Questions & answers about what is the best way to vent cathedral ceilings?

Here are replies to frequently-asked questions about ventilation of cathedral ceilings.

This article series describes the best methods for ventilation in a cathedral ceiling and the effects on heat loss of air movement through a ventilated attic or cathedral ceiling.

FAQs on Venting Cathedral Ceilings

These questions & answers about ventilation in cathedral ceilings were posted originally at CATHEDRAL CEILING VENTILATION - be sure to review the advice given there.

On 2017-01-21 by (mod) - Summary of why under-roof ventilation is better than letting the water in, letting condensate form, then hoping it'll run harmlessly outdoors

Regarding the prior home in which you lived for 10 years, the combination of soffit vents and gable-end vents doesn't work very well, even if the amount of venting intake and outlet areas seems adequate.

One can find by observation that air tends to move from the soffits up and across the attic to the gable outlet - leaving the center areas of the roof un-vented, collecting moisture, frost and worse.

OPINION: "Never any issues" with such a house can be misleading and maybe mistaken. For example, water dripping off of a roof underside, moving on to wet insulation below or to wet the attic side of ceiling drywall is inviting mold contamination.

Water dripping onto insulation is just a bad idea. The thought that the foil facing will protect the insulation from getting wet and thus moldy and that it will keep water from passing through the insulation to the ceiling below is reasonable, and one that I share in part. Certainly anyone who has inspected many attics in a climate that produces attic moisture and sometimes frost will report having seen little drip marks as spots on the foil facing of attic floor insulation.

Nothing in that scenario promises us that the quantity of water dripping onto insulation will, over the life of the building, never be enough to pass through a stapled or loose insulation joint and into the area below, nor are we promised that all of the insulation kraft facing is perfectly intact, properly sloped, sealed, or otherwise placed to protect against moisture intrusion or water intrusion from above.

In sum it's a fragile solution to a problem that can lead to expensive trouble.

Sure, the fragile solution doesn't always fail - if it did everybody should have learned the lesson long ago. But predicting that a given home will never have a water, mold, leak, rot problem from the moisture you describe is in my view not reliable and may not be feasible.

Let's next look up: for problems I've found including rot and mold on roof sheathing or furring strips and on wood framing. Then we might look outwards to the shingle warranty: the manufacturer promises shingle life based on working under-roof ventilation as moisture moving into roof shingles (obviously that's not the metal-roof case) causes fishmouthing and wear.

Now lets' stare off into space in the attic area - if we are sometimes keeping high humidity in this space that alone can be enough to penetrate insulation and to ultimately give us trouble with mold or moisture accumulating in insulation or below it on the ceiling, independently of those little drip spots.

Let's step down into the basement to look at the furnace or boiler and then amble back upstairs to look in our desk at last year's heating bills.

Notice those high winter heating bills? If our roof or attic outlet venting is not properly sized we might, in winter, see warm air from the heated interior being sucked out through the attic and blown outdoors. Our local birds appreciate that, as does our oil company.

Finally let's look at the roof from outside in the dead of winter in a snow and ice climate zone: see those thick ice dams that are pushing off gutters, breaking snow guards, or that are causing water to back up under shingles, under flashings, and to leak into the building interior?

We could prevent those by a properly-vented cold roof design or by a meticulously-constructed hot roof design using bullet-proof materials like a metal roof and closed cell foam at an adequate R-value for the climate.

I don't count on water from seasonal condensation or frost in an attic to drip harmlessly into an attic, then to run harmlessly outside on its own .

Sometimes ventilation fans are required to make roof or attic venting work, but in my opinion a better design is one that uses passive and properly balanced (inlet at eaves, outlet at ridge not gable ends) ventilation that doesn't depend on fan s and that is adequate in capacity that I never have in-attic condensation forming.

See ROOF VENTILATION SPECIFICATIONS for details .

At last, regarding "... it would be unnecessary to place anything directly against the underside of the metal. "

I argued above that I don't want water accumulating in an attic nor on the underside of any roof and I don't trust that we can reliably promise it runs harmlessly outside. It won't. It'll misbehave. The water will find a protrusion and drip off into the ceililng below. The water will humidify the air and promote mold growth.

The water will accumulate around a chimney and cause enough rot that carpenter ants move in. The water will, if it escapes all of those fantasies and if it makes it to the outdoor roof edge, will freeze solid where it meets icy cold conditions. Eventually that indoor under-roof ice dam will pay us back with leaks into the structure.

At least that's the problem as I see it from the U.S. where global warming is a Chinese plot.

If one believes in the value of facts and objective data, then probably soon winter will be enough warmer that we'll worry less about frost and ice in attics and we'll be worrying more about roofs that tolerate heavy rainstorms and depending on where one lives, buildings tolerating rising levels of floodwaters.

On 2017-01-21 by Joe M

Thanks very much for your response. I value your expert opinion. I had to ask that question because I lived in a house for 10 years that had a gable roof and an attic. There was a soffit and end vents. The roof was simply bare metal sheets placed on purlins. In the winter time frost would form on the bottom side of the sheets.

When it thawed, it would drip down on the paper backing of the fiberglass batts that layered the floor. That house was built in 1987 and there was never any issues with it. So it is my thinking that maybe the same concept is applying to the cathedral ceiling venting.

Sort of a tiny "attic" being made underneath the metal sheeting. With a rigid foam barrier and batts underneath. Although, you didn't respond specifically to my post regarding the idea that water will drip down off the metal sheeting and onto the foil facing of the rigid foam.

With adequate ventilation in there, would this be an acceptable way of working? Would the water not just run down through the soffit and/or evaporate?

