Insulated cathedral ceiling/roof layers
I am wanting to figure out the best build-up for an insulated cathedral ceiling/roof. Here are some of the basics:
1. Zone 6B – so should be R49 or higher.
2. Roof pitch 5:12.
3. The roof structure will be BCI rafters over the top of a ridge beam and two purlins (one on each side).
4. The layers I am thinking for the roof will be as follows (listed in order from the interior to the exterior):
a. 1x tongue and groove pine ceiling attached to bottoms of rafters.
b. Maybe drywall (is this needed?)
c. 12″ or 14″ BCI rafters – with either of these sizes, there is not enough depth for R49 cellulose if 2 inches is needed for ventilation baffles.
d. 2x4s attached horizontally to the top of the rafters (at 90 degrees to the rafters) to allow for some horizontal airflow along with the vertical airflow in the rafter bay baffles.
e. 1/2″ or 5/8″ Zip System roof sheathing with Zip taped or Zip liquid flashed joints.
f. Peel-and-stick ice and water shield for lower portions of roof and valleys (is this needed? How far up from the facia should it extend? Any advantage to adding it to the whole roof?)
g. Standing seam metal roofing.
5. What am I missing in the above?
6. This roof won’t be super complex (no dormers), but will have several valleys (see attachment for something similiar).
7. How can I get this to R49+ affordably?
8. Do BCI rafters have much thermal bridging? It seems like it would be less with them vs. standard 2x rafters.
9. I appreciate the sample roof buildups presented in this document by Martin: http://conference2016.eeba.org/Data/Sites/2/media/presentations/Holladay-Getting-Cathedral-Ceilings-Right.pdf.
10. Of course, adding rigid foam above the sheathing has some benefits. But I’m concerned about cost as well as the added complexity of attaching everything. And adding strapping and a second layer of sheathing over the foam is something I would like to avoid. Adding foam between the rafters and interior T&G pine appears to be another option. But if BCI rafters don’t have much issue with thermal bridging, maybe the rigid foam is overkill?
11. One option to get the R factor up high enough in a 12″ to 14″ cavity appears to be using closed-cell spray foam with cellulose below it. In that case, does the spray foam get sprayed on the bottom of the vent baffle product? If going this approach, can the cellulose be dense-packed below the spray foam? Or does that risk breaking the air seal of the spray foam?
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