From Putty to Silicone - How Caulk Changed Architecture

The invention of the arch, the use of steel, reinforced concrete, glass, and the elevator are well understood as innovations that changed architecture in often dramatic ways.

Only few people, though, would count caulk among the disruptive inventions and game changers in architecture. But that may be an oversight. For understanding what really enables the type of  urban architecture we take for granted today, the role of joint fillers (commonly known as caulk) can't be underestimated. In fact, the story of caulk isn't interesting for the nerdy details of such mundane materials as caulk themselves but for the implications they have on style, architecture and even structural design.  Here is the story:

The view of the 9/11 site in lower Manhattan in 2006 seen through a
full glass wall (Photo: K. Philipsen)

In the small bit of literature there is about caulk it is usually noted that joint fillers have been around forever, for example, as mud and pitch. Examples of early uses includelog cabins where the logs weren't carefully hewn for good fit and using mud between rocks. Mud between layers of stone or early bricks were also common. One could even go as far as to call the mortar used in masonry a joint filler,and that wouldn't be entirely wrong. So what is so special about modern caulk?

Of those historic precedents the only durable joint filler was mortar. It bonds well with brick and stone and becomes a part of the wall. Everything else didn't last. The materials would harden and fall out of the joints. Joint fillers between different materials such as stone and metal, wood and glass or brick and wood would be a lost cause and were usually not even attempted, because there was no good way to make them durable enough to keep wind and rain out..

Golden Plow Tavern, York PA: Mudfilled timber joints, putty at the glass,
laps at the wood shingles (Creative Commons)

If you don't care for caulk, you still may care about architectural style. As a consequence of the poor properties of joint fillers, the art of architecture went to great length to cover up joints creating an entire set of looks we would identify as "traditional architecture". For example batten strips which cover the gap between two wood boards give certain types of wood construction a distinctive look. Exposed joints can also be avoided by overlapping materials in the fashion of clapboard siding or roof shingles. The overlap ensures that water (and to some extent: wind)  couldn't penetrate.

Another style forming method is the use of ornament to cover the gaps. Freezes and cornices often originate in the need to cover the gaps where different building materials and components meet, for example, the roof and the wall. Even inside buildings, decorative elements such as moldings and baseboards cover joints between unequal materials at the wall and ceiling or the wall and floor.

When the industrial revolution changed how things were made, it also changed aesthetics and sensibilities. Modernism was born as the art of reduction which emphasized utility and function over decor and tended to express the more naked truth of a building by showing instead of hiding how it was put together ("form follows function"). In that view the ornamental cover became maligned as a deceit, fig leaf, and cover up. Buildings were to express lightness, transparency and dynamism, the ideals of the evolving democratic societies, instead of power, permanence and stability preferred by autocratic societies predating the industrial revolution.

This window detail could be part of a curtain wall

A new way of doing the joint was needed that could connect and seal light materials and have properties of adhesion, cohesion, elasticity and weather resistance. Those requirements are no small challenge, especially elasticity. The reason mortar works so beautifully with brick and stone is that there isn't much expansion and contraction going on between two equal materials in general, and with brick and stone specifically.

As noted, uneven materials such as glass and metal are much more difficult to join than stone or brick. Metals and glass, for example, not only contract and expand differently they also offer different surfaces to adhere to. Plus, in a style that exposes the joint, it has to withstand sun, the rain, and all kinds of temperatures so it will reliably keep wind and rain out.

After the age of mud and pitch putty for a time was the choice joint filler. Putty is a mixture of calcium carbonate with some kind of oil (anything from linseed to fish oil), a mix that has good plasticity when first applied but once hardened, allows for very little movement. Its weather resistance was poor but could be improved with paint, which in turn, made it even more brittle. Putty was used particularly for glazing between glass and the frame. Putty was the material of choice for greenhouses and even theirgrandiose off spring, the Crystal Palace. But putty did not really meet the challenge.

Not surprisingly, the material that came to the rescue was a chemical product and it took a while before it hit the market.  The Thiokol Chemical Corporation was formed subsequent to the discovery of a two component polysulfide polymer that became the joint filler of choice for decades.

