[Vertebrate Pests] Pest-proofing small holes

In elementary level pest management classes, we learn that we should advise our clients to keep holes in buildings with diameters of 6mm/¼ inch and larger sealed to exclude rodents. Certainly, pest proofing is the best method of pest control. In fact, ideally, all holes of any size should be sealed to exclude not only rodents, but invading insects as well.

But what do we actually mean when we advise our clients (or perform the work ourselves) to repair the holes, cracks and crevices of buildings? "Plugging a hole" to keep pests out may be a menial job, but it is important that it be done correctly. Too often, this seemingly simple task is not given the attention it deserves.

"Pest-proofing" small holes ranges from the simple stuffing of some metal mesh into a hole to a more detailed process of selecting a specific sealant for the particular hole or opening and then ensuring the sealant is applied with attention to detail and good workmanship. This column addresses pest-proofing small holes from simple to the more elaborate situations to keep rodent (and insect) pests out in a manner that offers cost-effective permanency for your clients.

CAULKS VS. SEALANTS. The terms "caulk" and "sealant" are sometimes used interchangeably, but they are actually two different materials. Both caulk and sealant have their place in structural repairs (and thus in pest-proofing). Unfortunately, it is common to hear from the lay public and pest professionals alike the generic phrase of "caulking up" holes and cracks. But for persons involved in building maintenance (e.g., contractors, maintenance personnel, building supers, pest professionals), using the correct material and terminology is important.

With one of my pharmaceutical clients, for example, their pest management professional recommended the pharmaceutical company "caulk" up a large wall to keep clover mites from entering the filling room. The pharmaceutical company followed the pest professional’s direction and caulked a large section of an exterior wall. The result: clover mites later contaminated a significant amount of their product because the mites penetrated beneath the brand new caulk bead. This was a very, very expensive mistake in terminology.

The key to using either a caulk or a sealant to repair a break lies with the word "elastomericness." This technical word simply refers to whether or not a material will return to its original size and shape after being stretched or compressed.

Most caulks or caulking compounds are non-elastomeric. Caulk materials are used for filling small joints (¼ inch [6 mm] or less) where little or no movement is expected. Thus, for many pest-proofing situations, caulking compounds are not appropriate materials.

Sealants, on the other hand, are elastomeric materials used to seal joints where movement between the elements ranging from 25 to 50 percent is expected. (Sometimes, a "sealant" is confused with a "sealer," another material that is occasionally used for certain types of pest exclusion. A sealer, however, is a liquid coating applied to surfaces for filling pores and hairline cracks.)

But there are differences among sealants and selecting the right sealant for the job is important. The factors that should be considered when choosing a sealant include: 1) the amount of movement expected at the joint; 2) compatibility between the surface and the sealant materials; 3) durability (e.g., resistance to moisture, wear, oxidation, high temperatures and UV light); 4) temperature ranges; 5) appearance (especially paintability); and 6) ease of application.

Sealants that contain siliconized acrylic latex or ethylene copolymers rate high for the above factors and thus are excellent choices when high-quality pest exclusion work is necessary.

For example, both of these sealant groups allow for up 25 percent movement between joints, can last up to 30 years and maintain their seals within temperatures ranging from -20 to 180°F. These sealants also can be painted and they are compatible with nearly all surfaces except plastics. Some of the common label brands of high quality sealants in these groups include: NP-1™; Geocel™; and Rustoleum’s Industrial-Grade Sealant Compound. But there are others; a little Web surfing will offer up other name brands.

FOAM FILLERS. One of the more common materials used for "plugging a hole" by both the public and by many pest professionals is expanding foam fillers. Because these foams are applied directly from a can and they expand into a gap right before the user’s eyes, foam fillers are popular. But foam fillers do not provide for much elastomericness, nor due to the nature of their chemistry do they provide complete seals as can true sealants.

