Very sad begonias
 

I think I figured out what these want. At first I thought I just wasn't watering them enough, and more humidity would help, but I thought about it a bit more.

I got them back in the summer, and had them outside, and they were so beautiful.

http://i.imgur.com/MY1QQ1b.jpg

http://i.imgur.com/7U7RhUm.jpg

But then I had to bring them in for fall/winter, and since then they've gotten like this

http://i.imgur.com/WznLwB3.jpg

http://i.imgur.com/rz1nxmJ.jpg

And it just hit me that they're not only being underwatered. They liked the outdoor summer conditions, and the house is much cooler and less lighted.

The red one is obviously the more forgiving of the two, and I think it will bounce back once it gets higher light.

The green one, though... I don't even know whether or not it can be saved. The end of the rhizome is still green and fleshy, and the leaf stems aren't shriveled, so I guess that's something?

I've placed them in a spare 5.5 gallon tank for now, and then I'll work on getting them some lighting. I have an extra clamp lamp lying around somewhere. I don't think I can do much about the temperature, though.

http://i.imgur.com/aPluVvN.jpg

Depending on the kind of light you use, most lights give of a moderate amount of heat.

I have a couple of seedling heater mats that I use under my plants in the winter, because I keep my heat turned pretty low to save money.

One mat just has individual pots sitting on it (Phals and Paphs), and the other is under my glass globe terrarium (with Lepanthopsis and Haraella) and a small glass aquarium (with angraecoids).

The mats really help a lot. They raise the temp immediately around the plants by as much as 10 degrees from the surrounding room. Not too expensive, either, I think they were about $15 on Amazon.

Moisture and humidity are probably the big issues. Begonias should tolerate low light, and unless the temp regularly falls well below 60F (remembering it could be much cooler near a window) it is not too cold. Don't fertilize at all until they perk up and start growing actively.

and in case, they don't make it, make some leaf cutting at this point.
Use vermiculite to minimize any rotting.
Keep them covered with something clear so the humidity is maintained and it stays bright.
Also, it's important to keep them warm until the roots sprout. At least they respond faster that way.

Keep them at room temperature if possible. I don't think these plants like it too cool at all as mentioned.

Good luck!

There are begonias and there are begonias. The rhizomatous ones like this require high temperatures and high humidity and won't compromise.

Since cool and warm can mean very different things to different people or in different situations, I checked my memory against a couple sources. American Begonia Society and several others recommended 58-60F to 70-72F as the ideal range for rhizomatous begonias. Like many orchids, they can probably thrive in warmer day temps as long as most nights get down into that ideal range. And most seem to agree that they can be slow to adapt to changes in environment, and the big killers are low humidity and over-watering.

And I agree, it is definitely worth taking some leaf cuttings if the plants have sentimental value or would be hard to replace.

My house's temperature is usually around 70, but it dips into the 60s when it gets really cold outside because our heating doesn't work right.

I've just ordered a seedling mat (with a thermostat because I found a mat/thermostat set on Amazon for $30, and that's a good deal for an extra safety precaution, especially since this will be in the same room with my birds).