Yep, and that's how I initially for to this point. Armor doesn't stack, but reason tells me that two layers of insulation would dual up to a maximum level of efficiency.
Are you just doing this for Insulation? Or Non-conductivity and fire resistance, too?
An analog I can give you from real-life (since it sounds like you're trying to apply RL logic to this abstract game mechanic) is that for job-sites where you are required to wear Fire-Resistant Clothing, they have several different increasing ratings, a rating 2 might be adequate for an open-air gas-production facility, a rating 4 requirement might be mandatory in, say, a sub-surface mine where the atmosphere can be explosive. You cannot wear 2 layers of rating 2 FRC and still meet the OSHA requirement - you have to have the rating 4 FRC. Which is to say, I think these things (levels of protection) tend to be graded on an exponential scale, not linear. Stacking is linear.
Ultimately, as with all of these unclear rules, you should do what works best for your table. If you allow stacking, you could be slightly compromising (i.e. providing a loop-hole) to armor modification rules, by essentially letting low capacity armors do more (i.e. take some of the load off of higher capacity armors), which in turn will ultimately largely just benefit Mundanes vs. Awakened (not a bad thing if you think it's already Magic-run). Don't allow the stacking and you're applying Occam's Razor and, IMO, following the closest analog.
