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What’s the deal with dense pack cellulose?

rhl_ | Posted in General Questions on

I’ve been planning to dense pack my 2×4 walls in my old 1930 era Tudor. It has the original stucco exterior.

there are all these websites out there that discuss “cellulose” and paint peeling on old walls.

They aren’t being specific as to if this happens with dense pack or not. Does it matter?

This means there is air leakage through a cavity, I’m guessing mostly the exterior wall?

What are the details here for retrofitting an old house when we aren’t going to complete replace the exterior stucco?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Ryan.

    If the walls have sheathing and a WRB, even felt paper, it is my understanding that this a relatively safe retrofit that will give you some thermal and some air sealing value, plus a moisture buffer (the cellulose). Of course all water management issues (leaks) should be fixed first. If you don't have a WRB, sheathing, roof overhangs, or are concerned about unknown leaks, a consrvative approach would be to use the technique described in this article--Insulating Walls with no Sheathing--to give the wall the potential for what Lstiburek calls "hygric redistribution."

  2. rhl_ | | #2

    In my house I have the shiplap as identified by Joe, I believe immediately behind that is a black felt paper, then there is the stucco siding. There are no roof overhangs. Since I have the WRB and something for sheathing, it's not clear to me if this is clearly risky? Following the conservative approach certainly means removing the interior walls, you can't hope to retrofit by just dense packing through a hole in the wall.

    It seems like I should use this method? Or do you think the risk is actually pretty low with the WRB paper on the exterior? In that article it shows the felt paper on the interior. I don't have that.

    It seems like you don't really suffer much on a thermal loss front, the air gap is small, and it affords us the ability to really make sure the insulation job is done well?

    1. GBA Editor
      Brian Pontolilo | | #8

      Hey Ryan.

      I won't give a firm answer on this because I have seen failures and retrofits with no problem, but don't always know what makes the difference. Working from the inside and creating the air space inside the wall is a safe bet and as you noted, that air space can be as minimal as 3/8 inch and still be effective. You will also have the opportunity to air seal from the interior and you could consider some level of interior vapor control, both of which would be helpful.

  3. Expert Member
    Michael Maines | | #3

    Ryan, what is your climate zone? In cold climates, insulating walls reduces the heat energy available to dry walls by pushing moisture through to the exterior so adding any vapor-permeable insulation increases the risk of paint failure. You can reduce the risk by keeping indoor relative humidity levels low, and you might consider using vapor-retarding paint.

    This article covers the physics:

  4. rhl_ | | #4

    Zone 4A, New York City Area. The walls are 2x4. No double stud, no OSB, no modern anything.

  5. Expert Member
    Akos | | #5

    Insulating painted clapboard is not the easiest, it is impossible to say upfront if there will be issues. There are too many variables to say that the paint will hold. If the paint is water based, you have a WRB and decent flashing details, there is very little risk.

    Generally, if it is older clapboard with oil based finish there is a hihger chance of issues.

    One option would be to insulate part of a wall and monitor it for a year or two before insulating the whole house.

  6. Expert Member
    Dana Dorsett | | #6

    >"In my house I have the shiplap as identified by Joe, I believe immediately behind that is a black felt paper, then there is the stucco siding. "

    If there isn't a vented air gap between the felt and stucco the paint is almost certainly going to fail no matter what cavity insulation is used, whether vapor permeable or not, though a vapor permeable cavity fill such as cellulose would lower that risk slightly, at the risk of getting waterlogged if there isn't adequate flashing at the windows, and other bulk-moisture management details.

    If there IS a vented air gap (weep screeds at the bottom, the cavity vented to the outdoors or into a vented attic at the top) the paint should hang in there just fine (no matter what insulation is used.

    When the cavities get insulated, the average wintertime temperature of the stucco goes down, and it retains more moisture than it would previously. When the morning sun hits the now damper stucco the sun cooks out some of the moisture as water vapor in very intense bursts. With nowhere else to go, the water vapor pressure causes the (comparatively low permeance) paint to separate from the stucco. If there's a vented air gap, the moisture goes into the gap.

    This problem isn't isolated to stucco- painted clapboards and shingles can and do suffer paint failures post-insulation. But the effect is more pronounced with stucco than with wood siding due to the comparatively high moisture buffering capacity of stucco and relative to wood- it retains a lot more water. Painted brick/masonry has it as bad as stucco, sometimes worse.

  7. rhl_ | | #7

    Thank you all for the clear explanations.

    It's clear that it would be safer to follow the conservative approach and add an intello barrier or something in the wall.

    For whatever its worth, I did this sort of job in my daughters room 2 years ago, it wasn't dense pack, but big box store mineral wool with kraft paper.

    So far no paint peeling, but, it is also walls on the north/west side of the building.

  8. maine_tyler | | #9

    What's the fundamental difference between an 'old' structure that's been retrofitted with insulation (assume sheathing and basic wrb, like felt) and a new, 2x4/6 stick-quick-built modern home that's also insulated with basic details (no rainscreen, standard wrb, osb sheathing, fiberglass or cellulose insulation, non -airtight drywall, etc.)?

    I know some people say, 'oh there's no vapor barrier in the old one.' But certainly we're not (always) putting VB's in modern builds and we're not always getting airtight construction, no?

    1. Expert Member
      Peter Engle | | #10


      There's no fundamental difference between the two. Stucco fails dramatically on modern stick-built homes with basic details all the time, in all climates as does adhered stone veneer, which might even be worse. The use of an air gap material is becoming more common for this reason, though still not required by US building codes.

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