The short answer
Spray foam insulates more per inch and seals air leaks, which fiberglass does not, but it costs more. Closed cell spray foam is roughly R 6 to 7 per inch, open cell roughly R 3.5 to 3.8 per inch, and fiberglass batts roughly R 3 to 4.3 per inch. Spray foam makes the most sense in open walls during a remodel, rim joists and spaces where air leaks matter; fiberglass is the budget friendly choice for standard walls and attics.
R value per inch
Closed cell spray foam packs the most insulation into a wall cavity, roughly R 6 to 7 per inch. Open cell foam is lower, around R 3.5 to 3.8 per inch. Fiberglass batts run roughly R 3 to 4.3 per inch depending on the product.
Air sealing is the real difference
Fiberglass slows heat moving through a wall, but air can still leak around and through it. Spray foam expands to fill gaps and seals air leaks at the same time. In a drafty older Michigan home, stopping air leaks can matter as much as adding R value.
Moisture
Closed cell foam also acts as a vapor retarder at typical thicknesses. Fiberglass needs a proper vapor retarder and air sealing to perform well in a cold climate like ours.
Cost
Spray foam costs noticeably more per square foot than fiberglass. The extra cost is easiest to justify when walls are already open during a remodel, because that is the only time you can get foam into every bay.
What code asks for in our climate
Southwest Michigan sits in climate zone 5. As a rough guide, current energy codes for this zone call for around R 49 in attics and R 20 in wood framed walls. Check with your local building department for what applies to your project.
Where we use each
On our bathroom remodel with new windows, the walls were open, so we sprayed foam into every bay before the shower went in. For a quick attic top up or standard walls, fiberglass is often the practical choice.
When to call a pro
If your walls are already open for a remodel, that is the moment to decide. Quality Finish insulates homes, shops and outbuildings across Southwest Michigan. Call or text for a free estimate.
Insulation in Southwest MichiganCommon questions
Is spray foam better than fiberglass?+
It insulates more per inch and seals air leaks, but costs more. Which is better depends on the space and your budget.
Can you spray foam existing closed walls?+
Not the way it is sprayed in open walls. Spray foam works best when walls are open, such as during a remodel or addition.
What R value do I need in Michigan?+
Southwest Michigan is climate zone 5, where codes call for around R 49 in attics and R 20 in walls as a rough guide. Confirm with your building department.