): work out the temperature at each layer boundary, work out the vapour pressure from each layer's resistance to vapour, and see whether the actual vapour pressure ever exceeds the saturation pressure. The resistance is written as sd, a layer's thickness times its vapour-resistance factor µ, in metres of still air. A coat of paint is a fraction of a metre; a polyethylene film is tens of metres.

With no vapour layer, the wool's cold face sits at about −4 °C, where the air can hold only 439 Pa of vapour, but the vapour pressure arriving there is about 993 Pa. Water condenses. A smart membrane of sd 2 m cuts the vapour pressure to 578 Pa, still too high. A polyethylene film brings it down to 377 Pa, safely under the limit. Two lessons stand out. First, the layer that stops vapour belongs on the warm side. Put the film on the cold side and it traps the water. Second, the sheathing outside is itself a barrier: this wall is dry only if the inside is more resistant than the outside. That is the "perfect wall" principle: be more vapour-open to the outside than to the inside, so the wall can dry out.

Fire: time is the product

Fire design in buildings starts from an uncomfortable fact: you cannot make a building that does not burn. You make one that stays standing long enough for people to get out and firefighters to work. The product is time. National codes ask for a fire-resistance rating, commonly stated as 30, 60, 90 or 120 minutes depending on use and height, and tested in a furnace along a standard time-temperature curve. The best-known one, in ISO 834, is a logarithm: the furnace gas passes 840 °C in the first half hour, 945 °C by 60 minutes and 1,049 °C by two hours, and never cools.

Materials fail differently. Steel does not burn, but its strength falls away: it keeps about 78% of its room-temperature yield strength at 500 °C, 47% at 600 °C and 23% at 700 °C (the Eurocode 3 reduction factors). A thin steel section heats fast because there is lots of surface for each kilogram. Heavy timber chars on the outside, at roughly 0.65 to 0.7 mm a minute; the charred layer insulates, and the core keeps its strength. A thick beam can outlast a thin steel one.

A medium steel section reaches 550 °C in about twelve minutes of a standard fire, far short of a 60-minute rating, which is why structural steel is wrapped, sprayed or boxed in board. A heavy section takes about 22 minutes. The timber beam after an hour is 116 × 358 mm, with about 46% of its bending capacity left. This is one reason mass timber towers are now permitted in several countries, though codes and attitudes differ. The other part of fire safety is compartmentation: floors and walls that hold fire in one place for the rated time.

The lesson of the Grenfell Tower fire (London, 14 June 2017, 72 deaths) is about the envelope. The inquiry's Phase 1 report, published in October 2019, found that combustible materials in the external wall, principally the aluminium composite material rainscreen cladding, were the main reason the fire spread so quickly up the outside of the building. A skin chosen for appearance and insulation had become a fire path. The rainscreen cavity that drains rain also draws flame upward like a chimney.

Joints: everything moves

Materials grow in heat and shrink in cold. A building also settles, shakes in wind, creeps under load and takes up moisture. If you stiffen every connection against all of that, something cracks. The designer's answer is to decide where the movement will go and to put a joint there, with a sealant or gasket that can stretch and compress.

A 6 m aluminium panel that swings 80 K changes length by about 11 mm, so a ±25% sealant joint must be about 44 mm wide. A steel beam 30 m long and 60 K warmer is 21.6 mm longer, and nothing stops it. This is why a long concrete floor has saw-cut or built-in joints, why tiles have movement gaps, and why bridges have expansion joints whose teeth you can hear when a car crosses.

What a graduate just sees

• A wall is layers with jobs: shed rain, stop air, control vapour, slow heat, resist fire. Find each layer.
• Rain is shed by gravity and overlap, not sealed. A gap that is too narrow draws water in.
• Heat takes the easy path: studs, slab edges and window frames matter more than their area suggests.
• Windows and air leakage often lose more heat than the wall.
• Tall buildings breathe: the stack effect pulls air in low and pushes it out high in winter.
• Put the vapour barrier on the warm side, and let the wall dry to the outside.
• Fire is a race against time, and bare steel loses the race in minutes while thick timber survives it.
• Everything moves. Put the joint where you want the movement, or the material will choose.

Where it shows up next

• Environmental Systems in Buildings (34/94): the envelope you detail here sets the heating and cooling load there. The comfort of a window seat is its inside surface temperature.
• The Design Studio (34/96): a wall section at 1:20 is an exam of this course.
• Construction Management (34/70): who builds the airtight tape line, in what order, and who inspects it.
• In the world: retrofits of old housing, cladding regulations after Grenfell, the mass-timber towers, and every leaking flat roof.

If you want to go deeper

• 📘 Textbook. Francis D.K. Ching, Building Construction Illustrated (Wiley, 6th edition, 2020). Drawings of every common assembly, and the book most students keep.
• 📘 Textbook. Edward Allen and Joseph Iano, Fundamentals of Building Construction: Materials and Methods (Wiley). Longer, text-led, and thorough on codes and process; check the current edition.
• 📘 Gentler. Edward Allen, How Buildings Work: The Natural Order of Architecture (Oxford University Press). Plain explanations of rain, heat and air with simple sketches. Also, Stewart Brand's How Buildings Learn (1994) on how buildings change.
• 🌐 Free course. MIT OpenCourseWare 4.401 Introduction to Building Technology (Spring 2006, undergraduate, Prof. Marilyne Andersen).