Majority of problems with cathedral ceiling ventilation develop in structures where the ceiling framing, and roof framing are the same. What it means, that there’s no accessible attic between the ceiling and roof surface, only narrow, rafter / truss space which should be partially filled with insulation. Sometimes there’s partial attic, and you can see / look into the cathedral ceiling structure from accessible part.
“Safer” (from the cathedral ceiling ventilation point of view) types of cathedral ceilings are the ones, which have a separate from roof framing structure, which allows air to circulate more freely (image). But let’s talk about this first type. Depending on severity of the cathedral ceiling ventilation problem, you may, or may not see its consequences for a long time.
Some of the conditions, you’d be noticing are:
All of the above conditions are curable, but depending on the extent of damage, it might get expensive (lack of proper attic ventilation is a main cause of attic black mold growth and ice damming). Basic principle for cathedral ceiling ventilation is pretty much the same as for any attic with a “twist” to it.
To prevent possibility of moisture condensation, you need constant air movement between the bottom and top of the roof, through the space between rafters / above the insulation layer installed on top of the finished ceiling. “Twist” is for extreme conditions, which in regular attic are easier to spot, and much cheaper to correct.
If properly installed between the rafters, on top of the ceiling material (drywall, plaster, paneling sheets, etc.), there should be approximately 2″ of space left above it, which allows air to flow between the bottom and top of the roof. Ideal installation would also include vent chutes / baffles, secured to roof decking, which ensures proper insulation spacing and undisturbed air flow.
Check here your attic insulation amount recommendations.
Contractors who misunderstand principles of proper attic ventilation, never use vent chutes, use more or thicker insulation than required, packing it tightly into the rafter space, blocking completely air flow, and causing… costly “headaches”.
Because each rafter space is like a small, individual attic, it needs fresh air feed, which in most cases comes from roof overhang – soffit. With a variety of roof soffit designs, it is hard to suggest any particular type of vent, but continues one would probably cover all possibilities and ensure that each rafter space will receive fresh air. If there’s no overhang / soffit, “vented drip edge” can be utilized.
Some roofers prefer static roof louver vents, and if installed between every two rafters, it will do the job – but it doesn’t look nice, especially when you have a long ridge line. If your cathedral ceiling ventilation ports / space between the decking and the top surface of insulation resting on the ceiling drywall is visible from an accessible attic, and there are no obstructions, any type of properly installed attic vent will do, as long as it provides adequate output.
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This article was written by Dariusz Rudnicki
I'm a retired Illinois home inspector, founder and editor of checkthishouse.com, a blog which attracts around 2 thousand readers daily and is dedicated to answering the many questions of home owners and home buyers. Connect with me on Google+ Find me on Google+ Local