ASHRAE Journal - May 2012 - 68

building sciences
50% Relative Humidity

0% Relative Humidity

100% Relative Humidity

Desiccant

Water

Figure 4: “Cup” Terminology. Vapor transmission properties of materials are tested with both the “dry cup” and “wet cup” method. With the “dry cup” method the “cup” is kept dry with a desiccant. With the “wet cup” method the “cup” is kept wet by filling it with water. Both “cups” are placed in an environmental chamber kept at 50% RH. Note that both dry cup testing and wet cup testing establish a 50% RH difference across the test specimen. However, with dry cup testing one side is at 0% RH and the other side is at 50% RH. With wet cup testing one side is at 50% RH and the other side is at 100% RH. Over time the weights of the cups change. The dry cup weighs more over time. The wet cup weighs less over time. The rate of weight change is tracked and, presto!, we have a metric. Welcome the “perm.”

Photo 1: Kraft-Faced Batt. Humble and ubiquitous with a “dry cup” value of 1 perm so it meets the code requirement of a Class II vapor retarder as tested under ASTM E-96 Test Method A (the desiccant method). Its wet cup value is around 10 perms. tracked and, presto!, we have a metric. The “perm” was born. So what material is available to us that we can use for our workaround in our Houston roof? The humble and ubiquitous kraft-faced fiberglass batt (Photo 1). It’s dry cup value is 1 perm, so it meets the code requirement of a Class II vapor retarder as tested under ASTM E-96 Test Method A (the desiccant method). Its wet cup value is around 10 perms. So most of the year it is “vapor open,” especially during the most important part of the year for Houston, the hot and humid summer. Are we done with Houston yet? Nope. The batts don’t fit very well in horizontal applications. All kinds of gaps occur
68 ASHRAE Journal

Photo 2: Mind the Gap. Batts don’t fit very well in horizontal applications. Blow loose insulation over the top of them; use blown fiberglass or blown cellulose. Guess what we call it? Batt and blow. Photo courtesy of Steven Winter Associates. between them (Photo 2). The gaps are not an air leakage issue, but they are a thermal performance issue. What to do? Easy, blow loose insulation over the top of them; use blown fiberglass or blown cellulose. Guess what we call it? Batt and blow. Are we clever or what? What could we do if the code allowed? What type of insulation and vapor control strategy should we use if the code permitted? It seems that the “residential code” has better language. If we were operating under the 2012 International Residential Code rather than the 2012 International Building Code, we could just blow loose insulation on top of the ceiling gypsum board until we get to Climate Zone 6. So, Climate Zones 1 though 5, “blow is the
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Photo 3: Tenting. Many “authorities having jurisdiction” insist that sprinklers and the sprinkler lines have insulation only installed above them—no insulation under them—and that they be inspected with the insulation in place before gypsum board is installed. The only practical way to do this is by “tenting” kraft-faced batts over them. Don’t forget to blow loose fill insulation over the “tented” batt insulation. Photo courtesy of Steven Winter Associates. way to go.” Zone 7 and higher you actually need some type of vapor retarder. Guess what works? The kraft-faced batt. You see, in the winter, the interior RH is low, and we get the 1 perm we need. Now, let’s get to the next workaround. If we are operating under the 2012 International Building Code, we probably have to install sprinklers. Ugh. The “ugh” is not because of the sprinklers per say, the “ugh” is because we are now going to run into a fairly complicated and typically stubborn creature: the Fire Inspector.
May 2012

ASHRAE Journal - May 2012

Table of Contents for the Digital Edition of ASHRAE Journal - May 2012