Hybrid Insulation Zone 3 – Exterior vs. Interior Ratio
I’m building a new house in the Dallas TX and I want to use a hybrid insulation strategy of poly-iso and mineral wool batts, but I’m struggling to understand the ratios that are in this article https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
My confusion is whether the ratio is a minimum, maximum, or a target value? The article just speaks to “not violating” the ratio, without clearly articulating whether the violation occurs.
Zone – 3 warm/humid
Roof – 2×10 rafters minimum, mostly unvented conditioned attic with cathedral ceiling in some areas
Roof material – standing seam metal, sheathing over the rafters is taped Zip sheathing
Walls – 2 x 6, 16″ OC
Wall cladding – mix of stone, wood, and fiber cement board
Walls – I’m planning to do a hybrid of Zip-R6 and Rockwool Comfortbatt R21 or R23. That’s a ratio of 28-31% (6.6/21 or 23), which is 2x the ratio of 15% for Zone 3. Do I need to reduce the Zip-R to R3 to make this work or is exceeding the ratio better?h
Unvented Attic Roof – I’m planning to use the Hunter Cool-vent panels on the roof deck with a Rockwool Comfortbatt R38. I was initially thinking of using the R11.4 panels which would give a ratio of 22-30% (8.6 or 11.4/38) which is 2-3x the ratio of 10%. Same question as above, do I need to reduce the exterior insulation to make this work?
Cathedral Ceiling – I was planning to do the exact same detail on the cathedral ceiling (interior ceiling is tongue and groove), but after reading some other Q&As should I instead use closed cell?
I’m trying to avoid spray foam for a bunch of reasons – fire, off-gassing, installation issues, ease of remodeling – but I’m not 100% opposed if it’s the only practical solution.
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Replies
You can't have too much exterior insulation. Exceeding the ratio of exterior to interior is a good thing.
For both the unvented non-cathedral roof and the cathedral roof, the problem is the same. There is a chance if you have too little exterior insulation that humidity from inside the house could migrate through the permeable rockwool batts and condense on the cold underside of the roof decking. Humidity is usually highest at the highest point in the house because of buoyancy (water vapor is lighter than air). Even though you have enough exterior insulation to meet code, you may want to think about a vapor diffusion port or a dehumidifier for this attic space to reduce the risk of this condensation occurring. Either of these can work and be an alternative to closed-cell spray foam.
As I recall the ratio does not apply in zones 1-4.
My guess is in your climate insulating the walls beyond R19 seems unlikely to recover the high cost in your lifetime.
Consider making room inside the conditioned space for your HVAC equipment and ductwork and making the ceiling flat and covering the much smaller area with a cheap thick blanket of cheap fluffy insulation. If you want high ceiling make the walls taller.
I see conditioned attics, spray foam, cathedral ceiling and recessed lighting as self-inflected wounds and are best avoided.
If you want to understand what option in your build will pay off in dollars over time build a BEopt model of your home and enter the costs for all the options
You may find these articles interesting.
green building advisor how to design walls
green building advisor 5 cathedral ceiling
https://www.energyvanguard.com/blog/buried-ducts-allowed-2018-building-code/
https://www.nrel.gov/buildings/beopt.html
Walta
Thanks, these replies were helpful and I found some other GBA articles that addressed this:
1) Ratios are minimums, so you can increase the exterior insulation without causing issues. "In Climate Zones 1 through 3, you’ll need at least R-5 of rigid foam (about 1.5 inch of EPS, 1 inch of XPS, or 1 inch of polyiso)...If you are planning to thicken the insulation installed under the sheathing in order to achieve a total R-value that exceeds code-minimum requirements, you’ll need to also thicken your above-sheathing foam layer to keep the ratio of above-sheathing insulation to below-sheathing insulation in the proper proportion."
https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing
2) Other articles saying the same thing
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation
The last time I was paying attention to this issue was a few years ago so it is possible that new data has changed things.
The link below is your link #2 please read comment #23. The attachment shows the comment in question.
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing.
PS I think my Zip sheeting is great but in my opinion Zip+R is a poor choice I think it weaken the structure of the building and is not worth the upcharge.
Walta