Slab and Basement Foundations

There are many different options for foundations. Concrete stem walls and footings, piers, monolithic concrete slabs, rubble trench with concrete bond beam, and sand bag and rip-rap stacks are all possibilities. In general, foundation systems contain one or more of the following: footings, piers, stem walls, and/or slabs. The two most common systems are monolithic slabs and stem walls with footings. For straw bale construction, the most popular system is a slab on grade. This can be a stem wall with footings and a floating slab of concrete or earthen floor, or a monolithic concrete slab which incorporates the foundation and finish floor in one system. On the other hand, stem walls are especially useful in areas with a deep frost line. If your code requires you to excavate below frost line and your frost line is deep, you may as well pour foundation stem walls and add a basement to your plan.

Slab on Grade

A floating slab can work in conjunction with basement walls; however, if you are not required to excavate below deep frost lines, a monolithic slab on grade will be much more practical. A monolithic slab provides for a floor system, foundation system, and heat sink all in one step. A heat sink is created when materials with high thermal mass are installed in an area that receives direct sunlight. The sun heats the mass and then over time the stored heat is slowly released into the room. This is one of the major principles of passive solar design. A concrete slab makes for an excellent heat sink. This is also true when using a radiant floor heating system. The large mass of the concrete slab provides for excellent thermal storage, lowering heating costs.

As with any material, there are a number of downsides to using concrete. The production of the product itself is very damaging to the environment. Concrete, when used as a new product, has a huge embodied energy (EE) cost. The process of making Portland cement, an ingredient in concrete, creates a 100% embodied energy ratio. In other words, for every one pound of Portland cement, one pound of carbon dioxide is produced. As much waste material as product is created, yielding a 100% EE Ratio. The addition of rip-rap, old pieces of cured concrete broken up into small slabs, reduces the need for new materials. Minimal amounts of fly ash, incinerator ash, can be added to the mix in place of Portland cement. Some concrete companies do not offer this option, so check it out ahead of time.

Slabs are so popular because of the availability of the material, and ease of construction (once you have figured it all out). With a little practice, anyone can pour an attractive slab foundation. A basic slab consists of wide footings at the perimeter or wherever extra bearing is required, sub-slab gravel inside of the footings, 6 mil plastic, insulation, and a 2″ sand bed. See diagram A below. It is very important to get proper compaction of the sub slab material in order to reduce cracking in the finish slab. Once the slab is formed, leveled, and squared, the installation of the above materials can be relatively quick and easy. For example, a simple 200 square foot slab can be prepared and poured by one person in about four days. Once poured, as with any concrete, you must wait a minimum of 45 days to allow for proper curing before you begin construction on the concrete. The addition of acid staining or broadcast color is another plus for concrete slabs. The timing of the application of these materials depends on the materials you choose. Broadcast colors are added during the steel toweling. Acid stain is added after the forms are stripped.

Basement Walls

In areas where deep excavation is essential to meet code, a basement may be the best option. Because you may be required to pour deep stem walls to get below frost line, it makes perfect sense to finish these walls and create a basement. The added cost is minimal for the square footage you can realize. Bale walls can be placed on a wood floor joist system spanning the basement stem walls. If the code in your area states that bearing is required for the full width of the bales, you can minimize the use of concrete by adding a framed wall at the inner edge of the bales that bears on the basement floor slab rather than increasing the width of the concrete stem wall. The framed walls also allow for insulation to be placed between the heated basement room and the foundation walls, creating a physically and emotionally warmer room.

Waterproofing the exterior of the basement walls is essential. There are many ways to accomplish this. The following system is economical and provides excellent waterproofing. A waterproof membrane is placed against the foundation wall on the exterior of the building. On top of the membrane, use a sheet drain material to channel water away from the foundation walls. Sheet drains are designed specifically for this and consist of a formed polystyrene board covered on one side with filter fabric. The fabric is placed towards the soils and allows water to flow into the polystyrene board while minimizing the impact of the soils on the system. The sheet drain then wraps around a perforated pipe which carries any water away from the foundation. See diagram B below.

With the walls properly waterproofed, you can move your attention to the interior walls and insulation. By framing out interior walls, you create not only bearing for the bales, but also a place for insulation and backing for sheetrock or other finish materials.

Whether you choose a basement wall or monolithic slab system, straw bales can work with your project. Check with your codes, decide on your design and plan for some hard labor. No matter what system you use, concrete is physically challenging to work with. Set up your pours for mornings to allow yourself a full day to work with the concrete. Pay attention to the weather as rain will cause many problems when working with concrete. Finally, make sure you are ready for the concrete when the truck arrives. There is a time limit to how long concrete can spin before it starts to fire off. Don’t have a truck on site until you are sure that you are ready to go. Above all, be open to the process of learning and try to have fun.

To construct your own monolithic concrete slab I recommend the instructional DVD I created. Please click here for more details on our concrete slab foundation dvd.

ABOUT THE AUTHOR

Andrew Morrison has a passion for straw bale construction that is matched only by his desire to teach his knowledge to others. He has a wealth of experience in designing and building both conventional and straw bale homes. After years of building, he has moved his practice entirely to consulting and teaching. He shares his knowledge with thousands of people via his DVD series and this website and teaches roughly six hands on workshops each year. For more on his workshops, please visit www.StrawBaleworkshops.com.  Andrew received a BA degree from Hampshire College in 1995 for Glacial Geology. He also has a degree in construction technology.

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