In Part I of this series on designing for high performance, we discussed the control of moisture flow at foundations. We showed you this detail (below) from the Proud Green Home at Serenbe and reviewed the practices that we and the builder are employing for keeping unwanted moisture out of the building.

Looking back, we said the biggest opportunity in designing for high performance is in controlling the flow of heat, air and moisture. Today, we will focus on controlling heat. In other words, we want to keep heat in it’s place. Out in the summer, and inside in the winter. We’ve designed fairly simple strategies that will go a long way in keeping the homeowners in this house comfortable. Controlling air flow will be discussed in Part III.

From Hot to Cold

A little physics primer before we get too far. Whether we’re dealing with air, moisture or building materials hot always moves to cold. One version of how this works is described in the second law of thermodynamics. It says that if you have to regions of space, that are not in equilibrium with each other, are in contact with each other, each of them will eventually reach a state of equilibrium with the other. In other words, the hot region and cold region will eventually be the same state (hot, cold, warm…).

Again, it doesn’t matter if we’re dealing with air, moisture or a material, this phenomenon will occur if it is not stopped. In the case of this slab-on-grade, the two regions of space are the inside and the outside, and what connects them is the slab.

Control That Heat!

The other point to make about this heat transfer, is that it can happen up, down, sideways or anyways. With a slab-on-grade in the Southeast United States (Climate Zone 3), like the one in theSernebe Residence (near Palmetto, GA), the greatest heat transfer (or loss) happens SIDEWAYS.

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.

Though you might wonder why we’re even worried about heating when we live in the South, where there is so much cooling going on, we actually have more heating days than we do cooling. At least here in North Georgia, and the surrounding Mixed-Humid climates. You need to go further south for Cooling dominated climates, like most of Florida.

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 (another physics concept), 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 might make more sense to invest more in air sealing a home than in the insulation of it in the climate zones 1-3 (warm to hot). That’s not at all to say that air sealing is not as important in cold climates. In fact, I want to be very clear that air sealing is just as important in cold climates as it is warm and hot climates. The only thing more important in building a high performance, healthy home, is keeping the water out (Part I).

Beyond The Slab

In the above grade walls, we are filling the 2×6 framed cavities with open cell spray foam, with an R-Value of of about 3.6-3.7 per inch (total R-20), 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, 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.

Tags: controlling, design, efficient, energy, flow, heat, high, performance, slab-on-grade

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Replies to This Discussion

Hi Chris.  I have read part 1 and 2 and look forward to part three.  It is great that you are sharing these details with everyone, as this level of performance will eventually become the norm. 

I do have a couple of details I would like to discuss related to air flow.  Several years ago in my first Energy Auditing class I struggled with the presentation on attic venting, chimney draft, stack effect and CAZ.  The movement of air was there, but the explanation lacked the details I needed to be comfortable with my understanding. 

About a year ago I went looking for some numbers I could apply to these natural venting applications and not finding what I wanted I developed my own explanation.  After going public with my thoughts I discovered, as often happens, that others had already resolved my numbers problems, they just didn't publish them where we would find them.  However, my approach to explaining why we see hot air going up is still easy to understand and unfortunately, totally differs from what you have presented.  Since my objective was to bring clarity to an issue that has grown to be confusing, if you are interested, I would like to discuss it with you, either here or via email.


I would love to discuss it with you, Bud. Can you start by explaining what the differences are between the way you understood it and the way I presented it?

I have heard of and seen a lot of cases where heat flow is not understood. Money is wasted, and damage done using strategies that are not appropriate because of location, application or simple mis-use. It's so important to understand why we do what we do, before we do it. That's not always easy, .

Look forward to hearing from you.

Thanks for asking Chris,

An open mind is a wonderful thing!

My training fell far short of my curiosity and has led me to dig into what powers our stack effect and passive ventilation.  The answer is gravity.  The reason hot air goes up is because cold air is denser and heavier, thus pushing down with a greater force resulting in the warm lighter air being squeezed up.  Hot air moves up for the same reason a bubble rises in the ocean (buoyancy), each is lighter than their surrounding fluid.

Now, to apply this to our homes.  During heating season, the warm air inside our homes is lighter than the cold air outside resulting in the cold air pushing in the lower portions of our homes and forcing the warm air up and out the upper portions.  To understand how the pressures set up to do this takes a bit more explanation, but it is the foundation of so many aspects of our trade that it is worth understanding.

"Hot air Rising" has been hotly debated and the reason it moves up is still challenged by many.  Some are just stubborn and others haven't reviewed this information or simply want to avoid confusing people with the facts.  To support my position I put up a quick web page.  It needs more work (in progress) but it is a good starting point.

Welcome to the topic,



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