Considering Different Types of Insulation

Choosing where and what we insulate with is an integral part of the design and construction process. While insulation may seem like a mundane topic, the ramifications of our decisions can have a huge impact on project cost, the long term cost of owning a home, climate change, and the health of workers and anyone living in a home. 

What is R-Value?

Insulation is most typically discussed in terms of “R-value” which means how thermally resistant a material is. In the simplest interpretation, the higher the R-value, the better the material will keep a house cool in the summer and warm in the winter. Materials with a high R-value are great, but we also need to consider things like cost, availability, the source of the material, degradation over time, energy consumed in production, toxicity, fire resistance, and behavior in the wet & humid Mid-Atlantic climate.

Energy Code Requirements

The minimum insulation requirements for a home are dictated by the construction codes of the local government. Residential construction codes in most places in the United States, (including Alexandria, Arlington, and Washington, DC), are pulled from the International Residential Code (IRC) which references the International Energy Conservation Code (IECC) or simply “energy code.” The code gets updated every three years but jurisdictions adopt updates to the IRC/IECC at different times, so the insulation requirements in Washington, DC might be different than in Arlington, which might be different than the City of Alexandria or Falls Church, etc. 

Energy code requirements are broken up by climate zone. The Mid-Atlantic is in Climate Zone 4. Below is a chart of R-value requirements for walls and ceilings (source this article).   


As of this writing in June 2024, most local jurisdictions still require R-49 for roofs/ceilings and R-20 for exterior walls. Note that batt fiberglass or mineral wool insulations can achieve at best R-15 in 2×4 walls. This is one reason projects now have 2×6 exterior walls. We can still meet code with 2×4 walls as long as we also install R-5 or better insulation on the exterior of the 2×4 walls (see foam board insulations).

Below we breakdown the pros and cons (as we understand them at Rust Co today) using a rough rating criteria in the following categories:

Installed cost per unit R-value

This is how expensive a material is for us to purchase and install to meet a required R-value. 

R-value per inch

How thick the insulation needs to be to meet any given R-value. 

Fire resistance

Is the material flammable or not?

R-value degradation with moisture

Does the material maintain R-value after exposure to moisture?

Vapor resistance

Whether the material allows vapor or moisture to pass through it (depending on the situation/design 

requirements this can be a good or bad thing)

Life cycle energy

Namely, does the material take a lot of energy/limited resources to produce and/or reuse?

Greenhouse gas (GHG) emissions

Some insulations off-gas GHGs during their manufacture, installation in the home, or even after they are installed


The toxicity of the insulation from manufacturer through disposal including how the material impacts the health of the home and people installing it.


Fiberglass Batt

  •       Materials: Made from sand and recycled glass.
  •       Production Impact: Energy-intensive production, but modern methods have improved efficiency.
  •       Installation: Can cause irritation to skin, eyes, and lungs; requires protective gear.
  •       Durability: Long-lasting, up to 50 years but can droop and sag over time. Loses almost all insulation capability if it gets wet. 
  •       Recyclability: Can be recycled, but it’s usually disposed of in landfills.
  •       Overall Impact: Moderate due to high energy use in production and potential health hazards.


Fiberglass insulation is the most common insulation in homes. It is made from sand and recycled glass. In production, fiberglass insulation requires a lot of energy to create, but this has been improved upon in recent years as modern methods have proven to be more efficient. Exposure can cause irritation to your skin, eyes, and lungs – especially during installation or demolition. Because of this, it requires some protective gear to handle. Fiberglass is the most well-rounded insulation type in terms of pricing and efficiency of installation, but it can sag over time in walls leaving gaps, and doesn’t work if it gets wet. We stay away from it when budgets allow, or if we are working in a damp-prone area like a basement.


Mineral Wool (Rock Wool/Slag Wool) Batt 

  •       Materials: Made from basalt rock and industrial slag, a byproduct produced in the smelting of iron and steel.
  •       Production Impact: High energy consumption in production.
  •       Installation: Less irritating than fiberglass, but still requires protection.
  •       Durability: Very durable and fire-resistant.
  •       Recyclability: Recyclable, but often ends up in landfills.
  •       Overall Impact: Moderate, primarily due to energy-intensive production.


Mineral wool or rock/slag wool is made from basalt rock and industrial slag, a waste product of molten metal. It is comparable to fiberglass, but retains its R-value when wet, is less prone to sagging over time, is better at sound dampening, and is less irritating on the skin, lungs, & eyes compared to fiberglass insulation. The downside is that it’s quite a bit more expensive than fiberglass. When the project allows for it, we prefer mineral wool insulation.


