We're just a few weeks away from a computer generated, self-guided interactive virtual tour of the Proud Green Home at Serenbe, a design-build collaboration with The Imery Group. We're using REVIT software to generate the 3D model, then  VIMTREK will animate the tour.

What you will see in the tour is the home's modern design, its 3 bedrooms and 2.5 baths, its great views of the central Georgia landscape, and its outdoor kitchen and fire pit. What you won't necessarily see are some of the details that make it a high performance home, where we employ some of the best practices of building science and energy efficient design.

Details, Details, Details...

The biggest opportunity in designing for high performance is in controlling the flow of heat, air and moisture. Do this well, and the home will be comfortable, healthy, efficient, and will last a long time. Here is the slab-on-grade detail for the Serenbe home. We're going to walk through the moisture control methods we employed. In Part II, we'll focus on controlling heat. Finally, in Part III, concentrate on controlling air. Moisture is first, because it should be.

Designing for High Performance, Part I, Controlling Moisture Flow

Keeping water out is perhaps the most important thing you can do in a building because prevents mold, condensation, water damage, indoor comfort issues, and more.

For the first layer of control (bulk moisture) on this above grade wall, we have Nichiha® cementitious siding. Just behind that, we've chosen a layer of 1/4" rain screen, called Home Slicker, to create a gap which keeps any moisture that develops behind the cladding flowing down the wall via the drainage plane and out.

Designing for High Performance, Part I, Controlling Moisture Flow

For our drainage plane, we're using the OSB layer (with its built-in water resistive barrier) of the 1" Zip System® R-Panel. At the base of the wall, and at all openings, is continuous aluminum flashing, which is also taped to the R-Panel with Zip System® tape to maintain a continuous drainage plane. In the event that moisture does get in to the wall, which would most likely be airborne, not bulk, moisture, the materials have been selected to have a permeability that allows that moisture to get out of the cavity.

This is a critical point about designing building assemblies. They MUST have drying potential, either inward or outward or both. It's why the industry has tried to get away from putting vapor retarders, and other impermeable materials in walls (including vinyl wall coverings).

At grade level you will see that we have called out for a minimum 5% slope away from the building with an impermeable back-fill layer to prevent saturation of ground adjacent to foundation. The free-draining back-fill layer allows a free flow of any moisture toward the even more free-draining layer of stone. The continuous drainage tile is set below the top of the footing, and well below the bottom of the slab, to collect water that makes its way down and takes it to daylight, and somewhere downhill and away from the foundation.

The foundation wall is coated in damproofing, and then another drainage plane, in this case Delta-Drain®, is used to allow any bulk water that happens to make its way to the foundation wall, down to the drainage tile and away from the wall and footing. The purpose of the filter fabric is to prevent dirt and debris from clogging the perforated drain tile.

Designing for High Performance, Part I, Controlling Moisture Flow

At the base of the foundation wall, we will install Delta-Footing Barrier®, a capillary break, to prevent the wicking of moisture from the continuous concrete footing in to the foundation wall, that has the potential to wick up the wall and in to the home. The granular fill layer is not only a good bed for the concrete slab to set on to minimize movement and cracking, it also works allows any moisture to freely flow away from the slab in to the soil. The 6-mil polyethylene vapor barrier helps prevent any moisture from making contact with the concrete slab that could wick up and in to the home.

Designing for High Performance, Part I, Controlling Moisture Flow

The Cellofoam PermaBG Expanded Polystyrene (EPS) foam board acts not only as a thermal and insect/termite barrier, it's also acting as a vapor barrier for any moisture that may get in to the wall. Since we're well protected at the outside and base of the wall, the foams primary function is for controlling heat flow (which is covered in Part II of this series). Finally, as a last line of defense, the continuous sill gasket (while also helping control air - Part II) will prevent the wicking of any unlikely moisture from the foundation wall in to the frame wall.

Tags: controlling, design, details, efficient, energy, flow, high, moisture, performance

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Replies to This Discussion

You got some great details in their Chris, but I got to ask why no foam board underneath the slab? For those areas where it is required, got any thoughts for them? Thanks

 I just left a job site where the builder is convinced that he followed the IECC 2009 heated slab on grade specifications for Zone 6 (Montana).  He passed local inspections so I am now second guessing myself.  He has 2 inch EPS  installed 2 feet down and 2 feet out from the perimeter of the mono-slab, but there is no insulation beneath the floor area.  I agree with the perimeter insulation configuration, but I have always recommended a minimum of 1" eps (R5) to provide a thermal break and can model heat loss benefits for 2" eps (R10) it the soil conductivity is high .  As I read the 2009 code it calls for perimeter insulation for a heated slab to be R15, but I can no longer find the under slab requirement. 

The concrete contractor told him that slabs that are poured over EPS will crack, as all of the moisture lost during the curing process must travel to the surface of the slab, creating small air channels that promote cracking.

I have fully insulated slabs that are over 27 years old, with radiant heat, that do not even have a hairline crack. 

Can anyone clarify the ACTUAL code requirement for Zone 6??

It all depends on the AHJ as to how they interupt the code but 402.8 says

402.2.8 Slab-on-grade floors: Slab-on-grade floors with a floor surface less than 12 inches (305 mm) below grade shall be insulated in accordance with Table 402.1.1. The insulation shall extend downward from the top of the slab on the outside or inside of the foundation wall. Insulation located belowgrade shall be extended the distance provided in Table 402.1.1 by any combination of vertical insulation,insulation extending under the slab or insulation extending out from the building. Insulation extending away from the building shall be protected by pavement or by a minimum of 10 inches (254 mm) of soil. The top edge of the insulation installed between the exterior wall and the edge of the interior slab shall be permitted to be cut at a 45-degree (0.79 rad) angle away from the exterior wall. Slab-edge insulation is not required in jurisdictions designated by the code official as having a very heavy termite infestation.

The table reads R10 & 4Ft

Basement walls are R15 continuous or R19 batt while crawls are 10/13 while floors should be 38


The only time I have found that it make sense is in colder climates (CZ 4 maybe, and higher), or when you're installing radiant floor heating. Otherwise, the payback is not justified.

Slab edge insulation is way more effective in this area at reducing heating loads, because that is the primary point of heat loss.

The calculations for a 2,800 square foot slab on grade house in Georgia show a decrease in the heating load of less than 700 btu/h when we add full underslab insulation (R-10), and a decrease of 7,750 btu/h when we add just slab edge (R-10).

Chris, you mention slab edge r-10, yet the drawing shows 1" of foam.  Did you opt to go with r-5 instead of r-10?

Another issue I have seen is the, foundation in my case, being dug into impervious soils.  The result was a swimming pool with no drainage from under the slab.  If the soils are as bad as I encountered, then drainage would need to be installed below the footing level.  The soil guy called these glacier compacted soils, so doubt you have the same, but clay can be as bad.


Thanks, Bud.

The 2,800 s.f. home was one I worked on several months ago, and we modeled it with and without R-10. I find that the difference between R-5 and R-10 on any house in this climate zone is negligible. Maybe 500 btu/h, so I usually go with R-5 or even R-3. It's amazing what that little amount of insulation does.

I've had to deal with those types of soils before, as well. We also put tile in the inside of the foundation wall, and ran it out to daylight. For the little expense, it was well worth it.

We're actually still waiting on the soils test. May have to revisit this detail if we encounter too much clay. Thanks for the tip!


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