Insulating a Slab-On-Grade in Hotlanta

Why in the world are we insulating a slab-on-grade in Hotlanta?

Reason #1: Atlanta is not, I repeat, NOT a cooling dominated climate. Wait...What?!?!?

Yes, it gets plenty hot here in the Summer, but it also gets downright cold. Which is why it's one of the more difficult climates to design and build in. We get both extremes.

I'm not saying we know what cold is, like all y'all do up yonder in them northern states, like Vermont and South Carolina. In fact, some of us can be pretty clueless about winter weather, which is comically represented by a fictitious Atlanta-resident, Buford Calloway, in this SNL Weekend Update skit shortly after the infamous Snowpocalypse hit Atlanta in 2014 (click on image below):

How Cold Can it Possibly Get?

The night Jodi and I returned from our Florida Christmas, this year, it got down to a blistering 9 degrees. Fahrenheit, not Celcius! For all my building science or home performance geek friends and followers, Atlanta gets between 3,000 and 3,500 heating degree days (HDD) each year, and our 99% design temperature (DT)  ((ASHRAE's 99% Design Temp (DT) is the maximum Winter temperature a given location will experience for 99% of the hours in a year. This number is based on a 30-year average, and it equates to about 88 hours. A 1% DT refers to the Summer temperature that a given location goes over 1% of the year, based on the 30 year average))  is between 23 and 26 degrees Fahrenheit, depending on the chosen weather station. In comparison; Burlington, Vermont gets anywhere between 7,200 and 7,400 HDD, with a Winter 99% DT of around -4, and Miami, ,Florida rarely ever gets to 200 HDD, or goes below 52 degrees F. Atlanta is pretty much in the middle.

When it does gets cold, most of us put on our coats. That's fairly universal across all climate zones. Preferably we wear one that also blocks the nasty wind that can make nine (9) degrees feel like it's minus ninety (90). OK, maybe that's a bit extreme, but anything below 10 is painful for anyone living south of the Sweet Tea line.  ((Admittedly, I am a Winter Wimp))

When we design and build a home, we do the same thing. We give it a coat. And, just like most of us wear our coats on the outside of our bodies, we're putting the coats on the outside of our homes. In other words, we're insulating from the outside, to protect everything that's inside that coat. No exceptions.

Insulating from the Ground, Up.

Unlike the coat we wear on our bodies, the coat we put on our homes, the "housecoat", is more like a kind of "onesie", where it's absolutely continuous. The fewer the interruptions, like gaps, cracks or holes in it, the warmer and more cozy the home and homeowner are going to be inside that coat, and the longer the home is going to last by being shielded from the elements.

When we designed and built the High Performance Bungalow, we started the coat under the house, more specifically under, and at the edge of the entire slab-on-grade, with 2" of continuous rigid Extruded Polystyrene (XPS) board insulation. It started with the foundation form-work, and then the pouring of the stem walls and turn down slab edge.

Without slab insulation, I calculated and estimated almost 11,000 btu/h of heat loss, for a 99% design temperature condition, which means at least 1% of the year. 90% of that heat loss happens horizontally, through the slab edge, and the rest is happening vertically nearer to the center of the slab.

The reason there is not as much heat loss at the center, is that the temperature difference (delta T) between inside (70 degrees) and the soil (55-60 degrees F in north Georgia) on the other side of the slab, stays between 10-15 degrees F there, while the delta T can get between 44 and 47 at the slab's edge, on a winter design (99% DT) day.

With 2" of XPS installed, I calculated and estimated a total heat loss throughout the entire slab to be less than 3,000 btu/h. Worth it! For sure. That doesn't just equate to energy savings, but lots of added comfort and durability! Wait until you see what we did with the roof and walls, and the impact it had on the total heat loss of the home!

QUESTION: Why bother insulating near the middle of the slab-on-grade?

There is still heat loss, and remember, we don't want any interruption in the coat. Plus, the owner prefers a "warm" floor without the expense of radiant floors, which aren't very practical in high performance homes, anyway. Keeping the slab completely inside the continuous thermal envelope, gives it a better chance of being the same temperature as the rest of the inside.