Thiokol owes its origins to two chemists, Joseph C. Patrick and Nathan Mnookin, who were trying to invent an inexpensive antifreeze. In 1926, in the course of an experiment involving ethylene dichloride and sodium polysulfide, they created a gum whose outstanding characteristic was a terrible odor. The substance clogged a sink in the laboratory, and none of the solvents used to remove it were successful. Then the frustrated chemists realized that the resistance of the material to any kind of solvent was a useful property. They had invented synthetic rubber, which they christened "Thiokol," from the Greek words for sulfur (theion) and glue (kolla). Thiokol Chemical Corporation was subsequently founded on December 5, 1929. (Source)

The urgent need for innovative joint fillers arose not only from distaste for the ornamental but from buildings growing dramatically in size from the other innovations such as steel construction and the elevator.  The standard solid load bearing masonry and stone wall was dissolved into a beam and column skeleton with panels filling in in between, and the need for elastic joint fillers grew exponentially especially with the introduction of the curtain wall and with the insight that large buildings require expansion joints, even if they use low movement brick. Expansion joints cannot be rigid like mortar, so they often became caulk joints as well.

Structural glazing detail

The two-component polysulfide polymer and silicone where eventually followed by single component acrylic sealants and silicones, latex and butyl gaskets, by which time we have arrived in the postwar periodaround the time when John F. Kennedy became president.

The curtain wall and eventually the full glass walls are what characterizes the modern American downtown with its high-rises aspiring to achieve a unique skyline. For high rises that exceed the height of stone veneer structures such as the Empire State Building the curtain and the glass wall have become the two inevitable methods of construction. They are not universally liked but unavoidable for anything tall. Even in Europe with its generally lower downtown buildings or in Washington DC with its height limits, the curtain wall has all but replaced any other method of construction in office construction. Increasingly, glass and curtain walls also have been used for residential towers.

Glass on the entire facade: UB building Baltimore

The modern office building has become synonymous with the term curtain wall, because it uses non-load-bearing exterior walls which are constructed as glass and "solid" panels (non-transparent or non-translucent panels made form a variety of relatively light-weight materials), installed into aluminum frames and hung like a curtain on the front of the structural framing system (Steel or concrete) that holds up the building. It was this division of labor, i.e. separating the task of structure from the task of keeping the weather out that allowed really tall structures.

Modern commercial structures rely heavily on joint sealants to prevent water damage to buildings and their contents. While residential buildings use water-shedding techniques such as sloped roofs, lap siding, and overlapping flashings, many commercial designs don't; if a joint sealant fails, there is little or no barrier to leakage.(Architect)

The desire for light and transparent structures has culminated in full glass cladding where glass is no longer mounted into visible frames. Instead it is mounted on these frames with the glass panels abutting each other. Since glass expands and contracts quite a bit and has itself no elasticity, the joints between glass panels are quite substantial and they need to be filled. Once again, caulk (usually silicone) comes to the rescue. In the application called structural glazing caulk may have reached the zenith in its ascent by becoming a full blown part of the facade, equal to and literally on the same level as the glass and like it the only line of defense against rain, wind and uncomfortable temperatures. As such caulk is a ticking time bomb because caulk joints, no matter how well executed, don't have the same life span as brick, glass or pretty much any of the materials they connect.

Structural glazing: Caulk is the only line of defense
(Photo: Philipsen)

Architects and purists would like to see construction that does away with workers running around during construction with caulk-guns, squirting goo into cracks. Initial curtain wall designs tried to accomplish a good seal with preformed compression gaskets between glass and frames alone (neoprene or rubber), but in the end they had to rely on caulk as well to be really watertight. The search for a more elegant seal has not subsided, though. Recently pre-formed and pre-cured silicone that can be rolled out and squeezed into joints is making inroads. While it uses less goo, the cured silicone still needs to be embedded in thin beads of liquid caulk for adhesion. Either way, caulk joints while allowing previously unknown building transparency, remains a maintenance item.

A design strategy that reduces caulking and relegates it to a much less prominent position is the installation of so called rain screens, which means a rain barrier as an outer layer in front of the actual building envelope. Those rain screens are designed to keep the bulk of the rain away from the barrier facade but are typically designed with open joints, i.e. without caulk. The idea is that these screens "breathe", they let air and small amounts of water into the gap where it can't do any harm. Rain screens can be any thin non-bearing materials such as glass, metal panels, terracotta panels or cement board. If they are not transparent they allow the inner barrier to be made air and watertight with cheap plastic sheets, tape, and other unsightly but effective means. Whatever caulk the inner barrier wall would use, it would be well protected and behind the first line of defense, giving it a much better lifespan.

Dual facade with rain screen as the first line of
defense protects caulk and gaskets on the  inner
barrier wall (Klaus Philipsen)

In spite of all technological advance, the caulk joint remains the weakest part of any facade and it is relatively easy to predict that a day will come when building managers will curse the prolific use of caulk, especially as the first and only line of defense. Although there is little information given by manufacturers about the lifespan of caulk, even the best caulks will not last much longer than 20 years, often much less. The labor intensive removal of old caulk and installation of new beads will soon be a thriving cottage industry especially on structures with structural glazing.

