Building Science for the Rest of Us

An energy expert who spends her career checking weatherization work explains why homes get uncomfortable, moldy, or wasteful, and it usually comes down to a handful of physics basics. Heat moves from warm to cold spaces, so rooms over garages or behind attic knee walls stay uncomfortable until you seal air gaps and add insulation that breaks the thermal path between conditioned and unconditioned areas. Moisture behaves the same way, moving from wet to dry — which is why a damp basement or crawlspace, even one you never use, can send water vapor into the rest of the house unless it's sealed off with a vapor barrier and the water source is addressed.

Air leaks are described as one of the biggest drains on comfort and energy use. Insulation like fiberglass doesn't stop air movement, so a continuous air barrier (sealed seams, taped joints, foam-filled gaps) matters more than just piling on insulation. The article also covers water intrusion behind siding, noting that housewrap or felt paper needs to be installed shingle-style, overlapping downward, to shed water properly.

One point worth knowing if you've upgraded your furnace: swapping in a high-efficiency, direct-vent model can leave an old atmospherically vented water heater without enough draft to safely vent its exhaust, since it was sized to share a flue with the old furnace. Fixes include a chimney liner or switching to a power-vented water heater. The larger point is that a house works as one connected system, so a change in one place can create a problem somewhere else.