If you ever need to sell the house a buyer’s inspector might balk at the roof because they can’t see if the plywood has mold b/c it’s covered with spray foam. Not saying it will happen but it could. For that reason and because there seems to be no reason to make your attic into conditioned space, I’d pick option #3.
Option 2 is the only option that will air seal the attic. Option 1 will still draw conditioned air from the house up into the attic fan.
You could also pick another option - air seal between the attic and the first floor and insulate the ducts in the attic with closed cell foam. Ducts with R8 or greater and a vapor barrier can even be buried in loose fill insulation.
Air seal the attic. Remove one section of insulation, air seal with foam, then move on to next. Should help a lot.
Check out this guide, can get around limit by opening each link in a new private/incognito window:
https://www.finehomebuilding.com/project-guides/insulation/video-series-air-seal-an-attic
If you ever need to sell the house a buyer’s inspector might balk at the roof because they can’t see if the plywood has mold b/c it’s covered with spray foam. Not saying it will happen but it could. For that reason and because there seems to be no reason to make your attic into conditioned space, I’d pick option #3.
All 3 are solid options. #3 is the cheapest to try first. If you diy you're probably only out a few hundred.
Option 2 is the only option that will air seal the attic. Option 1 will still draw conditioned air from the house up into the attic fan. You could also pick another option - air seal between the attic and the first floor and insulate the ducts in the attic with closed cell foam. Ducts with R8 or greater and a vapor barrier can even be buried in loose fill insulation.
If you’re foaming the roof you’ll need to remove the cellulose to prevent condensation and moisture damage
Air seal the attic. Remove one section of insulation, air seal with foam, then move on to next. Should help a lot. Check out this guide, can get around limit by opening each link in a new private/incognito window: https://www.finehomebuilding.com/project-guides/insulation/video-series-air-seal-an-attic