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Insulation dilemma: Mineral wool batts, encapsulated fiberglass, and rigid boards

Cz4a_Giovanni | Posted in GBA Pro Help on

The Insulation Situation:
(Mixed Humid region) Asheville NC

Older 1968 800 sq ft house; prior owner had roof leak. Shingles replaced 1 yr ago, no more roof leak. Currently doing mold remediation on framing.Gutted all ceiling & most of sheetrock walls. All old fiberglass insul removed in attic & most walls due to mold on dust, some blackboard sheathing removed due to mold growth. House has aluminum siding exterior.

Approach: I only have access from interior to replace Insulation. I will first do thorough air sealing. Due to removal of old/damaged blackboard sheathing removefrom 2×4 studs, i now have thermal bridging to address on one side of the home.

I would like to use a rigid board (whether Xps Foam or Rockwool ComfortBoard 80) directly onto the Ceiling joists and the wall joists (whereby, i then would have non-paperbacked sheetrock eventually installed by a contractor). I want to install the insulation board across the studs, in order to address thermal bridging from the INSIDE walls and/or to give extra insulative value that i cannot obtain simply with one layer of the cavity insulation ( in attic for example) whether Poly encap FG, or Rock wool).

And into the interior cavities, before i install the board and sheetrock,  i am trying to decide between Poly encapsulated Fiberglass an/or Rockwool batts. Obviously, these both have very different permeance/permeability.

No matter how much i study, research, and ask people, i am getting mixed messages as to what is safe, considering the mixed humid climate here… is the XPS rigid board ( is it a Category III?) unwise to use on the interior rafters due to its less permeable features, compared to say, Rockwool Board? Which is the better to prevent moisture problems in my area? Yes, we will be installing a heat pump, so the house will have AC as well.

Please advise as to the best approach with the options i am considering as outlined in the discussion above, why one combination would or would not work over the other. Problems i might have with a particular material or assembly. Apparently, in my Mixed humid region,( area 4) i DONT need a vapor barrier beyond a simple category 3… so would using the XPS rigid board om  inside of house create a problem? I am needing to do the insulation install DIY, due to other pricey hvac we do have to hire out… one of the main reasons why i am considering these materials.

Practical & time tested advice on these specific issues at hand would be appreciated.
Thanks much

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  1. KeithH | | #1


    I'm a DIYer not a contractor so take it with a grain of salt and no warranty :)

    First, I'd strongly recommend you read this blog by Allison Baines about vapor barriers. I believe he contributes here or used to but has his own blog.

    The uptake of this blog entry is that (probably) what you need is a good air barrier, not a vapor barrier. Unfortunately for you, that's harder to execute well as all your penetrations (electrical, plumbing, etc) will need to well air sealed. There are a lot of approaches to air sealing that I'm confident you can details to on the GBA website.

    Vapor permeability is complicated. If you are really concerned, you could consider having a WUFI model of your house performed. Yes, that will cost money (though the software is available freely it is pretty complex). Perhaps you could get HVAC sizing and WUFI model bundled in by one provider (if you are having your hvac replaced, you definitely want a good sizing performed). If you want to get an idea of the complexity of permeability and the impact of even your interior paint choice, check out this powerpoint:

    Regarding your foam board inside idea:
    I wasn't totally clear, but it sounds like you are thinking of putting the foam board interior to the studs? If so, what's going in the studs? You should be aware that installing a non wood layer interior to the studs can create some challenges for affixing drywall in corners. Plan ahead for blocking for longer screws. I assume if you are on a budget, you are doing your drywall yourself. I'd also be sure your inspector won't be unhappy about that. You'll also need to plan box extensions for your electrical installations as you can't expose the foam to the outlets etc.

