Ventilating a Low-Slope Roof
We have a 1:12 pitch metal roof in zone 3 (charlotte NC). The architect has cathedral ceilings with a continuous ridge but no other details on insulating or ventilating the roof. The framing is done and the roof is on and we’re trying to come up with the most effective solution to ventilating or not ventilating and insulating it. I’ve already read “insulating a low slope roof” and I’m left with more questions. For a vented roof: I understand cupolas are recommended in the article but a ridge vent was already installed. I understand that is a bad idea due to wind driven rains, capillary action and possibility of sitting snow. I’ve seen some low slope ridge vent products out there. Has anyone ever used? Should we avoid them all together? 2. I understand 6″ of ventilation space is necessary; however, that would lower our ceiling height on the exterior parts of the ceiling to less than 8″ ( i know poor design). Is 6″ required the entire distance up the rafter bay or could we do 4″ at the exterior and get up to 6″ as the ceiling height increases? 3. If the insulation is tucked in each rafter (2×8) with furring strips, then won’t each bay be isolated from the next and a cupola would be ineffective anyway since it will only ventilate air from the rafter bays to which it connects? If we absolutely have to go with cupola instead of ridge vent, then we have to figure out a way for the air to converge into a common space under the peak in order to exit the cupola, correct? We cannot have the air staying contained to each bay, correct? 4. I’m trying to avoid any ductwork in the ceiling space to avoid air leaks- is this a good idea? 5. do we need to use a vapor barrier above sheetrock/below the insulation or can we just use a vapor barrier type paint on ceiling? 6. How do we determine how many vents we need in soffits? The ridge vent runs the length of the roof. 7. Looking at rockwool comfortbatt 10″ insulation- thoughts on this? 8. how do you insulate under the 2×8 rafters thenselves? Is our best alternative an unvented ceiling using closed cell spray foam? Is the air tightness of the ceiling just as important in an unvented roof assembly as a vented roof?
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