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Exterior Log Home Care

- Surface treatments should include borate -

Summary: Insects and rot fungi will attack the exterior of log homes if it is not protected. An application of borate wood preservative, which controls both insects and fungi, plus some type of exterior sealer is all that is generally needed.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

Log homes need special exterior treatments that conventionally-built homes do not. Logs are prone to attack by rot fungi and a variety of insects that will damage the surface if steps are not taken to prevent infestation. Fortunately treatments to prevent this damage are relatively inexpensive if done during construction and before water sealant finishes are applied.

Borates stop wood damage

There are a number of manufacturers that supply products to protect log homes from wood destroying insects and fungi. Many of these products are based on insecticidal borates. Bora-Care (Nisus Corporation) and Shell-Guard (Perma-Chink Systems) are popular glycol-based** borate products used to treat unfinished log homes (see Bora Care Wood Preservative).

Timbor, also from Nisus Corporation, is a pure borate powder without any glycol additive and is mixed with water to apply. Some studies indicate that the glycol in Bora-Care/Shell-Guard helps to carry the borate into the wood, but other studies found no advantage from the addition of glycol (see Timbor Wood Preservative).

building logs

Logs being prepared for building. Photo by William M. Ciesla, Forest Health Management International, Bugwood.org


All borate products should be applied before wood is stained and sealed for best results. Bora-Care is a 40% solution whereas Shell-Guard is a 25.3% solution. Keep this difference in mind when comparing costs. Timbor is somewhat cheaper than either of the glycol-based borates but does not have the possible benefits of the glycol penetrant. All borates are water soluble so wood must be stained or painted to seal the surface following treatment.

Bora-Care and other borate-based wood preservatives are available here, but are not generally available in home and garden stores.

Fused borate

Another form of borate that is useful against rot fungi and insect pests is called fused borate. Fused borate is a solid rod of borate or borate/copper mixture that is implanted in a wooden timber or log. Fused borate is commonly used in utility poles. Moisture in the wood releases the preservatives which diffuse into the wood fibers. Two popular fused borate products are Impel Rods and Cobra Rods. Fused borate can be used when treating finished woods and log ends.


** Ethylene glycol vs. Propylene glycol

Borates are sometimes formulated in a liquid called glycol. Glycol is described as a "penetrant" in that the liquid is supposed to carry the borate deeper into wood fibers. Manufacturers of glycol-based borate products, such as Shell-Guard and Bora Care, use two different chemical forms of glycol called ethylene and propylene glycol. While both forms are effective at carrying borate into wood fibers the compounds are radically different in terms of their toxicity to animals and humans.

Ethylene glycol is an odorless, colorless, syrupy liquid that is widely used as an engine coolant, or antifreeze. Ethylene glycol has a sweet taste but is highly toxic and accidental ingestion is a medical emergency. Bora-Care uses ethylene glycol as its glycol base.

Propylene glycol also is an odorless, colorless liquid but it is tasteless and much less toxic than ethylene glycol. Because it lacks the sweet taste of ethylene glycol, propylene glycol is also less likely to be accidentally ingested by children or pets. There is at least one brand of antifreeze (Sierra Brands) that uses this less toxic form of glycol and Shell-Guard is also based on this less toxic form of glycol.


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Jack DeAngelis, Ph.D.

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