The Importance of Good Caulking
Caulking materials are an important component of exterior walls. They seal expansion and contraction joints, joints between dissimilar materials such as glass, steel, masonry and concrete and joints in window frames, metal curtainwall and balcony enclosures. Their main function is to prevent rain and dirt from entering the joints and, depending on the purpose of the joint, prevent air leakage. Caulking is also used to seal cracks in exposed cast-in-place concrete and in precast concrete panels which are subject to thermal movement.
Its ability to provide the required seal is dependent both on its ability to adhere strongly to the building surfaces and on its flexibility to enable it to accommodate changes in width of the joint without failing. It must retain these properties over the normal range of temperatures experienced in Canada and should retain sufficient ductility as time passes that it will perform properly for its expected service life which, for many caulking materials, is assumed to be approximately 15 years.
Caulking material with the right properties in terms of adhesion, elongation and flexibility must be selected with care with the joint design and the substrate material to which the sealant must adhere being taken into account. When selecting a material, good past experience with a particular material under similar service conditions is the best guide.
Caulking materials must be applied to clean dry surfaces within the correct temperature range for the particular material being used. Its sensitivity to moisture is also important since both masonry and concrete are porous such that the surface to which the caulking material is adhering will periodically become damp.
Replacement of caulking should include removing all of the existing material, cleaning the surfaces to which the new material will adhere and then installing a new bead with the correct cross sectional shape and with a bond breaker at the back unless the future changes in width of the joint are going to be so small that a bond breaker is not required.
The shape of the bead of caulking material is important to its successful performance in its service environment. The bead should be concave on both the outside and inside surfaces, should have a width to depth ratio of approximately 2 and its sides should be fully adhered to the sides of the joint.
The curvature on the inner side of the bead is typically achieved by inserting a compressible plastic rope shaped material that will not adhere to the caulking material. The back-up rope is also used to control the depth of the bead and for this reason, great care should be taken to position it at the correct depth. Too deep or too shallow will result in the wrong shaped bead and this will cause premature failure by splitting or separation.
The depth of the caulking bead can be increased in relation to its width in those joints which are expected to undergo only small percentage changes in width. Fillet beads at corners subjected to significant movement should be convex and have the bond broken in the corner to avoid a concentration of stress at the corner which could result in premature failure. Where little or no movement is expected in a corner joint, the shape of the fillet bead becomes less important.
Bubbles of air trapped in the caulking bead as a result of poor application procedures create weaknesses and are often the points of origin of localized splitting failure.
Caulking beads fail by splitting as a result of inadequate cohesion of the bead and by separation from the substrate material due to loss of adhesion.
What Not To Do
Caulking material is sometimes left in place and a new thin bead applied over the top. This procedure will not result in a durable repair since the split in the underlying caulking, or the crack where it has separated from the substrate, will concentrate the stresses in the thin overlying bead and initiate early failure of the new material at the same location.
The installation of an overlay bead at failure locations will, however, buy time and maintain water tightness until a general replacement program is undertaken.
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