Does Rockwool Insulation, Un-faced or Faced Still Require Vapor Barrier Protection?
I need to insulate the low-ceilinged 400 sq.ft. attic above an unheated gambrel roofed garage located in Rhode Island. The gambrel roof makes adding insulation something of a challenge. The attic will be climate controlled by way of a mini-split HVAC system.
The garage has soffit vents at the base of its gambrel roof, there is a ridge vent running the full length of the roof and I am considering adding gable end vents to assist natural ventilation.
My research indicates Rockwool insulation seems to have some useful properties, like the ability to dry out if exposed to moisture. But this same research has come up empty regarding whether providing natural ventilation space behind unfaced Rockwool allows this project to move forward without a vapor barrier. Whether using faced Rockwool might be a viable alternative also remains an unanswered question.
At the end of the day I would prefer to not create a situation where the proper materials were installed in the wrong or incomplete way. Any viable suggestions are appreciated.
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You don’t need a vapor barrier when insulating a ceiling between conditioned space and a vented attic. You sometimes need a vapor retarder (you usually don’t want a vapor barrier) in some types of walls. I think what you have is something like the “Devil’s Triangle” area of story and a half type homes. If that’s the case, you want to cover the attic side of the wall insulation to prevent convection currents from reducing your insulation’s effectiveness. Sometimes house wrap is used, but I prefer to use polyiso. If you use polyiso the same thickness as is required in your climate zone (most likely R5), then you don’t need an interior side vapor barrier. That would mean 1” polyiso on the attic side of the studs, then mineral wool in the wall (I’m assuming 2x4 walls here, if you have 2x6s you need thicker foam to maintain the same ratio of batts to foam and the ratio is the important part) and you’re good to go.
If you don’t want to use rigid foam on the attic side of the walls, then I’d tack up some breathable housewrap (don’t use poly here!), then batts, and I’d use a smart vapor retarder like MemBrain on the interior under the drywall.
If this doesn’t sound like what you’re trying to do, post a drawing showing where you want to insulate to help everyone here know what we need to know to be able to help you.
Bill
Bill: Thank you for this comprehensive answer to my question. You have introduced several new concepts for me (polyiso and "smart vapor retarder like Membrain"). It may be worth noting that as part of this project I created a landing and "turned" the top three steps of the garage's original stairway. This has allowed me to install a solid core/insulated door and completely "isolate" the attic space from the unheated garage below. The two end walls are framed with 2x4s and all the roof trusses are 2x6. At your suggestion I will post a sketch (and photos) to better describe the roof and ceiling area I need to insulate.
Bill:
You clearly know a LOT about this insulation/vapor barrier issue, and I clearly know not very much. But I’ve now looked into your polyiso reference, and think it can be ruled out for this particular project. The supplier for the Rockwool insulation I am looking at tells me it does not come in a “faced” version. So now I am thinking seriously about both the “breathable house wrap” and “smart vapor retarder” references you raised. I am attaching several sketches and photos to help indicate the problem areas I need to address:
1) Ceiling and Sloped Walls: I want to retain the look of the 2X4 roof tie beams. To that end I plan to create a “poor man’s” drop ceiling above them by resting precisely-cut (removable) pieces of sheetrock on top of drop ceiling framework, fastened to the top of the beams. Above that is the shallow ridge-vented space provided by the gambrel roof. I envision 1” or 2” “baffles” holding the 5.5” Rockwell away from the plywood roof sheathing. Does “breathable house wrap” go between the baffles and the Rockwool? Once that is complete additional Rockwool will be placed on top of the removable pieces of sheetrock. Does your “smart vapor retarder like Membrane” go above the sheetrock and below the Rockwool? I do not expect this to be an easy process, but if it would work I’m up for it. The primary goal is to maintain the hard-earned natural air flow resulting from now-working soffit vents and the re-established ridge vent.
2) Stairwell Isolation Walls: There are now four 2X6 quasi-knee walls that, along with an insulated solid core door, will isolate the attic from the unheated garage below. I have begun to imagine these walls as vertical versions of the attic’s roof, with “breathable house wrap” and “smart vapor retarder like Membrane” similarly sandwiched around the Rockwool insulation. Would these walls also benefit from a variant of a soffit vent at their base, a 1” baffle on their “outer”surface to allow natural ventilation and some form of ventilation tie-in to permit air to vent out of the attic’s ridge vent. Or would gable ends vent serve the same purpose? Or is the concept of “natural ventilation” not relevant because these are not actually “outside” walls?
3) North and South 2X4 End Walls: Finally, I am envisioning using the “breathable house wrap” and “smart vapor retarder like Membrane” similarly located on either side of the Rockwool insulation for the attic’s end walls. Would these thin walls also need a baffle to allow natural ventilation on their outside walls, or is that overkill?
Or am I overthinking this entire operation? During the age of COVID-19 some of us retired guys have time on our hands, not much in the way of “resources” and always little room for error. Given the additional documentation and above clarifications, please know that any additional recommendations you may have regarding how to properly insulate this gambrel-roofed attic would be greatly appreciated.
The first thing I'd do is get some cable staples and tack that run of NM cable up to the underside of the rafters. You're not supposed to free-span wiring like that. I like the plastic cable staples that have a nail on either end -- it's hard to hurt the cable when installing those kind, and you can install them with a smaller hammer which is easier to work with in tight spaces.
Vapor retarders go on the "warm in winter" side when you're in a heating dominated climate, so between the insulation and the drywall. Using something like housewrap on the other (attic) side of the insulation where the insulation would otherwise be exposed on a vertical wall helps to limit convection currents from reducing the R value of the insulation. You wouldn't need to do that on a roof, but I would consider using some kind of ventilation baffles, either site built (1/2" polyiso works great for that), or something premade like Duravent.
For those walls, I'd look into polyiso on the attic side again if I'm understanding your layout correctly. One of the tricky things with helping people on these forums is that it's difficult to visualize things sometimes. Construction is very hands-on, and it often helps to actually be present at a site so that you can crawl around and see how things go together. If you read through the various articles on GBA, you can get an idea of many of the basic concepts that will help you to understand where things should go for best performance. You can always ask in Q+A too if you get stumped of if you have a situation that doesn't quite fit any of the examples in the articles.
Bill
Bil:
Err, the FIRST thing I'm going to do is get that box of plastic cable staples and tidy up the NM cable as per your suggestion. The work you see in the photograph was at the very end of my electrician's week, and he was pretty fried. As his totally amateur assistant I was hustling like mad just to keep up with him, so thank you very much for your objective observation.
The rest of your recommendations ALL sound completely reasonable, very doable and will form an excellent "road map" for how to proceed.
Just to test my mettle, I recently received an estimate from a reputable insulation company complete an entire Johns Manville TempControl Mineral Wool version of my insulation project for $3K. They would also include insulating the garage ceiling with John Manville R-30 Kraft Fiber Glass. Being a bit of the stubborn DIY-type, I'm inclined to take every one of your well-thought-out recommendations and run with them! There's nothing more to say, except that your patience with my hairy little attic project is appreciated more than you'll ever imagine!
If you are building a vented cathedral ceiling, there is lots of information available about this with the search box. If you are making a vertical exterior wall, make sure it complies with Table 2 A or B here.
Jon R:
Thank you for the referral AND link. I'll definitely be checking it out before I proceed. This is a great forum, the Green Building Advisor site!