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Rigid Board Insulation: The Ultimate Guide

By Tobias Roberts Rise Writer
Apr 16, 2020

Properly insulating your home is one of the most important things that homeowners can do to improve the overall energy efficiency of their homes. The United States Department of Energy estimates that the energy that homeowners use for artificially heating and cooling their homes accounts for almost half of total energy use. Investing in more efficient heating and cooling technologies such as a heat pump or high-efficiency furnace is one way to but back on your heating and cooling bills. However, an unsealed building envelope or insufficient insulation in your walls, roof, or foundation will increase your energy use, regardless of your type of heating and cooling.

Rigid board insulation is a relatively new product. It is long-lasting, easy to install, and can improve the energy efficiency and thermal performance of even the oldest and leakiest homes. If you find that your energy bills are always high, even after adjusting the thermostat and upgrading to Energy Star appliances, a retrofit that focuses on improving your insulation might be in store. In the best of cases, this type of insulation can help homeowners save up to 40 BTUs of energy for each BTU of energy consumed by the home heating and cooling systems. For older homes, improving your home's thermal envelope with rigid board insulation could increase your home's energy efficiency by up to 70 percent. Below, Rise offers a complete guide to the pros, cons, and rigid board insulation installation process.

Rigid Board Insulation Guide from the US Department of Energy
Rigid Board Insulation Guide. Photo Credit: US Department of Energy

What Is Rigid Board Insulation?

The vast majority of houses across the country continue to use batts or mats of soft insulation. Fiberglass, cellulose, and even natural sheep wool insulation continue to be the most popular insulation alternatives for homeowners. Rigid board insulation is made from rugged, dense sheets of certain types of foam. The most commonly used foam products for rigid board insulation include polyurethane, polystyrene, or polyiso. Due to the chemical structure, these types of foams are considered to be "closed-cell." In contrast, the majority of insulation products are open cells. 

Closed-cell insulation products like the rigid board are generally stronger, more durable, and resistant to natural elements. In addition, they have a higher R-value than open-cell insulation products. R-values measure the level of insulation capacity of different products. The higher the R-value, the more insulated your home will be.

Rigid foam insulation products are generally sold as 4 foot by 8-foot sheets (the same size as plywood). They come in several different thicknesses ranging from ½ inch to 2 inches.

Insulfoam EPS
EPS Tongue and Groove Rigid Insulation. Photo Credit: Insulfoam

How to Use Rigid Foam Board Insulation?

Because rigid foam board is sold as solid sheets, many handy homeowners might be able to self-install these products in strategic areas around your home. For example, it is possible to enhance the insulation in an unfinished basement by installing sheets of rigid foam board between the exposed studs or along the basement's concrete wall. After attaching the foam boards to the wall, you can place furring strips over the board. These strips will give a secure base for finishing your basement with drywall, bamboo plywood, or other wall material. For homes with concrete walls in the basement, rigid foam board will be much more durable, especially when compared to the loose rolls of fiberglass or cellulose insulation. Concrete tends to seep moisture. So, most open-cell insulation products (like fiberglass) will absorb moisture and could lead to molds and mildew that compromise your indoor air quality.

Rigid board insulation is an excellent option for every other home area, including foundation walls, regular walls, roofs, and attics. Because they are naturally moisture resistant, however, this insulation alternative is usually recommended for external insulation or below-grade applications (such as basements).

Rmax Polyiso
Polyiso Rigid Insulation. Photo Credit: Rmax

How Thick Is Rigid Foam Insulation Board? 

Rigid board insulation generally comes in standard thicknesses ranging from ½ inch to 2 inches. It is possible for homes located in extreme climates to order specialty thicknesses of up to 6 inches directly from certain manufacturers. Obviously, the thicker the rigid board, the higher the insulation capacities. Depending on the type of foam used, homeowners can expect between 3.2 and 6.5 of R-Value per inch.

Where Can Rigid Foam Board Insulation Be Used?

Foam insulation board is applicable virtually anywhere that insulation is used in the home. You can use it on foundations, basements, regular walls, roofs, and attics. When used in above-grade applications, such as walls, ceilings, or attics, foam board insulation can reduce thermal bridging and improve the entire home's energy efficiency. When used for the interior or exterior of foundation walls or below-grade basements, this type of insulation can improve insulation while avoiding moisture buildup or leaks.

Foam board made from polyisocyanurate (polyiso) generally offers the best insulation capacity and 6.5 per inch. Water is the primary "blowing agent," making it one of the most environmentally-friendly foams used. Of course, it is still sourced from petrochemical materials, thus increasing the embodied energy footprint of the product. Fortunately, almost all rigid board insulation made from polyiso does not contain any brominated flame-retardant chemicals. Great news, since these chemicals can be hazardous when inhaled. This type of foam, however, will absorb water and is thus not suitable for below-grade applications.

ACH Below Slab XPS
Foam-Control PLUS Below Slab Rigid Insulation. Photo Cerdit: ACH Foam

Should Rigid Foam Insulation Board Go Under or Over The Slab? 

