If you blow HOT MOIST air into your roof and it doesn't go out it will become a haven for MOLD which will ultimately damage your wood.  

Dry mold is more dangerous than wet/moist mold, because it's airborne easier and at much higher levels.Mold grows because there is water and food present.

My present rehab has just that, but the boards are not rotted or moist, just surface "blackness" at this point.

I have a tenant that reported moist in bathroom on July 9, 2013.

It can even grow on moist metal surfaces with air passing over it. 

And due to the vapor barrier, (a huge plastic sheet) all the little (and not so little ) poopies stayed nice moist and fresh!

In general, you would need less of a gap in summer and moist months, and more of a gap in winter and dry months.

If you are insulating from the inside, then closed cell spray foam is your best option - it will prevent the warm moist air in the basement from making contact with the cool walls in the summer.

thanks Dan i'm in Signal Hill, CA but not any especially high latitude but whose exact coastal semi-arid (moist mild winters, bone-dry and warm rest of year) climate is lumped together with San Diego which is practically a baja california or coastal 'mexican' climate.

Second, while Rob K is correct that there could be cathodic corrosion occurring that usually requires a moist environment which inside a house would indicate other problems.