Sink or swim: A comparison of below-grade rigid foam insulation moisture performance

by nithya_caleb | July 13, 2019 12:00 am

By David Stassi

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Creating a strong building starts with the foundation. In the same way, designing an energy-efficient building starts with insulating from the foundation up. This is why savvy building professionals insulate below-grade. They include insulation on buried foundation walls and under floor slabs, knowing this is the first step toward maximizing a building’s efficiency. However, creating a building with a Leader in Energy and Environmental Design (LEED)-certified plaque on the wall does not come about by putting just any type of insulation on the foundation and below the floor.

Due to poor moisture performance, not all insulation types are suited for below-grade use. Taking the time to learn about the physical properties of insulation products and how they handle moisture is, therefore, key to ensuring the selected material will perform as intended below grade to help maximize a building’s energy efficiency. In a market flooded with options, making the most informed decision comes from understanding the products inside and out. For the purposes of this article, commonly used below-grade rigid foam insulation products will be examined.

Why below-grade insulation is important

It may seem counter-intuitive to install insulation below grade due to the historically high-moisture content of Canadian soil. After all, damp insulation is much less effective at blocking the flow of heat than dry insulation. However, the Expanded Polystyrene (EPS) Industry Alliance notes, a lack of insulation on below-grade foundations, crawlspaces, and under slabs accounts for up to 25 per cent of a building’s total energy loss (Read Expanded Polystyrene (EPS) Industry Alliance’s Technical Bulletin 103, “15-Year In-situ Research Shows EPS Outperforms XPS in R-value Retention.”). Additionally, uninsulated concrete creates a thermal bridge between a building’s heated interior and the relatively cooler earth surrounding its structure, as well as through any concrete foundation edges exposed to the outside air. Blocking this heat flow is critical to creating a comfortable, energy-efficient building. The dilemma, however, is determining the insulation that will not lose its insulating properties in the damp soil and effectively insulate a building’s below-grade spaces.

Moisture’s effect on insulation

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Imagine someone stepping out into the cold Canadian winter air with proper warm, dry clothing. Since their body is well insulated, it does not have to work hard to stay warm. Now, if the clothes get wet, their body would have to work harder to fight off the cold winter air. This is because water conducts heat better than textiles, and damp clothes provide an easier path for heat to escape from skin. These same principles apply to rigid-foam insulation.

When rigid-foam insulation is wet, the moisture causes the material to compress, reducing the space for insulating air trapped between layers. As the insulation continues to uptake moisture, the trapped air is displaced from the space. This reaction causes the compressed and wet insulation to experience a drop in its thermal performance rating (R-value). Insulation that is not well-rated for use below grade will uptake this moisture and not release it.

Additionally, water’s thermal conductivity is nearly 10 times higher than the conductivity of the average thermal insulation (Consult the presentation “Effects of Moisture on Thermal Insulation” by Steve Badger, PhD, at the International District Energy Association (IDEA) 2017 annual conference.). How does this effect insulation performance? According to Professional Service Industries (PSI), a division of construction materials testing firm, Intertek, “Absorption of only 20 per cent moisture can cause up to 55 per cent loss of insulation value.” Installing insulation in a moisture-rich environment below grade presents the risk of losing over half of the insulation’s engineered thermal capabilities. To prevent this from occurring and ensure the insulation resists heat flow from the surrounding earth and the foundation wall or slab, it is crucial to select a product that mitigates moisture’s impact on its thermal properties.

Below-grade rigid foam physical properties

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To determine which below-grade rigid foam insulation is best to use, it is important to first understand the material’s makeup. Here is a look at three common rigid foam insulation products and their ability to combat moisture.

Expanded polystyrene

Expanded polystyrene (EPS) is a polystyrene resin-based closed-cell insulation material. It is available alone or in combination with other rigid insulations, polymeric facers, and/or backer boards. Historically, EPS was used as a stable roof insulation. In recent years, it has gained wide acceptance in wall, below-grade, and under-slab applications because of its inherent moisture performance, strength, and stable long-term thermal qualities (R-value). Below grade, EPS is a viable option because of its high-density structure that can effectively withstand both backfilling and concrete slab pouring. It is lightweight, resilient, and can be custom-cut into a variety of shapes and sizes to meet a wide range of job specifications. For example, some manufacturers offer EPS board sizes up to 4880 x 1220 x 1020 mm (16 x 4 x 3.33 ft). While this larger size of a board is not typically used in below-grade construction, the customization options of EPS are vast.

One unique component of EPS is manufacturers expand it by using steam, rather than a chemical blowing agent. The natural expansion of the polystyrene foam beads make EPS adept at retaining its thermal insulation R-value over the long-term. This is a key benefit as other rigid foams expanded using a blowing agent can lose up to 30 per cent of its initial R-value over its lifespan. This is reflected in the EPS insulation’s long-term thermal resistant (LTTR) value listing. In application, EPS’ stable R-value helps support lower energy use and costs over a building’s time in service.

 Moisture performance of EPS

The steam expansion process bonds EPS’ polystyrene foam beads together with heat rather than chemicals. The heat expansion creates air pockets within the board allowing moisture to uptake into EPS faster than alternative materials, as well as releases it more quickly. In below-grade applications with consistent wet-dry cycles, EPS releases its moisture content rapidly to return to its original R-value.

