Converting a traditional ceiling to a vaulted ceiling is not simply a matter of removing ceiling joists and nailing wall board to the bottom of the rafters. Changing the structure of the ceiling has major ramifications for the building’s entire roof structure, and the elimination of attic space can have an effect on everything from your roof’s thermal performance to the load on your furnace.
Homes constructed in the past several decades typically have engineered roof truss assemblies that use a complex arrangement of support members to provide a stable roof structure while using a minimal amount of lumber. You can’t simply remove parts of the trusses to make room for a vaulted ceiling without compromising the stability of the roof, so installing a vaulted ceiling may require significant reconstruction of the roof. If the roof was built with traditional rafter-and-joist construction, the ceiling joists work to resist the lateral force of the rafters pushing outward on the walls. It’s possible that the joists may be removed if they’re replaced with collar ties, horizontal members higher up the rafters that keep the roof from pushing outward. In any case, you should employ a qualified structural engineer to determine what’s necessary to keep your roof safe and stable — especially when you live in an area of frequent seismic activity — and to get the required permits.
Ventilation and Insulation
When you eliminate attic space with a vaulted ceiling, you also eliminate space for insulation and ventilation of the roof. The conventional way to insulate a vaulted ceiling is by installing fiberglass batt insulation between the rafters, leaving at least an inch of space between the top side of the insulation and the bottom side of the roof sheathing. The space allows air to flow between soffit vents at the roof overhang and a ridge vent along the peak of the roof. This means your rafters must be deep enough to allow space for insulation and ventilation between the ceiling and the roof. Some builders favor unvented roof assemblies for vaulted ceilings. This typically requires that rafter bays are completely filled with spray foam insulation, or it may include a combination of batt or blanket insulation and rigid foam board added above or below the rafters.
Wiring, heating and air-conditioning ducts, vents and chimneys all commonly run through attics, and if any of these mechanical elements are in the attic space that you want to replace with a vaulted ceiling, you’ll have to move them elsewhere. Here again, you’ll want to consult the appropriately qualified electricians, plumbers or heating, ventilation and air-conditioning technicians before you make any changes to your home’s systems.
A vaulted ceiling in one part of your house may create an attic space elsewhere that’s isolated from the rest of the attic. The new attic plan will have to provide access to each of the discontiguous parts of the attic. A vaulted ceiling also turns a large volume of former attic space into living space that must be heated and cooled. An HVAC technician can tell you whether or not your existing heating and cooling systems are up to the new, bigger job they’ll be asked to do.