GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Moisture issue with improperly dense packed ceiling?

bsampson | Posted in GBA Pro Help on

Does anyone have any idea what the chances are of a moisture issue(s) occurring in an unvented vaulted ceiling where the cellulose insulation is not in contact with the roof sheathing? The home is located in a cold climate (New York) and there is anywhere between 1 to 3″ of air space between the cellulose insulation and the roof sheathing in the lower sections of the vaulted ceiling (as observed through an inspection hole that was drilled in the exterior soffit) while the cellulose in the higher parts of the ceiling appears to completely fill the rafter cavities. Just wondering how likely or unlikely it is for condensation to form in the areas with inadequate cellulose density. Any thoughts?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    The notion that "condensation will form" is technically inaccurate- both the wood and the cellulose will adsorb the water directly, and liquid water would only become evident after they become saturated.

    Where in NY are you? Got a Zip code? NY state covers quite a range of climate zones, and climate matters.

    So does the roof deck material type (OSB? Ship-lap planking?) and the finish roofing type/color.

    Dense packed cellulose is not sufficiently air-impermeable even at 4lbs density to keep wintertime moisture from migrating to the roof decking and would constitute a code violation. But whether it's a real problem that needs to be addressed immediately kinda depends.

    Cellulose can handle quite a bit of moisture by itself without losing function, and by buffering that moisture it is protective of the roof deck. But high seasonal moisture cycling is also what causes cellulose to settle, which may be why you're able to find sections where it no longer fills the cavity. At a specified moisture cycling level it's possible to define a density at which it won't settle, but in a roof assembly in US climate zone 6 that density is probably higher than most blowing machines can execute. Most installers run it to 3.5-4lbs and call it a day, which may work fine even for a decade in an unvented roof, but probably not a century.

    IIf we can determine that it's not on the urgent list, the long term solution is to add the IRC-prescriptive amounts of rigid foam over the exterior side of the roof deck for your US climate zone when it's time to re-roof, and topping off the cellulose by drilling from the exterior when that happens.

    But tell us more about the roof assembly- how thick is the cellulose, type of interior finish (including paint), how air-tight is the interior, any vapor retarders/barriers, etc.

  2. GBA Editor
    Martin Holladay | | #2

    This type of insulation doesn't meet code. If you want to insulate a cathedral ceiling with cellulose, the code requires a ventilation channel between the top of the insulation and the sheathing, from the soffit to the ridge. You need a baffle to create this channel.

    Whether or not you will get moisture problems with this error depends on many factors, including (a) whether or not your ceiling is airtight, and (b) the relative humidity levels in your house. (Lower RH is better than higher RH.)

  3. bsampson | | #3

    Hi Dana. I'm in climate zone 5. The roof decking appears to be 1x6 tongue & groove finsihed with architectural asphalt shingles (Foxhollow grey). The rafters are 2x8 and there is quite a bit of old batt fiberglass insulation underneath the cellulose. The interior is all 1/2" sheetrock with two coats of Benjamin Moore Regal Classic (100% acrylic) applied to the walls and ceiling about three years ago. With the exception of a woodburning fireplace in the room with the vaulted ceiling, the home (built in 1957) is relatively air-tight with all new energy efficient windows and doors. The bottom of the vaulted ceiling terminates in a 2' exterior soffit and there is alot of air leak into the soffit from between the outside wall sheathing and the top plate. The soffit itself is also quite leaky between the roof sheathing and the top edge of the facia board. What else can I tell you?

  4. bsampson | | #4

    Hi Bill. That's an interesting point. Would you happen to know whether or not the insulation contractor that installed the cellulose could be held liable for the error? This was a retrofit installation and a building permit was never obtained.

  5. Expert Member
    Dana Dorsett | | #5

    The air leakage on the interior side is moving the lions share of moisture into the space, and probably what is causing the cellulose to moisture-cycle to the point of settling, despite being nominally dense-packed.

    The interior acrylic paint is sufficiently vapor permeable so as not to create a moisture trap, but is modestly vapor retardent (both of which are good.) If there is any way to air-seal the interior better, it will lower the risk of problems getting started.

    Plank sheathing is a lot more tolerant of this situation than OSB, which would more than likely would have already turned into a mold-farm. If you can, snake a flexible inspection camera into your peep holes and assess whether mold/mildew is getting started on the decking and report back. If it still looks pretty good, you may have years before any real problems get going.

    With grey architectural shingles you get a decent amount of solar heating and roof temps much higher than the daily high temperature (even in winter) which is also somewhat protective, since it drives moisture accumulated overnight back out of the susceptible wood. The most susceptible pitches will be the shaded and north-facing sections that get less solar gain, so if you haven't already, that would be the more critical part to assess. South facing pitches can be remarkably robust, even in this non-code type of installation.

    FWIW: National Fiber (a regional cellulose insulation manufacturer in MA) swears by and will warranty dense-packed cathedral ceilings if installed by one of their approved contractors, even in US climate zone 6, (but I still wouldn't do it.) If you're on the warm edge of zone 5 the risks are much lower than on the cool edge of zone 6.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |