WHAT ARE THE KEY DIFFERENCES BETWEEN COPPER-BASED PRESERVATIVES AND BORATE?
Copper-based preservatives and borate are both effective at protecting wood from decay under the right circumstances, however they are very different chemicals. A copper-based preservative chemically bonds to the wood; in other words, it is “fixed” in the wood and cannot diffuse throughout the piece nor can it wash out. This means copper-based treated wood can be used outdoors or even submerged in water. Borate, on the other hand, is diffusible; in other words, it doesn’t lock on to the wood like copper-based preservatives. The advantage of diffusion is borate’s ability to keep moving deeper into the wood after pressure treatment. The disadvantage is that borate can leach out of treated wood that is continuously exposed to liquid water. Sodium borate is quite diffusible and zinc borate less so, however neither is recommended for outdoor use. A simple rule of thumb: use copper-based preservatives outside the moisture barrier (the building paper or housewrap), and use borate inside the b
