Q&A: Filling the Gaps - Literally. Batt Insulation, Trusses, and R-Value Real Talk
When a master electrician writes in to double-check your roof insulation strategy, you know you’ve struck a nerve and a shared frustration with spray foam.
Charlie P. reached out after watching one of our Rockwool Roundtables, where Travis walked through a high-performance roof assembly from a recent build with Joe Lstiburek. The project featured 5" of Rockwool Comfortboard 110 on top of the roof deck plus R-38 batts installed below the sheathing, between the trusses.
That combo sparked a question:
“If you’ve got trusses made from 2x4s 16” on center, and you’re installing R-38 batts between them-but they don’t fill the entire cavity-what do you do about the gap between the trusses? Do you leave it? Fill it? And how do you keep the batts from falling out?”
Let’s break it down.
First: How We Held the Batts in Place
We used good ol’ nylon strap, the kind you’d use to suspend flexible ductwork.
Staple it to one side of the truss, pop in the batt, then staple the other side. Done. The strap holds the batt snug up against the sheathing. It’s simple, low-cost, and solid.
Now About That Gap...
With 2x4 top chords and 14.5" wide batts, you’re left with a 1.5” void between the batt and each truss. That’s air space, not insulation.
We debated filling it too. But here’s the key: the entire roof assembly is already airtight, and we’ve got 5” of rigid Rockwool continuous insulation on top. In other words, the air isn’t moving in that 1.5"—it’s just sitting there. And still air has an R-value of about 1 per inch, just like wood.
So when you add it all up, here’s what we’ve got at the worst-performing part of the assembly (right at the truss):
R-20 (rigid Rockwool above)
1/2” roof sheathing
3.5” wood truss
~1.5” of still air below
R-38 batts in between
That works out to about R-30 every 16", and R-58 in between the framing. With that framing factor in play, the overall effective R-value lands about 11–12% lower than the full R-58. But we’ll take that tradeoff for durability and ease of install over a foam gamble any day.
Why We Steered Clear of Spray Foam
Charlie’s not alone in his skepticism. We’ve seen too many spray foam jobs go sideways:
Inconsistent application
Improper mix ratios
Fire risks during curing
Technicians who (let’s be honest) aren’t always dialing in the details
Even when the design calls for unvented “hot roofs,” foam often means multiple lifts, multiple crews, and multiple callbacks. It’s tricky, messy, and expensive to fix when it goes wrong.
And to match even the worst-performing section of our assembly using foam? You’d need:
5” of closed-cell (that’s three passes minimum), or
2” of closed-cell + 4-5” of open-cell, which means two different foams, trucks, and mixes
To hit R-58? Try nine lifts of closed-cell. No thanks.
Bottom Line:
If you’re going with Rockwool below the deck and continuous Comfortboard above, don’t stress the truss gap. You’ve already won the battle with air sealing and smart layering.
This isn’t a perfect lab scenario - it’s a real-world, high-performance roof. And it works.