Last Updated on October 9, 2022 by Barry Gray
Vaulted ceilings invariably make me think of the majestic old cathedrals built in Europe more than five hundred years ago. The marvels of the construction techniques of that era still astound me.
A vented cathedral ceiling has a ventilation space between the roof and the ceiling insulation, allowing airflow over the insulation. In an unvented ceiling, the ventilation space is filled with insulation preventing the flow of air over the insulation and out through the roof vents.
Roofing companies have developed a cathedral ceiling technique called a “Hot Roof,” or unvented vaulted ceiling. The roof, insulation, and ceiling boards are in contact and sealed to prevent any moisture intrusion from the top or through the ceiling.
Such unvented cathedral ceilings are possible, but they leave no room for error, and the risk of failure due to expansion and contraction due to temperature extremes is high. Let’s review both methods and discuss why installing vented cathedral ceilings is recommended.
How A Vented Cathedral Ceiling Construction Works
The majestic high ceilings of the old European cathedrals were well sealed from the roof with copper and lead plate, and the lack of insulation under the roof allowed for airflow to keep the rafters and ceilings dry.
The old cathedrals are not very cozy as they hardly had any form of insulation to keep in the warmth. Modern buildings and houses are designed with energy efficiency and comfort in mind.
We build our houses to be South facing to best absorb the sun’s energy to warm and provide natural light. The roof’s overhang is calculated to allow the winter sun to shine through the windows but to shade the windows from the summer sun.
The changing angle of the sun during summer and winter is an essential factor when designing the energy efficiency of a house. The overhanging eaves are fitted with vents in the soffits to allow cool, dry air to be drawn into the roof by thermal convection.
The air drawn in via the soffit vents has to flow upwards over the insulation layer on top of the ceiling board and out through a vent near the top of the roof. This airflow ensures that the roof cavity remains cool and dry and prevents moisture from causing rot or mold growth.
The same construction method is used in cathedral ceilings and normal attics to ensure the longevity of the construction materials and to dissipate heat out of the roof. There are no soffit vents in unvented cathedral ceilings, and the insulating material is packed and sealed tightly between the ceiling and the roof.
The sealed roof space created in an unvented cathedral ceiling must be built with no air gaps between the ceiling and the insulation. The heat generated by the sun on the roof will cause expansion of the roof materials, leading to cracks developing.
Warm moist air from the living space will rise and accumulate in the ceiling insulation through gaps in the ceiling boards and cornices. This moisture will cause mold growth in warm, moist, and dark conditions.
There are no significant cost advantages to building an unvented cathedral ceiling versus a vented ceiling. The extra insulation and sealing material costs will offset the additional cost for soffit vents and roof vents.
Despite the additional insulation to completely seal the cavity between the roof and the ceiling, this construction method will make the house warmer in summer. The insulation R-value selected must be very high and thus be more expensive.
There is no good reason for building an unvented cathedral ceiling. There are more costs involved and potentially much more than can go wrong versus the tried and tested construction of a properly vented cathedral ceiling.
What Are The Pros And Cons Of Cathedral Ceilings
A cathedral ceiling creates a high vaulted space along the roofline, making the room appear larger and airier. The problem with eliminating the attic space above the ceiling is that it results in the heat from the roof being in close proximity to the ceiling.
This may lead to heat being transferred into the living space below or the heat being lost through the ceiling and roof during winter. Eliminating this heat exchange requires that the space between the ceiling and the roof be insulated and sealed to prevent air and moisture flow.
The lack of space between the ceiling and the roof limits the amount of insulation that can be used and has to accommodate space to allow airflow between the roof and the insulation layer.
The roof overhangs are fitted with soffits to allow cool, dry air to be drawn in from under the overhang into the cavity created between the roof and the insulation layer. The roof has vents fitted near the apex to allow the air to circulate via natural heat convection.
Without the air flowing through the ventilated space above the insulation layer, the heat buildup from the roof will be transmitted through the insulation and ceiling and into the house. The risk of moisture building up in the insulation is also high without the venting to draw it out.
This built-up moisture can cause ice to form in the roof cavity, which can cause roof leaks or mold growth.
Cathedral Ceilings Pros & Cons:
- Creates spacious vaulted ceiling
- An excellent technique to expand small rooms
- More opportunities for natural lighting
- Allows you to expose raw wood beams as a feature
- Creates a hot roof
- More difficult to air condition
- More complex to install lighting
- Less energy efficient
- Additional construction costs
- More difficult to clean and maintain ceilings, lights, and wood
The design and installation of vaulted cathedral ceilings is a great technique to create the illusion of light and air in tiny houses. The most significant benefit is enjoying the additional sunlight and warmth in your living space.
The installation of cathedral ceilings will add cost to your project and should incorporate adequate ventilation to keep the roof cool and dry. A well-ventilated cathedral ceiling will cost less and eliminate the risks associated with an unvented ceiling.