This post originally appeared on the Green Building Advisor website. www.greenbuildingadvisor.com
I once heard Dr. Joseph Lstiburek use the term “pookie”, which made me chuckle. He was referring to a fluid type product used to seal something. In construction, we use a lot of different caulks, sealants, and adhesives, all of which are available in some sort of tube or bucket. A walk through the caulking isle at any hardware store or lumber yard can make your head spin, what to use when and where. 
The Difference Between Sealants and Adhesives
Let’s start with the difference between a sealant and adhesive. The purpose of sealant is to bridge a gap between two surfaces. Some sealants tend to be a cosmetic solution, making the joint look good at a low cost, such as making a small gap between two pieces of trim disappear. Other types of sealants are more effective at stopping air, water or other undesirable substances from moving in or out of the joint, these types of sealants tend to be more expensive than standard painter’s caulk. Sometimes these purposes are combined, the seal needs to both look good and perform. Sealants have properties with lower strength bonds but have high flexibility. Adhesives, on the other hand tend to have high bond strengths, usually with less flexibility. Adhesives are used to hold two or more surfaces together, a chemical bond if you will. There is a line of adhesives called hybrid adhesives that also act as a sealant, the best of two worlds.
Major Manufacturers
Most manufacturers of sealant also make adhesives. Titebond, Loctite, DAP, Sashco, Liquid Nails, Prosoco, and Sika all make quality sealants that can be found at local hardware stores, box stores and lumber yards.
Types of Sealants
Different sealants have different properties that have been designed to work with a variety of substrates, working temperatures and have a range of flexibility rates. No one sealant is the best at everything. It is crucial to understand the strengths and limitations of the different products. The manufacturers have very good websites describing these strengths and limitations. The different formulas include latex and acrylic, butyl, synthetic rubber, polyurethane, silicone and polyether. There are other types, but these are the most common used in construction.
Latex and Acrylic
Latex, which is a water-based formula and acrylic, which is solvent based, are typically the least expensive of all sealants. These products cure through the evaporation of water or solvents in the product which results in a shrinkage rate of one-quarter to one-third of the sealant. Application temperatures ranges from 40°F-120°F. Because of the low odor and minimal volatile organic compounds (VOC’s), latex sealants are a good choice for indoor applications, latex is also tooled easily. They work well with wood, masonry and concrete, vinyl and aluminum, but their use with polyethylene and polypropylene should be avoided. Sashco’s Big Stretch and DAP’s Dynaflex 230 are examples of latex/acrylic sealants.
Butyl
Butyls are some of the oldest “modern” sealants in use, they are a solvent-based product that, similar to the acrylics, use evaporation to cure. This evaporation results in shrinkage. Cure rates typically take more than one week with some butyl sealants never fully curing, they remain soft and sticky throughout their service life. Application temperatures range from 20°F to 100°F and they bond well to most common building materials. Butyls tend to be sticky and stringy, difficult to tool. Butyls are often used in air sealing, sound proofing and sometimes in below grade applications. Because of their solvent base, they tend to effect air quality and are best used outside. Tremco Acoustical Curtainwall Sealant (also known as black death) and Manus-Bond 50-A are examples.
Synthetic Rubber
Synthetic rubber sealants are similar to acrylic and butyl in that they are solvent based with the product shrinking during the cure, cure times can take up to one week. Synthetic
rubber is very flexible and will return to its original cured shape and size when stretched. Application temperatures are from 0° to 120° and many can be applied to wet surfaces, some formulas are designed for roof repair, even when it’s raining. Synthetic rubbers have higher VOCs and are recommended for outdoor use. These sealants bond well to most common building products but should not be used with polystyrene, polypropylene, polyethylene, silicone and waxed surfaces. Sashco’s Lexel and OSI Quad are examples.
Polyurethane
Polyurethane sealants are a moisture cure product, the sealant requires some moisture from either the air or substrate to start the curing process, which can take up to 7 days to fully cure. Application temperatures range from 32°F to 100°F. Polyurethane sealants are durable, long lasting and can be painted. Polyurethane can be a problem for people that have sensitivities to urethane chemicals, specifically isocyanates, PPE is recommended. This sealant works well with concrete and masonry products, metal, stone, vinyl and wood. Sikaflex from Sika and Loctite’s PL Window, Door and Siding Polyurethane Sealant are two examples.
