Appears you are in Zone 4; http://publicecodes.cyberregs.com/ic..._11_par002.htm
Code required basement insulation is R-10----13 insulation, see footnote "c"; http://publicecodes.cyberregs.com/ic..._11_sec002.htm
You will be losing heating energy if the furnace/ducting is down there, without insulating. Any insulation will help save energy dollars: http://www.quadlock.com/technical_li...Insulation.pdf
Though in your milder climate, not as quick of a pay-back with the cost of foamboard. It will reduce any incoming moisture, stop outgoing moisture from condensing on the concrete, and slow the heat loss. Pretty much a win-win.. Using your average low temp of Jan., Feb., and Dec.http://virginia.stateguidesusa.com/a...-in-virginia?/
, of 30*with 1/2" of foamboard (R-2.5 XPS) and R-13 in a frame wall, your relative humidity would be safe to 29% at grade, and 45% at six feet below grade at 70* room temp. With 1" (R-5) XPS= 34%RH at grade/50% below. As the frost line is only 10-20", depending on your location, this will change the condensation risk as well as the exposure time. http://www.soundfootings.com/pdf/US_...t_DepthAVG.pdf
As far as foam or not, stop using the dehumidifier and monitor the RH, then you will know to use some or not. OTH ,with your earth/water temps closer to 50* than 60*(location), and high basement humidity, you may want some f.b.- if not just for peace of mind; knowing it can handle moisture going both ways to keep the cavity insulation dry at heat loss minimal.http://www.epa.gov/athens/learn2mode...enrys_map.html
The moisture won't show on the concrete if exposed to basement air as it dissipates away before forming. Poly on a section is good for only that localized area (incoming moisture), check in numerous locations/times of year.