The "easiest" way to insulate skirting is to install insulated skirting panels. This can be expensive material, but you have to measure that price against standard skirting, separate insulation, mmisc stuff to attach with and the labor. Just do a google on insulated skirting or R-COTEC Insulated Skirting, R-Co Skirting or Rapid Wall. Rapid Wall is very cool stuff.
Mark, the owner of this web site, has made several posts on how he insulated his skirting, but I did not find them using the search feature. You can browse through all of Mark's posts, but that will be a long read. I think he used standard batt insulation and wire or something to hold it in place. Maybe the plastic wrapped insulation would work well for that. I know other members here have used foam board to make a backing on the skirting. I have never insulated skirting, so I don't have any first hand knowledge to share. Maybe other members will chime in here.
Mark does sell automatic (open/close) skirting vents here at this web site, which I would imagine would be an excellent choice in cold climate areas. I believe the standard ventilation factor for skirting is 1 square foot of vent for every 300 square feet of floor space. A 12" by 12" (hypothetical size) does not equal 1 square foot of ventilation. The fins of the vent and bug screen reduce the total amount of air flow. Vents usually have their air flow rating stamped on the vents themselves.
Hope this helps,
Today is PERFECT!
All information and advice given is for entertainment and informational purposes only. The person doing the work is solely responsible to insure that their work complies with their local building code and OSHA safety regulations.