Past Three Years
Cathedral Ceilings
From the beginning, we envisioned cathedral ceilings in lieu of an attic but was unsure as to how they should be constructed to maximize energy conservation. My first choice for the roof-ceiling complex was structural insulated panels but as discussed in the previous post, SIP discussion, they were not budget-able. So, that's OK -- we'll just throw up some 2 x 12s, sheath and waterproof the tops, drywall the bottoms and insulate in between, right? Whoa, I was soon to find out that it's not that simple.
Moisture Condensation
Moisture condensation occurs when the ceiling is not air tight, which includes most ceilings. As air moves from the
interior, it takes moisture with it that condenses as it approaches the cold side of the roof in winter, rotting the wood, encouraging mold and lowering the insulation R-value. And this phenomenon is much more critical for cathedral ceilings than for ceilings under attic space. The moisture reaching attic space is dried by attic ventilation before it can do damage. But not so with enclosed cathedral ceilings.
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| Download pic: 2 x 12 rafters for a cathedral ceiling |
The internet is rife with the pros and cons of using the typical sheathing-rafter-drywall approach for cathedral ceilings. My interpretation of the chatter is that 2 x 10s or 2 x 12s with sheathing and drywall will work under some circumstances and not others. When urethane-type spray foam fills the space between sheathing and drywall, air infiltration is nil, even without a sheet plastic moisture barrier, so moisture condensation is moot. But spray foam does not fit our budget. When it comes to dense pack cellulose or dense pack fiberglass insulation, arguments fly back and forth -- some say they inhibit air infiltration enough that condensation is not a problem and others warn against using them in a closed cathedral ceiling. Batt insulation is leaky enough that, even with a plastic moisture barrier, is probably not worth the risk.
Mini-Attic with 2 x 12s
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| Download pic: Ventilation for cathedral ceiling |
The nay-sayers advocate using what essentially is a mini-attic at the top of the rafters to allow any air passing through the ceiling to exit into ventilated space. Then, even if some condensation takes place, it readily drys because it is in contact with moving outside air. This is the cautious approach I have decided to adopt so we can use any type of insulation and not worry about condensation issues. And, to stop air from entering the ceiling in the first place, I plan to use at least one layer of carefully-detailed 6 mil plastic sheeting as a moisture barrier below the rafters before installing the tongue and groove natural wood ceiling which, unfortunately, has more potential for air infiltration than drywall unless it is backed up by some sort of solid sheet material like 3/8 drywall or Masonite. (Recent update: As voiced in subsequent posts, I learned that plastic sheeting above the drywall is ill-advised for our climate; it will not be included in our design.)
To create a mini-attic, I plan to use structural (construction) screws to fasten 2 x 4s on edge and on 24" centers at a 90 degree angle to the
rafters and fasten the sheathing to them. Then, by keeping the insulation flush with the tops of the rafters, the "attic" will comprise the 3 1/2 inch void between the insulation and the sheathing that will be ventilated through the soffets. I had already planned to use foil-faced OSB board for sheathing in order to reflect radiant heat in summer. It is typically installed with the foil side down, which should also allow it to handle better than the "raw" OSB side of the sheathing any moisture condensation that reaches the mini-attic and takes a while to dry.
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| Download pic: Natural wood ceiling |
R Factor
Since I plan to insulate both the walls and the ceiling with rice hulls that are rated at around R-3 per inch, a 2 x 12 roof gives at least an R-36. But, consistent with super-insulating, I would like to shoot for R-50. Consequently, I plan to secure with structural screws a 2 x 4 edgewise to the bottom of each rafter in order to provide for an extra three and a half inches of insulation.
However, the above modality does not address thermal bridging through the rafters. I plan to handle this problem by ripping 1" thick foam board into strips and sandwiching them between the bottom of the rafters and the edgewise 2 x 4s. They will also increase the height for the loose insulation by another inch. The roof will then be at least R-50 with thermal bridging controlled.
Mini-Attic with I-Joists
An alternative choice for a mini-attic system would be 15" I-joists instead of the stick-built 2 x 12 system described above. The advantages would be simple installation, fabrication from renewable sources and minimal thermal bridging. Their disadvantages would be greater difficulty mating them to the 15" exterior walls than with the 2 x 12s and higher cost. Not only are the I-joists more expensive than new 2 x 12s per linear foot but the stick-built approach will allow the use of materials recycled from tear-downs for an additional savings.
Recent Update
Recent Update
Much of the above design became moot late in 2016 when I decided in favor of 16" and 18" tall roof trusses in lieu of joists -- either 2 x 12s or I-joists -- and a double layer of roof sheathing with a 3 1/2" space between layers to serve as a dedicated mini-attic. Makes me grateful that the building inspector was happy with architectural drawings that were sufficiently nonspecific that I could improvise on the fly.
