Why hasn’t this cathedral ceiling failed?
Climate zone 5, northwest Ohio. Evaluating options for improving a cathedral ceiling in this 1970’s house.
What’s there now, outside in: asphalt shingles on 5/12 roof, original plywood sheathing, full-depth fiberglass between 2×6 rafters, foil facing on fiberglass, 2″ polyiso with that bumpy white facing. The polyiso is in big 4×8 sheets, snugged right up to the rafters and held in place by drop-ceiling-type rails. This gives about R-32 or so. Not exactly bad, but when the blower door test started, you could hear the polyiso squish downwards against its supports. NOT air tight, obviously.
Inspection of the underside of the sheathing shows no evidence of moisture. Nor do any other surfaces, such as the white polyiso facing. The shingle nail penetrations might have been damp once in a while, but there is literally NO trouble up there.
Questions: first of all, why does there seem to be so little condensation issues? Second, what might be a good way to add some insulation and make the assembly airtight? I figure maybe just drywall over battens, with another layer of polyiso or EPS in between the battens. Thanks for any suggestions!
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Sorry, one correction to my description above: the big 2" sheets are not polyiso, but actually that dense yellow fiberglass board typically used as sound insulation.
Andy,
Q. "Why hasn't this cathedral ceiling failed? Why do there seem to be so little condensation issues?"
A. Just because an insulation method is risky, doesn't mean that the method results in 100% failures.
For example, imagine an insulation method that resulted in a 20% failure rate. That would be disastrous for an insulation contractor, who would be bankrupted by the callbacks. But it would still mean that 80% of installations are problem-free.
There are at least two factors that may have saved your ceiling: low indoor humidity levels, and enough air flow from the soffit to the ridge to remove some of the moisture. (Note: this is speculation.)
Andy,
Q. "What might be a good way to add some insulation and make the assembly airtight?"
A. The safest way to add insulation to this assembly is to add rigid foam above the existing roof sheathing, followed by new roofing. For more information, see How to Build an Insulated Cathedral Ceiling.
The best way to make the assembly airtight is probably to install a drywall ceiling that is installed with attention to airtightness.
Thanks, Martin. Your advice and this site are indispensible! To someone new to green building and building science, it can be confounding to analyze a certain construction in terms of "risk" and "safest way" rather than in more absolute terms. But I'm seeing that it's the only way to assess an equation with many, many variables. A cathedral ceiling is not a closed system--its performance depends on many characteristics of the construction above and below it, not to mention the weather inside and out.
HOWEVER, in the case of the ceiling I described, don't the facts indicate that it has failed miserably? If a non-airtight, underinsulated cathedral ceiling is NOT damaged, does it not indicate a sufficient amount of drying air is moving through it, which probably means it is accomplishing something less than its intended purpose of insulating?
When budget and time are factored in, am I right in assuming that it is a relatively good strategy to improve this assembly by making it airtight with drywall, and putting off adding additional insulation for a future upgrade when the shingles needs to be replaced anyway?
Andy,
Q. "In the case of the ceiling I described, don't the facts indicate that it has failed miserably?"
A. It's impossible to answer that question without a site visit. A blower-door test would certainly be informative.
Your cathedral ceiling may be performing well, or it may not. It's certainly true that an R-32 ceiling assembly doesn't meet minimum code requirements.
Q. "Am I right in assuming that it is a relatively good strategy to improve this assembly by making it airtight with drywall, and putting off adding additional insulation for a future upgrade when the shingles needs to be replaced anyway?"
A. That's probably a safe strategy, if the condition of your roof sheathing is as you described. By you will still have an unvented roof assembly filled with fluffy insulation, which is risky. Moisture damage in this type of ceiling usually shows up near the ridge. Have you disassembled your roof assembly and examined the roof sheathing near your ridge?