Terms and Perms
A tight wall has the advantage that less air moves through it and since moving air carries energy with it, a building with tight walls requires less energy to keep the indoor spaces at the proper temperature. This is not rocket science but a tight wall also allows less moisture to sneak in (or out) of a wall and hence can contribute, both good and bad, to condensation, retention of odors and other building performance issues. The recent expanded use of air and water barriers in exterior walls can increase the tightness of walls and some people feel this may be part of the cause with various types of moisture-related problems in newer construction.
In my consulting practice, I get a lot of questions about the tightness of walls and part of the problem seems to stem from the terminology used to describe heat and moisture flow and particularly some of the basic terms used to define this type of behavior. This month's column will give some basic information about how to describe heat and moisture flow so that you can properly describe and understand what is going on in your walls.
Communication is the beginning of understandingThis '60s slogan from GE is a good one and applies well to the water vapor and heat terminology issue. The terms used to describe the properties of heat and moisture flow are so similar that it's no wonder they get confused. Consider the following chart, which I routinely provide to clients. Notice how some of the terms are almost the same but mean very different things. Note also the heat and moisture flow terms are analogous in the sense that they refer to similar forms of behavior and measurement. For example, the common term R-value, which refers to resistance to heat flow, has an analogous term for the resistance to water vapor flow, namely what I call the "Total (vapor) Resistance."
Note also that some terms are for a given thickness, such as drywall, for example, which comes only in fixed thicknesses, such as 1/2 and 5/8 inch. Some other terms refer to homogenous, solid materials (drywall is not a solid, single material but a series of different layers), such as concrete, which come in whatever thickness specified. In other words, concrete is measured per inch, not per a given thickness. Thus, be really careful about what exact wording is used in describing the properties of materials.
Homogeneous? You sure?Don't be fooled into thinking that some materials that appear to homogenous can have their properties extrapolated to give new values at different thicknesses. Published values often take into account in-place conditions (such as seams between pieces of sheet materials) and hence are not simply a function of simply the thickness of a material. This is why, for instance, that 8-mil poly does not have twice the permeance of 4-mil poly.
Similarly, extruded polystyrene insulation has "skins" on its surfaces that are higher in density than the core (these skins are the foam plastic itself, not a separate laminated material). Thus, 4-inch thick extruded polystyrene is not quite twice as vapor impermeable as 2 inch. EPS does not have skins, as it cut from a large block.
With EIFS, the "at a given thickness" makes a difference, because basecoats and finishes can be put on in a range of thickness. Hence their properties, especially regarding water vapor flow, depend on the application technique and the final thickness. This is why, for instance, when looking at data for coatings, that the permeability is sometimes quoted at a given thickness, say 10 mils (0.010 inch) and so on.
Temperature dependenceAlso, keep in mind that thermal values are sometimes temperature dependent, in the sense that the R-value depends on the temperature at which the test used to measure the R-value, is made; you get a different number at 75 degrees F vs. 40 degrees F and so on. This is why the government requires reporting R-values at a given temperature.
Producers of building materials should have these thermal and moisture properties available for their products. This data is used by architects and engineering in designing HVAC systems, determining if and where a vapor barrier should be installed and in complying with energy codes.
The water vapor permeability of the EIFS lamina is important. If water vapor flows outward through the EIFS insulation in cold climates, it might freeze between the lamina and the foam. Water expands when it freezes and this can dislodge the lamina from the foam. Conversely, in the summer, in hot, humid climates, vapor trying to permeate into the building from the outside, might condense in the stud cavity, reducing the insulation effectiveness and contributing to, possibly, mold problems.
For EIFS, there is some specific hope in this terminology and testing issue. The well-known ASTM organization has a new standard test method for measuring the permeability of EIFS coatings. This method uses another, existing ASTM test method, titled E96, as the basis determining the perm ratings of EIFS samples. This new EIFS-specific test method was developed to help improve the existing E96 method, which could be used for a variety of materials but needed to be updated be more accurate and clear about how exactly to do the test for EIFS.
The bottom line of this article is to be sure to inquire about the units of measurement of which a given bit of technical data is presented. The terms perm and permeance, for example, are often used interchangeably when they clearly are not the same thing. It's easy, if one uses the incorrect terms, to ruin any engineering calculations for water vapor and heat flow. This, in turn, could have a major impact of heating and cooling systems and other performance aspects of the walls. If you or the person you are discussing these issues with is unsure about what exactly is being discussed, send the pictured table to them and then discuss the matter. A free, full-page PDF version of this table can be found at www.eifs.com/heat-vapor-terms.pdf.