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What’s new in preservation? Solving the dry rot mystery.

Moisture in attics is a common concern for historic property owners. The interplay of warm interior air rising to meet cold attic framing can create all kinds of headaches, from mold growth to severe wood rot. During a spring inspection of the Phippen House attic, powdery dry rot was observed on framing in several areas. Initial assumptions were that the rot was caused by condensation in the attic that had long since dried. This rot looked to be old and inactive. Moisture meter readings showed the wood to be well below moisture tolerances. But the question remained, what was the source of the moisture that caused the rot in the first place?

Thanks to the help of one of our advisory council members, John Watne of Structures North, a simple answer was found. Sapwood, the soft, outer layer of wood of a living tree, tends to be more moisture laden than the tree’s inner structural heartwood. This outer layer also lacks the cellular density to prevent water absorption and fungal infections. Sometimes a fungal infection in the sapwood can occur before the tree is even cut down. Once the sapwood has softened from a fungal infection, there is little to do to stop the rot and attack by pests like powder post beetles. But the heartwood remains structurally safe if the initial source of moisture is removed.

This effect was confirmed during a follow-up site inspection by our preservation director and Structures North in the fall. With lumber moisture readings well below the 20 percent threshold, a sound heartwood structure, and no new signs of rot developing, the Phippen House roof is safe. In the meantime, we continue to inspect our historic properties for moisture issues that may affect their structures. To prevent moisture damage developing through means other than sapwood rot, roofs should be inspected for possible leaks and properly ventilated, interior humidity should be controlled, and warm interior air should be prevented from meeting cold attic surfaces. 

Date: November 26, 2022

Author: sperling

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