The differences between mass and insulation
The “I installed more attic insulation and now my AC runs more” thread got me thinking about the mass effect a bit.
Let’s say I have a lightweight wall insulated to R-10. Say, 2″ XPS sheathing over an empty framed wall. On the outside, it’s 90 degrees F. On the inside, it’s 70 degrees F. According to the calculations I can do from the wall’s R-value, each square foot of wall will transmit 2 BTUs per hour to the interior. Easy peasy. It’s slowing down the heat transfer such that only 10% of the heat that wants to enter every hour is able to. If I wait 10 hours, as much heat will have passed through the wall as in a single hour if the wall was only R-1: 20 BTUs. From the perspective of the occupant, heat flow has been slowed.
But don’t massive materials like concrete and adobe also slow down heat flows through the building envelope? If I have a foot-thick concrete wall, it’s going to take hours for heat originating on one side to reach the other side, right? Maybe even 10 hours…
So that got me thinking about the actual differences between insulation and mass, because clearly they both slow down heat transfer, but in different ways. It seems kind of like insulation is akin to a funnel that reduces the size of the heat stream, and lets through a constant trickle proportional to the size of the funnel’s orifice, while mass makes the heat move through a tar pit–eventually all of it gets through all at once, even though it might take a long time, depending on how big a tar pit it is.
If we take both to their logical extremes… A super-insulated box would only admit or leak a negligible trickle of heat, allowing the occupant to heat or cool it at very little cost. By contrast, the interior of a cave or fortress with 20 foot thick stone walls would remain the same temperature year-round, because the walls would be so thick and dense that most of the heat transfer would take place within them, never fully reaching the interior or exterior. You couldn’t budge the interior temperature if it was uncomfortable.
So here’s my question: is the reason we don’t use mass as the sole thermal control material for energy-efficient construction because we would need so much of it that building the darn things would become impractical? Do we primarily use insulation because of its properties of being far cheaper, easier to build with, and more space-efficient for a given desired rate of heat slowness through the building envelope?
That is to say… if we had the magical ability to cheaply and easily build mass homes that kept the interior right at the average comfort levels year-round without requiring any HVAC equipment, then we would do that instead, right?