QUESTION: Why not use 2" at the edge, and 1" in the middle?

It would have added complexity to the installation, which adds labor. Plus, more insulation is not a bad thing, and there was very little material cost difference.

The Installation Process

Cutting the XPS foam was made easy with a circular saw, given its two-inch thickness. In a later post, when I talk about the rigid insulation for the walls and roof, I'll show that utility knife is more than adequate for the 1/2" thick pieces (in two layers) we used everywhere else.

There were two different foundation conditions where the slab edge insulation was installed. Image 4 shows where the steel reinforcement bars came straight up out of the stem wall. They were bent 90 degrees, to connect with the slab, just before we poured the concrete.

Where there was a turn-down footing, the steel was bent before the concrete was poured. This required punching holes in the foam, before feeding it over the bars.

We then filled the gaps between the bars and the XPS with closed cell spray foam.

Preparing for depressions in the slab (showers, thickened ares for point loads, etc.) was the most tedious part of installing the foam, but worth the extra effort to maintain continuity.

All seams were held together by a few pieces of tape. The vapor barrier was installed next, then the slab.

Now that we have a slab-on-grade, with a continuous thermal control layerwe need to carry it through, uninterrupted, to the rest of the building enclosure. If we don't, the homes winter coat will have a failure.

In a following post, I will explain how we keep it continuous, and how and why we built the exterior walls without any structural sheathing on the outside.

A Very Big Thank You

This is Adria Aldridge. She is a marketing development manager at Owens Corning. We worked closely with her and a top notch research and development team to develop the high performance building enclosure for the home, which they are now making in to case study, to highlight the benefits of making the extra effort to seal the enclosure on the outside, rather than waiting until a house is dried in. The project is also one of their many "Discovery Homes". This woman is tireless!

This is Niel Friendberg, Building Science Leader and Field Applications Engineer with Owens Corning, on one of the many training days he flew out from Texas for. Our crew had some experience working with foam, but Niel was there to offer best practice tips and tricks for preparing and installing the foam. He was a huge help throughout construction, guiding our installers to get the job done right. He's wicked smart, and patient!

Last, and certainly not least, this is Mikael Salonovaara, on the right. He is the Building Science Leader at Owens Corning that I spent countless hours with, fine tuning our perfect enclosure. He was very instrumental in getting the whole house monitoring equipment installed in the house, that, in part, measures heat loss and gain, as well as moisture content, in each of the assemblies, and for all different orientations. This is the geeky data stuff that we are excited to be sharing, later on. He's also wicked smart and patient.

Thank you.

Follow more on the #HighPerformanceBungalow, with this hashtag.

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Tags: Construction, architecture, atlanta, build, building, bungalow, design, high, insulation, performance, More…science


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Comment by Safe-R Insulation on January 2, 2018 at 6:39am

Hey, Your article is full of information about insulation. There are lots of ideas you are sharing here. I can see how you guys insulate with construction. That's how you can make your new house warm.

Insulation NewZealand 

Comment by Chris Laumer-Giddens on February 2, 2016 at 7:44am

Steven Lefler: For ground temperatures, check out the link that Graham Irwin posted, These are reasonable assumptions of the varying temperatures across the country. In Atlanta, we assume between 55-60 degrees, which goes to show that a local soil temperature profile is useful before starting calculations and making assumptions.

Chris Dorsi: Thank you, and I agree. The more we share our troubleshooting and successful experiences, the sooner more of us are applying best practices, and the better our buildings become. This will help drive demand up, and cost down, which is one of the many things we are working on with research and development teams at various manufacturers. This coming year will have a lot for us to share, as we continue to get the insulation out of the cavities, and put it outside the structure, with all the other control layers (water, air and vapor), like we did on this project. Continuous exterior insulation is something we definitely don't see a lot of in the Southeast, but the time is coming when we won't have a choice, as the IECC continues to crack down on low-performance building. The goal is to find solutions that stay way ahead of the code that is already way behind. The difference in all our projects moving forward is that we're going to stone wool (good hygrothermic properties, doesn't burn, etc.) for all insulation, cavity and continuous. In fact, we're getting away from foam, altogether, and only using tape and liquid applied materials for air sealing, flashing, etc. One of the goals, as it should be with all buildings, is to push durability and reliability to the highest priority in the enclosure for the sake of comfort and longevity.