 Extruded polystyrene

Extruded polystyrene (XPS) foam is another polystyrene resin-based closed-cell rigid foam. It is easily recognized by its bright colouring. During manufacturing, molten foam polystyrene beads are extruded through dye and expanded using blowing agents. After this process, the material is formed into blocks or insulation boards up to 2438 x 1220 x 76 mm (96 x 48 x 3 in.).

Moisture performance of XPS

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When XPS is extruded through dye, it expands into a uniform, closed-cell rigid foam without voids or pathways for water intrusion. While this allows the XPS foam to uptake water at a slow rate, it also causes the material to release moisture slowly after absorption. In turn, this causes XPS to slowly return to its R-value.

Polyisocyanurate

Polyisocyanurate (polyiso) is another common form of rigid foam insulation. Unlike EPS and XPS, it is not often considered for below-grade use.

Polyiso insulation boards are made with organic and inorganic facer materials such as paper, foil, or fibreglass. The use of a facer plays a role in the insulation’s moisture performance. Foil-faced polyiso boards have permeance (perm), or measure of its allowance to the flow of matter, rating of about 0.3, according to the Polyiso Insulation Manufacturers Association (PIMA). In part, because of their low perm ratings, polyiso products often are used in wall and roof applications.

Moisture performance of polyiso

Most polyiso wall insulation products are very moisture resistant. However, “because the polyiso facer can absorb water, polyiso is not recommended for use under slabs or on the exterior of foundation walls,” notes Green Building Advisor, a website dedicated to the residential construction industry. Although EPS and XPS also absorb moisture, these products do not need a facer, and are not subject to moisture accumulation and retention in the facer’s material. In the absence of a facer material, EPS and XPS can release moisture at their respective rates below grade.

Deciphering the results: Below-grade rigid foam moisture performance

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As evidenced throughout this article, virtually no insulation is immune to moisture reducing its thermal performance. However, the level to which an insulation readily absorbs and releases water is key to determining its below-grade efficiency. Since EPS and XPS perform significantly different in terms of moisture, much confusion exists about which one is better for below-grade use. Ultimately, the answer depends on how it is measured.

 ASTM C272

Moisture performance of rigid foam insulations, including XPS and EPS, is reported on product data sheets based on ASTM C272, Standard Test Method for Water Absorption of Core Materials for Sandwich Constructions. In this test, the lab submerges insulation samples in water for 24 hours, and then weighs the samples for moisture absorption immediately upon removal.

The ASTM C272 tests reveal, and product manufacturers typically agree, EPS absorbs small amounts of moisture more quickly than XPS does. Fortunately, virtually no building is subjected to 24 hours of continuous submersion, otherwise there are bigger problems at play. Most buildings go through periods of wetting and drying. This allows the insulation to release the moisture and return to efficiently insulating the foundation. Although the explicit results of the test would seem to imply XPS offers superior moisture performance, it is important to understand EPS releases moisture much faster during these wetting and drying cycles. This fact is especially significant when considering how both materials perform in the field.

 In-situ testing

Good coaches understand that, even if their team performs well in practice it does not necessarily mean that they will perform well in the game.
To properly prepare for a game, a coach should provide in-game situations before the team takes the ice. In the same way, relying only on the laboratory results of ASTM C272 without testing how a product performs in-situ is like playing for the Stanley Cup without proper preparations.

In one example of a real-world application versus a laboratory test, the Expanded Polystyrene Association of Canada (EPAC) and the National Research Council Canada (NRC) jointly conducted EPS moisture performance testing over a 30-month exposure period in an exterior below-grade application. The testing was conducted in an area of multiple freeze-thaw cycles and monitored using thermocouples attached to the insulation and concrete wall. During heavy rains and thaws, the monitoring detected moisture on the outside surface of the EPS foam, but moisture penetration was not observed on the surface of the structure’s foundation. Further, performance tests confirmed the EPS insulation did not lose any of its thermal insulating R-value after over two years in-situ. Moreover, the moisture content of the samples was less than 0.5 per cent on average by volume.

Additional in-situ testing from the independent lab Stork Twin City Testing evaluated the moisture content of EPS and XPS buried side-by-side for 15 years on a laboratory’s foundation in St. Paul, Minn. At the time the insulations were removed, the EPS was four times drier than the XPS. In fact, the EPS had only 4.8 per cent moisture by volume compared to 18.9 per cent moisture content for the XPS. After 30 days of drying time, the EPS had dried to only 0.7 per cent moisture by volume, while the XPS still contained 15.7 per cent moisture.

The Stork Twin City Testing insulation test also showed the degree to which moisture absorption impacts R-value. The XPS insulation lost 48 per cent of its R-value, compared to only a six per cent decrease for EPS.

Conclusion

While energy-efficient buildings are created by insulating from the bottom up, getting the best possible insulation for the project starts by understanding the product from the inside out. When conducting research of below-grade insulation, it is important to keep in mind laboratory results do not always tell the whole story when it comes to rigid foam insulation moisture performance.

[6]David Stassi is field technical support manager at Insulfoam, a leading manufacturer of expanded polystyrene (EPS) insulation products. A graduate of Colorado State University, Stassi worked as a field sales representative for OMG Roofing Products prior to joining Insulfoam. Stassi can be reached via e-mail at david.stassi@insulfoam.com[7].

Source URL: https://www.constructioncanada.net/sink-or-swim-a-comparison-of-below-grade-rigid-foam-insulation-moisture-performance/

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