Outdoor caulk seals the exterior seams of a house in order to keep moisture, cold and hot air and pests from coming inside. Your exterior caulking should be inspected and replaced as required once every year or two, but properly applied caulk of good grade should last for many years with little to no maintenance. There are caulks are designed just for indoors use, and there are other caulks meant for the exterior of buildings, so don’t get confused.
Although the term sealant is technically more correct for exterior use caulks, the terms caulk and sealant often refer to the same class of product. Caulk is a product meant for filling gaps, and a sealant is meant for providing long term sealing of a joint which has relative movement, and to prevent moisture and air penetration.
When you go to the hardware store, you’ll see all kinds of caulk for use on different materials, like special caulk for concrete, for brick, for glass, aluminum and so on. While it is important to be aware of the materials you are applying caulk to, for outdoor work it is best to stick with a good polyurethane caulk, or siliconized acrylic.
Definitely avoid latex and butyl rubber caulks, which are not durable enough for outdoor use. (Butyl rubber formulas also contain high levels of volatile organic compounds) Silicone based caulks are also poor choices for outdoor use, due to its poor adhesion to wood.
Siliconized Acrylic Caulk
Siliconized acrylic caulk is a little less durable than polyurethane, but can work well for caulking around vents, doors, windows, and trim, since this type of caulk is good to use on smaller gaps. It is easier than polyurethane to work with since it cleans up with water, and better tolerated by those with chemical sensitivities. Siliconized acrylic caulk can be painted over after is applied, so it is less noticeable. All of these features make this one of the most popular caulks for outdoor use among do-it-yourselfers.
Examples of siliconized acrylic caulk include:
• OSI Pro Series Pro 210 Acrylic Latex Caulk with Silicone
• OSI Pro Series SA 167 Siliconized Acrylic Latex Caulk
• Polyseamseal Paintable Acrylic Caulk with Silicone
• Red Devil PaintMaster Siliconized Acrylic 25 Year Caulk
Polyurethane caulks are paintable, flexible, and weather resistant. Although they are the most expensive of all caulk types, they are the standard of the construction industry for outdoor use, and for a good reason. They have superior bonding ability, to the extent that they can be used as an adhesive in certain applications. Because of this, polyurethane caulk works well in joints between dissimilar materials, for example window sill plate joints of wood to concrete, joints between flashing and masonry around chimneys, and joints in driveways and concrete slabs between different masonry types.
The downside to this type of caulk is that it is somewhat messy to work with, as it is stringy and sticks to your hands and clothes. It must be cleaned up with mineral spirits, as it is not water soluble. It can also be somewhat toxic, so it should not be applied around intake ducts or ventilation grilles, where siliconized acrylic latex caulk should be used.
Because they expand as they cure, polyurethane caulks are also a little tricky to apply; they come in low, medium, and high expansion formulations, with low expanding polyurethanes enlarging up to 300% when cured. Follow the instructions and warnings on the product label carefully. Wear a respirator and rubber gloves when applying these materials.
Examples of polyurethane caulk include:
• Sherwin-Williams SherCrete SW-2NS Polyurethane Sealant
• Sonolastic NP1 One-part Polyurethane Sealant
• Sikaflex 15LM Polyurethane Caulk
There is also a new class of exterior caulk formulations that combine properties and benefits of both silicone and polyurethane products. This includes polyethers like Sonolastic 150 VLM and modified-silicone polymers and such as DAP Side Winder and OSI Advantage. Although expensive, they are very durable, contain lower levels of Volatile Organic Compounds and cure easily. Since they are newer products, that are not yet in wide use so some contractors may not be familiar with them.
There are a few things to consider when shopping for an outdoor caulk.
-If the surface is to be painted, do not use silicone caulk, it is unpaintable.
-Caulk usually works best in a gap that is ¼ inch or less, so if you have a gap that is larger, it should be filled with some other material, such as a strip of wood, or a foam backer rod.
-Look for a caulk with a warranty as long as you can find, some can run up to 50 years.
-As their long life might indicate, polyurethane caulk can be difficult to remove once cured, so make sure that the area you are caulking will not need access or replacement.