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Replacing Fiberglass with Mineral Wool

kramttocs | Posted in General Questions on

Gable and roof vented Cape cod attic.Zone 4 SW MO
I believe it’s r19 between the ceiling joists with unfaced r30 rolled across it.
I want to replace the r19 with r30 roxul. The r30 fiberglass will go back over it.
Should I leave the Kraft facing in place and just pull the fiberglass from it or cut the facing out?

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  1. Expert Member


    I doubt it would really matter, it's vapor open enough that it's not going to stop vapor leaving in the winter.

    Having said that, this sounds like a no gain situation. With the amount of R value you have now, assuming it is installed well, and free of rodents / debris / gaps, adding 10 more R between the studs is unlikely to provide any noticeable difference in energy costs. For the money, you'd probably be better off blowing fiberglass on top of what's already there and boosting the R value 'globally' instead of adding it between the joists where it's already less effective.

    Alternatively, I'm not sure I can think of a more tedious job than pulling insulation, just to exchange it, and put more back on top.

  2. walta100 | | #2

    The problem with your plan is that your home is a story and a half building. That style building is almost impossible air seal and insulation is useless when air is blowing thru it. I hate spray foam but sadly it is likely your only prayer to keep the air inside such a building.


  3. Expert Member
    BILL WICHERS | | #3

    This is a lot of effort for very little gain, so little that you'll probably not even notice any difference in practice. You'd be going up from R49 to R60, which isn't very much, only around 10% reduction in BTU loss. you'd need to go up to R98 to cut the energy loss in half, and at R49, you're already pretty well insulated, so you are well past the point of diminishing returns here.

    You'll find it very difficult to try to remove and replace/reuse existing insulation. Usually the material comes apart as you try to do this, and you end up with a mess of a project. I've always just replaced old insulation after removing it for nearly any reason.

    If you really want to up the R value here, I would just roll out some unfaced R19 on top of what you already have, which will get you up to R68 for a lot less effort, and at much lower cost. Note that your insulation layer will be pretty heavy at that point, so you should consider the load on the drywall below if that's what is going to be supporting all this.


  4. kramttocs | | #4

    Thanks all. You all are right - plus the r30 roxul isn't cheap.
    I might ramble for a bit trying to explain my thoughts :)

    It was just the R-19 when we moved in a few years ago and I went up there soon after and rolled out the R-30 fiberglass. This was at the same time that I went behind the second floor kneewalls and installed xps into each of the floor joist cavities (beneath the kneewall), can spray foamed around them, added another layer of r-19 to the back of the kneewalls and then r-30 to the floor, backing myself out as I went. With dormers separating areas back there leaving only a small triangular area to fit myself while pushing the insulation ahead of me, I still have nightmares. I say that only to admit that I don't mind the work or messing with insulation, I like these projects and I don't have to pay myself much.

    Again though, I agree that the roxul swap doesn't make financial/roi sense. Here are some of my reasons/excuses for looking to do this.
    1. I made another thread about this but I do want to reinsulate/reseal the ductwork as during the winter I can feel cold air dropping from the ceiling vents. To do this I'll need to remove the r30 anyways and at least some of the r19 to get to the vents and seal around them.

    2. I don't like all the dust and debris (from water damage roof repair done last year) that is sitting on the insulation and I really like the clean look of roxul fit into a joist. Terrible reason and ridiculous I know.

    3. This one I was going to ask about in another thread and I know it can be a tough subject...
    The house has no soffits and thus no soffit vents. So the kneewall attic areas are essentially isolated, at least from a purposeful venting situation, meaning there is no airflow going through them from the eave up into the top attic and out the roof vents/gable vents. Not ideal in today's construction but it is what it is and all of the decking is solid/mold free. I do try to really think before I change too much and mess up the dynamic. During the winter it's the little 3' sections of the roof line between the kneewall and the attic (mini-cathedral-ish areas) that always show the joists melting off first. Those 2x6's have R-19 6.25". At some point pulling down the drywall from those sections, installing baffles to let the kneewalls breathe more, scabbing on a couple inches to the 2x6 with xps for a thermal break and increasing the rvalue would be preferred but not ready to tear into that yet. So... I was thinking dig out the R-19 from those and slide some R-23 (5.5") roxul down in them. This could be a trick with the shingle nails but I think I have an idea to help with that. I know this wouldn't do a thing for the thermal bridge problem that is the primary issue. Oh goodness. As I type this out, it does sound like a fool's errand....

    4. Removing it all will allow me to clean and airseal with confidence. Originally I was going to remove the kraft facing by running a knife along the joist-drywall joint and then put a bead of spray foam along it. Keeping the kraft would prevent this option though.

    @Walta - I did the spray foam thing in a converted bonus room cathedral ceiling (converted before my time) and don't think I'll ever do it again. I love the performance of it but am still dealing with fallout from the install. A little bitter that I spent so much time researching, thinking I got the right contractor, only to have to deal with what I am (avoiding details on purpose for my blood pressure and embarrassment :)).

    I'd be lying if I didn't say I kind of just "want" to do this and it's hard to mentally abandon it but I am also not refusing to listen to reason so please don't write me off as having already made up my mind. Forgetting the financial/labor of it, I don't want to be sitting around this winter fearing that I messed something up by making a bad choice for the house by doing this against better reason.

