I am trying to decide what to do about some old stairs in my home that are not inuslated.  The steps are narrow and not ADA compliant, they where covered in carpet which I removed and resecured and stained the stairs.  The underside of the steps is not insulated and opens up to the basement, which is partially insulated.  

I’m interested in getting some recommendations for insulating the stairs, what material would be best for this application? 

Note: I would like to avoid spray foam.