Therm-a-Rest - Z Lite Original

Most closed cell foam pads will be lower R value but can easily get 2 to reach about the same. They tend to not be as comfortable though. Thermarest Z lite has been one of the better that I've seen, but most closed cell foam pads are about the same. I'd make sure to check the dimensions so that they work with you/your pack system.

We have a similar setup, but my wife and I share a double sized down quilt (EE Accomplice) on two air pads and our dog has his cut down ZLite and synthetic Kifaru woobie. Our dog (heeler) is about 40lbs. We are vigilant to keep him from getting on our uncovered air pads. Our quilt more or less covers our two pads, so it adds a layer of protection, but we insist that he stays in his ZLite as much as possible. We've gone out dozens of times and only lost a pad once when he was young and tried to bolt after a goat.

Have you considered a camping cot? There are a lot of ultralight options now which are about the same price as a good sleeping pad for side sleepers. The only difference being is the weight, you’re looking at around 1.5kg for a cot bed which would be double the weight of a sleeping pad. For me, the extra weight is worth it for the comfort. For context I’ve tried Therm-a-Rest, Big Agnes and Nemo mats. If I’m going for comfort, the cot bed wins every time.

Z lite. Only about $50 and works great even in wet weather.

That's still not true though. An X-therm weighs 16oz and has a R value of 7.3. Whereas a z-lite weighs 14oz and has a R value of 2. Even if we assume that foam pads perform above their stated r value you would still need multiple to match the warmth of a single inflatable.

Second this. Ridgerest or zlite closed cell foam pads are great for kids and actually pretty comfortable and sturdy. 

Throw a thermarest on top of a Walmart foam pad lol save yourself some money on the process. Otherwise I would just stick to Thermarest.

For one year trip I would use something cheaper, more sturdy than ultralight. Thermarest often listed is great but £50 decathlon air mat is more sustainable, you'll have less stress. To increase R factor, you can add some CCF pad.

I just Sept on the rei campwell for 7 nights. Best sleep I've ever gotten camping. I've had the klymit static v, and a foam thermarest. This thing is pretty big and not heavy but not light. Had it strapped to my Ibex 450 for the the entire week riding the lower half of the MABDR. I use 2 rok straps and it never moved. Absolutely love this thing. Both of my riding buddies were on blow up mats one Nemo and one thermarest. Both were cold and not sleeping well the whole trip. I'm a stomach and side sleeper.