Field Monitoring of Embedded Wood Members in Insulated Masonry Walls in a Cold Climate

One durability risk when applying interior insulation to mass masonry walls is the hygrothermal behavior of wood beams embedded in the wall. With the retrofit of interior insulation, the embedded beam ends will spend longer periods at colder temperatures than their pre-retrofit condition. Therefore, these wood members will have lower drying potential (due to reduced heat flow), and experience higher relative humidity and moisture contents. This work involves the field monitoring of embedded wood joist ends in a solid brick building which is being retrofitted with interior insulation. Eleven joists are being monitored, with a variety of orientations and exposures; they are being monitored for wood moisture content, and temperature and relative humidity in the joist pocket. Results have been collected over two years and are continuing; one limitation is that construction is still ongoing, and some of the building’s interior is still unheated and unoccupied.
At many orientations (especially north), joist moisture contents are high (20-30% or higher), which is higher than the range considered conducive to long-term durability; RH levels often remained at 100%. On other, solar-heated orientations (south), the moisture content is in the safe, 10-13% range. However, installation of sensors and the removal of a joist end sample indicate that these wood members (dense, old-growth framing) can survive these moisture contents without damage. Seasonal MC cycles indicate that interior heating results in joist end drying (even with interior insulation), consistent with the change in moisture vapor gradient direction. The moisture contents at bottom part of the joist (near the beam “seat”) are consistently higher than the corresponding upper position.

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