As Mudmixer suggested, this is a tough question to answer. The dynamics of a straw bale home work differently than any other.
You will need to seal the entire underneath with a vapor barrier so the moist air does not migrate into the structure from underneath. I would also have an air exchange system in place to create positive pressure inside the building.
Even telling you this may not be good advice because we cannot see the home, or the conditions of where you live. If you live in a wet part of the country it will not matter what you do.
Closed cell organic and green spray foam is also available. This would be my first choice.
Someone to contact would be www.buildingscience.com for their suggestions on your particular house. You can even get them out for a paid consultation. I would suggest you do this and pay the money now for the right advice. Because it will absolutely cost more to repair it, and get their advice after the fact about how it should have been done. I have been a builder for 25 years, and used them more than once with excellent results.
Please tell us what you decide, and If you contact them as to what they say.