Using RockWool in between roof rafters?
RoelMMata
| Posted in GBA Pro Help on
Hi, my name Is Roel I’m no building science expert just a guy wanting to build a good home in the near future. From what I understand the conditioned space should be at the slope of the roof if my duct work will be up in the attic, would putting RockWool insulation in between the rafters be acceptable? The reason for RockWool is from what I learned it’ll be more cost effective if I buy and install it myself than if I pay for spray foam. The location would be “Zone 2/South Texas Area”. I’d really appreciate any help with this, thank you!
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Replies
Hi Roel,
There are two types of insulated roofs, vented and unvented, each with different possibilities for insulation. If you have an easy roof to vent and decide to go that route, you could use mineral wool insulation between the rafters and beneath the vent channels.
If you choose or need to go with an unvented roof, you would need either exterior insulation or a flash coat of closed-cell spray foam first, then you could round out the insulation package with mineral wool.
That's a brief answer. For much more detail, I suggest that you start with this article: How to Build an Insulated Cathedral Ceiling
Thank you for the fast Reply Brian, I will read that post.
Hi Brian,
I had open cell spray foam insulation in my attic and after 8 months it still had a chemical smell (I posed about this on this forum). The contractor removed it and suggested to replace it with a 3 inch hardboard insulation and 5.5 inch Rockwool on the rafters. Trying to keep the attic not vented since we have the HVAC system in there. Do you think that's a good idea? Many thanks
Hello John, I am in a similar situation. Persistent smell wafting down from the attic which has open cell foam on the rafters (a conditioned attic in a brand new construction home finished March 20203). I have tried an ERV, depressurizing the attic, sealing ducts, painting the foam, etc. I am at the point where I want it gone. Don’t know what to do though in terms of attic insulation if we have the spray foam removed. The attic would need to remain conditioned, I think. Did you go with what your contractor recommended? Has it worked? I would love to connect!! Did the smell lessen once you took out the foam from the attic?
Hi Brian,
Does your suggestion/answer change if there is no cathedral ceiling? I ask because in a cathedral ceiling I am assuming you are counting on the drywall and paint to act as a class 2/3 vapor barrier on the warm side.
So in short, I am asking if the same flash and batt detail as described for a cathedral ceiling is recommended for an unfinished attic (with no air barrier on the warm side). In my case, I am in zone 5.
If you want to build a “good home”, do NOT put ductwork or other mechanicals in the attic. Keep all your ductwork and plumbing, and as much of the electrical wiring as you can, in the INTERIOR walls and floors. Avoid using the attic space for anything. This lets you build a conventional vented attic with soffit and ridge vents.
If you’re able to keep everything out of the attic, and vent it as described above, then you can use blown cellulose on the attic floor which is about the cheapest insulation available, and one of the best options for this type of project. Just be sure to do a good air sealing job on the attic floor prior to installing any blown insulation, and put some baffles out at the eaves to prevent wind washing.
I’m a big proponent of mineral wool insulation, but it’s best in walls. Blown-in insulation is both better AND cheaper for most attic insulating projects.
Bill
I have never heard of ducts in the walls, I'll have to Google that 😁. How do you do good air sealing on the attic floor? Would the blown in cellulose sit on just the drywall? I'm looking at doing zip system on the exterior of the house, slab on grade.
>"I'm looking at doing zip system on the exterior of the house, slab on grade."
By using future tense it sounds like this house isn't built yet?
If that's the case, as little as 3/4" (R5) of foil faced polyiso above the roof deck in a continuous layer is sufficient protection for the roof deck in your climate in an unvented insulated roof design.
A cheaper solution would be to use a trussed roof with plenum trusses to keep the ducts 7 & air handlers inside the pressure & insulation boundary of the house:
https://www.greenbuildingadvisor.com/article/use-plenum-trusses-to-keep-ducts-out-of-your-attic
There are other solutions too, such as soffited duct chases, etc:
https://www.finehomebuilding.com/membership/pdf/72033/021233086.pdf
A big +1 on this Zephyr.
In Zone 2 Texas with dark shingles it's relatively safe to use (moisture buffering) cellulose between the rafters without venting even at R30, with a modest increase in risk if going with fiberglass or rock wool.
See Table 3 in this document the "2A Houston" row, the "Spray Fiberglass" and "Cellulose" columns:
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
Rock wool batts are very comparable to 1.8lbs density fiberglass, which was simulated at R30 in that study. Denim batts would be comparable to cellulose in moisture buffering.
If it's a tile roof, fuggedaboudit.
Go ahead and read the whole thing- it's fairly short, and helps understand those tables.
With a batt solution it will perform better with an interior side air barrier to limit air & moisture transfer via convection. It need not be a true air barrier that would keep air from moving under pressure (the roof deck provides that function), but sufficiently tight to limit air movement with the relatively low pressures of the air buoyancy. If that air barrier was say, PERFORATED aluminized fabric radiant barrier stapled to the under side of the rafters it would be somewhat vapor retardent too, at about 5 perms, and won't create a moisture trap the way foil, bubblepack, or unperforated aluminized fabric would. The low emissivity of the aluminum will lower both the peak and average heat gains of the air handler and ducts.
Be sure to block off any attic venting if you go this route, even if you also have insulation on the attic floor. In zone 2A Texas attic venting puts far more moisture INTO the attic than it removes. If the attic is air tight to the outdoors, air conditioning the spaces below the attic will still provide some drying benefit (via vapor diffusion through the ceiling).
That's correct Mr. Dorsett, I'm in the planning stage, construction hasn't started yet. I've had to re read your comments a couple to make sure I understand them 😂, thank you for the information I'll have to talk to my builder to see if we could do those plenum trusses. Do you have a image/video example of what the foil faced polyiso roof would look like, also could I use Zip R6? Does that do the same thing? Thank you
Are you in a hurricane-risk zone, or safely inland from the coast? If hurricanes are a risk, an unvented attic has two potential advantages over a vented roof: 1) the closed-cell spray foam provides some structural benefits by gluing the roof decking to the rafters; and 2) an unvented attic (regardless of insulation type) does not allow air intrusion and pressurization that a vented roof can suffer in a hurricane.
I installed R30 rock wool batts in my rafter bays with a vented roof and conditioned attic space. I would highly recommend a vented roof if you plan to use a metal through fastener roof like R or U-panel which is extremely common in Texas. I used 1/2” rigid foam to create 1-1/2 vent Chanel in each rafter bay from soffit to ridge vent sealed on the edges with caulk / exp foam. Then I installed the rock wool behind that. If I could do it again I would still create the vented roof but use open cell spray foam instead of rock wool. It would have been cheaper and way less labor intensive. Would have still used rock wool in the walls though.
I know this is an older post but I have a question. my roof is as follows, 5/8" zip followed up by wip barrier, with 2 layers of 1" polyiso with alternating seams sealed on each layer, with 3/4" battens laid at 45* on top of all that with standing seem metal roof on top. What do you recommend for a conditioned attic for that . My walls, soffits up and over roof and back down other side are all sealed up. walls have same build up except (1) 2" layer of rigid foam board on exterior walls with battens attached vertically over each stud to create rain channel for drying purposes , then siding. I will be using zip system all the way around my house. Inside on walls I will be using 5.5" of rockwool insulation for a r23 and r10 on exterior. I live in northern NC on wet side of 95 approx. 3.5 house from coast so hurricanes not as destructive here.