Part II: Designing for High Performance, Controlling Heat Flow in Slab-On-Grade

From High to Low, and From Hot to Cold

A little physics primer before we get too far. Hot air is simply air molecules that are under high pressure, and cold air molecules are under low pressure. Not unlike people, the molecules migrate from high pressure situations (say, at work) to low pressure situations (say, the bar). With me so far?

Let’s apply it to a real world situation in a home. Have you ever heard the phrase, “hot air rises”? It simply means that the high pressure (hot) air is going (rising) to where there is low pressure (cold) air. In the case of a home in winter, the hot air inside is migrating to the cold (low pressure) attic, and/or the ambient conditions on the other side of the roof. So, what seams like a phenomenon of rising heat is really almost drawing process, where the cold air draws the hot. High to low. Hot to cold. Got it?

Control That Heat!

At the foundation, this phenomenon occurs sideways, downward, upward, or all three ways. When you’re dealing with a slab-on-grade in the Southeast United States (Climate Zone 3), like the one in the Sernebe Residence (near Palmetto, GA), heat will travel SIDEWAYS. And, because we spend more money on heating than cooling in this part of Georgia (yes, it’s true), that’s a big concern.

Stopping this heat flow is one of the most often missed opportunities in these types of climate zones (mixed-humid), and it leads to unnecessarily high heating bills. Even the smallest amount of thermal “protection” can go a long way.

Designing for High Performance Foundation Detail Controlling Heat Flow

For the Proud Green Home, we’re stopping the sideways heat flow by putting a continuous layer of Cellofoam PermaBG Expanded Polystyrene (EPS) foam board. Because the movement of heat increases as the temperature difference increases, we loose more heat in winter than we gain in the summer. The delta T (temperature difference) in winter is as much as 60-70 degrees Fahrenheit, and in the Summer, the highest delta T may be 30-40. This is why homes in the Northeast and Canada are “super-insulated”. They have delta Ts in the 80s, 90s, and higher, and loose way more heat.

We’ll talk more about this in Part 3, but what I’ve just explained is the reason that it makes more sense to invest more in air sealing a home than insulating it in the climate zones 1-3 (warm to hot), maybe even 4. To be clear, though, air sealing is just as important in cold climates as it is warm and hot climates. In fact, the only thing more important in high performance, healthy homes, is keeping the water out (Part I).

Beyond The Slab

In the above grade walls, we are filling the 2×8 framed cavities with blown-in fiberglass insulation (B.I.B.S.), with an R-Value of of just over 4 per inch (total R-30), and using the integrated layer of polyisocyanurate foam (r-value = 3.6) in the Zip System® R-Panel wall sheathing for a continuous thermal barrier.

In a future post, perhaps, we will discuss how and why continuous insulation (like that in the R-Panel) should be used to provide a thermal break between ambient conditions and wood framing in an exterior wall, floor or roof assembly. The condition is called thermal bridging when you have a material that is in contact with ambient conditions on one side, and conditioned space on the other, and heat moves through that material like a car does on a bridge. In our case, we are stopping that bridge with the continuous insulation. The heat (during winter months) on the inside essentially hits a wall when it reaches the foam, and stays inside where it belongs.

- Written by Chris Laumer-Giddens, Architect, HVAC Designer, Building Science Professional, Certified HERS Rater.