Klaus Philipsen, FAIA
edited by Ben Groff, J.D.

The Short Course on Caulk
Wikipedia on Sealant
Twentieth Century Building Materials
National Institute for Building Sciences


From the National Institute of Building Sciences:

Liquid-Applied

·         Latex (water-based, including EVA, acrylic)

o    Used mainly in residential and light commercial construction applications

o    Interior and exterior uses

o    Premium products rated for movement meet (ASTM C920, Classes 12½ and 25)

o    Products not rated for movement meet (ASTM C834)

o    Excellent paintability (with latex paints)

o    Very good exterior durability

o    Exhibit some shrinkage after cure

o    Sometimes referred to as caulk

o    Not used for exterior applications on high rise construction or for applications undergoing cyclic movement greater than ±25%

Glass curtain wall: Johns Hopkins Hospital, Baltimore

·         Acrylic (solvent-based)

o    Used in residential and light commercial construction, mainly for exterior applications

o    Generally have a maximum of ± 7½% movement (ASTM C1311)

o    May need special handling for flammability and regulatory compliance

o    Can be painted

o    Short open time; difficult to tool

o    Exhibit some shrinkage after cure

o    Often used for perimeter sealing; low movement joints

o    Butyls (solvent-based)

o    Excellent adhesion to most substrates

o    Generally have a maximum of ± 7½% movement (ASTM C 1311)

o    Excellent weathering

o    Good use as adhesives in industrial and packaging applications

o    Sometimes used in curtain wall applications where adhesion to rubber compounds is required

o    Most are stringy and difficult to apply neatly

o    May show some shrinkage after cure; may harden and crack over time on exposed surfaces

Structural glazing with point fasteners seen from the
inside (photo: ArchPlan Inc)

·         Polysulfides

o    First "high-performance" sealant chemistry; mainly used in industrial applications

o    Products rated for movement meet (ASTM C920, Classes 12½ and 25)

o    Poor recovery limits their use in joints with moderate cyclic movements

o    Can be formulated for excellent chemical resistance (especially for aviation fuel)

o    Good performance in submerged applications

o    Require a primer on almost all substrates

·         Silicones

o    Structural sealant glazing of glass to metal framing systems

o    Excellent joint movement capabilities (ASTM C920, Classes 25, 35, 50, and 100/50)

o    Excellent UV resistance and heat stability

o    Good adhesion to many substrates especially glass; a primer is recommended on some substrates, particularly cementious substrates

o    Most formulations are not paintable; however, there are a few that are paintable

o    Used in vandal resistant, missile impact, and blast resistant glazing systems and to insulate glass to improve thermal performance (reduce heat loss).

o    Acetoxy chemistry based sealants have an acidic acid odor while curing, but newer chemistries have very low odor

o    High, medium and low modulus materials available

o    May stain some types of porous materials such as concrete and natural stone

o    Non-staining and non-bleeding formulations are available where aesthetic considerations are important

·         Polyurethanes

o    Used in industrial and commercial applications

o    Very good movement capabilities (ASTM C920, Classes 12½, 25, and 50)

o    Not used in structural glazing applications (avoid direct contact to glass)

o    Excellent adhesion, generally without a primer for many substrates (cementious substrates require a primer)

o    Can be formulated for good UV resistance

o    May stain some types of porous materials such as concrete and natural stone

o    Paintable

o    Some formulations may contain low levels of solvent

Full glass facade: MICA Brown Center Baltimore

Factory Preformed

·         Dense and Cellular Gaskets

o    Generally used to seal glass in openings and joints between metal panels and usually installed under compression

o    Dense and cellular rubber products are generally formulated from EPDM, neoprene, silicone, and thermoplastic polymers

o    Cellular non-silicone products meet (ASTM C864), For dense products most meet (ASTM C509), silicone products meet (ASTM C1115) and thermoplastics meet (ASTM E2203)

·         Extruded and Molded Seals

o    Fabricated primarily from silicone formulations, some polyurethane formulations are also available

o    Silicone formulations meet (ASTM C1518, Classes 12½ to 200)

o    Provided as extruded strips in various standard colors and widths from 1 to 6 inches (25 to 152 mm)

o    Provided as custom molded components for corners and other transition areas

·         Compression systems

o    Preformed, precompressed products usually fabricated of a polyurethane, acrylic impregnated cellular component with a silicone rubber exterior face

o    After insertion in the joint opening the foam expands to be compressed in the joint and the rubber face is sealed to the substrates

o    Typical products are capable of ±25% movement