Both packing peanuts and certain types of foam board insulation are made from expanded polystyrene (EPS). Yet, packing peanuts are cheaper to purchase, even in their hundreds or thousands. In addition, how often do you get them for free in your packages?
To understand why the substitution cannot be made, you need to know that the design features that make packing peanuts great for cushioning make them terrible for insulating houses. In addition to the thermal inefficiency of packing peanuts, there are also fire risks and issues with code compliance.
While packing peanuts and EPS foam boards are made from similar materials, packing peanuts are designed to fit together with gaps in between them. These gaps create airflow that severely limits the potential insulating properties of the foam.
Packing Peanuts Are Used for Cushioning
Packing peanuts are meant to cushion items inside a package, not to insulate them from heat or keep heat in. This means that their whole design is aimed at cushioning.
The design choices are all based on how objects move around in boxes, rather than how air flows and temperature changes.
The following qualities of packing peanuts make them good at cushioning:
- Small individual pieces. Unlike a foam block or sheet, the “peanut” form is intended to create gaps between units. These air-filled gaps create extra space for compression, absorbing the shock of items jostling against the peanuts.
- Soft and flexible. Like air-filled gaps between peanuts, air-filled pockets within the foam also helps them to absorb shock. Peanuts are bendable, not rigid, so they can squish and shift around certain items.
- Interlocking shape. The interlocking curves of the peanuts create a kind of structure that holds space for air in between peanuts (rather than having straight peanuts that get smashed together).
- Lightweight. Peanuts do not add significant weight to the package. This also allows them to get pushed around and make their way into nooks and crannies.
Materials of Packing Peanut Can Be Insulating
Packing peanuts are expanded polystyrene (EPS), which you probably think of as styrofoam.
Styrofoam actually refers to a trademarked type of extruded polystyrene (XPS), not what most people think of when they use the word “styrofoam”. That’s a whole other story, but it demonstrates how confusing the world of foam can be.
Expanded polystyrene also comes in the form of rigid foam boards for insulation.
Packing peanuts and some types of foam board insulation are made from the same materials, which can lead you to think that they will work in the same way.
However, some of the design features that make packing peanuts good for cushioning boxes actually make them bad for insulation.
The gaps between peanuts provide shock-absorbency. But when used as insulation, those gaps will allow for airflow. Air passing through the walls or ceiling will conduct heat in and out of the building.
Using peanuts as insulation is a controversial topic on homebuilding forums, where some contributors have suggested combining peanuts with blown-in cellulose to fill the voids.
However, even if you could perfectly fill in gaps with cellulose, the expanded polystyrene peanuts would not have the same R-value as expanded polystyrene board. You could not compress the peanuts enough to achieve the same effect as a board.
Another problem is that many packing peanuts are now made from cornstarch-based biodegradable foam. If you use cornstarch-based foam as insulation and it gets wet, it will melt and break down, get moldy, and provide food for insects.
It’s also worth noting that expanded polystyrene is not necessarily the best foam for insulation in the first place. Even in the form of rigid boards, expanded polystyrene has a lower R-value than XPS or polyurethane foam.
Code-Compliant Insulation Is Listed and Labeled
In addition to not being effective, packing peanuts are not approved as insulation. For insulation to meet the standards set by building codes, it has to be listed and labeled.
That means that it needs to be found suitable for use as insulation. Packing peanuts have not been found suitable for insulation, as they are not marketed, tested, or used by contractors in that way.
Code-compliant insulation is tested to establish its R-value. Building codes require minimum R-values depending on the part of your home (wall vs. ceiling) and your geographic location.
No R-value has been assigned to packing peanuts because they are not meant to be used as insulation.
Foam insulation also goes through very specific fire testing required by the International Residential Code Section R316. Packing peanuts are not tested for how they would behave in a house fire and most peanuts lack the flame retardants that are used in some foam insulation.
If you rely on packing peanuts as insulation, inspectors will consider your home insufficiently insulated and not code-compliant. Ultimately, packing peanuts are suitable for use as packing peanuts, not insulation.