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Community and Q&A

Attic Insulation Retrofit and Mold Mitigation

Justin_ | Posted in Energy Efficiency and Durability on

Hello, I’m hoping someone can offer some guidance here. In doing some recent air sealing work in my attic I came upon some blackish-gray mold in 3 stud bays on an exterior wall. I did a bit of investigating and it seems that the builder neglected the top plate in the wall below, just in these bays. It’s in a funny area, behind a closet and a nearby roof line above a cathedral ceiling below meets the wall in that general area. First, when I look down into the wall cavity by pulling the R19 batts back from the sheathing I can tell right away the the mold is not in the wall, just up above the wall insulation. I assume this is because I have warm moist air leaking up freely, hitting the cold sheathing thus giving the mold a place to thrive. Does this seem logical?

Once I have dealt with the mold I’m wondering how to best ensure I don’t see the black stuff creep back up the wall in the same area. I was thinking the easiest way I can address this problem is to build an air seal from above and tuck it down into the stud bay. I already have 2″ XPS boards and cans of spray foam on hand. My idea is that I’d recreate the missing top-plate using these materials much like way you air seal rim joists in the basement, just reorient the rigid to lay flat, seal the sides with spray foam then cover the top with regular fiberglass batts. This would be much easier than opening up the drywall below and fixing from the warm side of the wall assembly. Thoughts on this approach?

Lastly, I have loose fill cellulose in the attic now, maybe R50 in total if I can get everything raked back properly but no vapor barrier above the drywall. I’m pulling R30 faced batts out of my basement ceiling (not needed since I’m now heating the space and plan to finish it down the road). I’m wondering if I am better off peeling the craft paper off the batts and adding the insulation “over the top” vs retaining the vapor barrier and placing them down in joist bays, covering them with my left over loose fill after the fact. I won’t end up with enough to cover all of the attic with these, so I might just focus on adding insulation over the master. I’d rent the machine and add blown-in to level it out later. I’m concerned with creating another mold situation up there, hoping for some advice here as well.

I’m in Zone 5. The house was build in 2013. 3000 sq ft finished, 1300 unfinished in the basement. 2×6 walls, fairly tight construction as I had a blower door test done after the first year, 1650 CFM was the reading and I’ve only been buttoning things up as I go so I have to imagine the house is even more efficient now. Appreciate any feed back.


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  1. walta100 | | #1

    I am not there but my guess is what you think is mold is in fact dirt.

    I suspect lots of air had been moving thru the fiberglass over the years and the fiberglass filter the dirt out of the air and changed color from pink to black that looks something like this photo.


  2. user-2310254 | | #2

    I think Walta is correct. My first home had fiberglass batts in the attic, and they proved to be great filters for all the air leaking out of the conditioned space. If you have wall-to-wall carpet, you may see this sort of discoloration at the edges as well.

    Go here ( and scroll down to the "air sealing" section for articles that can help you to address this issue.

  3. Justin_ | | #3

    ok, so I've seen dirty batts before, to me that isn't what I'm looking at here. This is only on the sheathing and a bit of the 2x6s.

  4. user-2310254 | | #4


    Is this an isolated issue? Does the OSB feel wet? Is there a musty smell?

  5. Expert Member
    BILL WICHERS | | #5

    Justin, that recent pic does look like mold to me. What I’ve done in the past with this kind of thing is to treat the moldy areas with a mold killing solution, then paint with a mold killing primer as some insurance against future mold growth in trouble spots. After you’ve done that, make sure to address the issues that caused the mold growth in the first place. Usually you need to do some air sealing work, and sometimes add or repair attic vents.


  6. Justin_ | | #6

    Thanks for the replies. Yes the black stuff does seem to be isolated to this one area. I've been trying to avoid disturbing the area until I have a proper game plan, can't say that I've been able to smell anything and I've not touched the area to see if it's wet. I ordered some RMR-86 and RMR-141 to clean and treat the area. Speaking of attic vents, all that is up there besides the soffit vents are 3 gable end vents. I don't know if these are providing enough ventilation. The roof line is a bit complicated, two major pitches with varying heights, plus a few dormers. I was looking into having a ridge vent installed. One roofer I had out commented on some rust he was seeing on the visible points of the penetrating roofing nails in the attic... I'm not sure if that's part of the sales pitch or if that is really a problem.

  7. Expert Member
    BILL WICHERS | | #7

    Ridge vents used in combination with soffit vents are probably the best way to ventilate an attic. Gable vents aren't as good. Note that once you install a ridge vent, ideally you want to block off the gable vents.

    Rusty stuff in the attic can be a sign of high moisture levels or condensation, but it's not a guarantee of such conditions -- sometimes materials rusted at installation time (construction was going on during rainy weather, etc.), so you have to look more closely at the entire situation to figure out if you're seeing recent moisture issues.

    The mold is your biggest sign here. You probably have an air leak in the area of the old mold that you need to fix, and you may have insufficient attic ventilation too.


    1. oldbungalow | | #8

      What's the conflict between gable and ridge vents? Is the concern that air comes in from gable and goes up the ridge? wouldn't that still be circulatory flow though? Similarly can solar (low voltage fan) vents work in conjunction with ridge vents?

  8. oldbungalow | | #9

    There are some green mold killers that use salicylic (sp?) acid, they smell awful but work, can't recall the specific names.

    Re: order of placement from ceiling wall up, not sure it makes a difference- the cellulose retrofit folks pump loose fill over fiberglass (topping off) all the time.

    I thought kraft paper was a retarder but in any case my gut sense is your described approaches are backwards. I could be wrong.

    If the fiberglass goes first above the ceiling, the vapor barrier will trap moisture in the fiberglass and fiberglass has no moisture holding capacity. So seems to me it's better to use unfaced batts up against the ceiling, so the moisture can migrate up to the cellulose, and cellulose can absorb moisture and slowly dry it out.

    If the fiberglass goes on top of the cellulose, then the moisture migrates from ceiling into the cellulose which has moisture holding capacity and dries to the outside slowly. Faced or unfaced above it shouldn't matter if there's a thick enough layer of cellulose below.

  9. Expert Member
    BILL WICHERS | | #10

    You can use vinegar as a mold treatment too.

    Ridge vents are the “out”, with soffit vents being the “in” for circulating air. If you put other vents somewhere between those two, you short circuit some of the ventilation airflow. This results in less than ideal ventilation airflow. Solar vents aren’t needed at all when you have a ridge vent.

    Faced batts would normally be installed first, with the facer against the ceiling drywall. If you put additional batts in as higher layers, you’d use unfaced batts for that. You are correct that the Kraft facer acts as a vapor retarder, which is not the same as a vapor barrier. It shouldn’t be a problem to put a layer of cellulose over an existing layer of batts regardless of if they are Kraft faced or not.

    Note that you should not put in faced batts with the facer on the cold side in heating dominated climates. I see this done in crawl spaces all the time, where the facer helps support the batts between joists, but it’s not the right way to do things. I would not use faced batts in higher layers if a multi layer insulation assembly on an attic floor either.


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