Read the label on a tube of caulk and it’s likely
to promise the world: "50 years," "lifetime durability,"
"guaranteed." Superlatives abound, exclamation points are in
order. "Exceptional adhesion," "permanent flexibility," "all
Flip the tube over and read the small print, however, and
you’ll find that the warranty is practically worthless
because it only covers replacement cost for the caulk itself.
The stakes are a lot higher than that if you’re
relying on caulk to seal out the weather on a new home or
Faced with a shelf displaying ten different products, each
claiming "superior performance" and "adheres to most surfaces,"
how do you choose?
Researching this article was a chance to retest my old
prejudices and preferences, and see how the new formulations
compared with the products I used seven years ago when I was
still in the trades. I went through almost 50 tubes of sample
caulks on a variety of substrates. I concentrated mainly on the
high-performance caulks — the ones designed for moving
joints and that meet the ASTM C-920 standard (see ""). In the
end, my own simple testing did far more to inform me than all
of the hours spent talking to technical experts. I formed
definite opinions that would influence my buying if I had to
caulk a joint tomorrow.
Caulk Not a Cure-All
Before you buy a tube of high-performance caulk, ask yourself
why you’re buying it. Is the caulk your main line of
defense for keeping out the weather, or do you have some backup
protection underneath? Caulking is not a substitute for proper
flashing and weatherproofing. For example, no amount of
caulking will make a vinyl siding job waterproof —
that’s the work of the building paper underneath.
Before you buy into the hype and expect a caulk to provide a
"permanently flexible, watertight seal" that will "last a
lifetime," stop and think about those warranties.
All caulks are formulated starting with a base polymer,
sometimes called the "backbone polymer." To this polymer are
added assorted ingredients, including fillers, plasticizers,
solvents, adhesion promoters, UV absorbers, pigments, and
curing agents, among others. The proportions and types of
additives are varied depending on the properties the sealant
needs and the various substrates it needs to stick to.
The backbone polymer gives a certain identifiable character
to the caulk; the additives change the performance in various
ways, depending on the manufacturer’s intent. One
polyurethane formulation may have increased hardness for use as
a pavement control joint, where it will get walked over by
high-heel spikes; another one may have increased flexibility
but may be softer, making it better for vertical joints where a
lot of movement is expected. But both products will exhibit
essential polyurethane qualities.
There are four polymer types used to manufacture most
high-performance caulks: silicones, polyurethanes, latex
acrylics, and the solvent-based "block copolymers" —
synthetic rubbers, of which Shell’s Kraton is the most
Making Sense of Silicones
Next time you’re walking downtown, look up at one of
those sleek glass-clad skyscrapers and consider this: Those
huge glass panels may be resting in their frames restrained
only by beads of silicone sealant. It’s a scary
thought, especially on a windy day. But it’s a
testament to the strength of the so-called structural silicones
developed for high-rise curtain wall construction —
applications that go far beyond the demands of residential
construction. Yet the silicones that are available to builders
and homeowners have trickled down from the commercial market,
and share many of the characteristics of those super-silicones.
It’s no wonder that silicones are often touted by
caulk companies as "top of the line" and that many builders
look for "100% silicone" on the label when they want the
Silicones come in two types: neutral cure (the three on
the left) and acid cure (the righthand three). Performance
varies between the two types, but manufacturers don’t
always make the distinction clear.
But descend from the world of high-rise structural glazing,
and "best" becomes relative. Silicones don’t always
live up to expectation in the messy world of home building and
No paintable silicone.
For one thing, silicones won’t hold paint — a
huge strike against them. Tech support reps for silicone
sellers like to say this is because the paint is not as
flexible as the silicone, so it breaks down and falls off (in
other words, blaming it on the paint). Common sense and
experience suggest that the paint just doesn’t stick.
In fact, silicone giant GE seems to be giving up the quest for
a paintable silicone: For paintability, they’ve added
acrylic latexes to their new consumer product line. There are
still some paintable silicones on the market, but
they’re probably not worth the trouble.
Acid cure vs. neutral
Silicones also seem to be choosy about what
they’ll stick to. One issue is that silicones come in
two types: acid cure and neutral cure. Acid-curing silicones,
which produce acetic acid as the rubber cures, can corrode
metal and etch some plastics. Unfortunately, the manufacturers
don’t often make the distinction clear. If the tube
lists chemical ingredients, look for the word "acetoxy." If you
use a silicone and your eyes and nose get assaulted by the
Vinegar from Hell, you’ll know you’ve got an
Generally, acid-cure silicones work best on nonporous
surfaces like glass, glazed tile, and unfinished aluminum. With
most other metals, including anodized aluminum, you should
avoid acid-cure silicones (unless you’re willing to
locate and apply the right primer).