    Going back to #3. Let's say that R-19 is left alone. Regardless if the attic floor R-19 is left or replaced with Roxul, would there be any benefit to fitting foam at the top/attic end of those cavities? So preventing what little, if any, airflow there might be right now? Then in the future, I wouldn't put baffles in them but just fill them up. I.E. with no soffits, should I just go all out in stopping air flow? The image in the attachment (credit to for that image) but without the yellow Ventilation Baffles and the rigid foam block above the kneewall top plate would be on the attic end and seal the entire cavity.

    Thanks again all - your input is seriously appreciated.

  5. Expert Member
    BILL WICHERS | | #5

    I'm a big fan of mineral wool, but for walls. In attic areas, I like to go cheap -- so I prefer loose fill (blown) cellulose, fiberglass after that. Since you're already thinking about insulating the roofline in the future instead of the wall and floor of this "devil's triangle" area (aptly named, as I'm sure you'll agree!), I would just use the cheapest material possible in here since you're likely to pull it out in the future anyway. The cheapest way to go is usually an R19 roll of unfaced fiberglass.

    What I did in my somewhat similar area in my own home is to put R15 mineral wool in the vertical walls, with 3" polyiso on the attic side over the mineral wool. For the floor (my floor is sloped though, and enough so that blown cellulose would probably not stay even over time), I put R21 fiberglass batts (high density fiberglass for 2x6 cavities) in the floor between the joists, with the kraft side down, towards the interior. I rolled two layers of R19 fiberglass over that, each time going perpindicular to the previous layer. This gives R59 at fairly low cost. I used 23" wide R19 rolls too, which cuts down on how many passes I had to make to cover the area I was insulating. In my home, insulating the roof line wasn't really an option because of the way the attic ties into adjoining areas.

    I'd skip mineral wool in the floor here, but consider it for the wall if it will be permanent. Anything temporary I'd stick with fiberglass to save money. On the floor, I'd go with loose fill cellulose or fiberglass batts, with batts/rolls of fiberglass being easier to remove in the future if you change to insulating the roofline. Mineral wool in the floor doesn't gain you as much as it does in the wall, so I don't think it's worth the added expense in that application.


  6. kramttocs | | #6

    Thanks Bill. Sorry, my description wasn't very clear. Plus I threw a lot out there in that post.
    My devil's triangle (great description!), is actually just like the image minus the baffles. So I have the floor insulated with r-19+r30 an the wall with R-19+r-19 doubled up.

    The only roofline that is insulated in that case is the little stretch in the top right corner that connects the devil's triangle to the upmost attic.
    The primary question regarding it is: with no soffit and no baffles. Would it make sense to completely seal that stretch off? So like they did (and I copied) in the bottom right corner with the foam blocking, do that in the top just minus the baffle.
    With the R19 in that little chute right now I know there could be some air movement through the fiberglass from the devil to the upper attic but again, no soffit so I don't know where it'd be coming from.

    Also, I wish I'd have done like you with the 3" polyiso back there instead of the double layer of R-19 on the wall. Would have made airsealing really nice.

    1. Expert Member
      BILL WICHERS | | #7

      I would leave the triangle area connected to the upper attic to allow any moist air (which tends to rise) to get up to the upper attic, which I assumed is vented somewhere, since you mention air currents. Maybe there is a cable vent? Some type of roof vent on the roof deck somewhere? You don't want to create a situation where moisture may become trapped in the triangle area and cause rot/mold on the roof sheathing.

      One of the issues you have with vertical fiberglass batts is that convection airflow through the batt will greatly reduce it's effective R value. It's important (and is now also a code requirement) to have air barriers on ALL sides of batts installed that way. I use polyiso on the attic side of those walls for three reasons: more R value (obviously), which is continuous, so also helps with thermal bridging of the studs, but also as an air barrier for the batts and also an air barrier for air sealing purposes. Mineral wool, due to it's higher density compared to mineral wool, is less susceptible to convection currents, but should still really have that air barrier for best performance.

      In your case, you have R38 of fiberglass over the studs (presumably something IN the stud bays too, maybe R11 fiberglass batts?), which is a lot. You'll lose performance from air currents through those batts, but since you have so much R value to begin with, you aren't likely to notice as much difference as I did. I saw a significant improvement just moving up to mineral wool, even before I put up the polyiso.


  7. kramttocs | | #8

    Yes - there are two gable vents and 6 turtle vents.

    I'll forego sealing those channels off and someday add the baffles. They may be a road 'from' nowhere due to the lack of soffits but maybe air is like dinosaurs and it will 'find a way'.
    Plus it won't be really changing any dynamic regardless of what I do in the upper attic that way.

    Thanks again for the discussion.

    Just to make me look less crazy on the labor aspect of gutting the attic insulation and replacing-
    When I added the r30 that was a process of shoving the rolls through a 20x20 square hole in the closet ceiling on a ladder and climbing up after them. After the joy of that process when I tore off the siding last fall to add xps, housewrap, and new siding I cut a large hinged door in the gable. (see photos).
    The center cutout is filled with a gable vent of that shape so the door is the larger doghouse shape in the exterior photo. It may take more prep to put up the scaffolding, remove some vinyl siding and cut the housewrap/taped seams (retape it all when done) but it will beat the closet ceiling hole. Plus I won't be tracking insulation all through the house. :)

    A discussion for another day (or thread) would be how I could replace the upstairs (two bedrooms and a full bath) attic airhandler/ductwork with minisplits inside the envelope. I suspect that would be beneficial in a number of ways.

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