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EPS Expanded polystyrene insulating board (C) Daniel FriedmanAboveground & Underground Use of Polystyrene Foam Board

EPS vs XPS Foundation, Wall, Roof Insulation Comparisons & Installation Instructions

Expanded polystyrene insulation board properties, comparisons, uses, & installation instructions.

This article also discusses the question: which is the best foundation insulation to use below-ground. Portions of the text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

Uses & R-Values of Polystyrene Foam Insulating Board

Polystyrene foam insulation below concrete slab (C) Daniel FriedmanArticle Contents

 

Types of Polystyrene Foam Insulating Board

EPS compared with XPS

Both EPS and XPS polystyrene are forms of a closed cell plastic-based (styrene) insulation, made from the same resin (polystyrene) that can be formed, molded, or cut into a desired shape and size - in construction these are usually cut or molded into a board-form that is then applied to a surface to serve as insulation, or in the case of re-siding applications, as a leveling board.

EPS = Expanded Polystyrene Insulation (beadboard)

EPS or expanded polystyrene insulating board is manufactured by expanding spherical beads of polystyrene inside of a mold.

The polystyrene beads are then fused together using heat and pressure. The actual amount of plastic (polystyrene) is quite small - about 2%. The insulating value of EPS derives from its trapped-air.

XPS = Extruded Polystyrene Insulation

XPS or extruded polystyrene insulating board is manufactured of closed-cell foam using an extrusion process.

XPS has a greater compressive strength - that is, it's more dense or "stronger" than EPS.

Both EPS and XPS are water-resistant but not absolutely impermeable - that is, not absolutely waterproof.

Therefore, XPS is a vapor retarder but it is not considered a vapor barrier.

The very familiar "Styrofoam" insulating board is a Dow product, blue in color, and is an extruded (XPS) foam board product.

Comparing Features of EPS vs XPS Insulation
Feature EPS XPS Comment
Cellular structure Closed
Contains air		 Closed
Contains other gases		 Gas choice affects long term
R-value
Compressive Strength  ... 15 - 100 psi XPS varies by type V=100, X=15
15 psi for OC Formular 150 XPS
Cost Moderate Higher  
Density 110 ~ 120 kpa
140 kpa

150 - 700+ kpa

20.8 (1.3 pcf)

 for EPS @ 18 kg/m3 tensile
for EPS @ 20 kg/m3
for OC Formular 150 XPS
Dimensional Stability   2.0 % change max
Environmental Impact Low GWP
Produced using Pentane		 High GWP
Produced using HFCs		 GWP = global warming potential
Insulfoam
Flame Spread Index   < 75 ASTM E84 Flame Spread Index
Flexibility Moderately Flexible Brittle
Breaks easily		  
Flexural Strength (min)   40 - 100 psi XPS varies by type V=100, X=40
Heat preservation good    
History > 30 years use Europe & US    
Inner Structure Open cell? Closed cell
Closed pore		  
Moisture resistance Fair Good  
Permeability Moderate Low Low permeability can contribute
to condensation in building
cavities
R-Value   4.6 - 5.0 Varies by XPS type
POLYSTYRENE
R-Values
R-Value Stability Invariant over life Degrades over time thermal drift in XPS reduces its R-value
Smoke in Fire   < 450 ASTM E84 Smoke Developed Index
Standards     ASTM C578, Standard Specification for Rigid,
Cellular Polystyrene Thermal Insulation,
Surface Permeable Dense  
Tensile Strength

18 kg/m3
-
20 kg/m3

 25 kg ~ 45 kg/m3  
Thermal Conductivity   0.029 (k=0.20)
2" Thickness		 OC Formular XPS
Thermal Resistance Lower than XPS in long term
exterior studies		 1.77 (R-10)
2" Thickness

OC Formular XPS, XPS may absorb
more water in-situ
Insulfoam EPS
Water Absorption   0.3% Max volume percent
Water Retention Drying potential under severe
exposure conditions		 Long-term in-situ testing: XPS
traps water, lowers R-value		 Insulfoam
Water Vapor
Permeance		 2.5 to 5.0 perms /inch
facing lowers perm rating		 1.1 perms (un-faced)
= semi-permeable		 Perm

Notes to the table above

Aboveground Use of Polystyrene: Walls & Roofs + Foundation Insulation

Fanfold XPS re-siding underlayment board from Owens Corning, cited & discussed at InspectApedia.com

XPS refers to Extruded polystyrene insulating board, used often in re-siding applications but also in roof construction and as foundation insulation, described by these example specifications & installation instructions from Owens Corning - below.

Above: Owens Corning XPS Formular is a re-siding underlayment insulating board commonly used when installing siding over an existing siding surface such as asphalt shingles. The company also describes the product as a leveling board.

