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Q&A Spotlight

Right Idea, Wrong Result: A Cellulose Insulation Job Goes Off Track

Too much cellulose in the wrong place creates a winter nightmare

Too much of a good thing. Cellulose oozing between roof slates is soaking up winter moisture and creating problems inside the house.
Image Credit: Jason Shapiro

Jason Shapiro took the plunge and invested in more insulation for his house: blown cellulose for his attic and dense-packed cellulose in the exterior walls. No doubt he’d like to be enjoying a warmer house and lower energy bills. Instead, he’s dealing with a mess.

“We have now had significant snowfall and freezing rain, and today I noticed a leak from the top of a window casing in the wall that was dense-packed,” Shapiro writes in a Q&A post at GreenBuildingAdvisor. “When I went up on a ladder to look at the gutter, I found a significant amount of frozen cellulose protruding from a gap between the slate roof tiles and the sheathing underneath.”

It looks like the cellulose was wicking up moisture from the outside and allowing it to get into the wall, but why is the excess cellulose there in the first place? Shapiro says foam insulation baffles were placed between each rafter before the cellulose was blown in, and the top plate was sealed with spray foam.

“The soffits appear to be filled with cellulose that is poking out through the vents, and which can be easily seen from the outside of the house,” Shapiro says. “I am also able to reach up through and behind the gutter and under the sheathing and can feel a large amount of wet cellulose. The contractor is saying that there must have been a problem with the roof that allowed the cellulose to communicate to the exterior in this way. He is also saying that there is no need to remove the wet cellulose because it is treated with borate and will not mold.”

Source of the problem

GBA senior editor Martin Holladay points to one potential cause: baffles that weren’t designed for the application.

“Styrofoam baffles are flimsy and are the wrong type of ventilation baffle to use in this application, especially for a home with slate roofing over skip sheathing,” Holladay says. “I’m guessing that the cellulose is finding its way through the cracks at the sides of the flimsy baffles and is filling the spaces between the skip sheathing and slates. In addition, some of the cellulose filled your soffit.”

Daniel Ernst thinks the baffles didn’t completely span the gap between the top of the plate and the roof sheathing, creating a gap through which cellulose could escape.

“It sounds like the cellulose installer did not adequately investigate the building structure before blowing the cellulose,” he says. “Without some additional information, it’s hard to say how the cellulose made it into your soffit. It could have been placed there during either part of the installation — the wall OR attic blow. The fact that your contractor wasn’t able to figure out how this happened isn’t very encouraging.”

Skip sheathing also might be a contributing factor. Unlike typical roof sheathing, ship sheathing consists of boards with gaps between them, so the roof deck is not continuous. The sheathing is spaced to provide structural support and a nail base for wood or slate shingles, but also allows drying on the underside of the roof deck.

“This was fairly common in older houses,” Ernst says. “It might help explain this fact, if the cellulose arrived through the attic blow.”

In an old house, never assume

Shapiro wonders whether the cellulose came from the dense-packed cellulose in the wall rather than from the attic installation. But how could the insulation have migrated into the soffit unless there was some defect in the top plate?

“I think it’s very likely that the majority of the cellulose under the slates (and in the soffit) came from the wall blow,” Ernst says. “There’s just too much of it — and it appears to have been forced into place from the dense-pack operation.”

“Are you even sure that the walls had top plates?” he adds. “Older houses were often balloon framed, didn’t necessarily have continuous top plates.”

Charles Shade suggests another scenario: missing wall sheathing between the top plate and the soffit, creating a series of open voids through which insulation could easily be pushed out as the wall was insulated.

Adam Zielinski agrees: “I’ve seen this before where the outside soffit is lower than the inside interior ceiling,” he writes. “You can’t be sure that the sheathing is intact and insulation won’t get into the soffit.”

All of it points to the vagaries of framing and construction practices in older houses. Oddly spaced framing members, missing sheathing and inexplicable crevices and gaps all can be par for the course.

“When you are installing cellulose in an old house, you can’t make ANY assumptions,” says Holladay. “It’s ALWAYS possible that the cavity you are filling with cellulose communicates with an adjacent cavity.”

In the end, it’s the contractor’s responsibility

The roofing contractor apparently believes that because the cellulose has been treated with borates, excess water won’t be a problem. But chalk this up to wishful thinking.

“You don’t want to have sopping wet cellulose insulation hanging out in your wall cavities or your soffit,” Ernst says. “It will not dry quickly enough, and could lead to more serious damage. It’s likely that you will have to open up your wall cavities to remove the wet insulation. Some judicious probing and testing with a moisture meter would help determine if this is the right path, and the extent of the ‘leak.’ ”

Shapiro says the contractor has already made one trip over, and is due back to discuss what steps to take next. More than one poster thinks that’s where the responsibility should fall.

“What kind of rookie insulation contractor would blow a bunch of cellulose up under a slate roof like that?” asks David Meiland. “Just when you think you’ve seen it all…”

“This is a mess, and the cellulose installer is responsible for making it right,” Holladay says. “If he doesn’t come back to fix the mess, it’s time to go to small claims court.

“It may not have been preventable (although sometimes cellulose installers stop blowing when a cavity seems to require more insulation than expected, and sometimes cellulose installers have a helper keeping an eye out for examples of cellulose popping out where it doesn’t belong). But, whether or not it was preventable, the results are the insulation contractor’s responsibility.”


  1. Nicholaus Baxter | | #1

    Pull down the soffit
    I suggest pulling off a section of your soffit to see where the insulation is coming from. It should be evident if it came from the wall or the roof. It is the insulation contractor's responsibility to remove and replace wet insulation as well as repair any damages from the poorly installed insulation. Be careful in allowing them to do work beyond the insulation task, if you have insulation installers out there removing and replacing soffits, 9 times out of 10 they will do a shotty job and you will need to get it redone anyway.

    Get a second opinion. Find a general contractor that can assess the damages, the cause, and the solution for you. This will help you know what the insulation installers need to do. Get a quote from the contractor to fix the damages, if there are areas that need to be fixed or replaced such as soffits, siding, roofing, or gutters, outside of what the insulation installers are capable of, it will be the insulation installers responsibility to pay your contractor for the repairs.

    It is a messy situation dealing with poorly done work, and likely you will end up paying for something out of your pocket, but you most definitely do not want to leave any sort of water intrusion issue alone. It will not dry out, and nothing in the building world just goes away, it only gets worse. Down the road you can end up with rot, or water intrusion issues that make their way into your home.

  2. Matthew Amann | | #2

    Obvious previous leaks.
    If there weren't openings in the assembly to start with, there wouldn't be wet cellulose. Sounds like the baffles sealed the deal, either weren't wide enough, or not tall enough, and/or the installer was blowing sloppily. Cellulose obviously shouldn't be in soffits or against ANY skip sheathing.....missing fascia?

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