Durability of Douglas Fir heartwood and western red cedar heartwood

Based on USDA testing, 𝐃𝐨𝐮𝐠𝐥𝐚𝐬 𝐅𝐢𝐫 𝐡𝐞𝐚𝐫𝐭𝐰𝐨𝐨𝐝 and 𝐰𝐞𝐬𝐭𝐞𝐫𝐧 𝐫𝐞𝐝 𝐜𝐞𝐝𝐚𝐫 𝐡𝐞𝐚𝐫𝐭𝐰𝐨𝐨𝐝 and both are classified as “most resistant” to decay in above-ground conditions (≥20 years expected service life) https://lnkd.in/gxtR_jTw

However, the sapwood in both species does not contain the natural protections that the heartwood has and if left untreated will be prone to rot and decay in exterior applications.

𝐖𝐞𝐬𝐭𝐞𝐫𝐧 𝐫𝐞𝐝 𝐜𝐞𝐝𝐚𝐫 𝐡𝐞𝐚𝐫𝐭𝐰𝐨𝐨𝐝 𝐡𝐞𝐚𝐫𝐭𝐰𝐨𝐨𝐝 𝐞𝐚𝐫𝐧𝐬 𝐚 “𝐦𝐨𝐬𝐭 𝐫𝐞𝐬𝐢𝐬𝐭𝐚𝐧𝐭” 𝐥𝐚𝐛𝐞𝐥 𝐢𝐧 𝐚𝐛𝐨𝐯𝐞-𝐠𝐫𝐨𝐮𝐧𝐝 𝐭𝐞𝐬𝐭𝐬 𝐦𝐚𝐢𝐧𝐥𝐲 𝐛𝐞𝐜𝐚𝐮𝐬𝐞 𝐢𝐭 is 𝐫𝐢𝐜𝐡 𝐢𝐧 𝐚 𝐠𝐫𝐨𝐮𝐩 𝐨𝐟 𝐜𝐨𝐦𝐩𝐨𝐮𝐧𝐝𝐬 𝐜𝐚𝐥𝐥𝐞𝐝 𝐞𝐱𝐭𝐫𝐚𝐜𝐭𝐢𝐯𝐞𝐬, 𝐞𝐬𝐩𝐞𝐜𝐢𝐚𝐥𝐥𝐲:

𝘛𝘩𝘶𝘫𝘢𝘱𝘭𝘪𝘤𝘪𝘯𝘴 (𝘵𝘩𝘦 𝘩𝘦𝘢𝘷𝘺 𝘩𝘪𝘵𝘵𝘦𝘳𝘴)
• Also called tropolones
• The most important one: β-thujaplicin (hinokitiol)

𝘛𝘩𝘦𝘴𝘦 𝘤𝘰𝘮𝘱𝘰𝘶𝘯𝘥𝘴:
• Are strongly antifungal
• Inhibit the enzymes fungi use to break down wood (cellulose & lignin)
• Can even be toxic to decay fungi at low concentrations

𝐂𝐞𝐝𝐚𝐫’𝐬 𝐝𝐮𝐫𝐚𝐛𝐢𝐥𝐢𝐭𝐲 𝐫𝐞𝐩𝐮𝐭𝐚𝐭𝐢𝐨𝐧 𝐜𝐨𝐦𝐞𝐬 𝐚𝐥𝐦𝐨𝐬𝐭 𝐞𝐧𝐭𝐢𝐫𝐞𝐥𝐲 𝐟𝐫𝐨𝐦 𝐢𝐭𝐬 𝐡𝐞𝐚𝐫𝐭𝐰𝐨𝐨𝐝.
Sapwood doesn’t share that property and should be treated more like a non-durable softwood.

𝐃𝐨𝐮𝐠𝐥𝐚𝐬-𝐟𝐢𝐫 𝐡𝐞𝐚𝐫𝐭𝐰𝐨𝐨𝐝 𝐞𝐚𝐫𝐧𝐬 𝐚 “𝐦𝐨𝐬𝐭 𝐫𝐞𝐬𝐢𝐬𝐭𝐚𝐧𝐭” 𝐥𝐚𝐛𝐞𝐥 𝐢𝐧 𝐚𝐛𝐨𝐯𝐞-𝐠𝐫𝐨𝐮𝐧𝐝 𝐭𝐞𝐬𝐭𝐬 𝐦𝐚𝐢𝐧𝐥𝐲 𝐛𝐞𝐜𝐚𝐮𝐬𝐞 𝐢𝐭 𝐝𝐨𝐞𝐬𝐧’𝐭 𝐬𝐭𝐚𝐲 𝐰𝐞𝐭 𝐥𝐨𝐧𝐠 𝐞𝐧𝐨𝐮𝐠𝐡 𝐭𝐨 𝐝𝐞𝐜𝐚𝐲, 𝐧𝐨𝐭 𝐛𝐞𝐜𝐚𝐮𝐬𝐞 𝐢𝐭’𝐬 𝐢𝐧𝐡𝐞𝐫𝐞𝐧𝐭𝐥𝐲 𝐡𝐢𝐠𝐡𝐥𝐲 𝐝𝐞𝐜𝐚𝐲-𝐩𝐫𝐨𝐨𝐟.

Its structure helps it stay drier (this is the big one)

𝘋𝘰𝘶𝘨𝘭𝘢𝘴-𝘧𝘪𝘳 𝘩𝘢𝘴:
• Dense latewood bands
• Relatively low permeability

𝘛𝘩𝘢𝘵 𝘮𝘦𝘢𝘯𝘴:
• Water penetrates more slowly
• It dries out faster after getting wet

𝘈𝘯𝘥 𝘥𝘦𝘤𝘢𝘺 𝘧𝘶𝘯𝘨𝘪 𝘯𝘦𝘦𝘥:
• Sustained moisture (usually ~20–30%+ moisture content)

𝘈 𝘤𝘭𝘦𝘢𝘯 𝘸𝘢𝘺 𝘵𝘰 𝘵𝘩𝘪𝘯𝘬 𝘢𝘣𝘰𝘶𝘵 𝘪𝘵:
𝐂𝐞𝐝𝐚𝐫 → “𝐚𝐜𝐭𝐢𝐯𝐞𝐥𝐲 𝐫𝐞𝐬𝐢𝐬𝐭𝐬 𝐝𝐞𝐜𝐚𝐲” (𝐜𝐡𝐞𝐦𝐢𝐜𝐚𝐥 𝐝𝐞𝐟𝐞𝐧𝐬𝐞)
𝐃𝐨𝐮𝐠𝐥𝐚𝐬-𝐟𝐢𝐫 𝐡𝐞𝐚𝐫𝐭𝐰𝐨𝐨𝐝 → “𝐚𝐯𝐨𝐢𝐝𝐬 𝐝𝐞𝐜𝐚𝐲 𝐜𝐨𝐧𝐝𝐢𝐭𝐢𝐨𝐧𝐬” (𝐦𝐨𝐢𝐬𝐭𝐮𝐫𝐞 𝐝𝐲𝐧𝐚𝐦𝐢𝐜𝐬)