Hygrothermal Performance Testing

Our clients understand their hygrothermal models can be sensitive to certain material properties. Rather than choosing generic values, which can lead to poor predictions, our clients turn to us to provide actual test data for materials sampled directly from the wall assembly. These inputs include density, porosity, water absorption coefficient, thermal conductivity, heat capacity, vapor permeance, vapor diffusion resistance factor, and values related to moisture storage function. These refinements are particularly important in historical buildings, where materials like clay brick can be much more highly variable in property.

Vapor transmissivity testing is performed by creating a uniform vapor drive through prepared masonry samples. From this, we calculate vapor transmission rate, permeance, permeability, and vapor diffusion resistance factor. This may be of particular interest when assessing materials from the same assembly, such as brick and mortar, where permeabilities can differ greatly. When this testing is performed for conservation studies, it can be used to assess building component compatibility to ensure that the wall is breathing through the mortar joints rather than the masonry units.

Hygrothermal models will predict the saturation level of the building envelope when freezing occurs. This leaves the engineer having to decide whether the masonry is likely to fail at these levels. The interpretation is a primary goal of the modeling when the intent is to consider insulating an existing building envelope. Where an educated guess is insufficient, Highbridge can perform a frost dilatometry test to determine the critical degree of saturation at which the masonry will experience irrecoverable expansive damage due to freezing.

A moisture sorption isotherm is a curve that defines the equilibrium moisture content at one temperature for different relative humidities. WUFI allows for a fairly straightforward linear interpolation of the critical region by determining the 100% free water saturation and the equilibrium moisture content at 80% RH. Where our clients require a more refined solution, Highbridge determines both an adsorption and desorption curve in accordance with ASTM C1498.

Physical testing can yield all of the input values needed for a carefully designed hygrothermal model. However, the materials and microstructure of the masonry ultimately inform the behavior of heat and moisture. Our specialization in construction materials microscopy means that our team can further study the causes of any anomalous physical property that yields unexpected results in the model. Understanding these microstructural influences can help the engineering team establish more tightly-constrained confidence levels.

Hygrothermal models consider moisture and heat fluxes in one direction perpendicular to the wall surface. However, masonry materials including natural stone, clay brick, and terra cotta often have strong microstructural anisotropies that result in material properties that are directionally-dependent. While the models themselves may not consider this anisotropy, our laboratory can provide permeability and thermal conductivity values in three directions to help clients better understand and predict masonry performance features that may not be revealed by their models.