    Regarding cut and cobble foam board:
    I've done this. It does work and if you leave a small intentional gap to spray foam, you can make a very tight (air and vapor) installation. It's also a lot of work for not a lot more gain. I'm not sure why you would choose XPS though. I'd suggest polyiso (assuming your climate doesn't get really cold) for better R value or EPS for lower cost. The one big advantage would be that you have your air barrier and vapor barrier built into your insulation. I'd personally estimate the installation will take 5x as long as installing roxul or fiberglass.

    Regarding using mineral wool boards:
    Same problems with interior installation as foam board (though no fire risk) but I'd be very sure they are greenguard rated. I've used a commercial product on my exterior foundation and that product smells too bad to use indoors. I've successfully used comfortboard 80 in a crawlspace insulation project but I had to jenga stack it in the garage with big fans running through it for a few days before I felt good about putting it inside. Even a year later the crawl space with the exposed comfortboard insulation has a faint insulation order. Meh.

    Regarding your removed and destroyed blackboard:
    I assume you are talking about fiber sheathing. That material was likely only R-value 1.3. I'm also not clear how you would have removed it entirely from between the studs and siding without removing the siding. Without a picture, I'm not sure what the issue is. If you are missing 1/2" of sheathing material, you'll need to figure out whether you need sheathing strength (probably not if you only removed some and it was only fiberboard; consult a structural engineer) or if you are just looking for fill. I'm also not clear how your system prevents wind driven rain or insect penetration through the siding. What's under your siding? If you are just looking to fill 1/2" of void from removed fiberboard in a few bays, why not use 0.5" xps or 0.75" EPS cut to fit inside the stud bay then fill with a standard batt product (mineral wool is my preference but fiberglass works too)?

    Honestly, your writeup sounds very complicated. If it was my house, I'd do the following:
    - Replace any removed corner sheathing with 2x4 and plywood box interior of the studs. That will provide needed rigidity at the price of extra thermal bridging (don't worry about it, corner strength of the structure beats thermal bridging)
    - Replace any removed non-corner fiberboard sheathing with 1/2" or 3/4" foam board cut to fit (minus ~1/2" to provide a place to spray foam) spray foamed in place.
    - Put a small caulk or spray foam bead around the perimeter of each stud bay to try to tighten things up if you didn't install xps
    - Replace your inside the studs insulation with Roxul comfortbatt. Very DIY friendly.
    - Include a smart vapor barrier like Intello Plus. Use airtight electrical boxes, tape, and spray foam to insure your air barrier is tight. Or learn about airtight drywall.
    - Forget your foam board inside the studs idea.
    - Air seal your attic with canned foam (get a pro style gun that takes the big cans, the little plastic straw cans are a rip off for big jobs)
    - Insulate your attic with blown cellulose (very affordable and DIY friendly). Watch out for your electrical connections. Burying them requires thought (consult with an electrician)
    - Insure your passive attic and soffit ventilation is adequate.
    - Insure that your bathrooms are very air tight to the attic to make sure that you don't get warm wet air into the attic.
    - Insure that your bathroom vents are well-ducted (solid pipe turns, no constrictions) and well-sealed to the drywall (to prevent moisture from going between the fan unit and the drywall into the attic.
    - Consider whether your want a ventilation system (ERV).

    If you are desperate for higher performance, perhaps to prevent replacement of an AC unit or furnace etc, then you could consider installing an interior 2x4 wall (double stud wall). I'd be surprised if the payoff is there for your climate and I would definitely consider having a WUFI model made first or your might make your condensation problems worse. I'd guess it's not recommended in your climate and situation.

    I hope that helps and please remember I'm just a DIYer

  2. exeric | | #2

    Whatever you do, don't purchase unreclaimed XPS. It's a global threat to the planet due to the destructive blowing agents used in it. It's only been said about 500 times here but it's still true. I really don't understand how anyone reading this site could fail to understand that at this time.

  3. Expert Member
    Dana Dorsett | | #3

    Don't use XPS for anything here- it's the least-green commonly used insulation material there is due to the HFC blowing agents used. Those blowing agents are banned under the Kigali amendment to the Montreal Protocol, but the ban isn't in effect in North America yet.

