Johns Manville R30 vs Blow-In Insulation: A Quality Inspector's Honest Comparison

Why I'm Writing This Comparison

I'm a quality and brand compliance manager for a building products distributor. I review data sheets, submittals, and first articles before they go to contractors—roughly 200 product lines a year. In 2024, I rejected about 6% of first deliveries for spec mismatches. One of the most common debates I walk into: Johns Manville R30 batts vs. Johns Manville blow-in insulation for attic spaces.

The vendor failure in March 2023 changed how I think about this. A contractor had specified JM R30 batts, then submitted blow-in as an equal. The installer said it would perform "the same or better." I rejected the submittal because there was no density chart. The owner eventually switched to batts, but the process made me realize how few people actually know how to compare the two.

An R-value on a label is a laboratory number. The real number depends on the person who installs it.

The Comparison Framework

I compare insulation the same way I compare any building product: installed performance, labor sensitivity, moisture and air handling, cost, and what can go wrong on site. The label is only the starting point. Here's what I've learned.

Installed Performance: R30 Batts vs. Blow-In

JM R30 batts are a good product if the cavity is built for them. But R30 is thick—usually about 10 inches. If an installer stuffs an R30 batt into a 5.5-inch 2x6 cavity, that's compression. Compressed fiberglass loses R-value. I've measured compressed R30 batts performing closer to R22. That's not a product defect; it's an installation defect.

Blow-in is different. Johns Manville blow-in insulation achieves R30 only at a specific settled thickness, with a specific number of bags per square foot. The bag chart isn't optional. In March 2023, I inspected an attic where the crew stopped blowing when it "looked full." The settled depth was 9 inches instead of 12. The client paid for R30 and got R25. That's the event that changed how I think about bag counts.

Here's the counterintuitive part: in my inspections, blow-in installations usually end up closer to the rated R-value than batt installations—because batts get compressed, cut around wires, and shoved into corners. Blow-in fills those gaps naturally. But it's only true if someone measures the settled depth after the crew leaves.

Labor Sensitivity: Who Can Screw It Up

Batts are simpler. Roll it out, cut it, fit it. But simple doesn't mean foolproof. I've seen gaps around can lights, batts folded over electrical cable, and double layers stuffed into corners. Every gap is a thermal bypass.

Blow-in needs a machine and a trained crew. It's faster on open attics, but it's messy. I've seen loose fill on top of HVAC equipment because the contractor didn't mask ducts. To be fair, that's poor job management, not an insulation fault.

If you ask me, a typical crew is more likely to install blow-in within spec than batts within spec on an irregular ceiling. The material does the gap-filling for you. But you have to accept the machine setup and the mess.

Moisture, Air Sealing, and Retrofits

Fiberglass is not an air barrier. R30 batts and blow-in both allow air to pass through and around them. If you don't air-seal first, all you're doing is filtering the draft through insulation. I'd argue that's why so many "energy upgrades" don't feel as good as expected.

The fundamentals haven't changed: stop air movement before you install insulation. What has changed is the order of operations. In 2025, I expect an air-sealing checklist before any insulation submittal is approved.

Moisture kills both systems. Batts are easier to pull out and replace after a leak. Blow-in can be re-blown once the moisture source is fixed. Neither should stay wet. I've seen wet insulation that looked fine on the surface but the R-value had already dropped.

Cost, Supply, and the Bag Count Problem

I still kick myself for not counting bags on a 2021 job. If I'd checked the bag count before the crew started, we would have caught a 12-bag shortfall before the drywall went up. Now I count bags on every blow-in submittal.

Batts have predictable pricing. Blow-in has a lower material cost per R-value on a big open attic, but you have to factor in machine rental and labor. In my experience, for a 1,000-square-foot attic at R30, blow-in often comes in 10-20% cheaper in material before equipment. As of early 2025, that's roughly what I'm seeing at distributors in my region, but it changes fast.

Here's something vendors won't tell you: the first quote for blow-in usually includes a bit of breathing room. Once you're a steady customer, the machine rental is often negotiable. With batts, the margin is thinner, so the price is the price.

Under the FTC's R-Value Rule (ftc.gov), insulation labels must state the R-value and the installed thickness or bag count needed to get it. I've rejected product literature that omitted settled thickness. It's not a minor detail—it's a legal claim.

What About Tempered Glass, Shower Shoes, and Liquid Glass?

Tempered glass and shower shoes keep showing up in the same searches as insulation. On a job site, they're connected: a shower remodel often includes tempered glass panels, and the inspector's shoes can damage a new tile pan.

Tempered glass is required for shower doors and enclosures in most U.S. jurisdictions because it breaks into small granules instead of large shards. If you're comparing framed vs. frameless tempered glass, the frameless look is cleaner, but the wall flatness and hinge tolerances are stricter. I've rejected glass panels because the edge radius didn't match the approved submittal. Ask for the tempering certificate before installation, not after.

And yes, wear shower shoes over boots when walking on a freshly waterproofed shower pan. A single piece of grit between your boot and the tile can scratch a pan that costs thousands to replace. It sounds like a small detail until you're writing the non-conformance report.

Liquid glass is a separate thing. It's a nano-coating applied to glass, tile, and metal to repel water. If you're asking, "how to turn off liquid glass," here's the answer: it depends. If it's a smart-glass film, you turn it off electrically. If it's a cured coating, you don't "turn off" a coating—you remove it with the manufacturer's recommended stripper. On screen protectors, isopropyl alcohol and a microfiber cloth usually do the job. On a shower enclosure, call the coating supplier.

Which One Should You Spec?

• Spec R30 batts when: the framing is standard, the cavity is deep enough, the crew isn't going to compress the batts, and you can inspect after installation.
• Spec blow-in when: the attic is open, there are irregular gaps, you want a faster install on large areas, and you'll verify settled depth and bag count.

For a retrofit attic, I'd choose Johns Manville blow-in insulation. For new construction with clean cavities, R30 batts are usually fine. The material isn't the main risk. The installation is.