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Suggestions for retrofit insulation in a ceiling with scissor trusses in Very Cold Climate.

pjmeg | Posted in Energy Efficiency and Durability on

I live in Northern Wisconsin (Climate Zone 7).
Here’s my current configuration (exists from when I purchased the house).
Scissor Truss with Roof slope of 4/12 and ceiling slope of 2/12.
Roofing consists of ½” plywood, roofing felt, and asphalt shingles with a vent at the ridge.
Insulation consists of Kraft-faced Fiberglass (estimated at R-20 to R30) stapled to bottom chord of trusses.
Underside of roof deck is ventilated from soffit to ridge.
Ceiling is finished with T&G cedar and does not have a gypsum board backer for a fire block. I am aware of that these ceilings are notoriously leaky (although we haven’t experienced a drafty feeling in the one winter we have lived in this house) and are fraught with issues.
From reading around this site, it sounds like ceilings that were finished with T&G boards (30 years ago) routinely left out the gyp board backer. I have not had an opportunity to remove some of the ceiling boards to see the condition of the underside of the roof decking to know if there is any rot or mold growth occurring.
I need (and want) to better insulate the ceiling. To accomplish this I plan on pulling down all the T&G and FG insulation (about 500sf). I want to continue to ventilate the roof deck.
For insulation my initial thought was to utilize ccSPF because of its high R value and air sealing capabilities, but my pre-school age son is asthmatic and we have concerns based on the experiences of some people with chemical sensitivity to some component(s) of this product and the potential for the lingering fumes wreaking havoc with his asthma. Thus I am asking for ideas for alternative insulating methods/materials and methods for air sealing.
My thoughts for the insulation have consisted of:
1. Cutting Rigid Insulation Boards to width and gluing them tight to custom fabricated vent chutes consisting of polyiso board insulation. Due to the limited difference in roof pitch between the roof deck and the ceiling plane I anticipate needing to have the vent chute run nearly all the way to the ridge in the center of the room (room is 20’feet wide) which will allow me sufficient room for the insulation. The rigid insulation would fitted between the trusses, and I would foam the perimeter of these boards with canned foam. My only issue with this is getting a good seal around the perimeter of the insulation board with the expanding foam sealant. I would layer as many boards as I can into the cavities until I either run out of available truss depth or achieve around R-60 (whichever comes first)
2. Using Cellulose Insulation (Dense Pack maybe) with the aforementioned custom vent chutes. However I am imagining the logistical complexities of doing this as I do not have any access from above the area I want to insulate. And I don’t know if dense packing is done (or permitted) in attics given the climate which I live. This would be air sealed by installing gyp board with the Airtight Drywall Approach.
3. Leaving the FG in place, and then securing several inches of XPS rigid insulation (seams and perimeter air sealed) to the underside of the bottom chord followed by screwing 1×4 wood strapping to the trusses. The strapping would give me something to screw the new gyp board to as well as nail the T&G into. My resistance to do this is based on aesthetics as I have a connecting hallway that enters the room with the vaulted ceiling along the wall and the lower ceiling (as compared to the ceiling height in the hallway) would catch my eye every day and slowly drive me crazy.
Additional considerations…I don’t need a new roof (yet) to warrant removing the roofing and placing the insulation on the outside of the plywood roof deck and the new insulation I would need on the gable end of the room to effectively seal the truss space and bring it into the conditioned envelope.
Thank you in advance for your time and opinions.

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  1. user-1127834 | | #1

    In one of 4 pockets of near zone 6 in a humid wintering of HDD of 6600+ and many seasonal -15 below to peaks of -18 to record -22 in Jan 1994,,, and since insulating with cellulose packing contracts of 1989- on, it has been extremely successful in hiring a team of pros that had a 1hp blower-booster/regulated- at the end of their cellulose hose from the truck. We have been able to guarantee non-settling, for 2 owners of a building (about the wall packing). The had a 1.1/2" o.d/1.1/4" id stiff tubing, flexible enough to ram old fiberglass up a stud space and pack in cellulose.
    Years later removing a bathroom flush in wall cabinet, the insulation was tight and dry an did not move from the cavity. Packed vaulted areas of higher ceilings of 8" and to 12" spaces and low pitches, very snug, has worked out nicely, minimal 75% borate has been used and their material of UL- grade A has not had odors that others have experienced. (?-dry boric-acid product that others can expound about, some other co claim over 95% is necessary).

    Over fiberglass the cellulose has more than effectually-enough sealed attic air -through insulates, with just a 6-8" loose blow-on. Attic-plates at the tops of old stud and construction(s) may need more attention to air-sealing.

    ((forced-air?- having or adding)) Any high return-air modification (highly recommend HIGH RETURNS for such low cooling requirements at even zone 4 and up) in a wall cavity can be sealed at the inner top of the airway with any light cardboard cut-in as a 'plate' and caulked, if not near a plate of wood (you might reach/look to know) which should be caulked. (High returns can be blocked by sometimes older odd blocks of wood in the middle of a wall.)

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