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Old-House Online » Old-House Tips, Restoration Stories, & More » Repairs & How To » How To Use Caulk

How To Use Caulk

This primer on caulk types offers advice for identifying the best material and application method for common caulking jobs in old houses. By Noelle Lord

    One of the easiest ways to improve the thermal performance of old houses is to caulk exterior cracks with high-quality sealant. Polyurethane products have the strength for exterior conditions, but their viscosity means choosing a good grade of gun that delivers power without hand fatigue. (Photo: Peter Lord)

    One of the easiest ways to improve the thermal performance of old houses is to caulk exterior cracks with high-quality sealant. Polyurethane products have the strength for exterior conditions, but their viscosity means choosing a good grade of gun that delivers power without hand fatigue. (Photo: Peter Lord)

    To caulk or not to caulk is a big question around our old house. Peter, my husband and co-restorer, would caulk the window fans in place if he could. He’s all about stopping drafts and keeping moisture and debris out of cracks and joints. That sounds reasonable enough, but I continue to balk whenever I see a caulking gun. Try as I might to lay the product where it belongs with a nice clean finish, whenever I caulk the stuff oozes all over everything (especially me), and I don’t like the hairline cracks that inevitably appear after a few years. Beyond my issues with its aesthetics, I reluctantly admit that caulk is useful for many old-house conditions.

    Whether your mantra is if it gaps, caulk it, or like me, you avoid the stuff except for basic needs, using the right caulk type for every application is critical. You need to consider where you’re working (on the interior or exterior), what you’re trying to achieve by caulking (weatherproofing, blocking drafts, or closing up unsightly gaps), and how much movement you expect. There are a multitude of products on the market, many of which combine basic ingredients in different ways to enhance characteristics such as longevity, flexibility, cure times, and ease of cleanup. To help, here’s a refresher course on which caulks to consider for old houses and how to make the best use of them.

    Caulk Talk

    For viscous caulks or tough jobs, such as working overhead, several cordless caulk guns are on the market. What these power tools add in weight and cost, they make up for in speed and saved labor. (Photo: Ryobi Power Tools)

    For viscous caulks or tough jobs, such as working overhead, several cordless caulk guns are on the market. What these power tools add in weight and cost, they make up for in speed and saved labor. (Photo: Courtesy of Ryobi Power Tools)

    Caulking is not a new concept. For centuries, natural materials, such as pitch and bitumen, have been used to fill gaps in all kinds of structures. However, what really separates today’s building caulks, or joint sealants, from earlier materials is the use of synthetic polymers, first developed in the 1930s. Polymers are substances formed by inducing small molecules of one kind to link up and make large molecules of a similar nature; they are the chemistry behind the rubber-like characteristics of modern caulks. Though the dozens of different tubes on a lumberyard shelf may present a daunting selection, the residential caulk market can be boiled down to five common caulk chemistry types.

    Butyl. Based on a man-made rubber, butyl is one of the oldest and most affordable caulk types. Butyl caulk is solvent-based and characteristically stringy, which makes it difficult to apply in a finish-quality joint, but its admirable adhesion and weather resistance continue to make it popular for sealing gutters, chimney flashings, walks, and other exterior joints.

    Latex. A general term for a rubber-based caulk that is applied as a liquid, latex caulks are usually water-based. Because basic latex caulks have the least ability to stretch (rated around 7 to 10 percent elasticity), they work best in interior applications where little movement is expected. These days, latex is often combined with another caulk type, such as acrylic, to enhance performance.

    Acrylic. In caulks, acrylics are a family of synthetic resins that are clear as well as water-soluble. 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 and is quickly becoming a favorite in a market that values ease and speed.

    Generally, acrylic and latex caulks have a life span of five to 10 years, depending on environmental exposure, such as temperature shifts, ultraviolet light, weather, and building movement. These caulks can usually be removed by simply pulling them up or by using a utility knife to cut them away. Acrylic latex hybrids might include more solids, such as butyl, to offer greater product strength and integrity.

    Ceiling moldings are another subtle source of drafts, as well as unsightly cracks, that can be improved with a quick bead of interior-friendly caulk, such as acrylic or latex. (Photo: Peter Lord)

    Ceiling moldings are another subtle source of drafts, as well as unsightly cracks, that can be improved with a quick bead of interior-friendly caulk, such as acrylic or latex. (Photo: Peter Lord)

    Silicone. Silicone caulk is formulated from silicone elastomers, or in simple terms, linear polymers of silicone oils that can be coaxed to cross-link in a couple of ways. For residential use, this means incorporating chemicals that induce the polymers to “room temperature vulcanize” or RTV. (This process releases acetic acid as a by-product, producing a vinegar scent.) Silicone 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 (50 percent elasticity) but does not stick as well as other caulking and in its pure form, can’t be painted. There are now some hybrid silconized 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, which is based on the reaction of a glycol with an isocyanate to form a compound, 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 (25 percent elasticity), and can be painted. These traits also make these products great for filling indoor gaps in floorboards because polyurethane can take the high-traffic stresses of floors.

    The products are overkill for other indoor applications, however, because the material is much harder to control and takes longer to set up in situations where acrylic latex would be perfectly suitable. Polyurethane caulks have tremendous bonding ability—so much so, that they can also be used as adhesives in some circumstances—but this tenacity does make them more challenging to work with.

