Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: An architect builds a 1/10 scale model of a house to test the design for sources of heat loss in the winter. Which part of the model would be most important to test with heat-sensing equipment?
Walls and windows.