Johns Manville Insulation & Roofing: 7 Mistakes I Made So You Don’t Have To

You're here because you've got a Johns Manville order coming up — and you don't want to learn the hard way like I did.

I've been handling Johns Manville product orders for about 6 years now. In my first year alone (2019), I personally made (and documented) 8 significant mistakes, totaling roughly $11,000 in wasted material and labor. Now I maintain our team's pre-order checklist to prevent others from repeating my errors.

Here are the questions I wish someone had answered before I started ordering JM insulation and roofing products.

1. What’s the difference between R-13 and R-15 fiberglass batts — and why did I get it wrong?

My mistake: I ordered R-13 unfaced batts for a 2×4 wall cavity because the spec sheet said “R-13 for 2×4.” That's correct for standard depth. But the wall was actually 2×6 framing with a 1.5-inch continuous insulation layer on the exterior. We needed R-15 in the cavity to hit the total assembly R-value. I didn’t check the full wall build-up.

What I should have done: Look at the whole wall section, not just the cavity. Johns Manville makes R-13 (3.5″ thick) and R-15 (3.5″ but higher-density) batts for 2×4 and 2×6 cavities respectively. When you have exterior continuous insulation, the cavity insulation R-value matters for condensation control. I skipped that step.

Result: 48 batts had to be swapped. $360 in material cost + a 1-day delay. That was back in Q2 2020. I still check the full wall assembly every single time now.

2. “Johns Manville roofing details” — which ones actually matter for a low-slope roof?

When I started, I’d ask the distributor for “Johns Manville roofing details” and get a 200-page PDF. I’d skim it, pick one detail, and order. Big mistake.

The detail that bit me: Edge metal termination at the perimeter. On a 12,000 sq ft roof, I specified the wrong drip edge profile. The contractor installed it, then the inspector flagged it — JM’s detail required a specific cleat and hem at the fascia. We had to re-order 400 linear feet of edge metal and pay for rework.

Cost: $2,100 in materials + $1,800 in labor. I’d skimmed the detail header but didn’t scroll to the “optional” fastener schedule. Turns out, JM’s “recommended” fasteners are actually required for the warranty. That detail existed in the PDF, just not where I was looking.

Now: I pull the specific detail number from the JM Roofing Manual (it's free online), and I print the full page — not just the drawing. The notes are where the gotchas live.

3. Can I use Johns Manville fiberglass duct wrap indoors without a vapor barrier?

This one I'm honestly still not 100% sure about. I've seen conflicting advice from different JM reps. Here's what I learned the hard way:

We installed JM Durawrap (fiberglass duct wrap) on a retrofit project in 2022. The spec sheet said “FSK facing” (foil-scrim-kraft) is the standard facing. I assumed that facing was the vapor barrier. It is — but only if you seal all the seams with the correct tape. We used standard duct tape. The wrap got wet behind the facing, and we had to tear it out after 6 months.

I called JM tech support. Turns out, Durawrap with FSK facing does act as a vapor barrier when taped properly using JM’s approved tape (Part #TC-100). Using generic tape voids the performance. I didn't know that. Whoops.

My rule now: If you're using any duct wrap indoors, just assume you need a separate vapor barrier unless the facing is specifically listed as “vapor retarder” in the JM data sheet. (Source: JM Durawrap Technical Data Sheet, Rev. 2021).

4. What’s the deal with white kitchen cabinets? (Wait — not my question, but I’ll answer it anyway.)

Okay, this doesn't have much to do with Johns Manville, but a lot of remodelers buy both insulation and cabinets from the same supplier. If you're here for the RLW keyword — white kitchen cabinets — I can't help with that. But I can tell you: if you're insulating around a kitchen, use JM’s Sprayed Polyurethane Foam (Closed-Cell) for behind the cabinets. It gives you an air seal and R-6 per inch without adding bulk that complicates cabinet installation. That’s actually relevant because I once ordered fiberglass batts behind a cabinet run, and the cabinet installers had to cut every batt to fit around pipes. Took twice as long.

Not a cabinet expert. But I've learned that planning the insulation before the cabinets go in saves a ton of headache.

5. How do I fix a mistake with shower head hose length? (Staying on topic: moisture behind shower walls.)

Another off-topic query, but here’s the insulation connection: if you’re installing a shower and you use Johns Manville GoBoard (their waterproofing tile backer), the joint treatment is critical. I once installed GoBoard around a shower valve, but I didn't seal the cutout for the shower head hose perfectly. Water seeped behind the tile, and we had to rip out 4 sheets.

Cost: $1,400 in GoBoard + labor. I should have used JM’s recommended sealant and the specialized GoBoard tape. I thought “it's just a small hole.” Nope.

Lesson: GoBoard is great, but treat every penetration like a potential leak. Exceed the sealing requirements — your future self will thank you.

6. How to remove wallpaper glue from walls before insulation? (Yes, oddly specific.)

This one actually saved me. On a retrofit project, the old wall had wallpaper glue residue. I didn't think it mattered for insulation. I sprayed closed-cell foam directly over the glue. The foam didn’t bond well; a year later, the foam shrank and left gaps.

Johns Manville’s Spray Foam Installation Guide (I finally read it) says: “Surface must be clean, dry, and free of dust, grease, and adhesive residues.” Oops. I hadn't removed the glue. The fix: scrape the glue, then apply a primer. That would have cost me an extra 2 hours per room.

So if you're asking “how to remove wallpaper glue” — use a steamer or a chemical stripper, then wash with TSP. Don't skip it if you're foam-insulating after.

7. Is Johns Manville R-13 unfaced the same as R-13 kraft-faced?

No. And I made that assumption on a job in Q1 2023. I ordered unfaced because the wall already had a vapor barrier. But the contractor installed it on an exterior wall that was behind brick — no interior vapor barrier needed, but the kraft-faced would have acted as a vapor retarder. Unfaced allowed moisture vapor to pass through, which led to condensation inside the wall during winter. We caught it during inspection because the wall cavity showed moisture readings.

We tore out 600 sq ft of insulation and re-did it with Kraft-faced. Cost: $940 in material + $1,100 labor.

Rule of thumb: Use kraft-faced where you need a vapor retarder on the warm side of the wall. Use unfaced where the assembly is already vapor-tight. Check the local building code — actually, check it before you order. (Source: Johns Manville Insulation Installation Guide for Residential and Commercial, 2022).

This was accurate as of Q4 2024. Building codes and JM product specs change, so verify current requirements with your distributor or JM tech support. I learned these lessons the hard way — hope they save you some headache.