How to build an exposed brick facade and keep the building dry: the cavity wall

The brief: a factory in exposed brick

For a new factory block at Rooma, Kanpur, the client asked for an exposed brick facade: honest, warm, and timeless, with no paint to fade. On the front elevation the brick rises as tall piers through all three storeys, between plastered bays of windows.

The usual way to build that would be a 9-inch brick wall left unplastered on the outside. It is a familiar sight in Kanpur, and so is its weakness. Plaster is not only a finish: it is the wall’s raincoat. Take it off, and after a few days of monsoon rain the water works its way through, and damp patches appear on the inside.

Why exposed brick lets the rain in

Brick is porous. Fired clay is full of fine, connected pores, and fine pores pull water in by capillary action, the same force that draws water up a thin glass tube. The finer the pore, the stronger the pull. A wet brick does not need gravity to move water; it soaks it up like a sponge, sideways and even upwards.

The monsoon adds pressure. Rain that arrives with wind is pushed against the wall face and driven into the mortar joints and into the hairline cracks that open where mortar shrinks away from brick. Ching’s Building Construction Illustrated and the US Brick Industry Association’s technical notes both make the same point: a brick wall should be designed on the assumption that some water will get past its outer face. The question is where that water goes next.

In a solid 9-inch wall there is nowhere for it to go but inwards. Brick and mortar touch all the way through, so water keeps travelling until it reaches the inside face.

What continuous dampness does to a building

A wall that gets wet every monsoon and never fully dries does damage in several ways at once. Mould needs only damp surfaces and still air; it spreads across walls, into corners and onto anything stored against them. The World Health Organization’s guidelines on dampness and mould link damp indoor spaces to respiratory problems, allergies and asthma.

As water moves through brick it carries dissolved salts. When it evaporates on the inside face, the salts are left behind as a white, powdery crust, called efflorescence, that pushes paint and plaster off the wall. Paint blisters and peels, a brown tide mark climbs the wall, and steel in contact with the wet masonry, such as lintels and fixings, starts to rust and stain.

A wet wall is also a worse insulator. Water conducts heat more than twenty times better than still air, so damp brick lets more of the outside heat in. In a factory the cost is direct: the section through this wall shows storage behind it at ground floor, and stock, cartons and packaging kept against a damp wall spoil.

The answer: turn a 9-inch wall into an 11-inch cavity wall

Instead of one solid 9-inch wall, we built two thin walls with a gap between them. The outer leaf is 4½ inches of face brick, left exposed: it gives the facade its brick look. Behind it is a 2-inch air gap. Behind that is a second 4½-inch brick wall, plastered on the inside like any normal wall. Together they make an 11-inch cavity wall.

Face bricks are made for exactly this job. They are fired to be dense, even in colour and true in shape, so they look good without plaster and take up less water than common brick (in India they are covered by IS 2691). The inner leaf can be ordinary brick, since nobody sees it from outside.

Where the water goes: the air gap and the weep holes

The cavity does not try to stop every drop at the face. It accepts that the face brick will get wet, and gives the water an easier way out than in. Water that soaks through the outer leaf reaches its back face and meets the air gap. Capillary action can only carry water through material that touches; it cannot pull water across a 2-inch gap of air. So the water runs down the back of the face brick under gravity instead.

At the bottom of the wall, it needs a way out. We left weep holes in the bottom course of the face brick: open joints at intervals that let the collected water drain back outside. They also let air into the cavity, so the back of the face brick dries out after the rain. The inner leaf, and the plaster on the inside, stay dry.

Wrapping the brick piers

On this facade the exposed brick is used as tall piers that project from the wall between the windows. Each pier is wrapped the same way: the face brick and the 2-inch gap turn the corners and follow all three exposed sides, so there is no point where solid brick runs straight through from outside to inside.

That continuity is the whole trick. A cavity wall only works if the gap is unbroken; any place where brick or mortar bridges the two leaves becomes a path for water to cross. So mortar droppings must be kept out of the cavity during construction, and the bottom of the cavity and its weep holes kept clear.

A bonus: the air gap keeps the building cooler

Still air is a very poor conductor of heat, which is why it is the working part of most insulation. The 2-inch gap acts as a thermal break between the sun-heated face brick and the inner wall.

With standard textbook values (ISO 6946 surface and cavity resistances, and a typical conductivity for fired clay brick), a plastered solid 9-inch brick wall lets through about 2.1 watts per square metre for every degree of temperature difference (its U-value). The 11-inch cavity wall lets through about 1.5: roughly a quarter less heat. These are rough figures, but the direction is clear. And because the inner leaf stays dry, it keeps its insulating value through the monsoon too.

What it means for air-conditioning

Every watt of heat that comes through a wall is a watt the air-conditioning has to pump back out. The heat gain through a wall is simply its U-value times its area times the temperature difference across it. Take 100 square metres of sunlit wall with the outside face 15 °C warmer than the room, a fair figure for a summer afternoon in Kanpur once the sun has heated the brick. The solid 9-inch wall lets in about 3.1 kW; the cavity wall about 2.3 kW. That is about 0.8 kW less, roughly a quarter of a ton of refrigeration, for every 100 square metres of wall.

On a whole building that adds up: a smaller cooling load means smaller AC equipment, or the same equipment running for fewer hours, and lower electricity bills for as long as the building stands. The saving is largest on the walls that take the afternoon sun.

The dry wall helps in a second way. A damp wall gives off moisture into the room, and an air-conditioner spends a good part of its energy removing moisture from the air, not just cooling it. A wall that stays dry does not add to that load, and a dry room feels comfortable at a higher thermostat setting.

It is a step, not the whole answer. India’s Energy Conservation Building Code asks for much better wall U-values on fully air-conditioned commercial buildings. Where that applies, the same cavity can take insulation boards fixed against the inner leaf, as long as a clear air gap is kept in front of them so the wall still drains. Either way, the client got the timeless brick facade they asked for, and a building that is drier and cheaper to cool than a plain 9-inch wall would have given them.