Kraft faced insulation Va. poly vapor barrier
Hope to keep this simple.
Steel building, Climate Zone 4a, steel girts/purlins 6″ thick. Wall stackup from outside is: Steel ribbed siding, woven house wrap, 1″ XPS, Girts/Fiberglass batts, wallboard. (See picture).
I realize that the 1″ XPS acts as a vapor retarder so I want to be careful about a vapor barrier toward the inside of the building when installing cavity insulation. At one point, someone told me that Kraft paper vapor barrier insulation was ok to use but avoid poly sheeting vapor barriers. Is this true?
I would also ask this same question regarding the roof. Same steel as the girts, but has your typical 3″ fiberglass roll insulation with the white vinyl vapor barrier toward the inside. Do I stick with unfaced insulation for the cavities, or is Kraft paper ok and/or preferrable.
Best Regards,
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How much R-value are you aiming for, or what will you be using the building for? Thermal bridging through the steel girts will significantly undermine any insulation you add between them. With the steel siding being an excellent vapor barrier, you certainly don't want poly on the inside. In climate zone 4, you only need a class III vapor retarder, which can be provided by a couple coats of latex paint.
Have you seen this article? https://www.greenbuildingadvisor.com/article/insulating-a-metal-building
FrankD,
You are right, going from memory, the steel girts in my wall will kill about 60% of the insulation I put in the cavity. It is a 6" girt, so R19 turns into R7. Adding the R5 from the XPS, I am looking at around an R12 wall assembly.
The main reason for the XPS on the outside was to provide a thermal break from the steel siding to the girts, as well as provide an air barrier (it is glued and taped), as well as a "semi" permeable vapor barrier.
I did see the metal building insulation article.
I still want to insulate the cavities, but realize that the kraft paper may make installation a little easier, but I cannot tell if this violates the "vapor barrier on both sides of the wall" rule. I had one person mention that kraft paper was ok on a previous thread here on GBA, so I wanted to flesh it out.
The 2024 IRC allows you to use a "responsive vapor barrier" over a 2x6 wall in climate zone 4 if you have at least R-5 of continuous insulation. I believe the kraft facings would meet the definition of a responsive vapor barrier. The 1" of XPS should give you the R-5 continuous. And at 6", your wall cavity is a bit deeper than a 2x6 stud wall, but that shouldn't matter if you are putting in batts intended for 2x6 walls. So, I think kraft-faced batts would be borderline okay if you're turning the building into a residence. If it's just a low-occupancy shop, then no concerns at all.
So let me ask this, is it OK to use unfaced insulation in the cavity then cover with plywood (likely finished or painted) in the work areas and painted drywall in the more polished area (break room, etc)?
That would probably be fine if you kept the interior RH reasonably low, below say 40% in winter. There are different grades and types of plywood, so a definitive answer isn't possible. Plywood typically acts as a vapor retarder but it becomes more vapor open in damp conditions, allowing the wall to dry to the interior.
Let me as you this then which is better in this application, kraft paper, or unfaced?
Does the answer change depending on the wall covering?
I am surprised by this, simply because I simply recall being told to use no vapor barrier if I put continuous R5 XPS on the outside of the girts. You almost speak like kraft paper barrier is better than no barrier.
Kraft facing is a class II vapor retarder, not a vapor barrier. The building code categorizes vapor retarders into three classes. Class I vapor retarders, such as poly plastic and sheet metal, are the most effective. Because they allow almost no vapor to pass, class I vapor retarders are sometimes also called vapor barriers.
In climate zone 4, the IRC (which is intended for residential occupancies) requires framed walls to have a class II (ex. kraft facings) or class III (ex. latex paint) vapor retarder on the inside. A class I vapor retarder/vapor barrier/poly is not permitted.
The IRC also doesn't permit you to have a class I vapor retarder on both sides of a framed wall because any water that got in wouldn't be able to dry out. Since you have sheet metal on one side, that's a second reason for not using poly inside.
I think kraft-faced batts would be fine under painted drywall. With plywood finish, I would choose unfaced batts because the combination of kraft facing and plywood might not allow enough drying.
Thank you for your help. When you recommend un-faced behind the plywood and faced behind the drywall, are you taking into account that I have steel siding, house wrap (woven) and 1" of foam on the exterior of the building?
Midwayman,
Your bigger problem is the roof. Can you describe what you are suggesting? Vented or un-vented, foam, sheathing, underlay, etc.
Also, how is this building seen by your code? residential or commercial use?
I am concerned with this as well. The original plan was to use 2" of rigid on the roof and do something similar in the cavities. However, the logistics of installing the rigid foam was simply too difficult (and costly) so I opted for the fiberglass just to give some insulation with a vapor/air barrier (the vinyl backing).
The stackup right now is simply steel roof, fiberglass, reinforced vinyl backing, then purlin. The ridge cap has closure strips but the bottom is "open" although compressing the fiberglass against the eave strut.
I have toyed with a few ideas to finish the insulation:
1. Do the same thing I do on the walls, 6" fiberglass in the cavities, drywall against the purlins (least amount of work, least amount of R value - R12-R14).
2. Glue 2" foam furring strips along the bottom of the steel purlins, then installing R-30 insulation through the 6" girt cavities to the end of the foam, then cover with drywall. (seems reasonable work wise and would probably give somewhere around R18-r20)
3. Building some kind of "dropped vaulted" ceiling by spanning something like 2x4s 16" OC between roof girders allowing the use of blown in, or even really tall batts or rolls (r-30-r-38) underneath the purlins.
Option 2 is the most attractive from a cost/work/benefit tradeoff. Just not sure if that is enough insulation.
Do you have any clever solutions aside from these? This is a residential small-business use workshop, but I want to make it insulated enough I can heat, and potentially cool if need be.
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hey krackadile, I am not as concerned with the cost and labor of something like the second option, but I am concerned with handling moisture properly so I am not dealing with mold/rot etc down the road.
Malcolm_Taylor, are you hinting at how you would approach the ceiling?
Is there a building code there you have to follow? The IECC gives recommended/required values for insulation depending on whether the building is commercial or residential:
https://codes.iccsafe.org/content/IECC2021P1/chapter-4-ce-commercial-energy-efficiency#IECC2021P1_CE_Ch04_SecC402
I guess if there's not code that you are required to follow you might want to do a rough cost analysis and compare the added cost to insulate vs the added cost to heat/cool the space and see what kind of a payback period you'd have. If it's less than 10 years go for it and if it's more then it may or may not be worth your while. Might want to consider how long you'll use the building and what it's future purpose will be as well.