BEFORE reading and implementing the method addressed in this article, ALWAYS check with the manufacturer of your insulation product to make sure that the material has been tested and approved for contact with the roof sheathing.
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, and most typical method is to run a continuous vent (see blue ventilation in Figure 1) from the soffit plate to the ridge of the roof. Once the attic has ANY ventilation (eg. ridge vent), in place, then the continuous vent approach is what is needed.
Figure 1 shows a well installed vent that runs all the way to the ridge vent. The insulation properly fills the cavity and is vented correctly.
Make sure that your continuous vents are not collapsed during the install. DuroVents by ADO are great vents that have raised bumps to prevent collapses. (see Figure 2)
Is the ventilation necessary? Yes. When you have even one vent installed in a cavity, it will allow some air movement. Air movement reduces condensation.
How do you treat a cathedral ceiling that DOESN’T have ventilation? (Figure 3) Then treat is as a wall. Walls do not have vents. They are self contained. If your building has a cathedral ceiling that is not vented, then fill the entire cavity. You do this by either using batt insulation or dense packing the entire cavity to 3.5 psi (cellulose) 2.5 psi (fiberglass) [check manufacturer’s specs]. Here is why insulating the cathedral ceiling in this way is suggested.
- 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.
- ONLY use insulation that has been tested for this kind of application.
If possible, add a ridge vent along with soffit ventilation to create airflow. If your application doesn’t allow you to create a ventilated cavity, then dense packing the cavity is a great option. Just treat the cathedral ceiling like a wall by dense packing it to 3.5psi. Some inspectors will expect to see a barrier between the insulation and roof sheathing. Make sure, if you are going to challenge the official, that you have the technical data sheets from the manufacturer that your insulation product has been tested for contact.
Oh! Did I mention? We sell the PERFECT insulation machine and dense pack equipment for this application! 😀