This is my first post here, so I’d like to begin by mentioning that I subscribed a while back and very much appreciate the myriad of knowledge and guidance provided by those who contribute. It has been invaluable for my latest project – replacing my roof. I apologize in advance for the lengthy post, but I believe all relevant information is required for relevant answers and guidance.  

 

My existing 2 layers of shingles are in need of replacement so I decided to install a high-quality 24 gauge clip-style standing seam roof for longevity and aesthetics. Because of the significant cost of steel roofing, I’m motivated to correct any underlying problems so that I don’t have to mess with this for a long time, or ever.

I’ll begin with the specs of my home, in its current state:

·         Location: Mid-Michigan (climate zone 5)

·         “saltbox” design (unequal spans) with two separate roof assemblies, both of which are 7:12 pitch, just at different elevantions

·         Built around 1980

·         The lower roof assembly is 100% cathedral ceilings

·         The upper roof assembly is cathedral ceilings that transition to a small vented attic space above the top floor

·         The existing roof in its current condition is as follows, from outside in:

o   2 layers of asphalt shingles w/ underlayment, overdue for replacement.

o   ½” CDX, questionable condition.

o   2×8 rafters throughout, 24” o.c.

o   EPS foam site-built ventilation baffles in the cathedral ceiling portions. The as-built air space was only ¾ inch to begin with and as you can imagine, the foam is flimsy enough that is deflected by the underlying batt insulation, leaving little to no ventilation in the center of the cavities, and minimal ventilation toward the edges.

o   Kraft-faced fiberglass batts under the site-built baffles. There’s about 5.5” of available space with the existing baffles, so I assume it’s R-19, but can’t confirm.

o   There appears to be 1” of XPS foam on the underside of the rafters in the cathedral ceiling portions as well. Based on what I can see from the attic space (there’s no XPS in the attic space) and a couple other areas, it’s tongue-and-groove board, but it doesn’t look like much attention was given to the seams (i.e., there may be gaps, and the seams don’t appear to be taped).

o   ½” drywall under the 1” XPS

o   In the small unvented attic, kraft-faced fiberglass batts are between the ceiling joists with unfaced batts across the top of the joists. Drywall ceiling in this area as well.

·         At one point, vinyl siding with aluminum trim and soffit was installed over existing wood siding, thus the questionable original soffit vents are buried by another layer of aluminum soffit. I disassembled one of the eaves while removing a chimney and found that the original soffit was about 50% blocked off due to fiberglass insulation and EPS foam fragments as a result of critters and degradation with age.

·         At the edge of the wall, the vertical distance from the wall top plate to the top of the rafter is approximately 6 inches, meaning there’s only about 4.5” or so of fiberglass batt insulation at this critical location.

·         There is only one chimney which blocks ventilation in one rafter bay. I will be taking this chimney down regardless of the chosen method of roofing.       

·         There is no rake overhang and the eave overhangs are only about 6 inches. I’m planning to add rake overhang and extend the eave overhang only if I add exterior rigid insulation.         

To summarize the existing condition, the roof assembly has sub-par insulation, very poor venting, minor to moderate ice dams in the winter, and seems to warm the house substantially in the hot summer.

95% of our heating is from an outdoor wood boiler and chasing firewood is one of favorite past times, thus the motivation for a possible upgrade is based predominantly on durability/longevity, ice dam mitigation, and summer comfort than simply increasing efficiency via additional insulation.

 

Based on my analysis after tearing off the fascia and trim at one of the eaves, I’m planning to, at a minimum, replace all of the existing structural sheathing. This presents two opportunities:

Option 1: Make Improvements as allowed by existing constraints

While the rafters are exposed, carefully air seal all penetrations and add/remove/supplement fiberglass insulation to the extent that the rafter bays will allow with new site-built baffles (likely 1” XPS foam or foil-faced polyiso), followed by new sheathing, underlayment, and roofing. Obviously the R-value wouldn’t meet code minimum and its actual value is debatable, but would be approximately the sum of R-5 (1” XPS interior foam) + R-15 (f.g. batts compressed to about 4.75 inch) + R-5 (1” XPS ventilation baffles) while providing a much-improved 1.5” air space. This option might marginally increase the R-value of the assembly, but the careful air sealing, improved ventilation, carefully detailed rafter/top plate insulation and sealing, coupled with the high SRI metal roof should provide a substantial improvement.

With this approach, I’m wondering if I should attempt to foam seal the interior XPS foam joints to the best of my ability, and whether or not I should foam seal XPS/polyiso baffles to the rafters. Won’t the underside of the baffles be a condensing surface?  

Option 2: Major retrofit, continuous rigid insulation above sheathing

While the rafters are exposed, carefully air seal all penetrations and remove existing baffles. Supplement or replace existing f.g. batts to completely fill rafter bay. Install new sheathing, peel & stick membrane, 4”+ of polyiso to achieve required outside/inside ratio for condensation control, followed by a vented cold roof using typical 2×4 strapping and another layer of sheathing, peel & stick membrane, and finally the standing seam roofing. Obviously, this adds substantial cost which I’m okay with, but the added roof thickness means I need to remove and/or relocate a sidewall window near the lower portion of the roof. That, coupled with the other inherent complexities and the cost, makes wonder if it’s worth it. However, this would achieve or surpass the code minimum R-value and eliminate ice dams.

What I’m really hung up on with this approach is the existing XPS and kraft-faced batts on the interior side of the assembly. Once the structural sheathing is installed and covered it with peel & stick membrane, I’m concerned about the ability of this assembly to dry toward the interior. Does the presence of these items in the assembly preclude an exterior rigid foam application?

 

A couple final thoughts:

·         No need to bring up structural concerns, I’m an engineer and have already performed necessary calculations to verify the structure’s ability to bear the extra weight.

·         I’m opposed to doing any interior work since all ceilings are finished, which means the interior XPS under the rafters stays.

·         I’m not interested in spray foam insulation, or anything requiring outside contractors to do work (I have trust issues)

Looking forward to any input – and thanks for reading!