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Choosing the right caulk type for each application can be a challenge. The goal of this article is to help you simplify the process. First, you need to consider where you’re working (on the interior or exterior). Secondly, determine what you’re trying to achieve by caulking (weatherproofing, blocking drafts, or closing up unsightly gaps). There are a multitude of products on the market to choose from based on, longevity, flexibility, cure times, and ease of cleanup.
The residential caulk market can be boiled down to five common caulk types:

Butyl. Butyl caulk is popular for sealing gutters, chimney flashings, walks, and other exterior joints due to its admirable adhesion and weather resistance. It is solvent-based and characteristically stringy, which makes it difficult to apply in a finish-.

Latex. Latex caulks have the least ability to stretch which means they work best in interior applications where little movement is expected. Latex caulks are usually water-based.

Acrylic. Like latex caulks, acrylic caulks are easy to work with because they can be painted and cleaned up with water, making them good for touch-ups and for filling small gaps. Elastomeric caulk, a generic term for high-performance acrylic caulk, is designed for greater elasticity. Generally for interior use, acrylic and latex caulks have a life span of 5 to 10 years, depending on conditions.

Silicone. Silicone caulk is generally for interior use. It is virtually non-porous so its big advantage is to make something watertight, and it’s most often used in plumbing applications (shower and sink installations) and some glasswork. Silicone is extremely rubbery but does not stick as well as other caulking and in its pure form, can’t be painted. There are now some hybrid siliconized acrylics that offer greater elasticity and a paint-friendly surface; they may be worth a try in an indoor setting where flexibility is critical.

Polyurethane. Polyurethane caulk is preferred for outdoor applications, with high-quality products having an exterior life span of 10 to 20 years, depending on exposure. Polyurethane products bond to most surfaces, including masonry and metal, hold up to heavy movement, and can be painted. Setup and cure times are high.

Cure times and durability
Just as the makeup of exterior caulks varies, so do their cure times. In general, latex products can be painted within one to four hours, but take about a day to cure. Silicones and modified silicones take between overnight and a couple of days, while polyurethanes need 3 to 10 days. Always read the product labels carefully for proper application.

Where to Caulk
Caulking closes up the cracks and gaps that allow air and water to infiltrate your house. Even the smallest voids exposing the inside of your home to the outdoors can be a threat causing underlying moisture damage. The primary goal of exterior caulking is to shed water and to make your house more weather and draft-resistant. Interior caulking seals against drafts along exterior walls and at intersections and prevents water intrusion at plumbing fixtures. It is always best not to depend on caulking as a fallback for haphazard workmanship.

Perfecting the Perfect Caulk Job
The secret to an attractive and long-lasting caulk job, especially indoors, is good preparation and meticulousness. Always clean surfaces well before caulking so there is no dust or debris to compromise the bond.

Microorganisms are always growing on exterior surfaces, so an important preparation step is to wash them well with a bleach and water solution or a commercial house cleaner in a pump sprayer, using a bucket and brush. Power washing is okay if done carefully (don’t drive water up under the siding and trim work), but isn’t necessary. After washing, rinse surfaces with clean water, and allow them to dry completely. Like painting, caulking should be completed no more than one week after washing or the organisms will begin to grow again. When painting is involved, the best time to caulk is after you have applied primer to new wood or before applying the final coat of paint.

The most important tool for caulking is a caulking gun, which is worth the investment. The better models have far more mechanical advantage than cheaper models and are worth the extra money. Remember you get what you pay for.

There’s more to an effective caulk job than just pumping goo into a gap, too. When viewed in cross-section, the ideal caulking bead has an hourglass shape-in other words, the sides need maximum surface area for adhesion, and the center is kept thinner so that the caulk has maximum flexibility to move with building materials. Never depend upon caulk alone to fill a gap any wider than ¼. Caulk alone simply cannot stretch enough on large openings.

Caulking for Energy Savings
We’re all looking for ways to improve the thermal efficiency of our houses by keeping heat in or out, and in this quest caulking has the advantage of being low cost and virtually invisible.
Any time you make a building more vapor-tight, you risk trapping high moisture levels inside, which can lead to serious problems, from peeling paint to rotting wood. When caulking exteriors, remember that water primarily travels down, not up, so don’t caulk the undersides of window trim, door trim, or siding such as clapboards.

Summary
Obviously, caulk isn’t a cure-all, and it will do little to solve a draft problem if you have major structural failures, such as damaged or missing siding and trim, or if an open fireplace damper or an uninsulated attic is creating a chimney effect inside your house. However, along with insulating and ventilating your attic, and making sure your windows and doors are tight and fitted with proper weather seals, caulk can be a cost-effective way to improve your homes comfort and performance.

Resources: Old House Journal