How to maintain cathedral ceiling ventilation?
Depending on your house interior and roof framing design, cathedral ceiling ventilation can be treated as an independent, or attic ventilation related condition. As much as regular attic ventilation is an important part of your house maintenance routine and usually easy to perform, cathedral ceiling ventilation maintenance might be difficult or even impossible to do.
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:
- roof surface deformation above the cathedral ceiling section of your house,
- moisture stains on cathedral ceiling surface,
- moisture dripping from recessed light fixtures or electrical boxes on the cathedral ceiling,
- moisture / water stains stains along the top section of the window located in the wall supporting lower end of cathedral ceiling,
- dark stains (ghost marks).
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.
1. Cathedral ceiling ventilation / insulation
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”.
2. Cathedral ceiling ventilation / Roof bottom vents
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.
3. Cathedral ceiling ventilation / Roof top vents
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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About Dariusz Rudnicki
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I don’t understand how ridge vents can work as advertised. Ridge venting, placed at the peak of the roof, can not allow sufficient air to exhaust. We all agree that hot air rises, in fact it is a thermodynamic law. Has anybody ever proved that hot air rises, then goes down? No. So why do ridge vent companies expect consumers to believe that hot air rises to a peak, travels back down an inverted “V,” then escapes the attic? To me, this defies logic and physics. Doesn’t a ridge vent create a thermal check-valve that doesn’t allow the hot air to escape properly? (Ever take a bottle with small holes in the lid, then turn it upside down? The liquid doesn’t leave the bottle.) I put a smoke canister in my attic on a calm summer day and that’s where the smoke stayed – in my attic. I have continuous soffit venting around 100% of my home, with about 60ea 1/4″ x 1/2″ slits in each 6″x12″ section. Also, I have no gable venting, so dual-venting is not a factor. I’m thinking that my soffits should be providing enough intake air to create sufficient exhaust, but the attic doesn’t exhaust. Perhaps wind blowing across the ridge vent will create a “suction” effect and remove the hot air, but I’m not sure that’s accurate nor can I depend on windy days for proper attic ventilation. Perhaps I have some other factors affecting my attic exhaust, but my thought process leads me to believe that ridge vents can’t work as advertised and homeowner’s should go back to other, more effective exhaust modalities.
Hi James,
Attic ventilation is a lengthy subject and there are many publications explaining it in detail and from various angles. You can go over this one: http://www.airvent.com/pdf/literature/PAVbooklet.pdf for more information. I am not sure why is your attic’s ventilation performance suffering but I would start from making sure that those soffit vents and a ridge vent are fully open and that there is a proper ridge vent to soffit vents area ratio.
I am adding a fist story addition (expanding the living room 12′x15′) onto my current house with a cathedral ceiling. The HVAC unit is in the attic and the duct work comes down my outside wall and vents into the ceiling in my living room. My builder wants to use a 2′x10′ for the cathederal roof. The issue is with out any sub-roofing, we can not extend the duct work out the new outside wall. The builder wants to just have the duct work blow into the new addtion from the current inside wall. I am afraid that this room will be hot or cold depending on the season? Do I have an issue?
Thanks
Doug
Hello Doug,
If the HVAC unit BTU’s output is sufficient for conditioning of this additional area I would just make sure that the duct work and the register dimensions can compensate for delivering larger amount of air volume. I don’t know how limited space there is on that inside wall but if there’s no way to put registers in two opposite ends of the wall, install one at the lowest and one at the highest point of the same wall cavity. This will be critical during the hot season.
If you have an air return port in the same room (it might be difficult to maintain desired temperature without it) , try to relocate it to as far as possible from the air supplying register(s). Also, to improve air circulation install a ceiling fan in the highest point of the cathedral ceiling and whenever necessary run at the comfortable speed (remember to reverse rotation between the seasons). Ceilings fans work extremely well on cathedral ceilings.
Another option would be to run flexible, well insulated air ducts inside the new cathedral ceiling rafter space, all the way to the opposite side of the room, and open a couple of registers close to the wall top edge. Let me know if you have any other questions. Thanks.
I have an existing flat ceiling that i want to convert to a cathedral ceiling. My problem is the roof has a vent in the area i want to convert and I’m not sure what to do about insulating. What is the best solution?
I can’t really answer this question because you didn’t describe your roof’s structure / framing type, the size of the attic area that will be converted into the cathedral ceiling (small section, entire attic…), ventilation type (gable, soffit, ridge, etc). Very general answer would be that you have to properly insulate the new ceiling to prevent mold growth and that you need to redesign your current ventilation… to prevent mold growth. Give as much details as possible so I can tell you more.