The neutral-cure silicones I tested stuck well to both
aluminum and galvanized steel, while one acid-cure peeled right
off of the galvanized.
The neutral cures also adhered moderately well to wood while
the acid cures stripped easily. In general, none of the
silicones I tested adhered to wood as well as the other types
of caulk, with the exception of GE’s Silpruf.
Dirt and staining.
Silicone caulks pick up dirt by electrostatic attraction. They
are also known to stain stones like marble, limestone, and
light-colored granites, as well as light-colored concrete.
Silicones have the reputation of not sticking to concrete and
masonry. The samples I applied to porous concrete and brick
bore this out.
are messy to work with. Unfortunately, it is usually
recommended that they be dry-tooled. In other words, you
shouldn’t use water or a soapy solution on your finger
or whatever it is you like to tool with.
On the plus side, silicones have an amazing application
temperature range — from -35°F to 140°F,
making them a good cold-weather choice. Their service
temperature range is even greater — from -80°F
up to 400°F in some cases. Silicones are the sealant of
choice for sealing around metal chimneys; high-temperature
formulations are available with service temperatures up to
can get a silicone bead to stick where you want it,
you’ve probably got the best seal you can get. No
other sealant has the long-term UV and weather resistance, nor
the strength. This is a two-edged sword, however. If
you’ve ever had to separate a sink that was bedded in
silicone from the countertop beneath, you know what I mean.
When you use silicone that way, think "permanent." Also, once
you’ve used silicone on a surface, you’re
committed to silicones. No other sealant will stick to the oily
residue left behind, assuming you can clean the bead off. Until
recently, there was no known solvent for silicone. Now a
company called Amtex (800/562-6839) makes what it claims is a
nontoxic solvent for cleaning either cured or uncured silicone.
The stuff’s not cheap — around $27 a quart
— but it may be worth trying if all else fails.
Silicones are popular in kitchen and bath applications
because of their water resistance and superior adhesion to
nonporous surfaces. Also, being inorganic, silicone
doesn’t support mildew growth, though the additives
Because of their UV resistance, silicones are great for
special glazing applications, especially glass-to-aluminum
joints (keep in mind that you may need a primer with other
metals). And given their excellent flexibility, silicones are
the sealant of choice for EIFS manufacturers, many of whom
specify Dow Corning 790, which is available in 13 colors.
Primers are often required.
Although "window and door" silicones are now being marketed,
be careful: If you use silicone around a window or door
installation that’s later going to get painted, you
might make an enemy of the painter.
Polyurethanes are favorites of masons. They stick well to most
kinds of masonry without a primer and don’t have the
staining problems associated with silicones. In fact,
polyurethanes stick reliably to just about everything. This
makes them an ideal choice for joints between dissimilar
materials — wood to masonry, metal to wood, and so
forth. In the adhesion tests I ran, the polyurethanes showed
better adhesion in every case than any of the other caulks. In
most cases I could hardly get my fingernails under the bead to
pull it up.
Polyurethane excels as a rugged outdoor caulk.
It’s particularly useful in masonry joints and for
joining dissimilar materials.
Polyurethanes have typically been hard to come by for
builders and remodelers who buy through retail lumberyards.
This is changing, however, as more and more caulk companies add
urethanes to their product lines. ChemRex’s PL line of
polyurethanes is even available in Home Depot. One of my
favorite products, Sikaflex, is available by credit card in any
quantity directly from Sika.
Polyurethanes can be applied down to 20°F, depending
on the product. They are relatively stiff out of the tube, and
can be tooled nicely with a plastic spoon. Sika allows wet
tooling with a soapy solution, though many polyurethane
manufacturers recommend against it. Since polyurethanes cure by
exposure to moisture, wet tooling can facilitate cure in dry
climates. Polyurethanes have a typical shelf life of one year,
after which they get so stiff in the tube they can’t
be gunned. Geocel’s polyurethanes cure by exposure to
oxygen instead of moisture, and have a two-year shelf life.
Polyurethanes are not naturally UV resistant, but are
formulated with UV absorbers to give excellent resistance. They
will not corrode metal, and are ideal for general caulking on
metal roofs. Like silicones, polyurethanes shrink very little
during curing. They come in a limited number of colors, but can
be painted after cure.