Cathedral Ceiling/Roof Ventilation
I posted this in another thread, but let me start another to get a discussion going.
I came across a recently published article in the ASTM Journal, “Compact Asphalt Shingle Roof Systems: Should They be Vented?”. http://www.astm.org/DIGITAL_LIBRARY/JOURNALS/JAI/PAGES/JAI102057.htm
The abstract concludes:
“Our study showed better durability of vented roof assemblies with permeable insulation in cold climates due to redundancies that can tolerate incidental moisture and provide visual indicators of roof leakage; roof sheathing typically dries in 1-1/2 to 2 months. All of the unvented roof assemblies are intolerant of incidental water leakage and the moisture-sensitive layers (i.e., sheathing and gypsum wallboard (for open-cell polyurethane insulation)) exceed the threshold for decay. In hot, humid climates, the most durable roof assemblies are the vented, open-cell polyurethane systems with shorter drying time of the interior gypsum wallboard when compared to the unvented roof assembly; both the sheathing and gypsum wallboard dry out within 2-1/2 months. In an unvented assembly, the drying time for sheathing is similar but the drying time for gypsum wallboard increases to 6.5 months on average. Alternatively, unvented permeable shingled roofs are a viable option in hot, humid climates, although they are slightly less durable. The least tolerant roof assemblies in either climate are the unvented closed-cell polyurethane roof assembly due to trapped moisture and slow drying of the roof sheathing (up to 12 months in Miami, FL and 27 months in Boston, MA).”
Has anyone seen the full article? It certainly supports my long-standing belief in the importance of ventilating a roof, the value of permeable insulation, and the importance of allowing all thermal envelope assemblies to be able to breathe for drying the inevitable leak.
Responses?
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Replies
I got a copy of the full ASTM Journal article and below are the conclusions of the WUFI hygrothermal modeling study which examined 24 different roof assemblies for each of the two climates: Boston MA and Miami FL. The study was run for a 10-year period, using hourly local weather data, and examining only the last 8 years - after the construction moisture content reached equilibrium. They also introduced a single event leak (to simulate a wind-driven rain storm) of an average 0.20 lb water/ ft²h for a total duration of eight hours in the third summer of each simulation to examine the effects of incidental leakage.
Each roof assembly included: asphalt shingles; either felt or self-adhering membrane; either OSB or plywood; 9¼" of either fiberglass, open-cell spray foam or closed-cell spray foam; either poly VB (with fiberglass), or variable VB (with open-cell foam), or no VB (with closed-cell foam); and gypsum wallboard.
The following conclusions contradict the conventional wisdom that closed-cell spray foam does not require venting and protects from water damage and rot. This is one of only two studies I'm aware of that considers the inevitable leakage that will occur at one of more times during the life of a house (the other was a CNRC MEWS study of plywood vs foam sheathed walls, which also demonstrated dangerously poor drying potential with foam).
Conclusions
Our comparative moisture analyses show that important design considerations, such as providing ventilation and selection of insulation or water-resistive barrier, can significantly influence the robustness of moisture sensitive layers in roof assemblies due to incidental leakage in various climates. These design choices ultimately affect the durability of the roof system.
Based on our analysis, we present the following guidelines for determination of roof performance, taking ventilation and its effect on the drying due to incidental leakage as a major factor:
Vented roof systems with permeable insulation in cold climates are durable because they include redundancies that can tolerate incidental moisture and provide visual indicators of roof leakage.
The least tolerant roof assembly in either climate is the unvented closed-cell polyurethane insulation roof assembly with SRAM applied over the sheathing. This roof assembly creates a vapor trap and is slow to dry although the SRAM is supposed to prevent leakage from wetting the sheathing. Additionally, the closed-cell polyurethane foam will not allow leakage water to filter through and can promote deterioration of the wood roof structure with no visible indication of a roof leak.
In cold heating climates, vented roof assemblies clearly outperform unvented assemblies with respect to drying potential. Multiple types of vented assemblies in this climate are considered durable, including permeable insulations such as glass-fiber batt and open-cell polyurethane insulation.
In hot humid climates, the most durable roof assembly is the vented open-cell polyurethane with either felt or SRAM applied over the sheathing due to decreased drying time of the interior gypsum wallboard when compared to the unvented roof assembly. However the addition of venting in a hot, humid climate further complicates the location and construction of the air barriers and vapor retarders. Functionally, the foam needs to be applied to a second layer of sheathing, which also serves as a redundant air barrier. A layer of blocking would form a vented cavity between two layers of sheathing. The first layer of sheathing could also be changed to glass-fiber faced gypsum sheathing, to improve the fire resistance of the assembly.
Unvented permeable shingled roofs are also a viable consideration in hot humid climates, although it would be slightly less durable. The unvented roofs would be less expensive and simpler to construct that the vented roofs. These unvented roofs result in increased drying time of the inboard gypsum wallboard. The insulation must remain permeable to avoid a vapor trap.
Given the trend towards unvented compact steep-slope roof systems with various types of spray-applied foam insulation, designers should consider performing transient hygrothermal modeling to analyze the potential for condensation and accumulation within the roof assembly and to understand the limitations of the system as a result of incidental roof leakage. Transient hygrothermal models can provide designers with a relatively inexpensive method of performing a durability analysis using alternate building components in a variety of climates.
I don't have any answers, rather a lay question. I live in Monterey California (climate similar to SF), and wil be replacing my roof next Spring. I have a cathedral ceiling and plan on installing 2x4 sleepers above deck and fill in wih 3.5" rigid foam insulation. I was told that If I fill in the spaces tight, I don't need to ventilate. My alternative design would be to use a lower R value (2.5" rigid foam) and install ridge vents. I am interested in hearing your thoughts.
The conclusions published in this study support my 30 years of experience in energy-efficient building. Roof ventilation is always preferable to a "hot roof" (unless you're in wildfire territory).
But I would recommend installing 2 to 3 inches of XPS over a breatheable weather barrier (15# felt is fine) to create a thermal break over your rafters, screwing flat sleepers on top of that to create a vent channel, a secondary roof deck nailed to the sleepers, a breathable underlayment and whatever roofing you plan to use.
What is SRAM?
It's a simulated meat that comes in a can ;-)
Or it's Self-Repairing Adhesive Membrane (bituthane).
I usually refer to it as Ice & Water Shield. Never heard the term SRAM before. Doesn't exactly roll off the tongue.
Robert - I ran across your insulation embodied energy table you posted a while back in another string - can you give me your source for that? I can use the info for a textbook I am writing. Thanks
Carl,
I'm not sure which table you're refering to. Contact me at HouseWright at Ponds-Edge dot net.