I have a large cathedral attic space which gets very hot during the summer months. I have soffit vents with shuts installed along with ridge vents along the entire length of the roof. I was considering installing an attic fan or gable vent fan in addition to my other ventilation, but after some research it seems that more is not always better. I am unsure how it would act negatively with the ventilation in the attic. Would you recommend this additional ventilation? Also how would you recommend cleaning the ridge vents in the event they are clogged or dirty?
Hi Wayne,
Your current attic ventilation design model is perfect, of course, this is assuming that it’s been properly calculated, balanced and installed.
Installation of an additional gable vent(s) and / or power vent might disrupt proper air flow and create “dead air space”. This often happens because air will always flow through the area of lesser resistance. If you install two gable vents, the air might start flowing between those two new openings and there will be no suction created to pull air from the soffits.
With one additional gable vent installed there might be no or insufficient air movement in the attic area across from this vent (even with a functional ridge vent).
Power vent might just pull the air from the ridge vent / upper attic area instead of the soffit vents. It will depend which vents (ridge or soffit) have larger air flow area.
In general, mixing soffit and ridge (or any top of the roof vents) with gable vents is not recommended. Adding power vent might work but you have to make sure that its size (air flow) and existing vents air flow area will work together.
Cleaning the ridge vent would depend on its design. If it has any type of filters / mesh installed you may need to remove the entire assembly as well as shingles if they cover the vent, and replace the filter. Some design would require a brush or powerful vacuum to clean openings along its length (just be careful!).
Ihave an existing single story cathedral ceiling addition. I want to abut a two story addtion against the existing single story structure. On the East side of the existing single story structure there is a soffett whiah allows the air to flow up to and through the ridge vent. On the West side of the existing single story structure there also is a soffett that allows the necessary air flow up to and through the ridge vent. When I abut the new two story structure against the existing one story addition, I”ll have to remove the soffett on the West side, and the single story roof on that side will now abut the new two story wall – thus I lose my airflow. I have some possible ideas, but i want to “pick your brain.” How can I get airflow back on the West side of my existing one story structure’s roof?
Thanks,
Jim
Hi Jim,
What type of the roof you have on that single story addition (gable, mansard…)? Do you have cold winters? Snow? Because of the cathedral ceiling design you’ll have to provide air flow into the all rafter cavities. I’m not sure if I understand correctly what you’re attempting to do, but after removing the soffit and building a wall along that roof edge the most efficient way to provide air circulation along this section of the roof might be by installing the edge vent . However, with snow (if you have snow of course) this might not be a good idea – is it something like on the drawing below?.
Thanks for your reply. The drawing you created is accurate. Living near Wash. D.C. we average two or three snowfalls a winter – anywhere from several inches per snowfall up to 2 feet! Obviously, snow accumulation would happen where the first story roof meets the two story exterior wall and block the airflow.
Any other ideas?
Thanks
How about something like that:
Excellent! your response exactly reflects one of the ideas I had, but I didn’t have the drawing means to show it to you. But I appreciate the confirmation your experience provided me.
The one issue I still need to deal with involves the venting cover on the open end of the drawing that will allow the air flow to pass into and up through the air space and out the roof vent. I need some kind of vent that allows air flow in but will not allow wind to blow in water penetration. I won’t be putting vent covers on both ends due to the new two story addition and existing one story addition abuts the original house.
Thanks for your thoughts. Any additional thoughts greatly appreciated.
Jim
Thank you Jim.
It’s a vertical wall, so any louvered and screened vent should work. You’ll just have to find the right shape for what you’re going to create there. What might be difficult or rather impossible is to balance the air flow… Since there’s no way to put a vent on both sides, monitor that ceiling during the next winter. I would also make that low slope roof section pitched slightly towards the front to improve drainage. Maybe install heat cables to prevent snow and ice from accumulating…
Thanks again, Dariusz.
I wasn’t aware that balancing the air flow would be an issue. It would seem to me that air flow would take care of itself in finding each roof cavity. Your thought gives me pause. If I monitor the ceiling over the next winter and end up with a problem, I’m going to find myself between a rock and a hard place at that point. I’ have some reflecting to do on this point.
Concerning the slight pitch suggestion: I was planning on running a valley that would allowe for that pitch you suggested.
Thanks for your perspective,
Jim
Well, the air uses areas of lesser resistance to flow. Since the vent opening will be on the front side of the roof only, I wouldn’t expect the air to flow equally through the all rafter spaces. The back portion of the roof where it abuts the original house might develop some dead space with no air movement. This could result in condensation buildup on decking surface, dripping on the ceiling below, and possible mold growth – I’ve seen it in hundreds of regular attics with soffit vents partially blocked or non existing along some sections of the soffit. Would it be possible to make a small access panel to that new crawlspace under the low slope roof at the point where it abuts the original house. That way you’ll be able to monitor what’s happening there during the winter. If necessary, you could install a small air pushing fan with a humidistat controlled switch to help in air circulation.
.
The valley should be fine as far as my virtual imagination sees it