Vented or unvented attic in a large house – GreenBuildingAdvisor

Hello All

We’re developing floor plans for a large house in Climate Zone 5A.

First and second floor would be 2300 sq feet each, with second floor having less living space with duplex living room. There will be a basement, which I will insulate but not finish, around 1500 sq feet. A 4 car garage will be attached and hopefully decoupled from home. The garage area would be 1000 sq feet. Above the garage, we’re planning a conditioned unfinished space for a future ADU.

This house size would not be passive but I’m hoping for a pretty good house. I am hoping to get R40-50 walls, R90 at roof, and and ACH 50 score of 1 or less.

The builder I am working with has never done a conditioned attic. And also not done a completely air tight envelope with ZipR sheathing before. The builder does two HVACs with one is basement, and one in a “partially conditioned attic”, whatever that means. We haven’t done calculations with the Electrical person yet but we feel we’re gonna be better with two HVACs.

As you can imagine the roof would be large.

My concern is that the builder won’t be able to make a completely conditioned attic since he has never done it, and I dont have budget to hire a passive house project manager. Plus it will be way more expensive. And it will add a lot of load on the HVAC.

Is it a good idea to make a very good vented attic with close detail to air sealing of the ceiling,  and have the second HVAC in the the conditioned space above the garage?

Pros and cons? Any guidance is really appreciated.

Thank you.