My house is in Northern California and was built within the last 15 years. I'm pretty sure I still have the original siding, which is hardboard (a.k.a. engineered wood). I've read that house exterior typically has 3 layers -- sheathing, a water barrier like Tyvek house wrap, and the siding. But when I looked under the siding, there are four layers. Please see the attached photo, which was taken from the ground looking up. What's this pink layer (pointed by the blue arrow)?
You can see the concrete foundation and the sill (pressure treated wood) sitting on top of it. Going from inside to out, the very first layer looks like OSB sheathing, and then the unknown layer, and then a very thin layer (which I think is Tyvek house wrap), and then the hardboard siding.
I think this pink thing might be a rigid foam board for exterior insulation, but it's only about half an inch thick (just like the siding and the OSB). Aren't foam boards for exterior insulation usually at least 1" thick? Thank you.