Howard,
There are two types of insulation that we use at our park in Montana. The temperature get -20F to -35F over the last month before any wind chill. Our practice is to heat tape and pipe wrap the water supply line and do nothing with the waste line. We had a few problem with unoccupied units during the recent cold snap to trailers that we recently brought into out park. Interestingly it was the waste lines that froze.
Moving water does not freeze so waste line are normal no a issue but with an unoccupied units and limited water flow ice started to build up and eventually froze the 3" main waste line both under the trailer and the 2’ section that not under the trailer. We used a diesel bullet heater under the unit for a few hours and got the pipe unfroze enough to allow water to flow and then ran hot water to clear the rest of the line. This all happened when the high temperatures were -15F
As for insulation we have nothing, spray foam or fiber glass bats on the skirting. Spray foam is a very good solution because it stops drafts that, in my opinion, are more important than the level of insulation. The reason I say that is I can heat up under any trailer with an 80,000 btu heater in the middle of winter as long as I don’t have the wind blowing all that heat away.
Now for cost. This is a business after all. Spray foam will run you $.75 - $1.50 per sqft one inch thick. The price will vary across the country and with the price of oil. Open cell foam is R3” and the low price. Closed foam is R6.5” and the high price. Either will work and 1" is sufficient.
Corbond is the brand I like the most. It is also the insulation I have had done in my houses over the years.
If you are looking for the most cost effective way to air seal the skirting buy a foam gun. It is much much better than the classic great stuff $3 can. It allows you to use as much or little as you need. See amazon link: https://www.amazon.com/GREAT-STUFF-PRO-Dispensing-Gun/dp/B0002YOMJE For less than $100 you can have your maintenance man craw under the trailer and seal any spot where light is coming in. This should air seal the skirting.
If you are looking for a better solution and are willing to spend ~$700 ish you can have your local spray foam insulation company spray inside under the trailer skirting. The nice part about this is it works in retrofit situations where you are having problems. You can also address any missing insulation under the trailer at the same time. This is most beneficial around where the main water line enters the trailer. The $700 was for a closed cell foam 14x64 trailer 30” off the ground. You should be able to do open cell foam for about half the price.
One last thought. If your slab that your trailer is sitting on is not insulated then the cold will just travel through the ground and under the skirting and the temperature under the trailer will still be close to the temperature of the outside.
So for me airsealing is by far the most important issue and insulation just makes unfreezing frozen pipe quicker and easier. Typically our tenant don’t have a 80,000btu heater and contact the on site manager to borrow ours. That being said frozen pipes are the exception and not the rule. Say 2-3% of our tenants per year.