Do I need to treat interior millwork?

Not all interior millwork needs chemical treatment — in a climate-controlled home, the primary protection is proper finishing (sealing all faces, including the back). End-grain sealing is the most important treatment. Chemical preservatives are needed only for wood in direct contact with concrete, framing in wet areas, or wood in concealed spaces where moisture risk is elevated.

What is borate and is it safe?

Borate (disodium octaborate) is a naturally occurring mineral salt that is toxic to insects and fungi but has very low mammalian toxicity. It is the standard treatment for interior framing and structural wood. It is not waterproof, so it is not suitable for exterior use, but for interior applications it is effective and safe.

How do I seal end grain?

Apply an end-grain sealer (available at lumber yards) or a compatible finish (the same paint, stain or sealer you will use on the face) to every cut end of every board before installation. The goal is to prevent moisture from entering through the end grain, which is the most porous part of the board.

Materials

Wood Treatment and Preservation Guide

Updated July 25, 20263 min read

The short answer

Wood treatment in Calgary serves two purposes: protecting against moisture-driven rot and protecting against insect damage. The treatment appropriate for each component depends on its exposure — interior millwork in a climate-controlled home needs end-grain sealing; exterior trim needs preservative treatment; subfloor contact needs borate application. Over-treating is as wasteful as under-treating.

Treatment types and applications

Treatment types and applications
Treatment	Protects against	Best for	Notes
End-grain sealer	Moisture absorption	All cut lumber ends	Apply to every cut end immediately
Borate (disodium octaborate)	Insects, fungal decay	Subfloor, framing, hidden millwork	Low toxicity, long-lasting, not waterproof
Copper-based preservative	Rot, insects	Exterior trim, ground contact	Not for interior use — green tint, toxic
Wax-based preservative	Moisture, moderate rot risk	Window sills, exterior joinery	Penetrating, breathable finish
Clear preservative (Tim-bor)	Fungal decay, insects	Hidden structural wood, attic framing	Apply before installation
Moisture barrier membrane	Standing water, ground moisture	Below-grade framing, deck joists	Physical barrier, not a chemical treatment

Calgary-specific treatment priorities

End-grain sealing: Every cut end of every piece of lumber is a moisture entry point. In Calgary's freeze-thaw cycles, unsealed end grain absorbs water, freezes, expands and splits. Seal every cut end with a purpose-made end-grain sealer or a compatible finish before installation. This is the single most impactful treatment for interior millwork.
Borates for framing: Borate treatment (disodium octaborate) protects framing lumber and subfloor from insect damage and fungal decay. It is low-toxicity, long-lasting, and effective. Apply to framing before drywall is installed, especially in areas with plumbing (kitchens, bathrooms, laundry rooms).
Below-grade protection: Any wood in contact with concrete or below grade needs a moisture barrier. Concrete wicks moisture, and wood in direct contact with concrete will rot. Use a capillary break (plastic or metal flashing) between concrete and wood framing.
Window and door frames: Window sills and door frames in exterior walls are exposed to condensation and weather. Treat these components with a preservative before installation. The time investment is small; the protection is long-term.

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Interior millwork treatment schedule

Before installation: Seal all cut ends. Prime or seal all six faces of MDF components (including the back and edges). Apply a moisture barrier to any wood in contact with concrete.
During installation: Use stainless steel or coated fasteners in areas with any moisture exposure. Apply borate to framing in wet areas before drywall is installed.
After installation: Apply finish coats as specified. Maintain humidity at 30–40% through winter to minimize wood movement. Re-seal end grain on any newly exposed surfaces (e.g., after on-site modifications).

Answers

Frequently asked questions

Do I need to treat interior millwork?: Not all interior millwork needs chemical treatment — in a climate-controlled home, the primary protection is proper finishing (sealing all faces, including the back). End-grain sealing is the most important treatment. Chemical preservatives are needed only for wood in direct contact with concrete, framing in wet areas, or wood in concealed spaces where moisture risk is elevated.
What is borate and is it safe?: Borate (disodium octaborate) is a naturally occurring mineral salt that is toxic to insects and fungi but has very low mammalian toxicity. It is the standard treatment for interior framing and structural wood. It is not waterproof, so it is not suitable for exterior use, but for interior applications it is effective and safe.
How do I seal end grain?: Apply an end-grain sealer (available at lumber yards) or a compatible finish (the same paint, stain or sealer you will use on the face) to every cut end of every board before installation. The goal is to prevent moisture from entering through the end grain, which is the most porous part of the board.

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