Cathedral ceiling and roofing design for great room
I wish to submit a design for cathedral ceiling and roofing for review. I did submit a prior question regarding the venting of eaves with this design back on 26-Jul, however, never took the opportunity to submit the complete design. Following are the details:
1. Location: Suburb north of Chicago, therefore Zone 5 . . . we are ~ 15 miles south of the Wisconsin border which is Zone 6.
2. Drawing: Attached is a cross-section of our Great Room design.
3. Great Room Size: Span of 40′ wide (as viewed in drawing) and a depth (into the page) of 36′. The room is symmetrical along the 36′ axis.
4. Roof Pitch: 9:12.
5. Construction: Supporting the roof deck are two end gables and 3 intermediate structural timber scissors trusses, all spaced at ~ 9′ on center. Atop each truss will be a 5/8″ plywood strip (see Ceiling Material below). Atop these strips, 2 x 12 Douglas Fir purlins will run perpendicular to the end gables and intermediate trusses. 7/16″ thick OSB sheathing will be installed atop the purlins (see Sheathing Material below).
6. Insulation: 7-1/2” of closed cell foam . . . while the insulation area has an R-value of 48.8, the effective R-value of the system is 34.5 (considering thermal bridging). The 2 x 12 purlins give us 11-1/4″ of depth for insulation, low profile insulated cans [with LED lamping], and miscellaneous electrical boxes for pendants and fans.
7. Ceiling Material: 1/2″ gypsum board will be installed to the underside of the purlins, with each drywall end able to be ‘tucked’ into the 5/8″ space above each truss . . . this will eliminate drywall taping against the trusses and enable efficient painting and ‘clean’ lines. Importantly, there will be no seal between the living space and deck cavity below the insulation.
8. Sheathing Material: To mitigate the effects of rain between the time of roof sheathing and roofing installation, we are planning to use the Huber ZIP System sheathing and tape to address this potential wet time period.
9. Roofing Build-up:
· Grace Ice and Water Shield over entire roof surface
· Atlas StormMaster Class 4 asphalt shingles
10. Other Considerations:
· Environmental Implications of Closed-Cell Foam – We are aware of the environmental impact of the HFC-245fa blowing agent and its GWP of 950 at 100 years. We are reviewing alternatives, but may not be able to change our design.
· Consideration of SIP Panels – We did a thorough review of this option, however, the need to accurately spot locations for electricals ahead of factory fabrication was deemed too challenging.
· We are considering the addition of 15# felt on top of the ice and water shield to facilitate a future tear-off.
· Upper Window Note – In case anyone is wondering, the upper window is not blocked by the truss as viewed at 6’ + elevation from either end of the room.
11. Our Questions:
· I am looking for any feedback on the technical soundness of our design with respect to moisture mitigation, energy efficiency, structural integrity, etc.
· Soffit Ventilation – we have soffit spaces that will be boxed with LP SmartSide. My question is do we need to vent the soffit space if all the insulation is terminated at the outside of the wall top plate? On one hand, I would expect it to be impossible to seal the boxed soffit, thereby seasonal air will make its way into the space . . . on the other hand, venting the space with a pre-vented soffit product such as LP SmartSide would invite dust and insects.
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