Peltier heat-pump, insulation between hot and cold chambers

[mozobata]

I have made a decent heat pump, with 4 x peltier elements each at 48W. Each element will be switched with a separate relay. I will later install DS18B20 (here is the datesheet of ds18b20)
sensors in the heatsinks, embedded in thermal paste. Right now my problem is to choose the best, most workable insulation material. It will be placed as a thermal barrier between the hot and cold chambers, which each have their own airflow running.

I chose PU-foam, which is popularly used for e.g. water heaters. The label says it is a cancer risk, so I was wondering what you lot would recommend as an alternative. PU-foam will harden, and make it impossible to remove my pump for repairs etc.

What's the best practice here?

[K3rryBlue]

Aerogel (R-5.5),
Fibreglass (R-3.1)