The Art of Basement Wall Insulation: Turning the Corner with InSoFast

I think I’m finally turning the corner.

After months of planning the retrofit of the basement—people are finally showing up—and the project is starting to resemble something finished, or at least finish-able. The floor, originally dreamed up by my friend and architect Derk (whose enthusiasm is contagious, but not budget-conscious), went through several design iterations before landing where it probably should have started. Something you can walk on, sweep, even rest—now that it has new in-floor heating, insulation and a vapor barrier underneath.

But the real prize so far in this retrofit story is the walls.

In terms of energy efficiency, insulating the basement walls is the biggest single move I could make. In Minnesota, basement wall insulation tops the list of energy upgrades by potential impact.

How do you insulate basement walls without encouraging moisture—and everything that comes with it?

Enter InSoFast.

As a retrofit solution, it’s particularly appealing. You need relatively clear walls to start, but beyond that, it checks off just about every concern a homeowner might have about insulating a basement—which is a long list.

The panels themselves are lightweight and click together in a satisfying way—think adult jigsaw puzzle meets vapor barrier. Each one is 2.5 inches thick, with an R-value of 10.5 built in. But thanks to the continuous insulation—top to bottom, side to side, corner to corner—the performance is even better in practice.

Our home is now currently at a 7 out of 10 according to the unofficial Home Energy Score I ran.

"Are you doing this work yourself?" people ask. I’m tempted to say yes, given the number of hours spent planning every step, drafting scopes for contractors, and waking up at 5 a.m. to consult AI with my latest worry. Fortunately, I have an A+ general contractor to help.

My goal is nearly reached, a 50% carbon emission savings, but more importantly, the core systems of wiring, heating and insulation are now up to modern standards as outlined in Green Home Club’s homeowner retrofit course. This is not easy to do for any old home. Things must be done correctly.

“Are we sealing the tops?” I ask the general contractor, Better Builds Minnesota in regards to the InSoFast panels.

“Yes,” they reply.  

“What about the bottoms?” I ask.

They answer yes to that as well.

“Sealing the seams?”  (You almost don’t have to, the panels fit together that well.)

I also expressed concerns about InSoFast’s durability to Keith, the lead carpenter at Better Builds, and he came up with the idea of adding subfloor panels to the area below.

What We’ve Got Now

Base layer: Cinder block (uninsulated, vapor-open).
Insulation: InSoFast EPS panels (2.5"), providing insulation, drainage, air sealing, and built-in stud strips.
Lower half: Covered with AdvanTech subfloor panels—not necessary, but I wanted the durability and impact protection.

So because of the AdvanTech, there will be a ¾" step between the upper and lower drywall after it is applied over the InSoFast.

To fix this, we will use a horizontal trim board or ledge as a transition—think chair rail or wainscoting. Bonus: I plan to use the trim as a marker for the InSoFast built-in wall studs, so if someone wants to hang a picture later, they’ll know exactly where to find those structures. Call it practical design.

Continuous insulation: it's a beautiful thing in a basement.

Moisture, however, is not.

In Minnesota, when it hits sixteen below, the basement walls are going to get cold. That’s not hypothetical—it’s reality. So the insulation needs to do more than insulate. It needs to manage moisture, or at least play nicely with it. InSoFast panels have built-in drainage channels on the back, so if moisture gets in—and it will with laundry, showers, maybe someday, cooking—there’s a path for it to dry out, drain, move on.

I still think the best defense is offense so we will be extra careful with sealing the panels, but just in case, I’ll add a dehumidifier (Energy Star, of course) and an ERV running continuously.