Spray Foam vs. FiberglassWhat the Data Shows

Same wall cavity, two very different materials.

Fiberglass batts and spray foam both carry an R-value, so it is tempting to compare them on that number alone. The published research says that is only part of the story. Fiberglass slows heat moving through it. Spray foam does that too, but it also stops air from moving through the cavity in the first place, and in a cold, windy climate like Minnesota's, that second part carries a lot of the actual energy savings.

The figures below come from the U.S. Department of Energy, EPA ENERGY STAR, and a 2011 Oak Ridge National Laboratory (ORNL) field study of spray foam retrofits. Every number links to its source. None of it describes a Trust Works project; it is the industry research that shapes how we recommend one material over another.

Where the raw insulating power comes from.

R-value per inch measures how well a material resists heat flow, independent of air sealing. This is the number most often printed on a bag or a spec sheet.

Closed-cell foam roughly doubles the R-value per inch of fiberglass, which matters most where cavity depth is limited, like a rim joist or a shallow roofline.

Sources: U.S. DOE, Guide to Home Insulation (DOE/EE-0340, 2010), Oncor Electric Delivery, Insulation R-Values per Inch (sourced to DOE.gov), ORNL/TM-2011/437, Spray Foam in Accessible Spaces, Table 1 (ASHRAE 2009 Handbook of Fundamentals)

Fiberglass does not stop moving air.

A fiberglass batt is a filter, not a barrier. Push air through it or around its edges, gaps at electrical boxes, top plates, rim joists, and the batt's R-value stops mattering much, because the heat is leaving with the moving air rather than passing through the material itself. Spray foam expands to fill the cavity and bonds to the framing, so it insulates and air-seals in the same pass.

Oak Ridge National Laboratory measured this directly with blower-door tests, which depressurize a house and measure how much air leaks through the envelope, reported as air changes per hour at 50 pascals (ACH50). Lower is tighter.

Between 2009 and 2011, ORNL and the Tennessee Valley Authority tracked blower-door results across a group of older East Tennessee homes as spray foam was added.

A lower ACH50 means less conditioned air is leaking out, and less cold outside air is leaking in, which is the mechanism behind most of the winter comfort improvement homeowners report after a spray foam retrofit.

Sources: ORNL/TM-2011/437, Spray Foam in Accessible Spaces (Christian & Gant, Oct. 2011)

Where the 15 percent savings figure comes from.

You will see energy-savings claims from 10 to 20 percent attached to insulation projects, and the range exists because different studies model different starting points. EPA's original 2009 guidance put the ceiling at up to 20 percent on heating and cooling costs, or up to 10 percent on total annual energy bills, from sealing air leaks and adding insulation together. ENERGY STAR's current methodology, modeled against a typical 1970 to 1989 home, states an average of 15 percent on heating and cooling costs and 11 percent on total energy costs.

Both figures describe sealing and insulating together, not spray foam specifically. That is the point: batt insulation alone usually only delivers the insulating half of that equation, since it does not address the air leakage EPA and ENERGY STAR are counting. A material that air-seals and insulates in one step, like spray foam, is built to capture more of that combined savings figure in a single job.

Sources: ENERGY STAR, Methodology for Estimated Energy Savings, U.S. EPA, Seal and Insulate with ENERGY STAR (EPA 430-F-09-052, 2009)

Why the gap widens in our winters.

Savage sits in IECC Climate Zone 6A, a cold, heating-dominated zone where the outdoor design temperature drops well below zero and heating degree days run high all winter. The bigger the temperature difference between inside and outside, the harder both conducted heat loss and air leakage work against a house, which is why the same batt-versus-foam comparison shows a larger real-world gap here than it would in a mild climate.

This page is a summary of published research, not a claim about any specific Trust Works job. What it does is explain the reasoning behind a recommendation: in this climate, air sealing is not optional extra credit, it is where a large share of the available savings actually lives.

Sources: U.S. DOE Building America, Climate Region Guide