What happens to outdoor douglas fir if protected but untreated?
Since it will be covered it won’t rot away but it will turn gray. I also live in the Pacific Northwest south of you , condensation is the biggest issue I see and occasional condensation is not a problem. 
My riding arena  120’ with poly the entire length for light it is 30 + years old with no issues but is 22’ up so the color is far enough away we don’t notice the gray. I did paint the back porch because I thought the bare grey wood looked bad after a few years. So since it will be covered it won’t rot away but will turn gray.