Cellulose Insulation

  •       Materials: Made from recycled paper products, like newspapers.
  •       Production Impact: Low energy consumption, eco-friendly.
  •       Installation: Can be dusty, but generally safe to handle with the right protection.
  •       Durability: Good durability, but can settle over time like fiberglass (although less so).
  •       Recyclability: Made from recycled materials and can be composted.
  •       Overall Impact: Low, due to recycled content and low production energy.


Cellulose insulation is one of the most environmentally-friendly forms of insulation. It is made from recycled paper products, such as cardboard and newspaper. Because most of the compound of cellulose is sourced from trees and recycled materials, it is the only known type of insulation that has negative upfront emissions. Other insulations made from materials such as denim or natural fibers carry similar characteristics to cellulose, but they are not as common. Cellulose can be chemically treated to resist water, which can complicate the recycling process and increase the insulations toxicity levels. Cellulose (or any loose fill insulation) needs to be held in place with netting before installing drywall, so we tend to opt for other insulations that stay in place without support before drywall is installed. 


Spray Foam Insulation (Open and Closed Cell Polyurethane Insulation)

  •       Materials: Made from petroleum-based polyurethane.
  •       Production Impact: High energy and chemical use; produces greenhouse gasses and off-gasses volatile organic compounds (VOCs) during and after installation.
  •       Installation: Requires specialized installation and protective gear.
  •       Durability: Highly effective and long-lasting, excellent air seal.
  •       Recyclability: None.
  •       Overall Impact: High, due to chemical use and environmental persistence.


Spray foam insulation is an efficient and effective option for remodeling projects where existing and hard to reach spaces can simply be insulated with a hose  – especially in areas that are prone to dampness (basements) or walls and ceilings where the framing is too thin to meet the energy code with batt insulations. Spray foam insulation is discussed in two forms: 1) open cell and 2) closed cell. Open cell insulation has an R-value per inch that is about 50% of closed cell foam and allows water and vapor to pass through it, unlike closed cell insulation. Both insulations off gas VOCs/ GHGs during and after insulation and are flammable. There are two other really important considerations when using spray foam. First, the stuff is essentially glue and will make it nearly impossible to remodel in the future or fix whatever pipe or wires are embedded in the foam. Second, the vapor and water impermeable characteristics of closed cell foam can cause wood framing to rot if water or water vapor gets trapped within it.


 Rigid Foam Board- Polystyrene (EPS and XPS) Insulation

  •       Materials: Made from petroleum-based polystyrene.
  •       Production Impact: High energy and chemical use; can produce harmful emissions (especially XPS)
  •       Installation: Safe to handle, but cutting can release small particles.
  •       Durability: Very durable and water-resistant.
  •       Recyclability: Technically recyclable, but rarely recycled.
  •       Overall Impact: High, due to chemical use and disposal issues but the manufacturing process for EPS produces far fewer GHGs than the XPS process.


Foam Board- Polyisocyanurate (Polyiso) Insulation

  •       Materials: Made from polyisocyanurate, a type of thermoset plastic.
  •       Production Impact: High energy and chemical use.
  •       Installation: Requires protective gear for closed spaces; off-gassing can occur.
  •       Durability: Very durable, high R-value per inch.
  •       Recyclability: Difficult to recycle; often ends up in landfills.
  •       Overall Impact: High, due to chemical use and environmental persistence.


Foam board insulations are petroleum products with R-value densities and water/vapor permeability characteristics similar to spray foams. However, foam boards are much less toxic to install and can be effectively designed into construction materials, whereas spray foam essentially encapsulates and glues in any space after the fact. Some rigid foam products are structurally rated, like Ox-Is Structural Insulation, or integrated with structural plywood, like Huber’s Zip System R-Sheathing. We also use foam board in situations such as under and against concrete floors in basements, where rigid foam insulation is required. While foam boards have myriad uses, they can be prohibitively expensive for some projects.

Want to learn more about insulation?

Here are additional reference sources:

Upfront Emissions in Home Construction

Environmental Impacts of Spray Foam Insulation

Specifics of Insulation Types

The Carbon Story of Cellulose Insulation

Environmental Considerations in Insulation

Health Concerns of Spray Foam Insulation

Health Effects of UFFI: Evidence of Causation

Health Risks in Homes Insulated With UFFI

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