    After laying in a bead of caulk, tool it to shape. Flexible latex caulk fitted this interior window stool project and was easy to work with and clean up using a water-soaked rag. (Photo: Peter Lord)

    After laying in a bead of caulk, tool it to shape. Flexible latex caulk fitted this interior window stool project and was easy to work with and clean up using a water-soaked rag. (Photo: Peter Lord)

    Polyurethane is solvent-based, meaning that you must use paint thinner for cleanup. Early polyurethanes had to be mixed from two parts in the field like epoxy, but the new products come in one-part formula tubes ready to use. Due to their adhesive strength, removal usually involves cutting out or sanding off unwanted caulk.

    Always read the product label carefully for application purposes, and take advantage of product websites and contact numbers before making a sticky mistake. Like most purchases, you get what you pay for, so the cheaper the caulking is, the less solid components it will contain, resulting in a poorer performance and more shrinkage. Because we’re talking about a jump from $2 to $7 a tube here, the investment is well worth it.

    Where to Caulk

    Caulking closes up the cracks and gaps that allow air and water to infiltrate your old house. Even the smallest voids exposing the inside of your home to the outdoors can be a threat to building materials and permit a tremendous amount of air flow. Remember not to go caulk crazy, though; houses do need to breathe to provide healthy air exchange and accommodate heating system and appliance requirements. 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, but it also has an aesthetic purpose. A thin bead of caulk can hide unsightly gaps and make joints easier to keep clean.

    It is always best not to depend on caulking as a fallback for haphazard workmanship. Cut trim and clapboard joints tightly, or scarf them with overlapping miters so they don’t gap excessively. Also, never caulk around panels in woodwork, such as a raised panel door or wainscot. These panels are designed to move freely in their framework during seasonal changes-as much as 1/8″ for a 12″ panel. Caulking them in place could spell disaster because today’s stronger caulks will often allow the wood to split before they break their bond. If you absolutely cannot stand to look at a gap that is expected to move seasonally, run just a slight bead of caulk along the surface, but never inject it underneath the joints of the paneling.

    Perfecting the Perfect Caulk Job

    Plugging half-used cartridges of caulk until the next project has long been the job of tape or screws, but for those who like accessories there are now various manufactured caps on the market. (Photo: Peter Lord)

    Plugging half-used cartridges of caulk until the next project has long been the job of tape or screws, but for those who like accessories there are now various manufactured caps on the market. (Photo: Peter Lord)

    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. Brush or vacuum interior gaps. Around bathtubs and kitchen sinks, wash carefully to remove any soap residue, and then follow with an alcohol wipe to catch any water.

    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.

    Caulking may not be brain surgery, but it isn’t child’s play either. As I have complained, this sticky stuff sure gets around, and if you don’t stay on top of the cleanup as you go, you’ll have a permanent mess. To help produce a clean, even job, don’t hesitate to mask off the surrounding area with tape and paper. Rushing never pays off because getting excess caulk out of wood grain and cracks is tough to do. Caulks that are solvent-based, such as polyurethane, need to be cleaned with paint thinner, which can leave stains on nearby finished surfaces. Always have cleaning materials ready: a bucket of water and rag for latex and acrylic products, or a cup of paint thinner and a rag for polyurethane.

    A nice, tight bead is sufficient for sealing the joints between building materials, such as wood trim and stucco. Many polyurethane products now come in tints that can be chosen to blend with exteriors, such as this stucco, eliminating the need for painting. (Photo: Peter Lord)

    A nice, tight bead is sufficient for sealing the joints between building materials, such as wood trim and stucco. Many polyurethane products now come in tints that can be chosen to blend with exteriors, such as this stucco, eliminating the need for painting. (Photo: Peter Lord)

    The most important tools for caulking are your fingers and a caulking gun, which is worth the investment. Whether you opt for a frame-style gun with double bars or an open-cylinder-style gun, the better models have far more mechanical advantage than cheaper models and are worth the extra money. If you are using stiff caulking, purchase a gun designed for applying polyurethane, or your fingers will get mighty tired. If you have a lot of caulking ahead of you, consider one of the cordless caulking guns on the market. They’re heavier and a little bulkier than a manual gun, but the ability to select (and change on the fly) bead size and control the caulking for precise starts and stops, all finger-fatigue-free, almost makes caulking a pleasure.

    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. You achieve this shape by using a tool or your finger to create a concave surface when you clean excess caulk off the bead surface.

    Never depend upon caulk alone to fill a gap any wider than 1/4″. If the joint is bigger, first insert backer rod (foam cording) in the gap, and then fill to the surface with the caulk of your choice. Caulk alone simply cannot stretch enough on large openings, and the crack will inevitably open up if you skip this step.

    Caulking for Energy Savings

    Always keep cleanup materials handy, especially for polyurethane and caulks that require solvents. Test beforehand to ensure that thinner will not discolor the finished surface. (Photo: Peter Lord)

    Always keep cleanup materials handy, especially for polyurethane and caulks that require solvents. Test beforehand to ensure that thinner will not discolor the finished surface. (Photo: Peter Lord)

    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. According to the U.S. Department of Energy, caulking drafty areas of a building can cut energy costs by 10 percent; furthermore, about 50 percent of the average fuel bill is the result of heat loss from air infiltration. If that sounds hard to believe, Old House Journalonce speculated that if all the cracks in a typical old house were combined into a single opening, it would total nine square feet or more, about the area of a window opening!

    Any time you make a building more vapor-tight, you risk trapping high moisture levels inside, which we all know have the potential to 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. This practice creates a path for some moisture migration out of the structure.

    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 storms and weather seals, caulk can be a cost-effective way to improve old-house comfort and performance.

    Published in: Old-House Journal January/February 2007



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