Dear Michael: Go to the Reflectix website and read their product info carefully. First, "reflective insulation" can reflect radiant heat, e.g.t from a hot metal roof in summer. Cold cannot be reflected, so the R value is near zero. Reflectix claims R7 with a cathedral ceiling in summer, for example, but only R1 in winter. Second, reflective insulation only works if you build / create a sealed air pocket on one side. This would be very difficult with your trailer.
Here are a few recommendations:
(1) Foam board is a good solution to create an insulated skirt under your trailer, with high R ratings, easy installation and water resistance. Home Depot carries a number of products. Foamular XPS foam board is tough and water resistant.. See: Owens Corning, Home Depot model # 24DD Internet # 100320335 Store SKU # 528022 Avoid foam board made from polyisocyanurate, which will absorb water.
(2) Skirt vs floor: Insulating the skirt will work only if you can seal all of the air leaks. An air seal around the entire skirt will also be difficult to maintain. A good alternative would be to fasten foam board to the bottom (floor joists) of your mobile home. If it gets very cold where you live, you may need to do both.
(3) Multiple layers: Depending on where you live and how cold it gets, you may need to add multiple layers of foam board. (This will only work if you pick the 'floor' option.) R13 to R30 is recommended for floors, and foam board offers R4 to R5 per inch. Run each layer at a 90 degree angle to the previous layer, to improve the air seal. Do a Google search for "Recommended levels of insulation" on the EnergyStar website, to find the R recommendations for your area.
(4) Carefully seal air leaks: Whether you choose the 'skirt' or 'floor' approach (or both), be sure to carefully seal all of the edges and joints to block air leaks. The 'Windows and Doors' version of Great Stuff works well; it remains flexible after it cures, to prevent cracks as things expand, contract and shift. For flat surfaces and joints, a thick layer of DAP 230 is best. (If you choose the 'skirt' approach, the bottom edge will be the most difficult; you would need to dig a small trench around the perimeter and bury the edge of the foam board in the soil.)
(5) Insulate exposed pipes: Whether you choose the 'skirt' or 'floor' approach (or both), you should begin by insulating your exposed pipes. For small water lines, start with rubber foam insulation, e.g. Pratt Retail Specialties, Home Depot Model # PI16RSS Internet # 204759170. If you live in an area with cold winters, add a second layer of fiberglass pipe insulation that will fit over the first foam layer, e.g. Home Depot Model # F15XAD Internet # 100162619 Store SKU # 170795 Each layer provides ~R3 insulation. Be sure to seal all of the slits to block air leaks with HVAC foil tape; the built in glue will not last.. Special pieces are available for 90 degree angles, e.g. Frost King Model, Home Depot # ELB1H Internet # 202262370 Store SKU # 632932
(6) Foam board is flammable, so be sure to avoid any sources of heat, e.g. a furnace, exhaust duct, etc. Use Roxul rock wool around any heat sources. Look for: Roxul Model # RXCB351525 Internet # 202090820 Store SKU # 974419
I hope this is helpful.