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Interior insulation options

Jon_Lawrence | Posted in Energy Efficiency and Durability on

I have been soliciting bids for the interior insulation of my current build. I originally spec’d dense pack cellulose in the walls and ceiling and closed cell Lapolla 4G in the rim joists.  The dense pack was to be held in place with a netting (smart vapor retarder), 2×3 horizontal furring strips placed on edge which would also create a service cavity for electrical.  The exterior insulation is 4” of roofing polyiso on the gable roofs, an even thicker layer on the low-slope roof and 3” of ROXUL on the walls. The exterior insulation thickness was designed to insure that the ratio of exterior to total r-value was at lest 31% (CZ4).  The exterior sheathing is Zip. To my surprise, I was able to find subs who have already converted over to 4G.  I realize I don’t need CCSF in my rim joists, but none of the vendors will do open cell and trying to get dense pack in that area is difficult.  To my surprise, the subs were also using damp spray cellulose and recommending I use that over dense pack even though I would be installing the vapor retarder and furring strips myself.

So now I am contemplating using damp spray as it would solve a few issues for me.  First, it would make a netting unnecessary which saves me money and time.  I still plan on using a service cavity in the walls  regardless of which insulation method I go with.  Second, it would solve an issue I have at the ridge in my gable roofs.  The collar ties make it very difficult to get netting attached to the bottom of the rafters for the dense pack.  Basically I would have to come up the rafters starting at the floor and then go across the bottom of the collar ties.  This would put my exterior/total r-value ratio at less than 31% in the exact area I am most vulnerable to condensation.  I could use CCSF there, but the downside is that I will never find a leak (it will happen eventually) until it is too late.  However, the damp spray would allow me to fill the entire rafter space, even the area in the ridge above the collar ties, with the right amount of insulation without the issues mentioned above, plus more money and time saved by avoiding netting.  Lastly, it would be cheaper to use damp spray in the rim joists, obviously less r-value, but with r-12 continuous on the exterior, not sure I am losing much, especially when it comes to the 1st and 2nd floor rim joists.  It might still make sense to use CCSF in the basement rim joist.

Now for my concern.  Damp spray is going to add quite a bit of moisture into the assembly.  I don’t like adding moisture to OSB and I believe Zip sheathing is less than 1 perm (I think they advertise 1.2 for the OSB, but the don’t say what it is with the coating). Also my exterior roofing insulation is already in so no drying to the outside at the roof.  The good news is the house is dry now, the windows are installed so I can open when the weather is favorable for drying, I can do this in September which is typically warm with lower humidity, and I can wait about a month after installing the insulation to install the drywall.

My gut is telling me that damp spray throughout would be be the best choice given my situation.  Thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    There's nothing wrong with damp-spray cellulose, as long as you leave a drying path to the interior for a reasonable amount of time. Use common sense, and I'm sure everything will be fine.

    For more information on installing damp-spray cellulose, see "How to Install Cellulose Insulation."

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