Specifying insulation in below-grade applications and vegetative roof assemblies

Certain enclosure locations present extreme performance challenges for insulation. For example, below-grade applications and vegetative roof assemblies (VRAs) test insulation’s moisture resistance, thermal performance, compressive strength, and more. Meeting these unique needs begins with considering how manufacturers make rigid foam board insulation commonly specified in below-grade applications and VRAs. It’s also important to consider the traits of different insulation.

How rigid foam board insulation is made

Different manufacturing processes create extruded polystyrene, expanded polystyrene, and polyisocyanurate insulation.

Extruded polystyrene (XPS) insulation’s production involves mixing polystyrene with a blowing agent and forcing it through a die as the insulation cures. This process generates a consistent, closed-cell structure of entrapped insulating gas without leaving extraneous internal voids. The continuous extrusion process produces a matrix that is resistant to liquid and vapor moisture.

Expanded polystyrene (EPS) insulation is made of polystyrene beads that are placed in a mold before they expand into a solid. Since each bead is initially separate, the combination process can leave voids between beads.  

Polyisocyanurate (polyiso) insulation relies on a continuous lamination process that involves the mixing of isocyanate, polyol blends, and blowing agents. A facer material forms the top and bottom of the insulation. As the mixture moves through a laminator, the process subjects it to pressure and heat.

Insulation traits for below-grade applications

When specifying an insulation for below-grade applications, consider moisture resistance, thermal performance, and compressive strength.

Moisture resistance: Over time, without preventative measures, the likelihood of liquid or vapor moisture infiltrating below-grade applications is quite high. Studies have demonstrated that in the presence of liquid and vapor moisture, a closed-cell insulation, such as Owens Corning Foamular NGX XPS, retains its R-value (a measure of insulation’s effectiveness). The insulation’s continuous extrusion process prevents water movement that would occur between beads in EPS insulation or in the irregular voids and damaged facers that sometimes present problems with polyisocyanurate insulation.

A testing method known as ASTM C666 demonstrates XPS’s ability to resist liquid water absorption and damage from water freezing in multiple cycles compared to other insulating materials. While this test was designed for concrete, it exposes the material to liquid water for freeze/thaw cycles rather than just freezing temperatures and humidity. This is akin to other in-situ studies, such as ones involving the harsh conditions of Alaskan roadways and runways, where XPS has repeatedly demonstrated its ability to stay intact for decades. 

Thermal performance: Insulation’s primary function is reducing heat transfer. In below-grade applications, this means preventing heated foundation and slab edges from allowing heat to escape into the surrounding cold subsoil. XPS’s ability to retain its thermal performance helps mitigate this concern.

Compressive strength: The backfill process imposes a heavy burden on the facers attached to some rigid boards. If the concrete has not undergone adequate waterproofing or if the polyiso board joints are not tight, incidental damage to facers during installation can allow moisture to infiltrate. Facers can also impose additional material costs and make installation more complicated. Unless polyiso boards are specially designed for a specific below-grade application, they may provide less compressive strength than expected.

VRAs: What to consider  

Above-grade conditions also present challenges for VRAs. When specifying an insulation for VRA applications, consider:

Stormwater management: VRAs have become popular features to help manage stormwater. XPS is part of one of the nation’s largest VRAs, the Douglas A. Munro Coast Guard Headquarters near Washington, D.C., where it helps manage water flow from the roof through a system of drains, providing greenspace for local species.

Stormwater management will only become more important as the years go on. For instance, according to the National Oceanic and Atmospheric Administration’s Fifth National Climate Assessment, precipitation across the Northeast is increasing across all seasons, and extreme precipitation events have increased nearly 60% in the region.

Functional space: The high compressive strength of XPS supports VRAs providing usable space in urban areas. The Wharf in Washington, D.C., is a mixed-use development that relies on Owens Corning Foamular XPS insulation to support VRAs in dining and rooftop gathering areas. 

In protected roof membrane assemblies, the waterproofing membrane is placed directly on the structural deck. Topping this, a layer of rigid insulation protects the membrane, then pavers are added. The plaza deck at Dickies Arena in Fort Worth, Texas, is a PRMA with the compressive strength required to support vehicles, rooftop gardens, and concert performances.

Conclusion

When specifying insulation for below-grade and VRAs, it’s important to consider several factors. Choosing an insulation that delivers moisture resistance, thermal performance, compressive strength, and functional space helps optimize insulation and deliver value beyond thermal performance.

Learn more about Owens Corning Foamular XPS insulation for commercial enclosures here.

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