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Log Home Chinking - The Keys to Successful Sealant and Caulk Joint Design

With its thick walls of solid wood, a log home is one of the best-insulated buildings in which you could choose to live. But the nature of a log home's construction also presents a unique challenge: the opportunity for air to pass through the gaps between the logs.
Fortunately, air infiltration is a challenge that's easily met, thanks to caulking, the general term for the seals that are introduced between logs. With the right caulking method, or "caulk joint" as it is generally termed, a log home can achieve thermal efficiencies to rival those of any building method.
Two key elements are essential to effective caulking:

  • First, the caulk used must form a "wet" seal, one that adheres to the wood surfaces rather than just filling the gaps between them.
  • Second, it needs to be elastic (a quality sometimes referred to as "memory"), either stretching or compressing as the shapes of the logs themselves subtly move through natural expansion or contraction.
    Another way of thinking about a caulk's effectiveness is in terms of "adhesion" and "cohesion."
  • Good adhesion means that the caulk sticks to the log well.
  • Good cohesion means that it holds together well under stress. Obviously, a balance of both properties is necessary for a caulk to work properly.
    With those elements met, four principles govern the most efficient caulk joint design:
  • caulk depth
  • points of adhesion
  • using a caulk "well"
  • conditions under which the caulk is applied
    Each principle is fairly simple to grasp, and understanding them all will help anyone buying, building, or restoring a log home make the right energy-efficient decisions.
    Principle 1: Apply the Right Caulk Depth
    In general, the depth of caulk in a joint is more important than the width of the caulk's application. Why? Put as simply as possible, the most critical factor is that the caulk have enough mass at any given point to absorb any movement in the logs. As a rule, whatever its width, the depth of a caulk joint does not need to be any more than 1/2 inch or any less than 1/4 inch. (See figure 1, which illustrates how a thinner cross-section results in less stress on the joint.). Also, a deeper joint is preferable to one that is too thin.

    Principle 2: Provide Two Caulk Adhesion Points
    Caulk works best when it adheres only to two elements, namely the two logs between which it is applied, so that the seal will hold if and when any movement occurs in the logs. (See figure 2, which illustrates how this principle applies whether the joint is being stretched or compressed.)
    Adhesion to three or more points, by contrast, would cause the caulk to be pulled in multiple directions, thus increasing the likelihood that the seal will break away from one or more of the points of adhesion. (See figure 3, which illustrates how such ruptures can occur under three-point adhesion.)

    Placing a backing rod inside the well With this principle in mind, any backing materials included in a caulk joint should provide a surface to which the caulk does not adhere. Doing so allows the caulk to remain at its most elastic.
    Principle 3: Create a Caulk Well
    Superior caulks have enough elasticity to absorb 25 to 50 percent of the total movement in the joint they fill. To allow for that movement, caulk joints require "wells," spaces that allow the joint to absorb that movement. For a reliable seal, the caulk well should be two to four times larger than the movement that is anticipated for that joint, thus reflecting the 25 to 50 percent absorption properties of the caulk being used. (See figure 4, which illustrates how an anticipated 1/4-inch movement calls for a 1-inch caulk well.)

    Obviously, the greater the movement expected in a particular joint, the larger its caulk well should be. Log homeowners can also take comfort from the fact that high-quality caulks currently being sold are capable of absorbing far greater movement than that for which they are rated.
    Principle 4: Apply Caulk Under Optimum Conditions
    Because logs expand or contract in response to temperature and humidity, the climate conditions at the time caulk joints are applied will affect the seal formed. On a very hot, humid day, for example, a joint will be at its smallest, since wood expands under such conditions; so caulk applied at such a time will later be subject to the maximum stretching. By contrast, a joint will be at its largest on the coldest, driest of days; so caulk applied then will later be subject to the maximum compression. (See figure 5, which illustrates the tension on caulk joints under both extreme and average temperature conditions.)

    With this principle in mind, it is wise to select a caulk with properties that best suit the range of local climate conditions, and to apply it at the most stable, least extreme point in the climate cycle. But smart log home builders will also apply caulk at optimum times; for example, sealing caulk joints on the cooler side of a home will work most effectively on a warmer day, while those on the side of a home that gets more direct and prolonged exposure to sunlight would best be sealed on a cooler day.
    By aiming to apply all four principles described above, you'll maximize the effectiveness of your caulk joint seal. That may require you and your builder or contractor to implement sealing techniques that go beyond the most obvious and simple solutions. (Figures 6 and 7, for example, illustrate how two logs conventionally sealed with caulk applied at their meeting point lead to a triangular, three-point adhesion that can easily rupture; but adding a caulk well with a backing rod to the same log cross-section results in much more secure two-point adhesion.)

    Keep these guidelines in mind, and you'll thus enjoy the maximum benefits of log homes' superior insulation and energy-saving capabilities.