{"id":107574,"date":"2020-03-04T14:58:42","date_gmt":"2020-03-04T19:58:42","guid":{"rendered":"https:\/\/www.constructionspecifier.com\/?p=107574"},"modified":"2020-02-25T15:17:48","modified_gmt":"2020-02-25T20:17:48","slug":"polyisocyanurate-insulation-and-the-vapor-retarder-paradox","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/polyisocyanurate-insulation-and-the-vapor-retarder-paradox\/","title":{"rendered":"Polyisocyanurate insulation and the vapor retarder paradox"},"content":{"rendered":"

by Tom Robertson<\/strong><\/p>\n

\"Photo<\/a>
Photo courtesy Atlas Roofing Corporation<\/span><\/em><\/figcaption><\/figure>\n

The world of construction is filled with contronyms\u2014words having two completely opposite meanings. Has a building weathered many seasons, or is it falling apart because it is so weathered? Is the contractor providing good oversight, or did an oversight set off an angry client? Anyone in the construction industry can vouch that almost nothing is ever quite as straightforward as one might hope.<\/p>\n

Like a contronym, here is another word that causes plenty of headaches\u2014vapor retarder. In a given application, it can have opposite effects depending on its properties and conditions of use.\u00a0 Employed correctly, it can help keep structures dry, and when used incorrectly, it can contribute to moisture accumulation and prevent structures from drying. This creates a paradox of sorts because some vapor retarder materials, like polyisocyanurate (ISO) insulation, may be perceived as a problem. In reality, such materials can provide a simple and effective solution to water, air, vapor, and thermal control for building enclosures. The solution is proper application in coordination with the climate conditions and the overall design of any building envelope assembly (i.e.<\/em> above-grade walls, below-grade basement and crawlspace walls, and roofs).<\/p>\n

\"Figure<\/a>
Figure 1<\/strong>: \u2018The perfect wall\u2019 has all of the control layers (water, air, thermal, and vapor) located on the exterior side of the building envelope assembly and thus entails the use of continuous insulation (ci) materials like polyisocyanurate (ISO).<\/span>
Images courtesy Applied Building Technology Group<\/span><\/em><\/figcaption><\/figure>\n

In its different forms (liquid, vapor, and solid), water is the great enemy of long-lasting, efficient construction. Water can cause sound structures to crumble from corrosion, rot, and rust. It can provide an environment for harmful mold that can be difficult and expensive to eradicate. It can completely ruin a building\u2019s aesthetics. Water is, quite literally, a relentless force of nature for good or bad. This natural dichotomy is why it must be managed or controlled in any reasonable design approach and can never be completely eliminated.<\/p>\n

Above-grade vapor control methods<\/strong><\/p>\n

The ideal wall is one that provides an uninterrupted thermal barrier to keep the building envelope protected and ensures long-term energy efficiency. To that end, a building must be resistant to water and air, manage vapor, and limit heat transfer, prioritized in that order, as detailed by Dr. Joseph Lsitburek of Building Science Corporation in his description of \u2018the perfect wall<\/a>.\u2019 As shown in Figure 1, the perfect wall has all of the control layers (water, air, thermal, and vapor) located on the exterior side of the building envelope assembly and thus entails the use of continuous insulation (ci) materials like ISO. For example, an interior vapor retarder is not required and instead the vapor control layer is provided on the exterior side (e.g.<\/em> by the inner facing of the foil-faced ISO). This maximizes the ability of the assembly to dry to the interior. While the perfect wall may indeed be the ideal solution, there are other wall configurations that will perform well for a given application.<\/p>\n

One example of an \u2018optimal\u2019 variant from the perfect wall is an above-grade wall with a combination of cavity insulation and exterior ci for thermal control to satisfy the energy code. This is also known as a hybrid wall (Figure 2). A key design consideration is the insulation ratio which compares the exterior ci R-value to the cavity insulation R-value\u2014material properties that are readily available and familiar to building professionals. When exterior ci materials like foil-faced ISO are used for this purpose, it provides thermal control, thereby, saving energy and preventing thermal bridging through wall framing. It also prevents moisture accumulation by shielding the wall from outdoor water-vapor sources in the humid summer while keeping the interior of the wall warm during the winter to prevent condensation or high humidity. This is particularly true when the ci is also approved for use as a water control layer (i.e.<\/em> water-resistive barrier [WRB]) and air barrier. For any wall design, it is of primary importance to control water and air movement. In other words, it is pertinent to never overlook the perfect wall priorities mentioned earlier, even if specifying a variant like the hybrid wall.<\/p>\n

\"Figure<\/a>
Figure 2<\/strong>: Hybrid wall assembly (cavity and ci).<\/span><\/figcaption><\/figure>\n

The \u2018warm wall\u2019 design approach described above is simple to implement and robust in the field. It controls the temperature of the assembly and promotes drying to the interior through the selection of an appropriate interior vapor retarder, such as interior latex paint, kraft paper, or various types of \u2018smart\u2019 vapor retarders. The warm wall design is based on a temperature-controlled design approach and, as such, removes the headache of attempting to control moisture purely by a traditional \u2018water vapor permeance\u2019 design approach. The downfall of the traditional method is it requires careful specification of the vapor retarder properties of each material layer on the interior and exterior of the assembly. For many materials, this property may be unknown or uncertain. It also attempts to walk a fine line between outward (wintertime) and inward (summertime) water vapor movement, which ends up cycling the assembly through seasonal periods of higher and lower moisture. Conversely, a warm wall design results in a stable and dry assembly year-round.<\/p>\n

As energy codes continue to advance, the traditional vapor retarder design approach has become more and more challenging to reliably implement. Increasing cavity insulation levels can worsen the vulnerability to condensation or moisture accumulation by causing wood or gypsum sheathing materials to become colder (not warmer), resulting in condensation on surfaces.<\/p>\n

\"A<\/a>
A construction project in California utilizing polyisocyanurate (ISO) insulation to protect against vapor and moisture intrusion.<\/span>
Photos courtesy Atlas Roofing Corporation<\/span><\/em><\/figcaption><\/figure>\n

However, the use of ci for a warm wall design completely changes this challenge into an opportunity to leverage newer energy code insulation requirements in a way that also improves moisture performance. Fortunately, energy codes always afford specifiers the ability to use ci (alone, as in the perfect wall, or together with cavity insulation) whether it is offered in the code as a prescriptive option or not. Also, building codes are in the process of updating vapor control provisions<\/a> to better coordinate with advancements in the energy code, such as in the case of the upcoming 2021 editions of the International Building Code<\/em> (IBC<\/em>), International Residential Code<\/em> (IRC<\/em>), and International Energy Conservation Code<\/em> (IECC<\/em>).<\/p>\n

Below-grade vapor control methods<\/strong><\/p>\n

The discussion so far has been focused on above-grade wall applications, but what about below-grade walls? Well, the same principles apply, but in many ways, basements are easier because the climate below-grade does not change as much with the seasons. Basically, the ground is always moist and moderates the outside temperature year-round. The result is water vapor drives are always inward. Of course, the same high priority for liquid water control applies. All high performing basement walls start with:<\/p>\n