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By: Wayne Summons, Sashco Sealant Inc.

(Note: For the purposes of this article, a "sealer" is like a water-thing coating, brushed or sprayed onto log surfaces for the purpose of preventing wood decay and water penetration. A "sealant" is like a caulk, applied to the joints between log surfaces for the purpose of stopping air, water, and insect infiltration. This article is devoted to "sealants" rather than "sealers". The importance of tightly sealing the many joints in a log home is well recognized by everyone in the log home industry: but, the craft and science of sealing these joints is much more important than has generally been realized. Involved in providing a quality, durable seal are five factors, all of which should be carefully considered, and any one of which, if overlooked, could cause failure. For the log home manufacturer or builder, understanding these five factors is very important to the production of a well-sealed and, therefore, more saleable home. (It should be noted, however, that in the presence of extreme log movement, which can occasionally occur, failure is inevitable for even the most conscientiously designed sealant system. In these rare cases, proper repair will then solve the problem for many years thereafter. The five controlling factors of a good sealant system are as follows:

I Type of Caulk or Sealant

II Joint Design

III Tooling

IV Application Conditions

V Sealer-Sealant Compatibility

The following presentation explores each area sufficiently to prompt good basic practices and stimulate further enquiry.


The choices are many, and each product class has several "pros" and "cons." Moreover, even though several products may be based on similar chemical backbones, the overall differences in their respective formulas can be enough to make one or more products vastly outperform the others. In other words, one should not rely solely on the "generic" name of any sealant to precisely characterize its performance. Each product should be considered on its own merits. Nevertheless, the following major classed of caulks and sealants, in alphabetical order, have these properties:

A Acrylic – This class of sealant is available in either water or solvent clean-up systems. Both the water and solvent systems share these properties: excellent long-term durability; excellent elasticity; very good performance in expansion-contraction joints (which are typical of log homes); very good adhesive and cohesive strength; moderate cost; paintable-stainable; long shelf life in unopened packages (typically 1-2 years); and, in good package freeze-thaw stability.

The water clean-up ‘pros’ are:

  1. Cleans up with water.
  2. Easily gunnable and toolable throughout its application temperature range (40 degrees F-120 degrees F).
  3. No obnoxious, toxic or flammable chemicals to contend with.
  4. A new generation of these products can meet the performance requirements of Federal specifications.

Water clean-up ‘cons’:

  1. Most are vulnerable to wash out if rained on within 1-3 days after application.
  2. Should not be applied in sub-freezing weather.
  3. Most cannot meet the performance requirements of Federal specifications. Solvent cleanup acrylic ‘pros’:
  1. Immediate resistance to rain.
  2. Difficult to apply and tool because the product is very sticky.
  3. Low temperature gunnability is poor, unless the product is heated before applying.
  4. Because of long "skinning" time, dirt pickup is a problem.

(Note: The new group of synthetic mortar-look-alike chinking compounds are based on water cleanup acrylics.)

  1. Butyl – This class of product is available only in solvent cleanup form. Its advantages include:
  1. Excellent water resistance.
  2. Excellent lap joint sealant, as in gutter and downspout joints. (Log joints are not lap joints.)
  3. Low to moderate cost.
  4. Very good durability.
  5. Paintable – after several days’ cure.
  6. Good package and freeze-thaw stability.

The Butyl disadvantages are:

  1. The solvent content is flammable and toxic.
  2. Difficult to apply, tool, and cleanup because it is solvent based and extremely sticky and stringy.
  3. Because of inherently poor cohesive strength and little elasticity, it cannot withstand expansion-contraction. Therefore, it is not suitable for the typical log home joint.
  1. Oil Base – This class of product is solvent based and has essentially three main disadvantages:
  1. Low cost.
  2. It has immediate resistance to rain.
  3. It can be applied during cold weather.

The ‘cons’:

  1. Poor durability – often failing within 1-2 years after application.
  2. These products usually become hard and brittle, thus accommodating little, if any, expansion or contraction.
  3. They can often stain surrounding wood.
  4. The solvents in them are flammable and toxic. Solvent cleanup is required.
  1. Polyurethane – Polyurethanes exhibit these main advantages:
  1. Excellent durability.
  2. Excellent adhesion-cohesion and the ability to withstand expansion and contraction.
  3. Immediate rain resistance.
  4. Can be applied in cold weather.
  5. Most meet Federal specifications. Disadvantages include:
  1. Moderate to high cost.
  2. These products contain solvents that are flammable or toxic.
  3. Solvent cleanup is required.
  4. One-component products have short lives – typically, six to nine months.
  5. Two-component products are difficult to mix and apply.
  6. Tooling can be difficult.
  7. One-component products, depending on geographical region, can cure slowly, thus picking up dirt on the surface.
  1. Silicone – Perhaps the best known of the quality sealants, silicone has these main advantages:
  1. Excellent durability, perhaps the most durable sealant known.
  2. Excellent sealant strength.
  3. Good for expansion-contraction joints.
  4. Immediate rain resistance.
  5. Can be applied in sub-zero weather.
  6. Long shelf life of unopened packages.
  7. Meets Federal specifications.

Some disadvantages are:

  1. High to very high cost.
  2. For adequate adhesion to wood, a primer is usually needed.
  3. Cleanup requires solvents.
  4. After cured, there is a strong tendency to dirt pickup.
  1. Vinyl –

This group of caulks is essentially water-based. In addition to water cleanup, other advantages include:

  1. Low to moderate price.
  2. Fast skinning to retard dirt pickup.
  3. Very good cohesive strength.
  4. Long shelf life in unopened containers.
  5. Paintable-stainable.