Graham Irwin: You're right on, and thank you for sharing the link. Good resource. Finding the balance is really what we spend most of our time doing in designing and construction, isn't it. Whether it's earth temperatures vs indoor/outdoor temperatures, or the clents wishlist vs. budget. As I said in my LinkedIn reply to your comment, this stuff ain't rocket science, it's way harder (disclaimer: I am referring to the proverbial difficulty of rocket science, here. I have never designed a rocket, nor do I actually know how difficult it is. Just sayin....)

Tony Hicks: I don't have experience with Insulslab, but it seems similar to Energy Edge, another slab edge form that attempts to make it simpler to insulate. Executing the transition between either one of these products is where a good thing can go south. Attention to the details will make or break it. The detail we used in this project was definitely somewhat tedious,but too much to deter us from attempting it. We would do a lot of things differently, if we were to do it again, but I would still attempt this detail again, if we ever build a house on a slab, which I'm getting away from and keeping contact with the ground to a minimum.

RE: 2x6 wall. Yes, we used advance framing techniques. All framing is stacked, at 24" o.c., including interior walls that are 2x4.

Comment by Walter Ahlgrim on February 2, 2016 at 12:09am

I see a lot of great ides.

Is there a termite barrier in the system?


Comment by Tony Hicks on February 1, 2016 at 2:29pm

Is that a 2X6 wall?

Comment by Tony Hicks on February 1, 2016 at 1:14pm

Great article. One of the biggest reasons I joined this organization is because I wanted to learn about slab insulation benefits and practices. I'm from northern Indiana where slab insulation is even more important. We've been trying to find a way to insulate slabs cost efficiently. We had a good experience with Insul-Slab, which works great for deep footers because of the pier beam system in place. The other thing we liked about Insul-Slab was how affordable it was at the end of the day. is their website, if anyone is interested. 

Since 90% of heat loss is through the edge and extruded foam is kind of expensive. I'm curious to know how much extra it cost to insulate the middle.

How much did the whole system cost at the end of the day? 

Comment by Graham Irwin on February 1, 2016 at 1:00pm

It's pretty hard to find a place in the US where the soil temperature is "perfect." In most cases, it's either too cold, or too hot, as the map here shows: The key is to find the right balance of under slab insulation for heating and cooling. Insulation at the slab edge and perimeter is nearly always crucial because that's exposed to temperatures closer to ambient (outdoor) air.

Comment by Christopher Steinhoff on February 1, 2016 at 12:31pm

Why is that poor man cutting XPS with a circular saw without proper PPE?

Comment by Chris Dorsi on February 1, 2016 at 11:17am

Thanks, Chris.

This is an excellent primer that shows both the rationale for your approach and enough details to whet the whistle of the hands-on practitioners among us. We also can also see how critical and sometimes complicated the details can be in high-performance assemblies. We NEED organizations like yours to troubleshoot and fine-tune the methods and materials that work best so we don't have to flounder around in the muddy trenches trying to reinvent the foundation and figure it out for ourselves.

As an aside -- to anyone who thinks that building codes alone are sufficient to guide the construction of high-performance buildings -- there are a ton of details in the assembly described here that have never been captured in code specifications or even program guides. This stuff needs to come directly from builders.

Good work -- I'll help spread this post around so others can see what you're doing.

Comment by Steven Lefler on February 1, 2016 at 10:59am

You mention ambient air temperature as the basis for your insulation usage. What is the ground temperature seasonally?

Would an ICF be the choice along the perimeter as your form?

Lastly, would a 12" raised foundation achieve the high performance (over the slab) for comfort and insulation goals since the foam would be in the floor?

Comment by Kevin Wood on February 1, 2016 at 10:58am

Wow! Thanks for sharing this info.

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