When installing insulation in foundation walls or basements, homeowners should look for products made from either expanded polystyrene (EPS) or extruded polystyrene (XPS). While both types of foam products are naturally water-resistant, the XPS foam insulation board offers enhanced R-values and water resistance. On the downside, all XPS products will include HBCD flame retardants, and the blowing agents will negatively affect the ozone layer and contribute to global warming. Choosing thicker applications of the EPS foam board is usually the most environmentally friendly option for below-grade insulation. EPS board is almost always made from pentane blowing agents, which do not affect the ozone layer of our atmosphere. 

How Long Does Rigid Foam Board Insulation Last?

Due to its firmness and water resistance, rigid foam insulation will generally last 100 years or more. The correct installation is essential to take advantage of the durability this product offers. For example, if you were to install a polyiso foam board in a basement or foundation, water absorption would quickly ruin the product, just as it would with cellulose or fiberglass batt insulation.

DOW EPS Insulation. Photo Credit: Dupont

Is Rigid Foam Board Waterproof?

Technically, EPS and XPS rigid foam board insulation are considered to be water-resistant. A product branded as waterproof means that it offers complete impermeability, even if permanently immersed in water. The two types of rigid foam boards mentioned above will resist the penetration of water to a particular grade. Most foam board products made from EPS and XPS will offer enough water resistance to deal with common basement and foundation moisture levels unless your home is located in a flood plain. Houses and buildings located in wet climates should prioritize high-density EPS boards for all below-grade applications. These products will have a density rating of at least two pounds per cubic foot and ensure proper water resistance levels.

Is Rigid Foam Board Fireproof?

One of the downsides of rigid foam board insulation is that they do not offer superior fire resistance. Other options are far superior, in this regard, like mineral wool insulation, which is often made from rock. For this reason, EPS, XPS, and polyiso foam insulation boards almost always come with some sort of fire retardant.

According to the National Institute of Environmental Health Sciences, some chemical flame retardants are known to cause adverse health effects, including:

  • Immune system impacts
  • Disruption of endocrine and thyroid
  • Reproductive toxicity
  • Cancer
  • Changes to neurologic functioning, and
  • Adverse effects on child and fetal development. 

When searching for the best rigid foam board insulation products, it is always good to prioritize products that use low-toxicity flame retardants like TCPP. You should certainly avoid any product that relies on polybrominated diphenyl ethers or PBDEs.

Quick Comparison of the Best Rigid Foam Board Insulation Products 

Expanded Polystyrene Board (EPS) Insulation

  • Pros: Cheapest option and doesn't deplete the ozone layer, water resistance at higher densities
  • Cons: Lower density products will not offer complete water resistance, lower R-Value than other types of foam
  • R-Value: 3.6 to 4.2 per inch
  • Cost: $0.31 per square foot for one-inch thickness

Extruded Polystyrene Board (XPS) Insulation

  • Pros: Stronger and more water-resistant than EPS, better for below-grade applications
  • Cons: Most products include strong chemical flame retardants. The blowing agents used will deplete the ozone layer
  • R-Value: 5 per inch
  • Cost: $0.47 per square foot for one-inch thickness

Fiberglass Rigid Board Insulation

  • Pros: Higher R-value per inch than traditional fiberglass batt insulation, easier to install
  • Cons: Not water-resistant and will require a vapor barrier installation
  • R-Value: R-11 for 3.5-inch board
  • Cost: $1.25 per square foot
HALO GPS Insulation
GPS Insulation. Photo Credit: HALO

Graphite Polystyrene Board (GPS) Insulation

  • Pros: Inclusion of graphite particles increase the thermal performance compared to EPS, increased vapor permeance to reduce the likelihood of trapped moisture
  • Cons: Costlier option
  • R-Value: 4.5-5 per inch
  • Cost: $14 to $16 per board
Rockwool COMFORTBOARD™ 110
Rockwool COMFORTBOARD™ 110. Photo Credit: Rockwool

Mineral Wool Rigid Board Insulation

  • Pros: Natural option, fireproof, no off-gassing
  • Cons: Dangers when installing. If not correctly installed, tiny slivers can become airborne and be inhaled
  • R-Value: 4.2 per inch
  • Cost: $0.50 per square foot

Polyiso Rigid Board (ISO) Insulation

  • Pros: Highest R-value among foam products, environmentally benign blowing agent, no brominated flame retardant chemicals
  • Cons: Cannot be used for below-grade applications due to a lack of water resistance
  • R-Value: 6.5 per inch of thickness
  • Cost: $0.60 per square foot at a one-inch thickness
Disclaimer: This article does not constitute a product endorsement however Rise does reserve the right to recommend relevant products based on the articles content to provide a more comprehensive experience for the reader.Last Modified: 2021-07-22T17:34:11+0000
Tobias Roberts

Article by:

Tobias Roberts

Tobias runs an agroecology farm and a natural building collective in the mountains of El Salvador. He specializes in earthen construction methods and uses permaculture design methods to integrate structures into the sustainability of the landscape.