Selecting Exterior Caulk
There are several general types of polymer used to manufacture most high-performance caulks: silicones, polyurethanes, latex-acrylics, and solvent-based “block copolymer” synthetic rubbers (which include butyl as well as some of the newer proprietary synthetic formulations).
The best type of caulk for the job depends largely on the materials to which the caulk must bond. For bonding dissimilar materials (for example, wood to masonry, look for sealant that can handle a lot of joint movement.
For the most part, polyurethane caulks provide the best all-around performance for exterior applications, but they are more expensive than most other caulks and can be difficult to clean up (below).
Exterior Caulk Selection Table
| Type | Maximum Joint Movement | Life Expectancy | Comments |
|---|---|---|---|
| Silicone | 50% | 20 years | Most flexible. Can be applied over the widest range of temperatures but doesn't stick well to wet surfaces or masonry; generally not paintable (though paintable products are available). |
| Modified Silicone Polymer | Combines the properties of polyurethane, silicone, and acrylic; easy to work with. | ||
| Polyurethane | 25% | 20-30 years | Good all-purpose caulk; excellent adhesion to most materials; difficult to clean up; paintable. |
| Polysulfide | 12-25% | 10-20 years | Difficult to handle; used mostly for sealing joints in concrete; may require primer on porous surfaces; wear gloves and maintain ventilation during application. |
| Acrylic Latex | 2% | 10-20 years interior only | Interior use only; easy to work with; cleans up with water. |
| Siliconized Acrylic Latex | 25% | 20-30 years | The addition of silicone makes this more flexible and water-resistant than standard acrylic latex; easy to work with and cleans up with water. |
| Butyl Rubber | 5-10% | 5-10 years | Poor adhesion on damp surfaces; attracts dirt; difficult to handle. |
| Synthetic Rubber | 50% | 5-20 years | Variety of proprietary formulas available; very flexible; can be applied in wet and cold weather; paintable. |
| Oil Based | 1-2% | 3-5 years interior; 1-2 exterior |
Not recommended. |
| Ethylene Copolymer | 25% | 10-20 years | Reasonable adhesion to most materials; can be painted. |
The table is meant only as a rough guide to help you narrow choices. (For instance, life expectancy and joint movement vary with the particular product.) While some caulks and sealants fit neatly into these broad categories, others, such as paintable silicone, are variations. There are also hybrid and specialty caulks with proprietary formulas that combine the properties of two or more types. As always, consult the manufacturer's specs on any particular product you are considering.
Joint Design
If the joint sealant is too shallow or deep, it will not expand and compress with the movement of the joint. Instead, it may split or rip away from one of the joint surfaces.
Shape
Ideally, caulk joints should be configured to span a gap that is half as deep as it is wide (below). The caulk should not bond to the back (three-sided bonding), but only to two sides, so it can flex as it expands and contracts.
Proper Joint Design
Corner fillets do not move freely and generally peel off the two surfaces. If a joint depends on a caulk sealant, open it up slightly to allow caulk to fill the gap. If a corner is tight, don’t caulk it; the caulk will fail faster than any finish applied to it and will only complicate refinishing.
Backer Rod
Backer rod should be used to fill large gaps and control the depth (Proper Joint Design, above). Most backer rod today is made out of foam and comes in diameters from 1/4 in. to 2 in. and up. Choose a backer rod that’s just a little larger than the joint so it will stay in place and allow you to control the depth of the sealant.
Foam backer rod comes in open-cell and closed-cell form. Open-cell backer rod compresses more and is easier to work with, but it’s like a sponge and can absorb water. In critical areas that may absorb moisture from the substrate, use a closed-cell material.
Bond-breaker tape. The main purpose of backer rod is to keep sealant off the back of the joint, thus preventing three-sided bonding. If the joint is too shallow to fit backer rod, apply bond-breaker tape to the back of the joint.
Applying Caulk
The fine print on the label of most caulk tubes reads: “Apply to clean, dry surfaces. Application temperature: 40°F to 80°F.” However, construction sites are never clean, seldom dry, and work doesn’t stop when the temperature drops below 40°F. To promote good adhesion, prepare the surface using the following strategies:
- Scrape, chip, brush with a stiff-bristle brush, and dust the inside of the joint, or even blow it out with compressed air.
- Wipe the surface with a strong solvent, such as acetone or methyl ethyl ketone (available in hardware stores and contractor supply houses). These solvents evaporate water and dissolve greasy substances that can retard adhesion.
- Every caulk joint should be tooled. The best tool is a steel spatula (or sculptor’s tool) that is slightly smaller than the width of the joint. Try to achieve a smooth, even appearance without squeeze-out on the sides.
- Cut the nozzle of the tube at an angle slightly smaller than the size of the joint.
- In cold weather, keep the caulk warm. If necessary, use an electric heat chest to store tubes. Also, it may be necessary to heat the surface with a hair dryer or a heat gun (just be careful not to strip the paint or start a fire).
