If you have vented soffits which used to provide ventilation for your attic, the discharged air from the exhaust vent will be pulled back into your attic.

Well, ok then I can vent out the side of the balcony soffit instead of the bottom? Still worried about what insulation to

Usualy R-8 is sufficient for duct insulation, you usualy wrap the 4-6' (depending on the length of the duct in unheated space) nearest area from the house out, because that is where condensation is most likely to happen.

It's a 10' run in the space. R8 is enough? The walls they usually do in r20

The temperature in that space is probably 5F in middle of

Yes, it's enough.

It would make some sense increasing the R value from the kitchen to the back draft damper, (are you installing one?) to reduce heat loss/gain in the house when the vent isn't in use. But after the damper to the outside cap the vent temperature will acclimate to the attic temperature with the dew point dictating condensation regardless of the R value around the vent. Once the inside vent is heated up by the exhausted air, condensation will cease and initial condensation will evaporate and exit. There is little or no benefit to raise the R value above 8 around the vent in this section.

From the kitchen hood to a back draft damper the vent should be insulated to a level equal to the homes thermal envelope if possible to reduce losses/gains. If you install a back draft Damper make sure the fan is powerful enough to open it and the damper is good quality. Also pushing more than 400 CFM out of the kitchen requires air makeup to the kitchen by code (IRC M1503.4). Sucking to much air from the house can cause issues with back drafting your open combustion furnace and water heater if you're using gas, so be careful.