Question

We are considering foam insulation for our two car garage. What are the costs associated with this and any pros and cons?

Asked by Kristi
Newnan, GA

We would like a green insulation solution and would like to compare the benefits and costs of products.

Answer

Spray polyurethane foam insulation offers great thermal performance when compared to fibrous insulation materials.

Foam insulation

The benefits of foam insulation can offset the increased upfront cost when climate offers high Delta-T's (temperature difference between outdoors and indoors).

  • Depending on location you can expect that foam insulation would be two to three times more expensive than cellulose or BIBS (dense-packed fiberglass).
  • Since you are south of Atlanta I would suspect that the Delta T's are lower than more northern or southern states. It would be difficult to justify a recommendation for spray-applied foam insulation.

Cellulose insulation

As an effective alternative you might want to air seal all ceiling penetrations with fire-rated foam sealant before installing cellulose (recycled newspaper).

  • Cellulose is also dense enough to reduce convective loops in the walls in your climate zone.
  • If you use damp cellulose in your walls make sure it doesn't get covered with drywall until the moisture content is below manufacturer's recommendation.
  • Any wood to concrete joints in the wall/floor structure and lumber joints should be sealed to prevent air penetration before insulating the walls.

Blown in blanket

An alternative to to cellulose would be a blown in blanket system. The installer installs netting over the wall studs before blowing fiberglass insulation at a density required to meet the R-value you contracted.

  • My recommendation would be R-15 for a 2x 4 wall assembly.
  • Certainteed Insul-Safe III is an effective insulation when installed at 2-pounds per cubic foot.

The secret to performance success is to install a material that is dense enough to reduce convection in the wall cavity. Fiberglass and other fibrous blanket material is not an ideal substitute.

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