    I assume the asphalted fiberboard sheathing was cut back parallel to the studs, but still has stud-width strips acting as furring between the exterior stud edges and aluminum siding?

    If yes, using bright foil faced polyiso that's at least 1/4" thicker than the original sheathing can be cut'n'cobbled into place as the exterior side air barrier for the cavity insulation. (eg if it was half inch fiberboard, use 3/4" or 1" polyiso). Tape any seams of the interior side facer with a temperature rated aluminum tape (Nashua 324A is available at most box stores), and use a polyurethane caulk or can-foam to seal the foam to the studs inside each stud bay.

    Rather than continuous sheet foam, build out the interior side of the wall or ceiling framing with Bonfiglioli strips. With 1x furring & 1" polyiso strips it will add 1.75" to the stud depth, and you'd have about 5" of total cavity depth.

    An R23 rock wool batt compressed to 5" still performs at R21 or better, and the R6 thermal break of the 1" polyiso on the studs roughly doubles the R value of the framing fraction compared to a milled 2x6. This will meet current IRC 2018 code min for the walls.

    For tips on Bonfiglioli strips see:

    For tips on cutting foil faced foam into 1.5" wide strips for making the Bonfiglioli strips see:

    Shiny foil facers on the cut'n'cobbled air barrier foam are also true vapor barrier, so the assembly now has to dry only toward the interior. But they also act as the weather resistant barrier and are a radiant barrier, which adds another R1 or so to the wall's performance when the foil is facing the air gap behind the aluminum siding.

    With an exterior side vapor barrier it's fine to use paper faced wallboard in your climate zone, even in air conditioned buildings. The asphalted fiberboard was about 20 perms, and which could lead to enough summertime moisture reaching the wallboard to be a problem. A foil facer runs about 0.05 perms, about 400x more vapor-tight than asphalted fiberboard. With 3-5 perm paint (standard interior latex) there will be no significant moisture buildup in the wall, winter or summer.

    1. Cz4a_Giovanni | | #7

      Thx Dana, thats a lot to chew on, will read thoroughly soon.

  4. Peter Yost | | #4

    Hi Giovanni -

    1. air tight - this is the one thing that needs to be at the top of every list, regardless of climate. Since you are DIY: invest in a blower door test to check on how well you did. Money well spent.

    2. I have a high performance building colleague in CZ4 who works on high-end retrofits. He has a ton of experience and case studies regarding wetting driven by condensation during the cooling season; very little wetting by condensation during the heating season. So, you do need to worry about both in CZ 4, but again, air sealing is the best defense, regardless of whether the condensation occurs in summer or winter.

    3. I think the best way to deal with corner drywall is to use drywall clips. I like this type (the Mighty Nailer -

    4. WUFI - I don't think you need to go this route. I do WUFI modeling and I don't think the answer will come through different from this: have your assembly dry in one direction or the other but focus your efforts on bulk water management and air leakage.

    5. Although this Q&A Spotlight was for Seattle (CZ 4B), I think it is still worth reading for CZ 4A... -

    Best - Peter

    1. Expert Member
      Dana Dorsett | | #5

      Seattle is zone 4C, not 4B, either of which have much lower summertime outdoor dew points than zone 4A, with mostly negative latent cooling loads. Zone 4C also has very low sensible cooling loads, and a longer but shallower heating season than 4A or 4B.

      Other than similar annual heating degree days zones 4A and 4C don't have a lot in common. In a typical year Seattle sees fewer than 200 cooling degree-days (the all time record was 350 CDD in 1967), averaging about 1/10 the CDD of Charlotte NC. With summertime dew point averages in the 40s F the risk of condensation even on uninsulated air conditioning DUCTS is mostly theoretical, let alone moisture accumulation in the wallboard of air conditioned houses due to the air conditioning. (Dew points north of 60F make front page news in Seattle!)