Nevertheless, foam fillers have their place for some types of pest-proofing. They can be used where the more permanent pest-proofing of a hole may not be necessary. Or they can be used where the area to be filled is not visible or is not in a sensitive building or area.

For example, if incoming rodents and large insects are using a hole around a plumbing fixture below the kitchen or bathroom sink cabinet, or via some similar gaps in basements, utility rooms and other out-of-sight areas, a quick foam plug or filling of a hole may be the most cost-efficient approach. Foams also can be used to fill in large holes before finishing the job off with an appropriate sealant.

But foam fillers should not be used when the job calls for a sealant, especially for sensitive accounts, such as food-handling establishments, health-care facilities, zoological gardens, pharmaceutical operations and similar facilities.

The disadvantages of using foam fillers include:

1) Foam fills are not cleanable. Once dry, the foam bead or plug creates hundreds, possibly thousands, of microscopic nooks and crannies in which dirt, dust, food dust and a range of other materials might become lodged. If the foam gets wet, the recipe exists for the growth of bacteria, fungi and/or mold.

2) Rodents and those inspect pests with chewing mouthparts (e.g., ants, cockroaches) can chew through and penetrate foam plugs and fills.

3) A true seal between two elements is not formed with foam. Expanding foams will fill a hole enough to exclude a rodent and some larger insects. But expanding foam is often not complete enough to exclude small arthropods such as mites, ants, psocids and other small insects pests from all areas.

4) If care is not exercised when applying expanding foams behind moldings, baseboards, fascia boards and similar structural elements, the foam can warp the structural elements out of shape. This in turn negates the primary function of these structural elements (which is to serve as closers of the gaps that are created where walls meet floors or roofs).

5) Urethane foam degrades with constant exposure to sunlight and thus should not be used for exterior holes and gaps.

Some contend that a foam "plug" can be used as a temporary plug until the hole can be sealed properly later. Possibly. But the foam plug will eventually have to be removed and the hole prepared properly (i.e., completely scraping out the foam residue) to hold a more permanent sealant. This makes for twice as much work as it would take to seal the hole using the proper materials in the first place.

METAL MESH PLUGS. Metal mesh plugs, such as copper mesh or steel wool, are commonly used by pest professionals to plug small holes in walls and around plumbing fixtures.

These materials are appropriate when the prevention of pest movement is desired, but moisture and air movement through the hole is also desired (e.g., weep holes in brick veneer walls). Construction adhesive or mortar can be used to "glue" plugs in place if rodents are removing them. In locations where high humidity or exposure to water is expected, steel wool will rust, which both reduces its effectiveness and causes rust stains. In these locations, copper mesh is the best choice of material.

But inside buildings, where holes exist in concrete, wood and masonry, metal mesh plugs should be used only as temporary plugs. Depending on the situation, a copper mesh plug stuffed tight into a hole beneath a cabinet may last anywhere from days to years depending on movement between the elements. Determined rodents can work out mesh plugs by pulling and/or gnawing on them. When a temporary plug is needed until a good sealant can be used, the use of copper mesh is a wiser choice than using a foam filler plug (i.e., the mesh plug can be cleaned and quickly removed).

DO IT RIGHT THE FIRST TIME. When a hole needs to be pest-proofed, it is best for a pest professional to err on the side of completeness and efficiency. Relative to the on-the-job costs, a high-quality sealant is not really more expensive than a low-quality caulk. And, a sealant will always suffice wherever a caulk could be used, but a caulk cannot suffice where a sealant is needed.

For pest professionals, perhaps it’s time to cease using terms like "caulks," "patches" and "plugs" as generic terms for pest-proofing materials. For quality pest exclusion efforts, sealed means sealed. It doesn’t mean partially, mostly or temporarily sealed. Be sure to choose right — seal it once and seal it tight.

The author is president of RMC Pest Management Consulting and can be reached at rcorrigan@giemedia.com or 765/939-2829.

June 2005
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