I've been sleeping in my 4runner regularly now.
I've noticed that there is little insulation inside of the 4runner and the cold creeps in fairly quick. I've been tempted to rip apart the walls and throw some insulation in there, but I'm not sure how well that would work.

Anyone else done it?

The glass is the problem. It doesn't provide any insulation at all from the cold. I have created some window shaped sections of the bubble foil insulation for the front windows to block out the summer heat. They are flexible and fit well without any thing else to hold them, just friction. Make them slightly larger than the glass section so you can squeeze them in. I've also seen people line their RV and travel trailer windows with this stuff for both cold and hot. I'd start there before I tore out the doors. Besides, with the power window mechanism in there you won't be able to put much insulation in.

https://www.homedepot.com/p/Everbilt...I-SE/315103185

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What King Arthur said. I've got the full weathertech window set and other than condensation buildup slept fine inside the truck in fairly miserable weather.

40ish with high winds and some light rain down on the beach, inside of the truck was closer to 60 overnight with no heat source besides the 2 people in sleeping bags. It also cuts down on external noise and gives you a little privacy if you need to change or anything.

Past that one, insulating the rear end's floor also made a huge difference in road noise for me and would probably help with cold too. The rear floor is a thin layer of foam/carpet over basically sheet metal, so lining it with dynamat or one of the knockoff versions was an immediate difference.

I forgot that when my 4Runner was new I ordered Catch-All custom mats for the front, middle and rear area. These are carpet, not the rubber ones like WeatherTech. Catch-All made them to not only fit exactly over the OEM carpet but they also catch any spills and keep them in the mat. There are slight ridges around the edge to hold any spills. The color matches the OEM color perfectly. Because they're carpet and not rubber they really help keep the noise down.

I've been very pleased with them and will probably order some replacements for the front.

Thank you for the response!
I will pick that stuff up. I hadn't thought about the power windows at all so the in-the-wall insulation is out.
When i get it I will post back!

I used this on my scion xb windows and it makes a huge heat difference compared to my hrv without. About to do the same with my 4runner. Pretty inexpensive as well.