Polyisocyanurate insulation and the vapor retarder paradox

March 4, 2020

by Tom Robertson

Photo courtesy Atlas Roofing Corporation[1]
Photo courtesy Atlas Roofing Corporation

The world of construction is filled with contronyms—words 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.

Like a contronym, here is another word that causes plenty of headaches—vapor retarder. In a given application, it can have opposite effects depending on its properties and conditions of use.  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. above-grade walls, below-grade basement and crawlspace walls, and roofs).

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 polyisocyanurate (ISO).  Images courtesy Applied Building Technology Group[2]
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 polyisocyanurate (ISO).
Images courtesy Applied Building Technology Group

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’s 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.

Above-grade vapor control methods

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 ‘the perfect wall[3].’ 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. 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.

One example of an ‘optimal’ 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—material 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. 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.

Figure 2: Hybrid wall assembly (cavity and ci).[4]
Figure 2: Hybrid wall assembly (cavity and ci).

The ‘warm wall’ 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 ‘smart’ 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 ‘water vapor permeance’ 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.

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.

A construction project in California utilizing polyisocyanurate (ISO) insulation to protect against vapor and moisture intrusion.   Photos courtesy Atlas Roofing Corporation[5]
A construction project in California utilizing polyisocyanurate (ISO) insulation to protect against vapor and moisture intrusion.
Photos courtesy Atlas Roofing Corporation

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[6] to better coordinate with advancements in the energy code, such as in the case of the upcoming 2021 editions of the International Building Code (IBC), International Residential Code (IRC), and International Energy Conservation Code (IECC).

Below-grade vapor control methods

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:

Nearly 90 percent of basement moisture problems are related to the failure to provide or maintain one or more of these crucial features.

For basements, the perfect wall concept also applies as the ideal solution. This simply requires the use of ci on the exterior side of the basement wall. However, for practical reasons, it is often optimal to use ci on the interior side of the assembly together with the water control strategies discussed above. These practical reasons include the need to protect the insulation (particularly above-grade portions) and the need to retrofit an existing basement (like the case study addressed in the next section). When insulating and finishing a basement on the interior side, it is important to be mindful of where the vapor retarder layer is placed. Ideally, vapor retarder should be layered directly on the interior surface of the block or concrete wall and should be water-resistant and insulating, like ISO ci, in order to protect moisture-sensitive materials. Consequently, wood framing, furring, gypsum, and water-absorbing insulation materials should be placed on the inside of the ci layer. This approach is consistent with the U.S. Department of Energy[7]’s (DOE’s) Building America Program best practices for foundations.

Nearing completion, this project in California includes ISO to protect against the elements and meet building codes.[8]
Nearing completion, this project in California includes ISO to protect against the elements and meet building codes.

Real-world application

Understanding the principles and methods for controlling water vapor is all well and good, but how does it work in a practical application? A recent residential project in Marietta, Georgia, provides a good example of how ISO insulation helps control moisture issues.

Homeowners Jon and Rachel recently remodeled their basement to create a comfortable suite for Jon’s mother. The previous structure of the basement made it difficult to comfortably control the basement temperature, and condensation issues had led to a strong, musty odor. The couple had already decided to install a second HVAC unit to help control the basement temperature, though they were concerned about increasing energy usage. They also knew they needed a better way to control moisture to rid the area of its musty smell.

Jon and Rachel decided to incorporate a better insulation solution to create a comfortable environment. They installed foil-faced continuous ISO insulation on the interior side and directly to the basement walls. Since the foil-faced ISO insulation is vapor impermeable and the basement is below grade, wood framing and drywall were placed on the interior side of the ISO. This protected the wood and interior finishes from moisture moving inward through the basement wall. There was no additional vapor retarder placed behind the finish on the wood frame, as the foil-faced ISO provided for insulation and a properly located vapor retarder for the basement wall. In the end, this approach solved their moisture-control issues and created a healthier, cleaner environment. As an added bonus, Jon and Rachel have noticed little change in their energy costs because the new basement space is so energy efficient.

In the case of this application, the insulation method was a simple and effective way to control moisture issues with few compromises. While each project requires builders to consider a variety of factors, such as the thickness of the wall design, extreme climate, or the client’s budget, the insulation method can be applicable in a variety of both residential and commercial applications.


While every project and its specifications will be different, the vapor control and insulation methods described above are available and generally applicable to optimize wall performance[9] and minimize moisture issues.

  1. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2020/02/DSC00003.jpg
  2. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2020/02/figure-1-insulation.jpg
  3. the perfect wall: http://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall
  4. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2020/02/figure-2-insulation.jpg
  5. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2020/02/DSC00261.jpg
  6. vapor control provisions: http://www.continuousinsulation.org
  7. Department of Energy: http://www.energy.gov/eere/buildings/building-america
  8. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2020/02/Untitled_Panorama.jpg
  9. optimize wall performance: http://www.continuousinsulation.org
  10. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2020/02/Tom-Robertson.jpg
  11. trobertson@atlasroofing.com: mailto:trobertson@atlasroofing.com

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