Also see our Adhesives, Penetration Seals and Roof Repair Sections for similar items.

Selecting the right sealant or caulk for your exterior application can be confusing. Over the past decades, many improvements have been made and new chemistries developed enabling greater performance and expanded applications.

Caulks vs. Sealants: Some manufacturers use the terms "Sealants & Caulks" interchangeably. Sealants are designed to seal an interface which has movement. Caulks are designed to fill a gap. Sealants are designed to create a long-term seal that stays intact and prevents moisture ingress and are UV resistant. Sealants deal with greater joint movement, extreme temperatures and have very low shrinkage. Class A sealants have +/-25% joint movement capability. Polyurethane (urethane) based sealants are now the most widely used product in the roofing and waterproofing industry due to their super adhesion and toughness. Today, polyurethane sealants (still called caulks by many) are specified for most roofing and waterproofing details. They are used extensively in flashings, termination bars, reglets, copings and pitch pans. Upgrades from this are polyether based sealants which can have +/- 50% to +100/-50% expansion capability. Examples include Sonolastic 150 VLM.

Exterior grade weatherproof sealants, such as NP1, TX1 and Sonolastic 150  are high performance polyurethanes which have excellent performance for sealing most interfaces.

"Painters caulks" are price-driven products serving markets where performance is subordinate to economy. Painters grade caulks have limited movement capability; usually less than 12 percent movement before cracking, delaminating, peeling or permanently deforming. Painters caulks shrink quickly over time and have limited (short-term) weathering properties (poor UV resistance). They are NOT DESIGNED FOR long term UV exposure. Painters grade caulks are are based on less expensive polymers such as linseed oil, polybutene, asphalt, butyl rubber, SBR rubber, vinyl acetate and acrylic latex. Do not use these products in exterior applications or where joint movement can be greater than 12%.

Sealants are a different class of product designed for exterior use and long term  performance. Very high-performance sealants are 100% solids and chemistries are based on Silyl-terminated polyethers.

Phil Georgeah, President of Chemlink, comments “(painters) caulk becomes hard & brittle and cracks, splits or severely chalks within three years. These caulks often shrink, sometimes losing up to 30 percent of their volume from solvent loss, condensation or oxidation. Shrinkage applies excessive stress at the interface between the construction material and the caulk mass. (Painters) caulks generally exhibit fair to poor adhesion on construction materials. Adhesion failure gets worse with increased volume shrinkage. Most caulks are designed for the “do-it-yourself” market, or should I say, the “do it to yourself” market."

Although silicone sealants generally have good adhesion, they do not always perform well in roofing or stucco applications where heat and oil migration from roofing substrates or house-wraps tend to degrade adhesion. Silicones also do not provide good long term paint adhesion and are difficult to repair (silicone does not like to stick to cured silicone). 

New on the market are sealant chemistries such as Polyether which have very aggressive bonding to most materials, skin over, cure faster than silicones and many older urethanes and are odor free. They exhibit very little dirt pickup. These products bond like traditional silicones but are both paintable and more cost effective. Polyether sealants with VERY low modulus (soft) are also now available. For example Sonolastic 150 VLM. These low-modulus materials are capable of up to 1,200 percent elongation and offer far greater durability in applications with great movement than either standard silicone or urethane. In addition, very little moisture is required to trigger a rapid polymerization reaction in polyether sealants, so they cure quickly too.

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