Foam is worth every penny to me.
When I built my shop 10 years ago I put a paper thin radiant barrier on the outside of the building under the steel. Then on the inside I put r-19 batts against the radiant barrier. The first year before I had the inside finished but the shop was heated I could reach through the r19 and scrap frost from the radiant barrier and the fiberglass was touching the frost. I had to pull the steel back off and put rolled foil/foam/foil under the steel. This stopped the frost transfer and your tyvek would frost just like my original radiant barrier.
Foam would have stopped this before it happened. I've regretted not foaming the shop originally. Wasn't popular yet and I didn't know anything about it.
Yep, what you are describing is what is discussed above - the thickness of the foam (since it is a vapor barrier) is important - super thin or missing you get frost, too thin and you get water condensing, thick enough - you are above the dew point and it stays dry.
If you would have had an effective vapor barrier under the fiberglass, the moist air would not have made it to the metal to condense and freeze into frost - where are you by the way?
Foam is indeed great, but it needs to be installed as a system based on the climate.
gogolf - I would either up the foam to the needed thickness for Mn, or just use it as a spot filler as you suggest and then fiberglass and then a vapor barrier to keep the moisture out of it. How critical it all is depends on how much you will be heating and how much moisture you will have in there. A full occupied house with cooking, showers, ventless heaters, etc. generates a lot of warm moist air. A workshop might not generate much to start with.