Johns Manville Insulation FAQ: Owens Corning vs. JM, Vapor Barriers, GoBoard Shower Niches, and More

Johns Manville product questions I've heard too many times to ignore

I spent twelve years at a distributor counter outside Boston, helping contractors order Johns Manville insulation, roofing, and GoBoard tile backer. I also made my own list of expensive lessons. This FAQ is the one that would have saved me a few phone calls and roughly $2,800 in wasted labor from one bad vapor-retarder call.

Owens Corning vs Johns Manville insulation: Which one is better?

There is no honest one-line answer to that question. Both Owens Corning and Johns Manville make quality insulation products, and each manufacturer has products engineered for a specific assembly. What matters on the job is whether the R-value, facer, and fire rating match the spec, not which brand name is printed on the bag.

I can only speak to what I saw from the distributor side. Product availability decided more jobs than brand loyalty ever did. A contractor who needed a full pallet yesterday chose the manufacturer with stock on the floor. Sometimes that was Johns Manville and sometimes it was Owens Corning. When a project required a foil-facing or a certain density, we read the data sheet instead of assuming the brands were interchangeable.

The honest answer: compare materials, not marketing. If the design names one manufacturer, get the substitution approved before the truck leaves, because that is way cheaper than a tear-off.

Where does a Johns Manville vapor barrier actually belong?

Start with this: 'vapor barrier' is a building function, not always a specific product. Some Johns Manville insulation products include a foil or kraft facing that acts as a vapor retarder. A polyethylene sheet can do the same job in a different location, but that does not mean you can treat them as interchangeable.

In most heating-dominated climates, a vapor retarder belongs on the warm-in-winter side of the wall cavity, usually the interior. In hot-humid climates, the local code may call for a different location or no vapor retarder at all. The simple rule I use for wood-frame walls is one vapor retarder on the warm side unless the designer says otherwise.

My own 2017 mistake involved a project that had a poly vapor barrier on the interior while I approved a foil-faced board for the exterior. Two vapor retarders on opposite sides of the same wall made a moisture sandwich. We caught it before the drywall went up, but only because the contractor stopped and asked why he had two materials doing the same job. That question cost the job roughly $2,800 in rescheduling labor and a three-day delay.

Not every assembly needs that level of caution. But if you see foil on one side of a wall and poly on the other, stop. Ask how the wall is supposed to dry out.

Do roof assemblies need the same vapor barrier as walls?

No, but they still need a moisture plan. In a commercial low-slope roof, the usual concern is warm interior moisture rising to a cold deck. Some specs place a vapor retarder on the deck, below the insulation. Others rely on the membrane system and ventilation details to move moisture out. The sequence is not the same as a wall assembly.

If I tried to name the one correct vapor barrier for every roof, I would be guessing. Roof specs depend on climate, building use, deck type, and membrane. My advice from experience: read the roof system data sheet and ask the membrane manufacturer for its recommended assembly. A vapor barrier in the wrong layer can trap moisture just as badly as no vapor barrier at all.

Shower niche trouble: Is GoBoard the right backer board?

If you are building a shower niche with Johns Manville GoBoard, the board is usually not the weak point. GoBoard is a waterproof tile backer board, so you can create the niche without stacking cement board and a separate liquid waterproofing layer on top. The mistakes start when the details get skipped: seams not treated, fastener heads left unsealed, or a niche floor that is dead flat.

Fasten the GoBoard to the framing and follow the current Johns Manville instructions for joints and penetrations. Set the niche floor with a slight pitch toward the shower side. Water is not polite; it will find the low spot.

One more thing: do not automatically add a plastic vapor barrier behind GoBoard in a normal shower installation. The board is already doing the water-control job. An extra poly layer can create a second moisture plane and make a future leak harder to trace. If an engineer requires a vapor retarder for a special assembly, make them explain why.

Why do roofers wear a Boston scally cap?

Fair warning: a Boston scally cap is not a Johns Manville product and it is not part of the roofing assembly. But it shows up in the same conversations, so here is the practical answer.

A scally cap is a flat wool cap with a short brim. Roofers like it in cold weather because it sits tight on the head, keeps the ears a little warmer, and does not get in the way. A warm head makes for a more careful roofer. It is not a hard hat and should never replace required head protection, but it is a decent piece of winter field gear.

If you are on a roof installing membrane in February, dress for the weather. The cap is comfort and style, not a specification item.

How to get rid of fleas in house?

That search phrase looks out of place on a Johns Manville insulation FAQ, but it lands here more often than you might think. The honest answer is: insulation does not create fleas and it does not cure them.

Fleas feed on animals. If fleas are in the house, the host is usually a pet, a stray animal, or wildlife that found a way into the crawl space or attic. A fiberglass batt has no food value to a flea. What it can do is hide an animal problem if it is laid over droppings, nesting material, or a dead animal.

• Remove any animal activity before installing new insulation. That includes droppings, nests, and carcasses.
• Treat pets and vacuum the house thoroughly. Flea pupae can survive an ordinary spray and keep hatching.
• Seal the structural gaps after the animal issue is handled.
• For a heavy infestation, call a licensed pest control company instead of relying on the insulation installer.

A sealed crawl space with a vapor barrier and the right insulation can make the space drier and less inviting to wildlife. But that layer is a moisture and air control system, not a pesticide. Johns Manville products are not designed to kill fleas. Replacing insulation without solving the host problem will not work.