Long story short, then:
Kapton insulation layer around CO2 gas is the insulation.
(I didn't find an equivalent R value for CO2 as that value is used for building materials and CO2 is not listed as such, IIRC)
Partial answer, according to a video:
Instead of using aerogel for insulation, the craft makes use of CO2 gaps between components. Even aerogel was too heavy!
(MiMi Aung) And so the enclosure itself we're using the CO2 gas as the insulation material
(Interviewer) Oh wow! No aerogel?
(MiMi Aung) No aerogel. We did consider it. It was in the game.
It was in the consideration in the beginning, and it turns out that just the CO2 as insulator itself was sufficient for us to close our thermal model.
the Ingenuity drone utilizes DuPont technology to protect it from the harsh Martian conditions. Ingenuity has a temperature control center onboard called the Helicopter Warm Electronics Box (HWEB). This control center houses all the electronic equipment needed to operate the helicopter drone, including pre-charged batteries, flight controller, and cameras and other sensors required for the laser altimeter and ground view system. The HWEB hangs between the helicopter’s landing legs and uses a combination of Kapton® polyimide thin-film heaters and a metallized Kapton® insulation layer for both absorbing as much solar heat as possible while, simultaneously minimizing heat loss from inside the box to the Martian atmosphere.
Without its thermal insulation, Ingenuity would freeze and would be unable to fly as temperature on Mars drop to -100°C at night. Thermal insulation is hugely important aspect and so the batteries are kept in centre of the helicopter surrounded by electronic circuit boards, further encapsulated with a shell containing CO2 gas, a greenhouse gas that further helps to keep it warm.
And so that particular vehicle went through all the test chambers we have at JPL, through vibration and shock and cold temperatures and so forth. So once we had success in both of those, they informed slight design modifications in the final flight vehicle. For example, we slightly increased the insulation gaps that we had along the fuselage.
There is a small solar panel on top. It turned out power would mostly be needed not to fly but to keep the drone’s electronics warm inside. Mission planners briefly considered insulating them with aerogel, an ultralight foam used on previous Mars probes that’s sometimes been called “solid smoke,” but they decided it was too heavy.