Silicone
Silicone sealants have been used in the construction industry since the 1930’s. This sealant bonds well to glass, plastics and other non-porous materials, but poorly to masonry and concrete products as well as to itself. Old silicone should be removed before reapplying. Modern silicones cure with moisture, taking up to 24 hours or longer to fully cure. Pure silicone will not accept paint. Application temps range from 20°F to 120°F. Titebond 100% Silicone Sealant and GE’s Supreme Silicone Kitchen & Bath Sealant are two examples.
Polyether
Also called sily-terminated polyether (STPE), these sealants have been used in Japan for
years but have recently seen an increase in popularity in both Europe and North America. Some STPE’s are also called fluid applied membranes and can be used to create a window pan flashing or a thin, tape-like seal for joints. STPE’s cure by a reaction with moisture, either from the air or substrate to which they are applied. In some cases, the cure time can be decrease by misting with water. Application temps range from 0°F to 120°F. STPE’s bond well to most building substrates. Examples are Prosoco’s Fast Flash and Titebond Weathermaster Sealant.
Types of joints
There are three common joints where a sealant may be required, a corner joint, or changes in plane or direction of the surfaces. There are also flat joints, a break or disconnect between two surfaces. These first two joints often include movement between the surfaces, expansion and contraction that need to be addressed. Lastly there is a bedding joint, this type of connection typically includes some sort of mechanical connection between two surfaces with the sealant between, think installing a flanged window to an exterior wall, using both a sealant and fastener.
When sealing either a corner or flat joint, it’s best if the seal only includes the two surfaces requiring the seal. For instance, let’s say we are needing to create a seal between two pieces of wood trim fastened to a substrate. The wood trim and the substrate has the potential to expand due to temperature changes or moisture content. Best practice is to only have the sealant span between the two pieces of wood, and not also bond to the substrate. This is accomplished by packing the joint with a backer rod or adding a bond-breaking tape to the substrate before installing the two pieces of trim. Corner joints would be treated similarly.
Illustration by Christopher Mills and Fine HomeBuilding Magazine
A bedding joint is simply a large bead of sealant that is squished between two surfaces, a way to keep water or air from moving through the joint of dissimilar materials.
Surface Preparation
Most sealants will require the surface(s) being sealed to be clean and free of loose debris. Some sealants will not adhere properly to oily or waxed surfaces, the oil or wax may need to be removed chemically or by sanding. Depending on the sealant being used, the surface may also need to be dry. Other sealants require some moisture to properly cure with the moisture coming from the air or from the surfaces which the sealant is being applied. Others may need a little misting of moisture when conditions or surfaces are very dry.
What to use when
As stated earlier, there is no one sealant that works for every application. Deciding which product to use will often require some research. Each of the major manufacturers have good websites to assist in product selection. Many also have live technical support during normal business hours. At a minimum, you should read the technical data sheet (TDS), often found on the manufacturer’s website. The TDS will cover everything from what the sealant is recommend and not recommended for, how to clean-up the product and any safety concerns that may arise in using the sealant.
Packaging
When using a caulk or sealant, the product is typically supplied in a 10-ounce tube and application of the product will traditionally be with a manual caulking gun. There are a few additional container and cartridge sizes available as well. The 30-ounce tube, 20-ounce sausage tube and bulk buckets of some sealants are available by some manufacturers.
The advantage of a sausage tube cartridge is the limited container waste produced. When the cartridge is empty, all that is left is a small cookie sized piece of foil, compared to the plastic tube left from the traditional caulk or sealant container. The sausage tube is my preferred application container, unfortunately, not many manufacturers offer the sausage tube option.
Traditionally, we use sealants to seal something out, like water or air. If you’ve had the opportunity to hear Steven Baczek speak, he makes a point that he is also concerned with what is sealed in, meaning how assemblies dry. An important consideration. When used in the right applications, sealants are critical parts of how water and air are managed in construction, the key is using the right product for the job.