Attic envelopes must be ventilated. That is absolutely critical. What happens when you no longer have an envelope? What about a cathedral ceiling which is typically 12″ deep and has no envelope across the length of the house? How should these ceilings be treated?
The common advice is to run a continuous vent (see blue ventilation in Figure 1) from the soffit plate to the ridge of the roof. For years, this has been the common approach to insulating a cathedral ceiling. In an ideal world, that is alright. Figure 1 shows a perfectly installed vent that runs all the way to the ridge vent. The insulation fills the cavity and it looks perfect. Those last two words, “looks perfect!” That is the idealistic approach.
In reality, the insulation looks more like Figure 2. Notice the red arrows pointing out the compression points, collapsing and impeding the airflow. One blockage stops the ventilation and the idealistic approach no longer works. Times this install by the 48 runs you have to insulate and there is no insulator on earth that can achieve what Figure 1 attempts.
Is the ventilation necessary? No. Just treat the cathedral ceiling just as you do a wall (which is not ventilated, either). Fill the entire cavity, but make sure it is filled. You do this by dense packing the entire cavity to 3.5 psi. This is a far better approach. Why? Here are at least a few reasons:
- You will never create airflow, even with a continuous vent. Just like a wall, it is a sealed cavity.
- There is no fear of insulation gaps. There will be plenty of insulation, packed at the right density. At 3.5psi, the insulation will not settle and create gaps. The insulation will continue to keep the cavity filled.
- You achieve the maximum insulating capacity. By filling the entire cavity with a dense pack, you get the highest insulation ratings fiberglass or cellulose can achieve.
- Figure 3 looks a whole lot better! Full of insulation and a well-insulated building.
So, don’t drill holes through the joists and fret over creating air movement. It will not happen. Just treat the cathedral ceiling like a wall by dense packing it to 3.5psi. Oh! Did I mention? We sell the PERFECT insulation machine and dense pack equipment for this application! 😀