As the chart shows, this difference in expansion rate must be considered when joining a Vinyl Window to brick, or pine, or even aluminum, for example. Some basic guidelines to follow: 1. Latex, Acrylic Latex and Urethane Acrylic have limited application range and about 25% shrinkage and should not be used during inclement weather or as a back bedding sealant. Excellent for interior applications. 2. Polyurethanes have excellent joint movement capability but are prone to UV degradation and become less aesthetic over time. 3. Low and medium Modulus Silicones have excellent joint movement and UV stability. Tendency to collect dirt and dust. 4. Silyl Modified polymers (MS Polymers) -best choice as a back-bedding sealant. Compatible with Flashing tape and weather resistive barriers (WRB). Excellent adhesion to common building materials. 5. Elastomeric Polymer have wide temperature range application and exhibits approximately 25% shrinkage. Best used as exterior primary sealant where aesthetics are important. Which is Best? Best is a relative term. With all the different types of materials that need to be caulked or sealed, it's difficult to choose among the variety of brands and types of sealants. To decide which type of caulk or sealant will work best, it is better to look first at what caulks are supposed to do, and why most sealant failures occur. In simple terms, caulks are supposed to seal a joint between two materials so that air and moisture cannot penetrate the joint and this joint must stay sealed under variable weather conditions and even if the two materials move. Therefore, caulks must stick to the two surfaces, have elasticity to withstand movement, and must not deteriorate due to the environment. To determine which is best for your application, consider the following: 1. Used inside or outside 2. Application and Service Temperatures 3. Materials to be joined Application is probably the most important part. The single most frequent cause of joint failure is in the application. Joints that are mis-matched as to width of substrates, joints that are narrow or thin, joints that are deep or at an angle, or joints that are three-sided need extra care. The illustrations show how basic some of the problems are and how to correct them. It is important to remember that the purpose of caulks and sealants is not to substitute for structural joining. No caulk or sealant will "tie" two materials together. They will only "seal" the joint against air or moisture penetration. These guidelines are how to determine the joint size, but consideration must be given to surface prepara- tion, adhesion, weather and UV resistance, and caulk shelf-life. To better explain, look at the diagrams. The three main causes of seal failure, regardless of proper width or flexibility are Adhesive Failure, Cohesive Failure, and Substrate Failure. 1. Adhesive Failure is the result of the caulk or sealant not sticking adequately to one of the sub- strates. This will happen if the movement exceeds the sealant's capability, if the surface isn't prepared properly, or if the bead isn't carefully placed. 2. Cohesive Failure is the result of the sealant itself failing to hold together. Splits, tears, or other ruptures can occur if the sealant dries out, is over-stretched, deteriorates due to time and weather, or is improperly mixed or has entrapped air. 3. Substrate Failure occurs most often when there is inadequate surface preparation. The surface that fails probably needed to be sealed, scraped, or otherwise prepared to accept the sealant selected. Caulks and sealants fail because they don't stick, won't move, or they dry out, crack, or shrink. The major considerations are Material Movement and Joint Design. The two materials being joined are subject to expansion and contraction due to temperature and moisture changes, and there may be movement due to use. It is necessary to design a joint that allows the movement without overtaxing the elasticity of the caulk. BOND BREAKER TAPE BACKER ROD BOND BREAKER TAPE BOND BREAKER TAPE FAILURE O.K. FAILURE O.K. FAILURE O.K. FAILURE O.K. FAILURE O.K. HOW TO MAKE A PROPER BOND Window Fitters Guide to Window Installation 10 ©2022 AWDI, LLC Do not reproduce without permission FIR 2.1 PINE 3.0 OAK 2.7 BRICK 3.6 CONCRETE MASONRY 5.2 STEEL 6.5 ALUMINUM 12.8 GLASS 5.0 PVC 40.0 COPPER 9.3 GYPSUM BOARD 9.0 MARBLE 7.3 COEFFICIENT OF THERMAL EXPANSION OF COMMON BUILDING MATERIALS in./in.-oF x10-6 SAMPLE MANUAL
View this content as a flipbook by clicking here.