    2. Deleted | | #8


    3. Cz4a_Giovanni | | #9

      Thx Peter, i will evaluate your suggestions more thoroughly soon.

  5. Cz4a_Giovanni | | #6

    Thx for the pointers. But i am looking for information specifically on the usage and appropriateness of those insulations in the heading ( and their associated properties) used together in assemblies in my mixed humid area. ( apologies for the lengthy writeup, felt like i needed to put the siuation in context). I should have been more direct with my questions. The actual questions are numbered below, after i give context. (Btw i have been checking out Allison Bailes website, good stuff...)

    My largest priority, is the potential of not-quickly detected moisture, to create mold problems again. Therefore, want to use insulation that isnt as easily damaged or as much of a sponge, and easier to install/keep clean. (Our region had the wettest year on record being 2018, its like a rainforest here. And we do get kinda cold in winter. Occasionally down to single digits.) Also, since i have all the joists and rafters exposed, why not use a large insulation board across the joists if i can, after cavity insulation? (yup, will accomodate with longer screws when the Contractors install the drywall on ceiling.)

    So, if you have a more clear understanding on proper wall assembly arrangement, than i myself have for a mixed humid area or advise on these specific insulations, please advise:

    1. Since I am going to choose between Rockwool comfortboard batts OR Poly Encapsulated Fiberglass batts for my inside ceiling & wall cavities. Give me your tech expertise as to WHY or why you wouldnt use one over the other.

    2. I am either going to use RockWool board OR a Rigid foam board that is least likely to collect water. If you are opposed to XPS, please make an alternative suggestion that is comparable in features yet guessing you may vote EPS. If so, how comparable in FUNCTION is it? (Not interested in PolyIso).

    Context/ further clarifications:
    (The question posed about my removal of the Fiber blackboard ( old JM weathertite). I tediously pryed out 10-12 foot by 8 ft high section of the entire bathroom wall plus another 4 foot by 8ft.. The only thing seen thru the cavity studs is the backside of aluminum siding. Nothing else. I also pryed it off the backside of the studs, yes from inside, carefully, and tediously. The nails that attached them are still in the exterior side of the 2x4 studs. So, whatever i install within the cavity insulation-wise, will never address the rear/exterior side of the 2x4 studs. Thus, want to use a board insulation across the 2x4 studs from the inside to deal with the thermal bridging from outside wall. 4x8 Board are also easy to install/cover alot of area. )

    3. Therefore, if you have a super understanding of wall assembly arrangements for my Area, what needs flow in what would any Foam board be too highly impermeable for an interior application/ operating as a vapor barrier?

    Thanks again folks,

    1. KeithH | | #10

      1. I've never seen roxul mold and I've tested some pieces outside for a long time. It is very resistant to damage from moisture. But it is still, despite it's density, air and vapor permeable. As long as you have a vapor barrier in one and only place, that's a good thing. I wouldn't put the poly wrapped fb behind another vapor barrier (multiple vapor barriers bad). I also think you can get a very good fit with roxul. My experience with poly wrapped batts is that fit is harder than roxul. Additionally if you have to cut the depth of roxul down, you can. For example, 2x4 or 2x6 batts cut in half. I've done it. It's not perfect but it works. Try that with fb.

      2. What thickness foam board? One thing to consider is that 1" foam board is a class III vapor retarder not a barrier (unless it is foil faced). Foil faced EPS is easy to find. I personally haven't seen faced XPS. Assuming you tape any damage during install to the foil face and seal the edges (tape, canned foam), it should be pretty invulnerable to moisture damage. Taping the edges is fast with a good HVAC tape that is usually available after the shelf from a big box store. (Look for the stuff that has backing paper you peel off).

      I think to fill your gap between your siding and your studs is going to be kind of awful. I'd probably use foam board (per Dana, and my, suggestions as your back later) taped/foam in but go one stud bay at a time. Back fill the stud face with canned foam using the just installed foam board as your backstop. Repeat. I don't see how you'll have meaningful thermal bridging then.

