Rigid Foam Insulation Pros and Cons

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R-values of rigid and liquid foam insulationThere are several types of foam insulation used in the walls and ceiling of residential and commercial buildings: rigid foam, rigid board made of mineral wool and liquid foam alternatives . Each has certain advantages and disadvantages. This article looks at the first option – rigid foam insulation.

What is it?

Rigid foam insulation, also called foam board or board-stock, is made from foam plastics and is a popular choice for insulating foundations and building exteriors. It can be used inside wall cavities as well. It is sold in rigid board-style sheets of varying sizes. Rigid foam can be supplied with a foil facing to further improve its insulating qualities.


  • Rigid foam offers very high R-values compared to loose-fill insulation, and some of the best among foam options. Ratings range from R-4 to R-6.5 per inch, even without a foil facing to improve matters.
  • Some rigid foam products are water resistant and can be buried to insulate foundation exteriors.
  • Expanded polystyrene board ( EPS or "beadboard" ) is the only type which does not use HCFCs in its production, so is the greenest choice.
  • EPS can be bought with foil and plastic facings so that it is water resistant and can be used underground.
  • Extruded polystyrene board (XPS or "blueboard") has a slightly higher R-value than EPS and is more resistant to moisture.
  • Polyisocyanurate board ("polyiso") offers even better insulating value, reaching from R-6.5 to a massive R-8 per inch. It is the least ecological option (see below).
  • Polyiso board is manufactured with various facings (plastic or aluminum, for example) to further improve its R-value.



  • Rigid foam insulation in wall cavities must be tightly fitted to stop air infiltration.
  • Joints between sheets and boards must be taped to prevent air flow.
  • Rigid foam is susceptible to sunlight. UV rays damage it, so it must be stored and installed appropriately.
  • The air bubbles inside expanded polystyrene board (EPS or beadboard) stop heat transfer but can accumulate moisture and thus become ineffective. A moisture barrier may be needed, depending on the installation location.
  • Basic beadboard is too brittle to be used underground.
  • Extruded polystyrene board (XPS or blueboard) uses HCFCs in its production, which deplete the ozone layer.
  • Polyisocyanurate board (polyiso) uses the worst HCFCs in its production.
  • Polyiso suffers from decreasing R-values over time. At installation it can reach R-9 per inch but, over the following 2 years or so, will reduce to R-7. Foil facing adds R-2 to that.