I designed a house a few years ago for some clients who were skeptical about using synthetics -- polystyrenes, polyurethanes, polyvinyls, polysulfides, and other such compounds -- on or within the exterior wall sandwich.

They believed that these new materials -- gigantic long-chain molecules of carbon, hydrogen, oxygen, nitrogen, sulfur, chlorine or silicon atoms -- have been around and tested for too little time. Would they endure for decades or even centuries the way good architecture built with traditional materials should endure? My clients knew that sunlight's ultraviolet radiation eventually can break down the molecular bonds of plastic and rubber compounds, causing them to become brittle, lose strength and decompose. Moreover, some emit poisonous fumes when burned.

Undaunted, I pointed out that, in certain respects, architecture had been revolutionized by such chemical innovations and that tens of thousands of buildings have been put together during this century utilizing synthetics. And they're still standing, although few are maintenance-free. Indeed, I suggested that many structures couldn't have been assembled at all without these special adhesives, sealants, coatings, insulations, gaskets, putties and paints.

Look closely at buildings, and you will see that synthetics are particularly indispensable to making latter-day facades. Modern technology and materials enable architects to design almost anything that doesn't violate the laws of physics or the client's budget. Compositional innovation and rapidly changing standards of taste have engendered new facade "freedoms" made feasible by 20th-century substances.

We can increase window area to the point of covering buildings entirely with gasketed glass, practically eliminating any trace of exterior mullions, trim or solid wall. Electric lighting and mechanical ventilation allow buildings to be sealed hermetically, without operable window sash. Or, in the Brave New World style, windows can be eliminated.

We can create structural spans and cantilevers of substantial mass and dimension, while slipping wall planes back and forth without needing to support them on bearing walls below. We can butt and glue together exotic and traditional materials to create unprecedented surface effects. Brick walls can be made to drape, fold, fly or otherwise defy gravity.

Nevertheless, certain facade conditions and requirements cannot be denied. Whenever there are penetrations, openings or discontinuities in exterior walls -- for windows, doors, emerging structural elements, ventilators, changes of material or expansion joints -- very careful detailing is necessary to ensure that the facade will do its duty, whether or not it looks good.

The control of moisture is the thorniest challenge. Rainwater -- especially from wind-driven rain -- always seeks to invade joints in walls or to penetrate porous surfaces. To keep it out, exterior waterproof coatings (e.g., silicone or acrylic) can be applied to seal large, permeable surfaces such as brick or exposed concrete block, which love to suck up water.

Inside the wall sandwich, horizontal wall flashing -- usually made of aluminum, galvanized metal, copper, rubber or plasticized fabric -- catches water forced inside and behind veneers. Installed over window and door heads, or at other critical points in the wall, L-shaped flashings divert water to the outside, where it drips away, sometimes through "weep" holes.

All exterior joints (excluding mortar joints between masonry units) must be gasketed tightly or caulked to seal -- hopefully forever -- the constructed cracks through which water easily can migrate. Critical joints occur between window glazing and sash, between window or door frames and masonry or concrete wall surfaces, between pre-cast concrete panels, and between abutting dissimilar materials. If joints are sizable (more than one-quarter inch), compressible fibrous fillers or backing "rods" of flexible polystyrene foam are used to pack the joint behind the caulking.

Having kept out the rain, walls also must provide thermal insulation, which is difficult to achieve with all-glass facades having little or no solid wall to contain batts of glass fiber or foam.

Special glazing has been invented to absorb or reflect infrared radiation -- though not all of it -- so that the summer cooling load on glassy buildings is reduced. Two or three parallel sheets of glass separated by a small, evacuated layer of space create an insulating window unit that resists heat transfer from inside to outside in the winter.

Instead of heat-absorbing glass, the architect can use clear insulating glass combined with summer sun-shading devices built into or onto the wall, thus admitting solar radiation into the building's interior in the wintertime, when it is desirable for space heating and energy conservation. Such shading devices, best installed outside the thermal envelope of the building, can include awnings, overhanging roofs, balconies, screens, louvers and other projections. Blinds, louvers or drapes on the interior are less effective because they deflect or absorb radiation only after the radiant heat already has entered the building's thermal envelope.

Window and door frames are crucial to the thermal picture during cold weather. Wood frames are natural insulators; their interior surface remains close to room temperature. But aluminum or steel frames conduct heat readily, staying frigid in winter despite the indoor temperature. This results in all-too-familiar condensation dripping down windows and jambs, eventually staining wall and sill finishes and causing deterioration inside the wall. On very cold days, ice can form inside the window.

Calling upon synthetics again, manufacturers now make metal frames with built-in rubber or plastic "thermal breaks." Thin gaskets divide the extruded frame into interior and exterior halves, with each half isolated thermally from the other.

In properly designed buildings, most of the exterior wall sandwich assembly -- including the insulation layer -- lies outside the column-beam-floor system. This keeps all of the building's primary structural components at or near interior room temperature, minimizing thermal expansion and contraction of the structural skeleton at the building's perimeter. This also prevents heat loss through the structural frame itself.

Each assembled material in a facade has its own unique rate of temperature expansion and contraction, as well as its own unique strength and elasticity (the ability of a material to deform without breaking and then return to its original shape). Intuition tells you that glued-together pieces of glass, rubber, aluminum, steel and concrete, under the same conditions of loading and climate, will change their respective dimensions differently.

Moreover, buildings as a whole move, flexing or twisting when the wind blows. On sunny days, south facades will expand while north facades contract and compress slightly. Foundation settlement at one corner may not be the same as another corner.

Therefore, buildings must accommodate differential movement. Fitted together too tightly, without compressible or stretchable sealants, or without attachments that permit slight slippage, facade components can be stressed to the point of fracture. Previously sealed joints can break open, or new ones can form, allowing water to penetrate. Panes of glass gripped too tightly by their frames can break. In large buildings, stress-relieving expansion joints cut through the entire structure.

The next time you're near a building's exterior wall, consider the many different pieces, materials, joints and openings that are visible, plus those you cannot see. You may gain new insight into how architects synthesize facades to provide "commodity, firmness and delight."