Drywall directly over semi-rigid rockwool panel?
We are in the planning phase of an addition to our home. For the exterior walls, I am planning to utilize 2X6 studs with 1/2” plywood panel on the outside, to be covered by a siding yet to be determined. The stud cavity will have 5.5” thickness mineral wool batts. Here’s the question: Does a rigid panel form of mineral wool exist that I can install over the interior of the batt filled stud wall? The plan is to directly cover the rigid panel with gypsum wall board. My concern is whether or not a “rigid” mineral wool panel is actually rigid enough to allow the drywall screws to hold firm without popping. The reason I am pondering a rigid type of mineral wool panel rather than polyiso is only because of flammability concerns. Would it be advisable for any reason to use furring strips atop the rigid panel before installing the drywall?
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There is *semi-rigid* mineral wool, the Rockwool ComfortBoard line. It's considerably more expensive for the R-value than their lower density stuff though. I'm sure it has the tensile strength to support the drywall sheets and break the thermal bridge with the studs, but I'm a lot less sure it has the tensile strength to support the joint compound in the drywall seams if a human being leans on the wall.
You can have furring strips involved in several constructs:
* For shear reinforcement in unsheathed walls
* On the exterior of the wall with the 'vented rainscreen' idea, which sounds like it improves the moisture resistance of the wall sufficiently well to recommend in all instances
* On the interior of the wall supporting the drywall with the 'service cavity' idea, which sounds like it makes airsealing layers safer against punctures & in a double batten form makes running electrical & plumbing much easier
* In a ceiling (this is standard in New England) to hold up the weight of attic insulation
* In several types of roof for much the same reason as vented rainscreens
* As a way to thermally break a wall by offsetting structural supports.
* In ceilings and walls by finish carpenters using thinner grades of drywall, to help straighten it.
https://foursevenfive.com/blog/the-service-cavity-making-airtight-construction-easy-2/
https://blog.rossroofgroup.com/tilcorusa/benefits-of-roofing-using-a-batten-system-over-sheathing-ventilation
Good point about whether or not the drywall seams could withstand force against them. Not having firsthand experience with the Rockwool comfortboard, I could only speculate as to how resistant it is to crushing under the weight of a person pushing against the wall. But, I do recall an online video where it is being used externally as sheathing, with furring strips used over it for siding to be attached. Siding, however, would be more forgiving to some flexing than drywall seams, too.
Late reply to an old post but I am thinking of doing the same thing only on an interior ceiling for sound insulation below some bathroom drain piping. We have already used cast iron for the piping but want the value of sound dampening that the rock wool provides. I don't see how the furring strips help with structural support for the drywall seams. I usually see the drywall nailed horizontally in a wall application so that most of the long seam is unsupported between studs. I would think that the rock wool board would actually provide more support than is typically available in the usual case. Am I missing something?