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Will blowing cellulose insulation into soffits cause problems?

PatDonahay | Posted in Energy Efficiency and Durability on

Sorry for the length of this question. . .I am trying to provide all the necessary background.

I am planning on adding cellulose on top of the blown fiberglass in my 1 story (brick veneer) house in Oklahoma (zone 3). The roof has gables in the front and a hip roof for the other three sides. (The roof was “stick built” rather than using trusses.) There are 2 large louvered/wood gable vents in the front of the house, and several very large vents (made from those fake plastic window shutters) that I added to the soffits on the rear & sides of the house. The are whirly bird turbines on the roof for exhaust. The HVAC supply ducts are in the slab, and the 3 return ducts (14″ insulated flex) run along the attic floor, partially buried in insulation. I have been spending many hours in the attic sealing the can lights, top plates, bathroom fans, etc. using gallons of duct mastic applied with a paint brush. Years ago (before I found this site) I stapled a radiant barrier to the rafters. The access ladder to the attic is in garage, so this is not an air leakage point. The furnace and hot water heater are in a utility closet connected to the garage and draw air from the attic for combustion.

I am confident that between the 2 gable vents and the large soffit vents that I added that the attic is well ventilated, although the air flow is not evenly distributed.

My question is whether I would create problems if I let the soffits (where I don’t have the large vents installed) fill up with cellulose when I blow in the added insulation? As the house was originally constructed in 1992, the “soffit ventilation” was a joke installed just to meet the letter of the code. There are these 4″ X 12″ screens installed about every 10 feet in the soffit. You can tell from dust clogging the screens there isn’t any real airflow. Even worse is the original insulation installer stapled tar paper at the roof-ceiling intersection to prevent the fiberglass from entering the soffit. I believe this is the accepted practice around here, even though it makes the soffit vents largely useless.

My concern is that the ceiling insulation at the floor/ceiling intersection can’t be very thick (due to the geometry). I don’t know the exact R value (in the section) of the current blown fiberglass, but I doubt it is very high. My idea is to remove the tar paper at ceiling/roof joint, stick the cellulose blower hose into the soffit, and pump it full with cellulose. After the soffit is full with cellulose, I would pull the hose out and pretty much fill the entire triangular section area inside the attic where the roof/ceiling meet as densely as a can. My feeling is that cellulose is cheap enough that pumping that area with cellulose would be reasonably cheap and greatly reduce the heat loss in this area.

I know some people will point out the lack of (desired) air space between the insulation and 1 X 8 roofing boards. However the same situation already exists in several rooms of the house where there is a section of “cathedral ceiling” where the 10′ ceiling intersects the roof line. In these sections there is blown fiberglass sandwiched between sheetrock and the roofing boards. I have been in the house for 20+ years and have not seen any problems from this, although the R value of this roof section is only about R-13. This configuration seems very common in this area and I am not aware of widespread problems resulting.

Anyway. . .any feedback would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Considering the situation you described, your plan is probably OK. Sometimes the best result we can hope for is based on a series of compromises.

  2. iLikeDirt | | #2

    My attic (5B, high desert) has the same laughable, mostly non-functional soffit vents with no evidence of condensation anywhere. I'm happy to hear that covering them with cellulose isn't likely to be a problem, as the roof pitch and truss layout make getting in there to staple baffles to the roof decking all but impossible.

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