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Comment by AngryData | Hacker News Reader

Yes this. It helps but it isn't a magic bullet because the roof is already airgapped from the roof and the house insulated from the attic. If the roof was directly insulated shingles would curl up and start failing within one or two summers due to the heat. That is why there are roof vents, gable vents, and soffit vents, to let air freely flow into and out of the attic/roof cavity.

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As far as I know, it’s important to have outdoor air that flows under the roof in cold climates to avoid ice dams. Otherwise insulating the roof deck is just fine and possibly even advantageous.

An unconditioned attic is not so great because it’s difficult or impossible to do a good job of avoiding leakage into the living space. Also, any mechanical equipment in the attic would like to be in conditioned space.

> If the roof was directly insulated shingles would curl up and start failing within one or two summers due to the heat.

This is an old school carpentry trope and yet I've never actually seen any statistical evidence indicating that roofs over vaulted ceilings which are directly insulated must be replaced more often than non-vaulted.

Anecdotally we have about 1/3 of our roof with a vaulted ceiling and the other 2/3 with an attic. The side with an attic is the side that would be suitable for solar panels (more sun at the right angle) or adjacent to the vaulted ceilings. The leaks that came up were all on the insulated vaulted ceiling side; starting about 25 years old, and we had another every couple years for about five years until replacing the roof. The attic parts never had any problems.

We have a vaulted ceiling and there is still an air gap under the roof. The ceiling "vault" is separately framed; there is still a foot or two of air space between the top of the vault and the roof deck.

In every vaulted ceiling ive ever seen or worked on it still has had a significant air gap between the insulation and the roof and it was still vented with roof and soffit vents. Also the areas that tend to leak before any other part of the roof because it is hard to vent enough air through such a small space.

And nearly every roof ive worked on that had curled shingles lacked proper roof ventilation. That isn't direct evidence, but ive never seen curled shingles on a well ventilated roof either and the only common factor in curled shingles is heat buildup in the attic/roof cavity.

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