Replies

1. 
   Doug McEvers | Nov 29, 2009 09:13am | #1

   Stop the roof sheathing short of the ridge beam to allow for an airspace, use a vented ridge like Shinglevent. When using TGI's I have attached 1/2" fiberboard sheathing to the bottom of the top flange to form a dedicated airspace, this allows for easier insulation, either batt or blown.

   12" ceiling insulation is kind of on the margin for real efficiency, you might consider a 16" TGI or a vaulted parallel chord truss.

2. 
   Michael Chandler | Nov 29, 2009 11:17am | #2

   I'd at least get a couple or three quotes on spraying R-38 spray foam up there and making it an un-vented roof assembly. Open cell at R 3.6 would take 10.5 inches to do this but you would probably be better off with closed cell in your area. You could still use blown-in fiberglass or cellulose in the walls and any flat ceilings if you wanted to cut costs.

   M

3. 
   Robert Riversong | Nov 29, 2009 02:02pm | #3

   With a wide LVL or GluLam ridge beam, a commercial ridge vent would be too narrow to span that and 1" sheathing gaps on each side. If this roof is not yet built, the ridge beam can be installed under the rafter pairs, with the top edge of the beam beveled to the roof angle and squash blocks added at the ridge, or dropped enough so that the top chord of the TJIs can span over the ridge and meet at the peak, raising the hangers to match.

   In either case, the interior peak of the ceiling may have to be flattened, which improves air movement and feng shui energy movement, as well as creating a surface to mount lights or fans.

   As Doug suggested, the TJIs make it easy to create a continuous vent cavity channel. The only two commercial ridge vents that function properly under all wind conditions are those with an external wind baffle, such as Airvent ShigleVent II and Lomanco OR-4.

4. 
   Doug McEvers | Nov 29, 2009 06:33pm | #4

   If the beam is the same depth as the TJI's, depending on the roof pitch, a plumb cut on the rafter could hide the beam and the top flange could ride over the beam allowing for the typical air vent opening. I have done this many times with as little as a 4/12 pitch, I hesitate to modify structural beams with notches, bevels and the like.

5. 
   Doug McEvers | Nov 29, 2009 06:44pm | #5

   One more warning, consider a a continuous membrane under the roofing if you live in a high snow area. Ice dams are a real problem in the mountains with the warm winter sun and rapidly changing temperatures. Metal roofs as an example are vulnerable to ice dams and need a waterproof underlayment.

6. 
   Robert Riversong | Nov 29, 2009 08:14pm | #6

   I would agree with Doug on the continuous roof membrane (I assume he means a self-sticking bituminous material like Ice & Water Shield), only if you DON'T ventilate the roof. With continuous soffit and wind-baffled ridge venting, I've never seen as much as an icicle, let along an ice dam. A fully vented roof needs only 15# felt as a roofing underlayment.

7. 
   Doug McEvers | Nov 30, 2009 09:59am | #7

   Complex roof designs with hips and valleys hold a lot of snow, even with steep roof pitches. Those areas with a southern exposure have the tendency to pool a lot of water, use a continuous underlayment, vented or otherwise. I have seen firsthand in the mountains west of Denver, roof leakage associated with ice dams in valleys even though the roofer used ice and water shield in the valleys only.

8. 
   Robert Riversong | Nov 30, 2009 12:45pm | #8

   Doug,

   You're right about the compounding problems of complex roofs. I'm so used to thinking in terms of simple, sensible roof designs that I completely missed that issue.

   Not only are complex roofs far more expensive to build and far more susceptible to leakage, but they're nearly impossible to vent thoroughly (maintain a cold roof).

   What advocates of "hot roofs" don't seem to understand or acknowledge is that even a highly-insulated roof with a good cover of insulating snow on it (such as in valleys) will move the temperature gradient out enough to bring the roof surface into the melt zone. For instance, a cathedral roof/ceiling assembly, with R-38 spray foam (ignoring thermal bridges) and 12" of snow (at R-1/in), with 70°F inside temperature would experience melting at the roofing surface when the outside temperature rises above 20°F. At the thermal bridging of 2x10 rafters, snow melt would occur even at 0°F.

   Additionally, as Doug correctly advises, such complex roofs might require protection with full covering of self-adhering bituminous sheathing, which protect from external moisture but don't allow drying of sheathing in the event of a leak from either above or through the air barrier below (SPF insulation, if installed in too cold an environment, can shrink away from framing and compromise both the thermal and air barriers).