Disadvantages are:

  1. Only fair durability – often failing after 2-4 years.
  2. Can wash out from rain within 1-2 days after application.
  3. The packaged product is often not freeze-thaw stable.
  4. Application in sub-freezing weather is not recommended.
  5. Expansion-contraction capabilities are poor.
  6. Cannot meet Federal specifications.


The next most critical factor is the design or configuration of the joint construction. As Les Burch, President of Sashco Sealants, pointed out in a recent paper presented to the North American Log Builders Association. (1), there are three main principles to good joint design:

A Depth Is One-Half Width – This is a general rule aimed at balancing the two factors of adhesion, (how well caulk sticks to the log) and cohesion. (how well it holds itself together.) Since most elastic caulks are cohesively strong, the thinner cross-section at mid-joint provides less resistance to movement and, therefore, less stress on the adhesive bond. However, the better caulks have good adhesive-cohesive balance. It is better, therefore, to fill a (1) "CAULK JOINT DESIGN" by Les Burch, President, Sashco Sealants, Inc. Presented at North American Log Builders Association annual meeting in Nashville, TN October 23, 1984 joint "too deep" than "too thin" since sufficient caulk mass is required to absorb movement. Practically speaking, caulking joints do not need to be greater that 1-2 inches deep; nor should they be less than 1-4 inches deep, regardless of width.

B Two Point Adhesion – Caulk must be free to move when the logs do. Whether the joint is in tension or compression, the caulk should only be adhered to the moving elements at two points. If it is adhered to three or more points or a non-moving element, the seal will likely break.

The job performed by backing materials is not only to take up space but provide a "third" surface to which the caulk will not adhere. It is then able to move over the backer rod freely and fulfill its elastic function."

C Proved a Caulk "Well"

Most elastic caulks are rated as to the total joint movement they will absorb. The better ones are rated at 25 % to 50% total movement. This means that a caulk "well" or space must be created which is two to four times larger than anticipated movement. If a 1-4 inch movement is anticipated, a 1-inch caulk "well" is needed. The good news is that these ratings are tied to Federal specifications, and caulks with good log home performance records are usually absorbing movement well above their specified rating.

The lesson here is to provide as large a caulk "well" as practical and realize that, in instances where movement is 100% or more of original joint size, some recaulking will probably be needed. There is great leverage gained in going, for example, from an initial joint design of 1-8 inch in width to 1-4 inch wide. A 200 percent movement would increase from 1-4 inches (for a 1-8 inch joint) to 1-2 inch (for a 1-4 joint). The larger the beginning joint size the less likely the log movement will exceed caulk capabilities.


Using good materials and installing them in well-designed joints could still lead to failure if the sealant is not force into intimate contact with the wood surface for good adhesion. This is simply a matter of patience and the "practice makes perfect" skill of tooling the caulk bead, using various techniques and equipment. Putty knives, cake spatulas, and fingers are often used; and usually, they are applied to the caulk surface after being wetted with water, soap and water or alcohol and water. In this way, the tool glides over the sealant leaving a smooth finished appearance while forcing the sealant deep into the wood surface.



Temperature and moisture are not only what caulk protects a home from, but they control a caulk’s application and its performance as well. The joints in any log structure are constantly moving in response to temperature change and swings in log moisture content. When logs shrink (as during dry, cold periods), the gap between them gets larger. If a sealant is applied during the coldest and driest conditions, then the gap will be filled while at its largest size. Any subsequent movement will make the gap smaller and squeeze the caulk strictly into compression. If the application conditions are reversed, all future movement will stretch the caulk strictly in tension. Ideally, sealant should be applied to a gap when at its median width so future movement will not be strictly compression or tension. Practically, this goal is impossible to achieve. Therefore, the best practice is to work with the weather and apply caulk to the cool side on a hot day and the warm side on a cold day. Also, certain products respond in unique ways to temperature and moisture.

For example, silicone, has the unusual ability to be applied in sub-zero weather and perform well upon cure. However, satisfactory results often are not achieved because, at these temperatures, there is usually frost (often microscopic and, therefore, unseen) on the surface being sealed. Since the silicone is thwarted from intimate contact with the surface, poor adhesion and failure can occur. Another example: Wet logs are not a good surface for any sealant. But, water cleanup acrylics perform very well when applied to damp logs; while most other sealants require the log surface to be dry.


The last main consideration is the compatibility of the caulk and the wood sealer. Several problems can arise if incompatibility exist. For instance, some wood sealers contain chemicals that virtually no caulk can adhere to. These chemicals (stearates and waxes) are excellent water repellants, but they form a powerful barrier to good adhesion. If this type of sealer is used, then the caulk must be applied to the log before the sealer is applied. The result is that deep fissures and cracks develop almost immediately in the uncured caulk, causing severe failure. Simply letting the caulk cure a few extra days solves the problem.

Other problems could occur as well, and the best advice is to test the caulk and sealer together before committing to their combined use.


Caulking log structures is not complicated, but it does involve more factors than is generally appreciated. By recognizing these factors and rationally applying common-sense techniques to overcome potential problems, highly successful, durable seals can easily be built into the already high quality of log homes.


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