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 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.

Views: 2478

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

Comment

You need to be a member of Home Energy Pros to add comments!

Join Home Energy Pros

Videos

  • Add Videos
  • View All

Twitter

Latest Activity

Diane Chojnowski replied to John Nicholas's discussion Slab Edge Insulation - A Side Thread to Melissa's Question
"Great videos Daniel. Would you please add them to our videos…"
9 hours ago
David Holtzclaw replied to Steve Mann's discussion Alnor 6200 LoFlo Capture Hood in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"Hi Steve I'm interested 402-213-3788 dholtzclaw@transductiontechnologies.com thanks david "
yesterday
Eric Kjelshus replied to Steve Mann's discussion Alnor 6200 LoFlo Capture Hood in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"call me Monday 816-537-5100 "
yesterday
Steve Mann added a discussion to the group Energy Auditing Equipment for Sale, Trade or to Purchase
Thumbnail

Alnor 6200 LoFlo Capture Hood

I've got an Alnor 6200 Balometer (Flowhood) I'd like to get rid of. No longer needed--I have a big…See More
yesterday
Paul Raymer posted an event
Thumbnail

MA Crew Chief/BPI Building Analyst Training at Bristol Community College at Commonwealth Landing

September 12, 2016 at 9am to September 16, 2016 at 4:30pm
Bristol Community College has created a hands-on training cabin in their laboratory.  This class…See More
yesterday
Daniel Morrison replied to John Nicholas's discussion Slab Edge Insulation - A Side Thread to Melissa's Question
"John, I just collaborated with Steve Baczek on a few videos for ProTradeCraft about slab insulation…"
yesterday
Profile IconMichael Sumpter and Bill Gartner joined Home Energy Pros
Friday
Malcolm Jackson joined allen p tanner's group
Thumbnail

Energy Auditing Equipment for Sale, Trade or to Purchase

Discuss the pros and cons of the equipment you are interested in prior to purchase. Post equipment…See More
Friday

© 2016   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service