Vaulted or cathedral ceilings can be a challenge to insulate. The lack of air space between the finished ceiling and the roof's decking material is only a matter of inches. There must be room between the insulation, and the roof decking for air to flow from the eaves of the lower roof-line up to the peak or ridge. Without this airflow, heat will conduct from the exterior roof's surface and into the interior rooms of the vaulted ceiling areas.
All homes, regardless of the construction, must have some form of uninterrupted airflow to exhaust hot air caused by the sun shining on roofs. Typically air vents are spaced along the underside of the roof's eaves. These vents allow air to enter into the rafter and attic space. Inside the attic or at the top of the roof's peak is a continuous vent called the ridge vent. This vent exhausts air from inside the confined space. Without a continuous airflow temperatures can reach in excess of 150 degrees F. This heat will eventually migrate into the lower rooms.
The roof of a vaulted or cathedral ceiling is typically made from rafters that are 10 inches to 12 inches deep. This depth allows thick insulation to be placed between the roof's supports and still allow for air to flow from the lower portion of the roof to the upper ridge area. Shallow roof framing, 2 by 6's and 2 by 8's, may not allow for any space once the thick insulation is installed. In these cases an air way or duct must be installed so the insulation does not block air from flowing up towards the ridge vent.
Roof Duct Material or Baffles
There are a number of methods for installing a roof duct system or continuous baffles between the rafters to create airflow. The most basic method is to install tar roofing paper between the rafters. The paper is cut wider than the distance between the rafters and then stapled to the framing members in such a way as to create an air channel. The insulation is then pushed up to the backside of the tar-paper. A gap of one to two inches is left between the paper and the bottom of the roof's decking material. A plastic insert is also available commercially that will serve the same function. The insert is secured between the roof's framing rafters by staples. The plastic baffle is held down from the roof's decking a predetermined distance to create airflow. Since the plastic insert is pre-made, installation is typically very fast. Various types of inserts are available depending upon the style and type of roof it is venting.
Once the baffle system is in place, the insulation can then be installed. Generally a fiberglass batting with a foil or paper backing is stapled on to the rafters. The thickness of the insulation will range between an R-19 and an R-49. Various parts of the country will require different R-values of insulation. The R-value of insulation relates to how well the material resists the transference of heat and cold. Consult local building regulations for the insulation requirements of your area. Un-faced fiberglass batts are not typically installed inside the vaulted ceiling. The bare fiberglass batts can be difficult to secure in place due to the slope of the overhead ceiling.
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