I'd be surprised if a 150 year old house was insulated and sealed as well as a 20 year old house, even in Texas. Though you're right that the old house likely

I'd be surprised if a 150 year old house was insulated and sealed as well as a 20 year old house, even in Texas. Though you're right that the old house likely had a wood burning heat source, but I'd imagine that there were times when wood was scarce. And that 150 year old fireplace would have sucked air out of the house, bringing in cold air through every air gap. I looked it up, and wood has an R-value of around 1.4 per inch, so a log cabin with 6" walls would have an R-value of around R8, current building codes in Texas specify R15 for walls, R38 for roofs. Single-pane windows have an R-value of around R1, energy efficient double panes, around R4 (triple paned can hit R9) so even though there are a lot more windows in a modern house, they have better insulation.