Insulating the underside of a trailer with spray polyurethane foam is much like insulating the underside of a home in the crawlspace. It is a good idea and should help with the energy bills especially if there is no insulation there presently. Work with the foam manufacturer or contractor to select the right product for your climate. Make sure you follow all applicable building codes.
Feel free to click on the "Contact The FAQ Administrator" to ask me a follow-up question.
Jason Hoerter, P.E.
Sr. Product Manager
Copyright © 2011 NCFI Polyurethanes