Temperature Overhaul:
The problem of temperature in the game was much harder to solve than you might think. The old model was based on a thermodynamic cellular simulation, which would supposedly allow for heat from fires to be captured in rooms and flow out open doors. The model was accurate, but it was based on thermal conduction, not convection (which is much harder to simulate), and the result was hot areas right around heat sources, and cold areas everywhere else, even in enclosed buildings. In other words, buildings were pretty useless for keeping warm.
Clothing also fit into this simulation, but in a bit of a strange way (it served as extra insulation in the tile that you were standing on). Clothing would amplify any heat source in your tile, turning fires into extreme heat death traps. Finally, biomes were also part of the simulation, adding small heat sources (or sinks for cold biomes) at every cell in the simulation grid. Again, clothing, which insulated the center cell of the simulation grid (where you were standing), would also amplify biome heat. And biome heat effects would blend at biome boundaries (a thermal grid simulation is actually a form of blurring between the grid cells). This meant that there were near perfect areas at the boundaries between hot and cold biomes.
Players, being the rational folks that they are, reacted to the peculiarities of this thermal simulation by avoiding buildings, founding towns along desert boundaries, wearing minimal clothing, and generally not depending on heat sources for warmth. This was never my intention for the game, of course, but that's where things stood. I envisioned a game were buildings, clothing, and heat sources brought crucial advantages to a civilization, and all of the more advanced civilizations would depend on all three.
So, how could I fix this? A different thermal model of course, but what model? And if I wanted both hot and cold biomes (which make a lot of sense), how could I prevent exploitation of the boundaries? I really wanted there to be no "perfect" spot on the map that would make temperature regulation technology irrelevant. If such a spot existed, the smart players would find that, and settle there, always. Cold biomes should be too cold. Hot biomes should be too hot. There should be no "middle ground" in between.
First of all, many thanks to all of the players who engaged in a lengthy discussion in the forums. Also thanks go to my local designer friend Casey, who stuck with me through at least three hours of in-depth discussion about this topic (at the end of our first two-hour discussion, we had pages full of notes, diagrams, and graphs, but still no workable solution to the biome boundary problem).
Okay, now the solutions.
I should mention that what I'm calling "R value" here is different than the standard term as used in the insulation industry. My R value is a fractional heat retention value between 0 (no insulation that loses all heat) and 1 (perfect insulation). This makes it easier to reason about and program for. I suppose I should call it something else, but I don't know what to call it, so I've been calling it R.
First, for walls, I really want to simulate some kind of convection, so that heat spreads more evenly in indoor spaces. Instead of a cellular simulation, I'm now walking through the entire airspace around the player, flood-fill style, until I hit a boundary of insulating walls (or the edge of the 8x8 simulation grid). After that, I find the insulating boundaries, and compute an average R value for those boundaries. The heat sources inside that airspace (which may be the entire 8x8 grid, if there are no walls) produce heat which is spread evenly throughout the tiles of the airspace. That heat is modulated by the R-value of the boundaries of the airspace (if the average R value is 0.5, then half the heat is lost, and the rest is spread evenly in the enclosed space). Floors themselves count as part of the boundary of the space (if there's no floor in a tile, that tile counts as one of the air boundaries, thus reducing the average R value).
So what happens in this new model when you open a door? Suddenly, your airspace gets much bigger (the inside of your house plus the area outside your house), and your airspace boundary also gets bigger---and likely includes some air boundaries at the edge of the 8x8 simulation grid---so the average R value of the boundary decreases. Thus, opening a door, if a fire is running inside, will cause the house to get colder. Closing the door causes it to warm up again.
Thus, we're essentially modeling perfectly even convection throughout the entire enclosed airspace.
But shouldn't standing next to a fire also warm you up, even if there are no walls at all? Yes, but that's not due to convection. There's also a radiant component in the new model, which is based on your distance from each heat source that is in your airspace (which might included everything in the 8x8 simulation grid, if you are outside). So, getting close to a heat source, indoors or out, warms you intensely (perhaps too intensely, depending on the heat source). In other words, up close, radiant. Further away in a house, convection. The effect of radiant heat becomes negligible beyond a few tiles away.
Next, the biome effect is based only on the tile that you're currently standing on, and it's added into the heat calculation after the heat at your tile is computed based on heat sources and walls. If you're in an enclosed airspace, the biome heat contribution is modulated by the average R value of the airspace boundary, but only if the entire airspace also has floors. This means that an enclosed house with a floor can make a hot biome cooler, and a colder-than-normal biome, like the polar biome, warmer.
Next, clothes are applied in a separate part of the code, and they slow the transition from your body heat level to the environmental heat level (as computed based on walls, heat sources, and biome). If you're naked, you change temperatures pretty quickly. If you're fully clothed, you change temperatures very slowly. Thus, you can warm up in a house, near a fire, until you are just right, and then put on clothes before a journey to "hold it in" for a long time, and keep yourself close to perfect along the way.
And finally, the hard part: biome boundaries. As the new system is described so far, the old boundary-blending issue is fixed (because only your current biome tile contributes to your heat equation, without blending), but an exploit is still possible: by jumping back and forth across a boundary, between a hot and cold biome, you could warm yourself up to perfect temperature without fire, clothes, or walls.
So, I added a system for thermal shocks. This occurs whenever you go from a too-cold biome into a too-hot biome, or vice versa. Your temperature instantly jumps from the cold side of the scale to the hot side, right to the new biome's target temperature (or from hot to cold, if crossing the other way). This shock effect is also modulated by clothing. More and better clothing reduces the magnitude of this shock. Furthermore, the shock is never allowed to bring you closer to perfect on the other side of the temperature scale than you were before crossing. So if perfect is 0.5, and you were at 0.3, you will jump to at least 0.7 when you cross into a hot biome, no matter what clothes you are wearing (if you're naked, you might jump all the way up to 0.9, though, so clothing still helps).
This means that you can never improve your food consumption rate by crossing between hot and cold biomes. In the very best case, your consumption rate will remain the same, but it will usually get a bit worse (and if you're naked, it might get a lot worse).
There is also still a small body heat effect inside clothing, so in a cold biome, clothing will gradually warm you up over time. This effect is somewhat larger than it was before. The general idea is that, in cold biomes, clothing gets you 1/3 of the way to perfect, while fire and walls take you the rest of the way there. If you actually want to work in one area and remain at a perfect temperature the entire time, you're going to need all three bits of technology.
One other problem in the old system was that the desert, while hot, was not as hot as the other biomes were cold. The jungle was too close to perfect, and the mosquitoes didn't offer enough of a trade-off. So the jungle is now as hot as the other biomes were cold (moving between prairie and jungle now results in no change to your hunger rate), while desert is now as hot as the polar biome is cold. You've always been freezing to death in the snow, and you are now cooking in the desert. Think of it like hot snow.
The other biomes remain unchanged for the naked player. Thus, the game isn't really any harder now than it was before, unless you count the loss of the desert-boundary exploit as making the game harder (yes, that was easy, but the game was never supposed to be easy like that). Clothing and walls are so much more helpful now, that the game might even be easier, ignoring the old exploit.
Here's hoping that the new system leads players toward advanced civilizations full of heated buildings and clothed residents.