Insulating a Slab On Grade in Hotlanta

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:

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)  a)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.  b)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.

Insulated Slab on Grade Detail

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.

High Performance Bungalow Insulating Slab-On-Grade Hotlanta

High Performance Bungalow Insulating Slab-On-Grade Hotlanta

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.

High Performance Bungalow Insulating Slab-On-Grade Hotlanta

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.

High Performance Bungalow Insulating Slab-On-Grade Hotlanta

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.

High Performance Bungalow Insulating Slab-On-Grade Hotlanta

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

High Performance Bungalow Insulating Slab-On-Grade Hotlanta

High Performance Bungalow Insulating Slab-On-Grade Hotlanta

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.

High Performance Bungalow Insulating Slab-On-Grade Hotlanta

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

High Performance Bungalow Insulating Slab-On-Grade Hotlanta

High Performance Bungalow Insulating Slab-On-Grade Hotlanta

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.

High Performance Bungalow Insulating Slab-On-Grade Hotlanta

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!

Insulating Slab-On-Grade Hotlanta Adria Aldridge Owens Corning

This is Neil 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 Neil 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!

Insulating Slab-On-Grade Hotlanta Niel Friedberg Owens Corning

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.

Footnotes   [ + ]

a. 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
b. Admittedly, I am a Winter Wimp

12 Responses so far.

  1. David Eakin says:

    You’re not going to use that nicely sealed/insulated slab for heating and cooling as per Robert Been ( Yes, you will also need dehumidification but that could also be integrated with the ventilation system.

  2. Graham Irwin says:

    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.

  3. David Eakin says:

    Sorry – I thought you may be well-acquainted with Robert Bean’s work (definitely worth it) as he is a very strong proponent of multiple influences on human comfort as well as radiant heating and cooling as the best way to accomplish both.

  4. Jay Freischlag says:

    Love it! Nice that you get some educated factory support for the project. It takes the right person to step outside of the box and start trying new things that only people later say, that makes complete sense……data, data, data. Everything can be measured.

  5. David Eakin: Yes, I’m well acquainted with Robert Bean. In fact, there is a great discussion on his website about radiant heating in high performance homes that should help explain our reasoning for not going that route. Specifically, the actual total heating load in this home, on the coldest day (about 1%, or 88 hours) of the year is between 10,000 and 12,000 btu/h. The rest of the winter, will be less and less, as the temps get milder. If there was a longer period of time spent at that load, or it was a much higher load, than it could start to make sense. As you will read in the discussion on Robert’s site, there are ways to make the system work for lower loads, an not overheat the house. The example he uses, is for a 10,080 btu/h load, which is about what this house is. Again, it’s only 1% of the year, and the insulation does a good job of preventing at least 80-90% of the heat loss, and keeps it within the thermal boundary, when under the entire floor and at the edge. floors in a low load building

    Comfort is king in any home, not just high performance ones. Radiant heating and cooling has its place in many homes, and it was considered here. In the end, we needed to move air for various reasons. A big reasons is fresh air distribution. The fresh air strategy uses the air handler and ductwork to distribute the air via an encapsulated attic, which acts as a plenum. Its a VRF air handler, so it runs constantly, and the fresh air is brought in through a simple inline fan, with filter, which dumps it in to the attic space where the air handler, with open return plenum, takes the fresh air, filters it again, conditions it, and distributes it.

  6. Tom B says:

    Wouldn’t perimeter insulation down to your footing get you 75-80% of the performance benefit for significantly less cost?

  7. David Eakin says:

    Are you relying exclusively on the AC air handler for conditioning or are you also employing a dehumidifier (like Matt Risinger recommends to his clients in Texas:

  8. Tom B: There is a significant amount of stem wall that extends above grade. It would have taken more insulation to cover the walls down to footing (a good detail), and we would need drainage mat for all areas below grade to minimize hyrdrostatic pressure and prevent bulk water intrusion. Also, there would be additional finishing system to install on top of the foam for the above grade areas of the stem wall.

  9. David Eakin: The primary purpose of a dehumidifier in a home is to remove the moisture that gets in to the house through infiltration, but it also helps remove moisture generated by internal latent loads, via mechanical fresh air, showers, cooking, human and pet activity, etc., when exhaust ventilation isn’t adequate.

    Two very effective ways, that also have less up front and operational cost, to control these moisture loads are:

    1. Minimize the infiltration
    2. As I eluded to above, effectively remove internal loads with exhaust ventilation.

    We did both in this house. The infiltration testing showed a leakage rate of 0.47 ach50, or 0.009 ELR, and we installed adequate ventilation to manage internal loads.
    In short, we don’t need a dehumidifier.

    The dehumidification being done by the cooling system, and the heat pump water heater in the encapsulated attic, is a redundant bonus. It’s not needed, but it’s there.

  10. David Eakin says:

    True about reducing internal loads, but in the Matt Risinger example he uses the stand-alone dehumidifier to also condition the air coming into the HRV so the internal latent loads are not increased via mechanical ventilation. I thought it might be very relevant in your build location.

  11. Meg Needle says:

    There are lots of good reasons to install perimeter insulation under slabs on grade. However, termites are a problem in the south and they use insulation as a path to tunnel to wood framing in walls. For this reason IBC prohibits under slab insulation in the southern states.
    Has there been new information/research that has overturned that opinion?

  12. David Eakin: As you will see in tomorrow’s blog post, about the fresh air and make-up air strategy, the semi-conditioned attic is “buffering” all incoming air, before the air handler picks it up to filter and condition it, prior to distribution. The amount of fresh air being brought in is based on the needs of the occupant, and doesn’t amount to very much additional latent (between 550 – 1,400 btu/h) or sensible load (between 450 – 1,200 btu/h), even on a design day. These loads are being managed partially through removal by exhaust fans, and the rest with condensation at the cooling coil of the air handler, which I sized accordingly.

Leave a Reply