Owens Corning FORMULAR NGX FOAM BOARD INSULATION GUIDES NGX = Next Generation Extruded Foam Board insulation used on both roofing and wall siding installations and also in below-grade applications as foundation or under-slab insulation are described by these product data sheets.

Find OC's complete set of NGX product descriptions at https://www.owenscorning.com/en-us/insulation/commercial/foamular-ngx/guide-specifications (Sept. 2022)

Underground Uses of Extruded Polystyrene

Is Expanded or Extruded Polystyrene Foam Insulation Better to Use for Underground Foundation Insulation?

Basement heat loss (C) Daniel FriedmanQuestion:

I am building an earth-sheltered home and discovered that I could save a great deal of money by insulating below grade with expanded rather than extruded polystyrene foam insulation

. Is it unwise to use the expanded polystyrene board, given its tendency to absorb moisture below grade? -- D.S., San Jose, CA

Our photo (above left) shows polystyrene foam insulating board below a concrete slab being poured in Two Harbors, MN.

Answer: 

Widely reported research by Dow Chemical Co. shows that their extruded polystyrene foam insulation, on average, outperforms expanded polystyrene (EPS) in below-grade insulation applications.

Due to the study's limitations, however, Dow cautions against using the findings to predict long-term performance of polystyrene foam insulation board in this application.

Interestingly, the foam board insulation testing found that the most common expanded EPS - 2-inch thick, low density (1 lb./ft3), material - held up about as well as extruded stock in both vertical and horizontal insulation board applications.

Both showed 2- to 13- percent increases in temperature conductivity after 6- and 18- month exposures.

Dow speculates that the poor performance of the high-density DPS was due to high void content and poor bead fusion. EPS manufacturers agree that good bead fusion is harder to achieve in high-density foam insulating board stock.

Here is a simple test of foam insulating board quality:

Polystyrene insulating foam board indoors (C) Daniel FriedmanEPS should break through the beads, not around them. Because over 175 small companies make EPS foam insulating board products, quality varies considerably.

Our photograph of white polystyrene foam board insulation (below) shows broken foam boards at an indoor foam board insulation retrofit project.

Photo at left: styrofoam insulating board used indoors.

With well-drained soil, a low water table, [protection also from roof drainage spillage against the foundation wall], and a waterproof outer membrane (e.g. polyethylene), a good-quality EPS foam insulation board at least 2 inches thick should perform adequately.

To play it safe, some underground builders use EPS toward the inside and extruded insulating board facing the backfill soil around the building foundation.

Below-Ground Insulation Research

Readers considering adding insulation inside or outside a basement foundation wall should also take a look

at BUCKLED FOUNDATIONS due to INSULATION?

See BASEMENT HEAT LOSS for a discussion of foundation and basement insulation methods.

See BUCKLED FOUNDATIONS due to INSULATION? for a discussion of insulation's role in foundation damage from freezing soils.

See TERMITE SHIELDS vs TERMITICIDE for a discussion of avoiding insect damage when foam insulating board is used below or at ground level.

See WET BASEMENT PREVENTION for other exterior treatments of building foundation walls to combine foundation insulation with basement waterproofing membranes and drainage systems.

Foundation Foam Insulation Protection Methods & Advice

Question: What finishes are available to protect Styrofoam insulating board used for foundation insulation?

I will be insulating a foundation with Styrofoam. I understand there are masonry finishes available that can be applied directly to the foam or to an intermediate mesh, and that such a surface will hold up to weather exposure.

What finishes are available to protect my foundation insulation? - Thomas F. Harter, Oakland CA

Reply: Several Ways to Protect Rigid Foam Foundation Boards

There are several ways to protect rigid foam foundation insulation boards, ranging from pressure-treated plywood to factory-applied stucco finishes.

One pre-finished foundation insulation panel is Thermboard, a 3/4-inch Styrofoam panel coated with a fiber-resin compound that has a stucco-like appearance. It costs [1980's prices] around $`.47 per square foot in the Wisconsin area where it is made. Thermboard is distributed by Georgia Pacific Corp.

Another pre-finished foundation insulation product is Styrofoam FP Panels, distributed by Dow Chemical and available at many lumber yards. The finish is a cementious coating that looks something like concrete.

For a field-applied finish to protect foundation insulation boards, you can trowel on your own stucco to the foam over expanded metal lath, or brush on a finish designed for this purpose, such as Styrofoam Foundation Brush-on Coating (also available from Dow Chemical).

It comes in a kit consisting of a dry polymer cement mixture, liquid additive, and fiberglass reinforcing tape [use at seams between foundation insulation boards].

You mix the components with water and apply directly to the foam foundation insulating board.

The insulating board is scratched-up to promote good adhesion. It dries to resemble concrete. The cost of this foundation insulation board coating is around 40 cents per square foot. [1986 prices].

[Solar Age 8/86]

At InspectApedia we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Original article


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