Understanding Outsulation: Top 6 FAQs for Rigid Foam Insulation

Outsulation is the construction practice of placing a layer of insulation on the exterior side of the building.  The generic term is rigid foam sheathing or rigid foam insulation, and it plays a direct role in a building’s energy efficiency and performance. Here’s what you need to know about this growing trend.

Frequently Asked Questions about Rigid Foam Insulation

1. What is rigid foam made of?

There are three types of rigid foam insulation:

  • Expanded polystyrene (EPS): This material is also known as beadboard. It has an R-value of 3.6 to 4.0 per inch. It has the least structural strength of the three types of rigid foam. It’s also the least expensive option and the most vapor-permeable of the three types of rigid foam materials.
  • Extruded polystyrene (XPS): This material has an R-value of 4.5 to 5.0. It is less water-absorbing than EPS or Polyiso and is used more often below grade applications under slabs, foundation walls, or basement walls. The cost lies between EPS and Polyiso.
  • Polyisocyanurate (Polyiso): Has an R-value of 6.5-6.8. Polyiso has the abnormal behavior of performing worse as the temperature gets colder.

One thing to note: R-values are determined at 75°F. This means material performance can vary if it’s not at these “ideal” conditions. The blowing agents that makeup polyiso start to condense at cold temperatures reducing its ability to prevent heat transfer (and thereby diminishing it’s stated R-value). Since EPS and XPS are made with different types of agents they maintain their list R-value at colder temperatures.

2. What are the benefits of outsulation?

More effective insulation, better at controlling moisture, and better at preventing air leaks. Read more about the pros and cons of rigid foam sheathing here


3. Do I use Outsulation in place of plywood or OSB sheathing?

It depends on design conditions for things such as wind, earthquake loads – aka “racking” or “shear” or “lateral” loads. Some builders place outsulation directly on top of the plywood or OSB. The rigid foam keeps the interior wood sheathing and framing both dryer and warmer. Other builders choose to forgo the plywood or OSB sheathing entirely. This could be a great cost-saving measure but remember that rigid foam does not have the same structural strength as wood sheathing. You have to compensate for the difference by adding diagonal bracing or inset shear panels.

4. How thick should my rigid foam be?

That depends on the region you’re building in. The Department of Energy offers a national climate zone map that you can reference. Zones 1-4 (with the exception of Zone 4 Marine) don’t need to worry about the thickness of their rigid foam. With colder areas (Zones 5-8) it’s important to choose the correct thickness. If the foam is too thin you run the risk of not warming the interior wall sufficiently while at the same time preventing it from drying to the exterior. The trapped moisture will slowly eat away at the wood framing or sheathing and cause rot. The International Residential Code offers a chart that lists minimum R-values by climate zone.

5. Do I still need to use interior insulation?

It depends entirely on meeting your area’s building codes.  You can do this entirely with outsulation, a hybrid of outsulation and interior insulation, or all interior insulation. Increasingly, building codes are requiring continuous insulation, which is a recent and significant change.  It is important not to trap moisture in the wall which can happen by placing a very low permeable product on the exterior and another very low product on the interior of the wall. To prevent moisture accumulation and subsequent rot, mold, or mildew, walls need to breathe. 

6. Do I still need to use a weather-resistive barrier?

Most likely, but again, that depends on the building code. As long as your rigid foam is the proper thickness for your climate zone and the board seams are properly sealed,  it will act as weather-resistive barrier.  When in doubt,  follow the International Code Council’s strict material and installation policies outlined in document AC71.