      3. I don't even understand this question. Your situation is weird and is as much about what is possible for you to execute as it is what is ideal. You aren't asking about the ideal wall. You are asking what to do with a weird situation. Decide where you want to put the vapor barrier. In your climate, that's typically behind the drywall. But you can't follow Dana's suggestion (or mine) to put 3/4" foam board in the bay (thus creating a vapor retarder at the outside of the assembly). Decide how you want to have a vapor barrier or retarder: smart membrane; poly sheeting; foam board; or even just 3 coats of semi gloss paint. Then make sure the rest of the assembly is vapor open in both directions. I'm trying to give you guidance for how to design it yourself since you are doing it yourself as I would as a DIYer. If that's not helpful to you, maybe it will be help to someone else reading this thread.

      1. Cz4a_Giovanni | | #12

        Keith, thanks for your advice & insights. Yours, along with Dana's
        guidance will help me figure out how to proceed in yes, this peculiar scenario.

  6. Expert Member
    Dana Dorsett | | #11

    >"Also, since i have all the joists and rafters exposed, why not use a large insulation board across the joists if i can, after cavity insulation? "

    Because the full width foam goods will impede drying of the assembly (a lot!), and rigid rock wool is very expensive, and more compressible, making it much harder to get a flat wall.

    >"1. Since I am going to choose between Rockwool comfortboard batts OR Poly Encapsulated Fiberglass batts for my inside ceiling & wall cavities. Give me your tech expertise as to WHY or why you wouldnt use one over the other."

    Encapsulated fiberglass is a vapor barrier, far less fireproof, more expensive and lower R than rock wool., and more likely to create a moisture trap. Adequately air sealing poly encapsulated fiberglass to the studs for the long seems like an impossible task to pull off. Making the exterior as air-tight as possible is key to avoiding moisture accumulation in the wall cavity during the cooling season, and that's easier to pull off with an exterior side rigid air barrier that's more vapor-tight than latex on wallboard.

    >"2. I am either going to use RockWool board OR a Rigid foam board that is least likely to collect water. If you are opposed to XPS, please make an alternative suggestion that is comparable in features yet guessing you may vote EPS. If so, how comparable in FUNCTION is it? (Not interested in PolyIso)."

    Polyiso is still your friend here, even if you're not interested. Yes, if BURIED or SUBMERGED FOR DAYS in a pond it can take on water, but unlike polystyrene it doesn't melt when burning, and and has a higher kindling temperature, and a higher R/inch that polystyrene. Even the polymer used has a lower environmental impact per R.

    XPS will eventually fall to the same R value as EPS of simliar density. EPS is more vapor-open than XPS so thickness and density matters. Type II EPS (1.5lbs nominal density) is already less than 3 perms @ 1" which is about as vapor tight as a few layers of latex paint, and sufficiently vapor tight for your climate and application. A full sheet of 1" Type II EPS on the interior with latex painted wallboard makes the assembly arguably TOO vapor tight, with only limited ability of the assembly to dry toward the interior. XPS runs about twice as vapor tight as Type-II EPS, and is usually a Class-II vapor retarder at 1.5" (which would meet Canadian code as a vapor barrier.)

    Like polyiso, EPS is blown with hydrocarbons, not HFCs. It is usually a variant of pentane, most of which leaves the foam and is recaptured at the factory, not released into the environment.

    In your climate it's fine to set it up to dry toward the interior, with a low-permeance layer on the exterior. After all, a layer of half-inch CDX plywood sheathing is a low permeance layer, less than 1 perm when dry. Most reasonably air tight CDX sheathed houses in your climate set up to dry toward the interior do GREAT, and only collect wall moisture in the wall cavities when an interior-side low-permeance layer is present.

    1. Cz4a_Giovanni | | #13

      Dana, thx for taking the time to explain the why behind some of my questions. These further details/guidance will help me devise the best approach. Will have to take some time to digest this new information... As for now, back to the remainder mold removal.

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