Pressure treated wood has been around since the 40s. Primarily used for posts, beams and structural members, it's typically for exterior use only. However, newer formulations of treated wood have been approved for interior use by the Environmental Protection Agency and may be used in exactly the same way you would use untreated wood.
In the past, lumber was injected with old-school chemicals such as chromated copper arsenate, chromium and copper, which are harmful to your health.
Since the early 1990s, one chemical -- borate -- has been commonly used to pressure-treat lumber. It's approved by homeowners, builders and carpenters. Borate is a naturally occurring trace mineral found in rocks, soil and water. Borate is one of the safest types of pressure-treated wood.
Contemporary alternatives, cyproconazole, propriconazole, copper azole, and alkaline copper quaternary have been used somewhat infrequently for interior use.
- Not defined as hazardous waste by the EPA.
- Essential to plant life, and environmentally benign.
- Inorganic salt, no off-gassing of vapors.
- Colorless, odorless, and non-corrosive to building materials and hardware.
- Slows down or prevents insects, microbes and fungus.
- Penetrates deep into the heartwood of the lumber.
Borate-treated lumber can be used in any interior application except cutting boards and countertops. Use it for:
- Sill plates
- Joist, rafters and trusses
- Furring strips
Interior trim made with borate-treated wood is also acceptable, but the use of it has somewhat mixed reviews. Borate-treated wood can warp or twist, particularly when milled into thinner pieces. Kiln-dried, borate treated lumber resists warping -- but also adds to the cost.
Hot-dipped galvanized or stainless-steel screws are recommended for borate treated lumber, but it can be used with galvanized steel, carbon steel, copper silicon bronze, and all other fasteners that are used on any other type of wood.
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Where termites rule the roost, such as in tropical environments like in Hawaii, it's possible to construct the shell of a home completely out of borate-treated wood. It works particularly well for repelling termites, and penetrating their nests.
Fir and Pine
Douglas fir and yellow pine are two of the common species typically pressure-treated with borate. But hemlock, spruce and western pine are also injected with borate. Specify wood type if you're concerned.
If borate-treated lumber is to be used in an interior application, and happens to get wet during construction, it should be allowed to dry before you cover it up.
The other pressure-treated lumber options -- cyproconazole, propriconazole, copper azole, and alkaline copper quaternary -- may be interchangeable with borate-treated wood. Check with the manufacturer or dealer for specific interior applications. One size doesn't fit all.