Choosing Interior or Exterior Insulation: The Pros and Cons

Insulation is one of the most complicated choices builders and architects face during construction. Interior insulation is cost effective, but can reduce usable space and doesn’t protect against water. Exterior insulation is expensive and susceptible to insects. Regardless of the insulation choice, efficiency, toxicity and resiliency must all be taken into account as well. The Mar-flex team has put together a quick “cheat sheet” to outline the differences, advantages and disadvantages of each type of insulation.

ADVANTAGES of insulation on the EXTERIOR of basement walls:

  • Minimizes thermal bridging and reducing heat loss through the foundation

  • Protects the waterproofing membrane from damage during backfilling

  • Serves as a capillary break to moisture intrusion

  • Protects the foundation from the effects of the freeze-thaw cycle

  • Reduces the potential for condensation on surfaces in the basement

  • Conserves indoor space, relative to installing insulation on the interior

DISADVANTAGES of insulation on the EXTERIOR of basement walls:

  • Costs may be high depending on materials and approach selected

  • Expensive for an existing building unless a perimeter drainage system is being installed at the same time

  • May be susceptible to insect infestation, however Mar-flex offers insect resistant materials

  • Contractors may be unfamiliar with proper detailing procedures that are critical to performance

  • Buildings with high radon will need a radon mitigations system to use exterior insulation

ADVANTAGES of insulation on the INTERIOR of basement walls:

  • Interior insulation is much less expensive to install than exterior insulation for existing buildings

  • Almost any insulation type can be used, however rigid foam EPS is recommended

  • The space is isolated from the colder earth more effectively than when using external methods

DISADVANTAGES of insulation on the INTERIOR of basement walls:

  • Many types require a fire-rated covering because they may release toxic gases when ignited

  • Reduces your usable interior space by a few inches

  • Doesn’t protect the waterproofing coating like exterior insulation

  • May become damp if perimeter drainage is poor