Dow Styrofoam

Residing is a great opportunity to increase a home’s value with a fresh, new look. It’s also an opportunity to increase energy efficiency and overall comfort with moisture-resistant, durable insulation.

For 60 years, BLUE™ STYROFOAM™ Brand Insulation has been a leading building insulation solution for residing contractors and homeowners alike. STYROFOAM™ Brand Extruded Polystyrene (XPS, ASTM C578 Type X) Insulation is valued for its exceptional insulation performance, excellent moisture resistance and the appropriate level of water vapor permeability under vinyl siding.

To appreciate the value of Dow’s extensive portfolio of rigid insulated sheathings, Dow recommends comparison shopping STYROFOAM™ Brand XPS and the competition – white bead board or expanded polystyrene (EPS, ASTM C578 Type I).

As part of a Type I foam-backed vinyl siding system, EPS is laminated or adhered to vinyl siding for ease of installation. When comparing STYROFOAM™ Brand XPS to this system, it is important to consider the following characteristics, including:

  • Foam structure
  • Compressive strength
  • Thermal performance
  • Moisture resistance
  • Breathability
  • Ability to block air infiltration

After you have learned more about – and compared – the key components of quality insulation, you will be in a better position to choose the right insulation for your application.

The first comparison is simple. STYROFOAM™ Brand XPS is BLUE™. EPS is white. Then when you look under a microscope, it is easy to see the structural difference.


Proprietary formulations and manufacturing processes for STYROFOAM™ Brand XPS ensures a closed-cell, high-density structure, providing a compressive strength of 15 psi (ASTM C578 Type X). This means XPS stands up to pressure and abuse at the job site much better than white EPS, with a compressive strength of 10 psi (ASTM C578 Type I).

STYROFOAM™ Brand XPS also resists moisture and maintains R-value.

When the white EPS backing on vinyl siding becomes damaged, it can create gaps, which can increase the risk of water absorption. Moisture absorption significantly impacts the R-value, reducing the overall energy efficiency and performance of the EPS-backed siding. The excess moisture can also increase the risk of mold and mildew and other moisture-related problems.

STYROFOAM™ Brand XPS Insulation is built on sound scientific principles and the proof is in the test results. Insulation from Dow outperforms white bead board in the following categories based on ASTM C578.


When it comes to R-value, the greater the number the better. BLUE™ STYROFOAM™ Brand XPS not only starts off with a greater value (5.0 per 1"), but its durable formulation and consistent thickness provide a long-term, stable R-value on the walls of a home. In contrast, EPS Type I has an R-value of 3.6 per 1".

Contoured vinyl siding adhered to contoured white bead board (EPS) may be a time-saver, but it comes at a cost: Contoured EPS foam has a significantly lower Rvalue when compared to the consistent thickness found in STYROFOAM™ Brand Extruded Polystyrene Insulation. For the best value, you cannot beat STYROFOAM™ Brand XPS for consistent and continuous protection around a building envelope.


White bead board (EPS, ASTM C578 Type I) absorbs 13 times more moisture than STYROFOAM™ Brand XPS Insulation. Water retention can rob the insulation of R-value and provide the necessary elements for moisture-related problems such as mold and mildew.

Help keep damaging moisture out of a home with STYROFOAM™ Brand XPS.

What happens when moisture, vapor and bulk water make their way into the siding/insulation system? If water in any form becomes trapped, it can potentially lead to mold, mildew and other moisture-related problems.

In a typical vinyl siding/adhered EPS system, water can become trapped on the one side by the vinyl siding. The only way out in this system is for the water to make its way into the EPS. And this EPS foam is typically applied directly against a relatively impermeable woodbased sheathing, increasing the potential for moisture damage.

As a separate layer, STYROFOAM™ Brand XPS provides an effective air space, which creates a drainage plane that allows water to drain to the bottom of the walls and escape from the building envelope through weep holes. This air space enhances the wall assembly’s drying potential, which can contribute to a more comfortable home and reduce the potential for moisture-related problems.

Think of permeability as a building material’s ability to breathe (allowing moisture in the vapor state to pass through the material). EPS seems to have a higher permeability (5 perms at 1" thick), but when attached to vinyl siding the overall perm rating drops to 0.05. And drying toward the interior is also limited because the typical wood-based sheathing has a perm rating of 0.88.

Bottom line – When looking at an EPSbacked system, the permeability of the overall offering is determined by the siding itself, not the EPS. Therefore, both XPS and EPS-backed vinyl siding systems have similar, if not identical, overall permeability ratings.


Air infiltration is one of a home’s biggest enemies. It enters a home throughcountless gaps and cracks, contributing up to 40 percent of a home’s energy loss. That means a typical 2,500 ft2 home can have over 1/2 mile of gaps and cracks.* And to make matters worse, air infiltration also increases the likelihood of moisture intrusion in the form of water vapor.

It takes a WRB to reduce moisture and air intrusion. Several STYROFOAM™ Brand Extruded Polystyrene Insulation products qualify as a WRB, eliminating the need for a separate housewrap, and saving material and installation costs.

White EPS products, however, require a facer with a low perm rating or a separate housewrap with taped seams to claim WRB properties. This adds time, money and material to the bottom line.

When installed properly, nearly all rigid foam insulations from Dow qualify as a WRB according to the International Residential Code. Examples include:

  • STYROFOAM™ Brand DURAMATE™ Insulation – Plus
  • STYROFOAM™ Brand Insulation – Tongue and Groove
  • STYROFOAM™ Brand Insulation – Residential Sheathing
  • STYROFOAM™ Brand Insulation – Residing Board
  • STYROFOAM™ Brand Insulation – Square Edge
  • STYROFOAM™ Brand Insulation – HPU

For the highest level of protection against moisture and air infiltration, select from Dow’s extensive portfolio, including:

  • STYROFOAM™ Brand Extruded Polystyrene Insulation
  • Dow polyisocyanurate insulation
  • STYROFOAM™ Brand Spray Polyurethane Foam (SPF) Insulation – RS
  • WEATHERMATE™ Housewraps
  • WEATHERMATE™ Straight and Flexible Flashings
  • FROTH-PAK™ FS Foam Insulation Kits and FROTH-PAK™ Foam Sealant Kits
  • GREAT STUFF PRO™ Gaps & Cracks Insulating Foam Sealant**
  • GREAT STUFF PRO™ Window & Door Insulating Foam Sealant**
  • DOW™ Window & Door 1000 Sealant