To recap, I put three cardstock models in my bathroom and left them there for a week to see if the steam from showering would warp them.
I don't time my showers, but here's how they go -- I shower for about 10 or even 15 minutes, get out, open the bathroom door so the fog will clear from the mirror. I took six showers in that seven days and my wife used the tub to take one bath. (She has her own bathroom with just a shower, just in case you think she only bathed once this week). When I take a shower, the humidity on that shelf goes from about 65% to 85% -- yep, I measured it.
The first little model was an onion dome from the Tower of London. It didn't warp at all. (No photos, but just trust me.)
The second was a trackside beanery I built to the specifications of a client -- and his measurements were all wrong, so I didn't care if the model was ruined.
It wasn't. There's a tiny bend up and down along the doors and windows of the long side, but it's barely noticeable -- it would certainly pass inspection under the three foot rule. The short side looks fine. The roof, which isn't reinforced with trusses, also looks fine. I expect this very minimal amount of warping will even out when it fully dries.
Here's what I think goes on: the tabs and folded edges of the model act as reinforcements. The chipboard base and the roof serve the same function. The model might tend to warp along the direction of the grain of the cardstock, but the tabs and edges resist that tendency, and it all evens out.
On the second day of the test, I added the third model. It's been subjected to five showers worth of damp air.
It's not really a model, per se; it's just a bit of O-scale wall and a window that I built as a reference so I can see all the layers required to make a sash window. The wall is chipboard laminated with a cardstock printout. It did warp quite a bit.
Notice how it warped: side to side. This could be due to the differences in expansion between the thin cardstock and the thick chipboard. (That's why it's always best to laminate with a non-water-based glue.) I expect with a roof glued on, this side to side warping would have minimized. If I'd included a simple tab along the bottom or top, I think it would have remained straight. It remains to be seen whether it will flatten out when it dries. (UPDATE: It did.)
My conclusion is: a complete cardstock model that has a lot of tabs and folded edges will be fine.
Also, if your basement or layout room is so consistently damp that it could warp a cardstock model, I suspect you'll have even worse problems with mildew, or corrosion of electrical connections.