Is this what dense-packed cellulose is supposed to look like?
Hi all,
I’d be interested in other folks’ opinion on this. I paid an insulation contractor to dense pack an unvented vaulted ceiling in my home a little over 2 years ago. I started experiencing some pretty bad moisture problems in the past month or so with the ceiling and exterior soffit. A couple of the seams of the sheetrock inside the house are starting to show on the ceiling and the outside soffit drips even when the sun is out.
I drilled several 3″ round holes in the exterior soffit this past weekend to inspect the rafter cavities and was shocked at how little cellulose insulation there was. I’ve attached a copy of the photo I took by sticking my cell phone up through one of the holes. The insulation contractor said that the reason it looks like there isn’t very much cellulose in there is because the moisture has likely caused it to settle over time.
Is this really what dense-packed cellulose would look like if it got wet and settled? Incidentally, this particular rafter cavity doesn’t appear to have any moisture problems.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Not a great photo, but from what I see it just looks like an under-filled space, not dense pack that has settled.
Where are the moisture problems coming from? As far as I'm concerned the jury is out on whether your can reliably dense-pack an unvented roof, depending on your climate zone and a few other factors.
Actually, I think it's more accurate to say that the jury is back and have ruled that you can't reliably dense-pack and unvented roof. Here is some reading material: http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems
Thanks for sharing that study David. As I recall, the installer made no attempt at air sealing anything, which might be part of the problem. I know for a fact that the exterior soffit is leaky because insulation was shooting out of the seams all over the place during the installation of the cellulose. I also have a bathroom vent pipe running up through part of the ceiling that was never sealed so I guess it's possible that moisture could be entering through there as well. Wish I had known then what I know now about air sealing!
Dense-packing a cathedralized ceiling in cooler climates still comes with some risks, but the risk is pretty low in US climate zones 1-3 where wintertime roof deck temps are higher, with lower adsorption of humidity from the interior air. The risk is highest on north pitches with OSB sheathing, since the average temp of the roof deck is then cooler (taking on and retaining more moisture, on average), and the material (OSB) is the most-susceptible to moisture damage & rot. For t & g or ship-lap roof decking the risk is quite a bit lower, with plywood somewhere in-between. Straube's WUFI simulations in RR-1001 show near-zero risk for the cellulose insulated assemblies with OSB roof decking in zone 3 & lower, but substantial risk in zone 4 & higher. But only (the most susceptible) OSB clad assemblies were simulated. The picture here shows (the least-susceptible) plank decking.
That doesn't look like dense pack. At 3+ lbs density pushing on it with a finger is like a firm to very firm mattress- it'll be quite springy and it takes signficant push to penetrate it with your whole finger. At <2.5lbs density you can just jam a finger in it with fairly minimal resistance.
Cellulose settling is a function of both density and seasonal humidity changes. If it's cycling a lot of moisture in/out of the material there will be mechanical creepage and it'll lose some loft over time, but for any given climate & moisture conditions there is a definable density above which it simply won't settle. (This was modeled and tested very carefully by a researcher at Aalborg University in Denmark named Torben Valdbjørn Rasmussen- some of his academic stuff on the subject are available online in English, if you really care about the details.) If this was truly dense-packed it would take several years or even decades to settle appreciably, unless it's getting massively soaked somehow. (Ice dams backing up, soaking the roof deck & cellulose, mayhaps? Roof leaks?)
If the cellulose isn't sufficiently dense and the interior side isn't air tight there could be substantial seasonal moisture stored in both the roof decking and cellulose, but it would have to have been totally sopping-saturated to still have dripping occurring in late July. There's probably a small roof leak saturating some section of the cellulose, but since the cellulose is redistributing it, little to none is being dripped inside the house.
That's very helpful Dana. The vaulted ceiling faces northeast. It is possible that rainwater is entering the exterior soffit via surface tension through one of the seams between the roof sheathing and the facia board. Since the soffit was filled with cellulose there may be some amount of wicking, although the line goes up the ceiling a good 5' above where the soffit begins and given the pitch of the roof it seems like gravity would win out over wicking. Do you happen to have any thoughts about the pros/cons of filling the exterior soffit with cellulose? Presumably, the installer did this because it was easier to fill the rafter bays from above in the attic than it was to open up the soffit and fill the bays from below and install blocking at the wall plate. However, I can't help but think that some of the moisture problems I'm experiencing are related to the leaky soffit.
Tony,
David and Dana have given you good feedback. Here are my reactions:
1. That is not a dense-pack job.
2. You never should have tried to dense-pack your cathedral ceiling in the first place unless you also included a ventilation channel between the top of your insulation and your roof sheathing. Insulating an unvented cathederal ceiling with cellulose is a code violation.
That said, Dana might well be correct that you have a roof leak. If you think that your ceiling leaks air, you have a whole bunch of issues that you need to deal with. Fix your roof leak (if you have one) first. And when you decide how you want to insulate your ceiling, address any air leaks in your ceiling as part of the insulation job.
More information here:
How to Build an Insulated Cathedral Ceiling
How to Install Cellulose Insulation