Cathedral Ceilings

Cathedral ceiling construction is inherently more prone to moisture damage than open attic construction, because cathedral ceiling construction creates isolated air spaces in rafter cavities. Although providing effective ventilation to attics with simple geometric, i.e. straight pitched, roofs is relatively easy and inexpensive, providing soffit and ridge ventilation to each individual cavity in today's cathedral ceilings may be impractical, and in many cases, impossible.

For many years now, all across the country, the filling of cathedral ceilings has become quite common. The original theory behind the air space for cathedrals was to evacuate moisture that would travel through the insulation. Now that there are enough test cases on the effectiveness of cellulose insulation in blocking air movement, the air space is not being used as much. Oak Ridge National Laboratory states in their publication "Moisture Control Handbook," October 1991, "Should cathedral ceilings be ventilated? Not if that space is tightly packed with insulation."

Packing insulation into cathedral ceilings with high-density insulation eliminates both the air pockets that cause moisture problems and the need for getting rid of condensation. The bulk of moisture moving into any insulated cavity is driven primarily by air. Stopping air movement should be the primary focus in insulating cathedral ceiling assemblies. High-density insulation, installed at greater than 3.0 lbs./ft.3, eliminates the voids and restricts the movement of air through the insulation.

Nu-Wool 's Cathedral Ceiling Guarantee

When Nu-Wool Premium Cellullose Insulation is installed to a density of 3.0 lbs/ft.3 or greater in cathedral ceilings which are at least 10 inches in depth and use gypsum wallboard, Nu-Wool Co., Inc. guarantees those dense packed cathedral ceilings will perform without the need for air spaces above the insulation. The Nu-Wool Co., Inc. warranty is in effect for any condition caused by properly installing Nu-Wool Premium Cellulose Insulation in this manner. Dense packed cathedral ceilings using gypsum wallboard, given adequate thickness, do not need to be ventilated.

Roof vents may make air leakage worse

Scientists at a number of research institutions have presented research findings that show that vented cathedral ceiling design does not provide adequate protection against moisture problems in roofs and is not appropriate for modern construction. The researchers summarized their results in a statement that urges the construction industry to develop new designs based on airtight cathedral ceilings. The most important point stressed by all the researchers is that although roof vents can and do dispel moisture from roof cavities, they can also cause increased air leakage and increased moisture transport through the roof assembly from the conditioned space below. In many situations, the increased air and moisture leakage into the roof may not be a problem. The roof may get wet, but then dries out again. However, that situation is unpredictable. Laboratory tests on full-scale roofs showed that, depending on pressure and indoor relative humidity, vented cathedral ceilings may suffer either "alarming condensation flow rates" or "sudden drying." If the interior ceiling surface is not airtight, the moisture flow will be four times as high as that for an airtight, unvented ceiling at normal house pressures.

Get rid of vents?

Anton Tenwolde, research physicist at the US Forest Products Laboratory in Madison, Wisconsin, presented the most forceful conclusions concerning roof vents in cathedral ceilings. Through presentation of laboratory test results, backed by computer simulations, Tenwolde demonstrated that ridge and soffit vents often couldn't provide enough outdoor ventilation to maintain dry conditions in roofs. Referring to tests performed on vented versus unvented wall cavities, Tenwolde explained that venting can cause increased air leakage and that in order to overcome the wetting caused by increased air exfiltration, vents would have to supply up to 23 air changes per hour of outdoor air into the cavity.