What is the optimum insulation and roofing solution for a low-slope cathedral ceiling/roof?
I need some advice, please. I own a mid-1950s rancher with a cathedral ceiling/roof in Hershey, PA. The slope of the ceiling is about 3 ft of rise in 11 feet of run. The ceiling is drywall, to which acoustical tile is attached. The character of the space from the ceiling to the roof is unclear. The gap between the ceiling and roof deck is 6 inches. The roof is unvented. The soffit does have some venting, but it does not seem like a lot. The “attic” has fiberglass batt insulation that likely is in failure. The problem is interior moisture. I do not know definitively whether the moisture is due to condensation or a leaky roof or both. In winter, substantial ice dams occur on both north and south sides of the house. The house is L-shaped, with long axis oriented W-E. Even though professional engineers and contractors have evaluated the moisture problem, I remain in search of a clear and satisfactory solution. I welcome any and all advice before embarking on a re-roof that ideally would include new insulation. Thank you.
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Replies
Have you read this article? https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling
Do you have any recessed lighting in your ceiling? Your issues may be a combination of air leaks and inadequate insulation.
Need to Learn,
You wrote, "The problem is interior moisture. I do not know definitively whether the moisture is due to condensation or a leaky roof or both."
Can you do a better job of describing the problem? I'm not sure whether you mean:
1. The problem is that the relative humidity of the interior air is too high.
2. The problem is that the ceiling is dripping.
3. The problem is that the roof sheathing boards are damp, which was discovered when the ceiling was opened up to inspect the area.
Or maybe there is a different problem. I'm just guessing.
Need to Learn,
You noted that you don't know how to distinguish between condensation and a leaky roof. Here's how you tell the difference:
1. If it's a leaky roof, the ceiling drips when it is raining.
2. If it's condensation, the ceiling drips during the winter (in cold weather) or during a spell of warm weather immediately after a cold spell (when the ice on the underside of your roof sheathing begins to drip).
Need to Learn,
If the roof is as you describe -- with just 6 inches between the ceiling and the roof sheathing, failing fiberglass insulation, and a lack of venting, along with ice dams -- then it is astonishing to me that "Even though professional engineers and contractors have evaluated the moisture problem, I remain in search of a clear and satisfactory solution."
It seems pretty clear that:
1. Your roof has less insulation than required by code.
2. Your roof lacks an adequate ventilation channel from soffits to ridge, even though codes require such a ventilation channel when the roof is insulated with fiberglass.
3. Enough heat is leaking through you roof assembly to cause ice dams.
The solution is to install an adequate thickness of rigid foam above the roof sheathing. Here are links to four articles that will help you understand your problems:
How to Build an Insulated Cathedral Ceiling
How to Install Rigid Foam On Top of Roof Sheathing
Prevent Ice Dams With Air Sealing and Insulation
Ice Dam Basics