Eliminating any possibility of rot? If so, this would mean it would be unnecessary to place anything directly against the underside of the metal. Thanks!

On 2017-01-21 by (mod)

Joe,

I agree that there's plenty of confusion about construction and building environment and IAQ topics, in part because the loudest-shouted opinion from the most aggressive person often carries the day, even if no actual expert data, no research, and no sense are involved in that view.

Generally you can build a hot roof, un-vented, well-insulated, successfully, but in my OPINION that success has a better chance if the roof covering is something very reliable - such as a metal roof, and, as you describe, you take care to prevent air leakage into the roof cavity. Insulating such a roof with a closed cell insulating foam (approved for use under a metal roof) can also significantly reduce the moisture penetration risk.

Joe Listriburek, a building scientist with whom I don't always agree, is a respected expert on hot roofs and un-vented roofs and building moisture - I cite his work in the references sections of many of these articles. I think he and you are in general agreement.

If from below you have no air leaks, a good moisture barrier, and adequate R-value, and if from above, over the life of the roof, the chances of a leak are absolutely minimized, that's a good design and one I've used myself.

On 2017-01-20 by Joe M

It's a bit confusing to me with all the mixed opinions regarding ventilation and insulation of a cathedral roof. I am about to build a pole building with a gambrel roof using rafters. I will be doing purlins an installing metal roofing. A standard practice for garages and barns.

Although, this will be lived in. it is my understanding (so far) that I could simply install the metal sheets on the purlins. Put furring strips a couple inches below the top edge of the rafters. Then carefully cut and attach rigid foam between the rafters (foil side up) and seal around that to create an air tight barrier between the outside and inside. So if any water forms or drips from the metal, itll land on the foil. Although there will be a vertical venting taking place from eave to ridge.

Finally, install fiberglass insulation between the rafters and choice of interior ceiling. Now I know the success of this plan must depend upon the R rating of the rigid foam acting as the barrier. Which I don't really know what that is for my Zone (KY). But does this sound about right?

I think its crazy that people go overboard and build up layers of foam sheets and strips on top of the roof, trying to detach the thermal bridge. Seems unnecessary to me. Any response?

On 2015-12-09 by (mod)

Anon:

Do not modify rafters and certainly not by drilling through their tops - you risk weakening the structure and causing damage or worse.

It is common to vent an un-insulated, un-heated garage only minimally, but if you are going to insulate the space you want either a cold roof - continuous soffit intake and ridge outlet venting and a clear air space from eaves to ridge (over the insulation) or you'd need to change to an un-vented hot roof design.

On 2015-12-08 by Anonymous

I am insulating a new garage. There an exposed laminated beem running the length of it and 2x12" exposed trusess. They put only two roof vents on either side of the roof.

They have quite a few of the round vents along the sides but I would that when I put baffles up to the top and insulate and cover that because of the laminated beem. There won't be adequate ventilation.
Would drilling holes a the top of the 2x12s help to circulate the venting
Thanks
Mike

On 2014-06-12 by Anonymous

We are looking to reroof a church with an extremely high pitch cathedral ceiling with no insulation. The rafters are exposed on the interior. Church was built in the late 1800's. When it was reroofed in 1999, plywood decing was added above the boards that are the interior ceiling.

Shingles were not nailed adequately and have been falling off since. We do not appear to have a moisture problem but we are considering adding rigid insulation, an air space, batten boards running vertically and ventilated drip edge and ridge vent to allow air movement through these pockets.

We are looking at new shingles or new metal shingles. The committee wants to do it correctly this time. We know the asphalt shingles will require 6 nails per shingle. What we are unsure about is the benefit of adding the insulation? We do not want to create a problem, but it seems like addition of insulation and ventilation would be a good idea.

What would be the best practice for reroofing? We are not in extreme climate (Kentucky) but it can get cold in the winter with snow and ice.

On 2012-02-09 by Dena - ventilation prob on low p

Hi, This is our first winter in a new home. We have a split shed metal roof with a standing seam. One roof is about 42'W x 40'L (1.5:12 pitch) and the other is 32'W x 25'L (2:12 pitch). The joists below are running perpendicular to the slope of the roof, not allowing for a stack effect air flow, from bottom to top.

There are 11-7/8" TJI's every 24" OC which have notched bracing around the perimeter, allowing for potential side-to-side ventilation since the insulation has been held down 2" in each TJI bay. There is a continuous soffit vent on the low and high soffits but again that vents just the one TJI bay that it is in, since the sheathing is sitting on the TJI's. They have recently added six randomly placed 6" dia vents, along each sloped side.

From the inside out we have cathedral ceilings: drywall, vapor barrier, R-38 fiberglass batting (10"), approx 2" air space, plywood sheathing, roof paper, metal roof with field crimped seam. We are getting condensation and water is dripping through the sheetrock in various places.

We opened a soffit vent and found ice build-up under the sheathing, mold, and damp insulation. The weather has been very cold and humid.

We have been trying to minimize interior relative humidity (keeping it at 35-40%) by raising the inside air temp, not taking showers upstairs (even though we have high cfm fans - they are exhausted under the eave and may be contributing?), and opening windows periodically. We have no can lights except in the master bath which has a dropped ceiling in the steam shower. How do we get better roof ventilation? The architect is proposing we turn it into a non-vented roof by blowing expandable foam into the ceiling from below.

The builder is proposing we add fascia vents (like a honeycomb cor-a-vent) along the sides of the roof, hoping for enough draft to ventilate under the roof expanse. We are also considering raising the roof in the spring and framing in a two inch air flow space under the sheathing, but would that even be effective on such a low pitch roof?

What is the best solution, for short term and long term? How can I test the solution is working?

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