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

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 two regions of space that are not in equilibrium with each other but are in contact 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 the Sernebe 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.

Designing for High Performance Slab On Grade Part II 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 at the edge of the slab. Because the movement of heat increases as the temperature difference increases (another physics concept), we lose 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 lose 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 in 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. 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

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2 Responses so far.

  1. Larry K. says:

    Looks like a good begining. I filled my 2 X 6 cavities with blown cellulose. sealed the vapor barrier after removing the hose. Tremco (accousitcal sealant) and poly work well together. I installed 2″ foam board on the exterior to provide a thermal break, I used 4″ of high density foam board under the slab for insulation and moisture control. One ceiling penetration for the soil stack, no ceiling light fixtures. Heat is provided by a 9,000 watt water heater and circulated with a 1/10 hp pump. heats the 1200 sq ft very nicely.

  2. […] conditions that effect the performance and comfort of the home. Specifically, we control heat, air, water and vapor, and it’s done with uninterrupted control layers. The wall section […]