exposed rock wool insualtion in a shop attic

Any thoughts on rock wool insulation in the rafters of a garage shop. rockwool so it is fire safe. I have thin plywood between the collar ties as a ceiling so the insulation is not going to see too much dust or oil fumes.
Bill D.

I think it will increase the heating bill if you remove the rock wool.

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They don’t like to insulate the rafters, the roof gets too hot and the shingles and felt don’t last. Instead they like to insulate the ceiling and ventilate the attic.


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You're in Modesto, so you won't have the typical cold-climate problem where the roof sheathing is cold and moisture condenses on the inside of the sheathing.

While vented attics with uninsulated roof and insulated floors are extremely common, there's no reason you can't have an unvented attic with insulated roof. It's not that unusual in modern construction for the attic (and crawlspace/basement) to be inside the conditioned envelope, and maintained at interior temperature and humidity. However, if you do that in a cold climate, you have to do at least one of two things: 1) prevent moisture from coming in contact with the sheathing (i.e., spray a layer of closed cell foam on the inside), or 2) keep the sheathing temperature above the dew point (i.e., insulate the outside of the sheathing sufficiently).

You are correct tthe underside of the sheathing never gets cold enough to visably condense water. I never considered that fact. My plan is just to insualate the west facing roof slope to reduce heat load ins ummer. I will see if that is enough improvement or if I need to do more. Next step would be reflective aluminum roll unde rthe rockwool to reduce radiant heating to the inside.

Bill D said:

You are correct the underside of the sheathing never gets cold enough to visably condense water.

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Here's the part that's unintuitive. If you insulate the inside of your sheathing (in a cold weather climate), the sheathing will get colder because there's less heat loss from inside keeping it warm. So you could go from uninsulated with big heat loss and no condensation (because the sheathing is at inside temperature), to insulated with low heat loss and hidden condensation leading to mold and rot (because the sheathing is at outside temperature).

If you wonder why ordinary insulated walls don't show this problem, they traditionally weren't insulated as heavily as roofs, so the wall sheathing didn't spend much time below the dew point, and a more vapor-permeable exterior finish (more so than roofing felt and a continuous layer of shingles) allowed the sheathing to dry to the outside. In modern "super-insulated" construction, this problem could show up in any wall, not just roofs.

Warm&wet-side condensation on cold sheathing also affects always-on-AC climates such as the muggy South, but the condensation forms on the outside of sheating, rather than the inside.

Again, you don't expect to see this in your location.

This is why we use vapor barriers on the "warm in winter" side of the insulation. People generate moisture. Processes generate moisture. So in the mass of the country, we use a vapor barrier over the insulation on the inside of the house. Even drywall helps, and can serve as a vapor barrier with some care regards penetrations like electrical boxes.
In say south Florida, it is reversed. The vapor barrier goes on the outside of the insulation.
I got a call down there one time to go look at a historic house owned by Dade Co. (Miami). The floors were buckling. I went inside and it was like an icebox. Why? "So we can preserve all the furniture from changes in humidity.". Under the house, the floor sheathing was dripping water.
Ranger asked what he could do. I suggested he unholster his side arm and take some practice shots on all the AC compressors, sell them for scrap, and buy some dehumidifiers.
There was no real way to insulate under a wood house and have an effective vapor barrier at that time. Closed cell foam would work now.
About the shingles, correct, do not insulate the underside of the roof sheathing. This will void the warranty on the shingles and make them die an early death. Insulate at ceiling joist level, and put in a vapor barrier. Ventilate the attic. If the plywood layer is tight, it might do for your vapor barrier.
If you must insulate under the sheathing, use metal roofing or low slope membrane roofing. But at that point, you can put continuous insulation on top of the sheathing and get about twice the bang for the buck.
In this climate, to comply with the Energy Code, I can do R-20 continuous on top of the deck, or R-38 between framing members. Both work out about the same in the calculator.
Note Modesto is in 3B zone. The B is for dry.
http://reca-codes.org/images/ieccmap.jpg

I'll wager that the exposed rock wool would make it much quieter inside the shop.

Not as good as Sonex Panels, but pretty darn good.

True. I used rockwool inside an OSB box to make the whine of my old blue PhasePerfect tolerable. Was quite effective. Turns out I'm now deaf enough that I can't hear that whine anymore. :-/