Background
Well before the building science community became interested in wood for its moisture buffering capabilities, the forest products industry was studying ways of preventing moisture from being absorbed by wood. A 1985 report from the national Forest Products Laboratory details the experimental procedure used to determine the moisture-excluding effectiveness (MEE) of 91 different finishes. The finishes were brushed onto 3″×5″×⅝″ samples of ponderosa pine sapwood, which were hung in a rack and placed in a room in which the air was kept at 80°F and at 90% relative humidity. The samples were weighed before being placed in the room and re-weighed one, seven, and fourteen days after the experiment had begun to determine how much moisture had been absorbed by each sample. Three coats of a sheathing epoxy finish yielded the highest MEE in the study, 91% after 14 days. Latex paint and nitrocellulose lacquer were found to have the lowest MEE values after 14 days, 16% and 19%, respectively.
A rack of ponderosa pine samples that were tested for the Forest Products Laboratory report. (Feist 1985, 3)
Tim Padfield, a conservation scientist, was among the first to suggest that buildings should take advantage of the absorptive properties of their materials to passively regulate their internal humidity. In his doctoral thesis, he describes the design and construction of a "flux chamber" for studying "the moisture absorbent properties of porous walls" (Padfield 1998, 23). It was important to Padfield that his chamber be capable of replicating the same kind of variations in humidity (which he refers to as "water vapor flux") that would occur in an actual interior environment over time. This requirement necessitated a significantly more complex setup than a traditional humidity box.
Padfield tested a wide variety of wall constructions in his flux chamber, including walls made of wood, clay tiles, unfired bricks, lime plaster, gypsum plaster, cellular concrete, and wool insulation. These tests revealed that "the best buffer of all is wood cut across the grain" (Padfield 1998, 59). A specially prepared mixture of bentonite clay and perlite was also found to perform quite well.
Padfield tested a wide variety of wall constructions in his flux chamber, including walls made of wood, clay tiles, unfired bricks, lime plaster, gypsum plaster, cellular concrete, and wool insulation. These tests revealed that "the best buffer of all is wood cut across the grain" (Padfield 1998, 59). A specially prepared mixture of bentonite clay and perlite was also found to perform quite well.
A cutaway diagram of Padfield's flux chamber, which consists of "a stainless steel box (A) without a base, that sits on a stainless steel plate (B). Electrical and other services (E) are sealed through the base. The experimental wall (D) is tilted back on a support so that it can be constructed of loose blocks. The air conditioning equipment (C) is within the box" (Padfield 1998, 23).
A wall assembly (composed of "wool insulation faced with vapour retarding paper and backed by aluminium foil and polyethylene film") being tested in the flux chamber. (Padfield 1998, 58)
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A diagram depicting the "flux generator" designed by Padfield to control the humidity inside his test chamber. (Padfield 1998, 28)
A graphical summary of material performance. Each curve represents changes in the relative humidity measured at a specific location within the flux chamber over time. (Padfield 1998, 91)
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Carsten Rode was one of Tim Padfield's advisors during his doctoral research at the Technical University of Denmark. In the mid 2000s, Rode saw “a need for a robust definition of a term for the moisture buffer effect of materials,” and set out to define such a term as well as a testing protocol to measure its value (Rode 2005, 2). His team developed the Moisture Buffer Value (MBV), which “indicates the amount of moisture uptake or release by a material when it is exposed to repeated daily variations in relative humidity between two given levels” (Rode 2005, 3). The MBV of a material is expressed in kilograms of water per square meter of surface area and per percent change in relative humidity.
The testing protocol for MBV specifies that material samples should be sealed on all but one or two sides and exposed to a minimum of three cycles consisting of 8 hours of high humidity followed by 16 hours of low humidity. This asymmetric cycle is meant to replicate the conditions observed in a “typical” room, such as a bedroom or office, in which the respiration and perspiration of its human occupants increases the humidity for approximately 8 hours each day. The test should be carried out at 23°C with the high portion of the cycle at 75% relative humidity and the low portion at 33% relative humidity.
The testing protocol for MBV specifies that material samples should be sealed on all but one or two sides and exposed to a minimum of three cycles consisting of 8 hours of high humidity followed by 16 hours of low humidity. This asymmetric cycle is meant to replicate the conditions observed in a “typical” room, such as a bedroom or office, in which the respiration and perspiration of its human occupants increases the humidity for approximately 8 hours each day. The test should be carried out at 23°C with the high portion of the cycle at 75% relative humidity and the low portion at 33% relative humidity.
One of the climatic chambers used to experimentally.determine the MBV of materials specimens at Denmark Technical University. (Rode 2005, 5)
At the University of Porto, Nuno Manuel Monteiro Ramos led an investigation of the effect of paint on the hygroscopic properties of building materials commonly used on walls and ceilings in Portugal. His team measured changes in sorption isotherms, vapor permeability, and Rode's moisture buffer value when four different types of paint were applied to gypsum board, gypsum plaster, and a plaster made from a mix of gypsum and lime. In their MBV tests, they found “that the base materials tested have a ‘moderate class’ (0.5–1.0 g/m² %RH) of moisture buffering efficiency, which can be reduced to a ‘limited class’ (0.2–0.5 g/m² %RH) by the effect of coatings” (Ramos 2010, 2594). Perhaps most interesting is the discovery that the primer applied to their materials before painting was “responsible for most of the coating’s vapour resistance” (Ramos 2010, 2596).
The MBV testing process used by Ramos, et. al. (Ramos 2010, 2592)
Sources
Feist, William C, James K Little, and Jill M Wennesheimer. December 1985. “The Moisture-Excluding Effectiveness of Finishes on Wood Surfaces.” https://www.fpl.fs.fed.us/documnts/fplrp/fplrp462.pdf.
Padfield, Tim. 1998. “The role of absorbent materials in moderating changes of relative humidity.” PhD Thesis, Technical University of Denmark. https://www.conservationphysics.org/phd/phd-indx.html.
Ramos, N. M. M., J. M. P. Q. Delgado, and V. P. de Freitas. 2010. “Influence of Finishing Coatings on Hygroscopic Moisture Buffering in Building Elements.” Construction and Building Materials, Special Issue on Fracture, Acoustic Emission and NDE in Concrete (KIFA-5), 24 (12): 2590–97. https://doi.org/10.1016/j.conbuildmat.2010.05.017.
Rode, Carsten, Ruut Peuhkuri, Kurt K. Hansen, Berit Time, Kaisa Svennberg, Jesper Arfvidsson, and Tuomo Ojanen. June 2005. “Moisture buffer value of materials in buildings.” Proceedings of the 7th Symposium on Building Physics in the Nordic Countries 1 (June 2005): 108–115.
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