The Foundation of your Home: Slabs, Basements, and Crawl Spaces

What is under your home?  I hope you have some solid ground.  If you do great!  Now how did your home builder get to solid ground?

The first of a multi-part topic.
Left: Slab on Grade ready for walls. (courtesy of Wikimedia Commons)

That can be done with a slab on grade construction. It can be done with a basement.  It can be done with a crawl space. A home can use a combination of methods. It is not at all uncommon to see homes with 3 of the 3 approaches.

Above Right: A slab on Grade shows heat loss through the slab. This is under the front door. Inside temperature is 72 degrees, outside is 14 degrees. The cold (blue) pile is snow.

Basements are generally a below ground space that contain living area and is capable of being heated.  This does not include below ground spaces that are for storage or storm shelter. Basements may be full, view out, or walk out types.

Left: Typical craw space, showing rim joist. This has about 30 inches of concrete and a 2×8 floor joist.

Crawl Spaces are below ground space with no living area.  They are generally low head room spaces, hence the name – Crawl Space.  Some you can actually walk in.  I have seen several crawl spaces that are only 12 – 15 inches high, most are 30 – 48 inches high.  Crawl spaces generally have duct work for the HVAC system, plumbing, and wiring. Occasionally the furnace will be in the crawl space.

Below Right: This addition has 2×10 floor joists with support beams. It provides about 8 inches of clearance between the floor and the beam.

Many homes have a basement with crawl space. Builders in one geographical area tend to build with the same approach.  Another geographic area will find another approach being common. In places where the water table is only a few feet below ground you would be hard pressed to find a basement.  In places with large rock formations near the surface, the builder could use any of these approaches depending on how deep the rock is on the building site.  In tornado alley, basements are always welcome for a storm shelter, and some families use their crawl space.

How do these spaces impact the energy use in your home?

Depending on the age of the home, the below ground walls of the house could be of stone, block, rubble fill or concrete.  These are hard materials that have similar physical properties.  They have high thermal capacity; they will stand extended periods below ground in contact with dirt and moisture.

Thermal Capacitance

Thermal Capacitance is the physical characteristic of a material to hold heat. How long does an object stay hot after being heated? The longer is retains heat, means a higher thermal capacitance.

Above Right: The Rim Joist is where the basement window is. Note the heat transfer through this area. It is somewhat diffused by the brick veneer. Outside temperature is 20 degrees, inside temperature is 68 degrees about 10 pm.

Growing up in Southern Nevada, I lived in a house constructed of concrete blocks.  My bedroom was on the east side.  An 8 foot fence stood 4 feet from my bedroom wall. The sun did not shine directly on my bedroom wall until about 11:00 AM and by 1:00 PM it had passed over. I would go to bed at between 10:00 PM or 11:00 PM and the wall would still be well over 100 degrees. So that wall loaded enough heat during that 2 hour period, to still be over 100 degrees 9 hours later. If a material can hold heat like that, then it can also hold the absence of heat, or cold, in the same way. Concrete, and stone have a high thermal capacitance.

Any basement or crawl space wall will extend both above and below ground. It is not uncommon to see a wall 1 and ½ feet above ground and the rest below ground.  If that portion of the wall is un-insulated, the heat will flow through the wall. Heat will flow in during the summer and out during the winter.

The Infrared Image (Above Left) shows a concrete basement wall. The upper part is showing relatively cooler and the bottom relatively warmer. The mid-point of this image is about 4 feet below the top of the concrete wall. The outside temperature is 45 degrees. Imagine the difference at 20 degrees?

What can the homeowner or the builder do to these walls for energy savings?

Insulation is the obvious answer. What kind of insulation is a better question. I would specify the insulation, you choose, to be installed correctly, that it be durable and properly protected from damage.

Before we actually get the insulation, we need to make a stop.  Air movement through and around the insulation will significantly impair the effectiveness of the insulation.  So lets tighten up the area.  The first area to look at is the Rim Joist.

This is on the Rim of the foundation wall. The floor joists, 2×6 or larger or a truss system rests on the top of wall. If you look at any of your rim joist area you will see a number of things.

(Right) An IR image of a Rim Joist. Outside temperature is 70 degrees. This is a south wall and the sun has been shining on it. Lots of solar loading.

Usually your outside faucets come through here. The gas line enters the house at this point, the air conditioning copper tubing, and perhaps other utilities.  I’ve seen dryer vents installed here, and even the flues of conventional hot water heaters.  Those are usually easy to spot because the flue then goes up the side of the house above the roof.  If you have a tankless hot water heater, the PVC flue may well exit the area through the rim joist.  Then you have joints, knot holes and it is not uncommon to see actual holes where something was there and has been removed. All these holes and gaps need to be sealed up.

Remember that insulation does not stop air movement. Caulk is great for stopping air leaks.  You can use caulk straight from the caulking gun.  Some gaps, may need  backer caulk also called rope caulk.  This is ½ inch strands of caulk like material that you can push in by hand. Then come back over it with the caulking gun.  Smooth the caulk with a tool, or your finger to make it go a little farther, make it look a little better and to eliminate any bubbles that might have formed.

If the gap is larger you may wish to use some of the foam in a can. This one part expanding foam uses moisture to cure. Use along a rim joist would probably have enough moisture to cure properly. Take seriously the caution, to not wash with water if you get in on your hands or elsewhere.  Water will cure it.  It will take a couple of weeks to wear off.  If you don’t ask how I know that, I will not turn red in the face!

You can also use rigid foam on larger openings.  Just remember to caulk the edges.  Also remember to caulk the joint between the sill plate and the concrete. New builders use a gasket now,  a bead of caulk is always helpful.

If the rim joist has insulation, you can remove it to do the air sealing.  You can replace it after air sealing, just be sure to install it properly. It is most likely a fiberglass batt, it may have a paper backing on it, it may not.  The paper backing may be facing the inside or the outside. I’ve observed all these in the same house.  The paper backing is a vapor retarder. It should face the warm in winter side of the wall in this area of Kansas.  (Climate Zone 4)  The batt insulation should fill the space between the floor joists, the concrete rim and the subfloor. That space is probably 8 inches deep, 16 – 24 inches wide and 6 ½ to  24 inches high.  Most common would be 8x16x 7.5 inches.

Since the Rim of the foundation wall is usually 8 inches wide, that would allow, at R-3 per inch, insulation of R-24.  I see R-19 batts commonly used here, older insulation jobs or homeowner installed jobs will show R-13, or even R-11.   If you have a ceiling in your basement, you may only have access to part of this area.  You can work in the accessible area and make a difference.

To remove a finished ceiling and then replace after the air sealing and insulation is completing will probably not be cost effective, even if you do not consider the effort of the Do It Yourself labor.

This is the first of a multi-part topic. We will look at Basements, both finished and unfinished, crawl spaces, and slab floors each in more detail.

One thought on “The Foundation of your Home: Slabs, Basements, and Crawl Spaces

  1. John Nicholas Post author

    The info speaks for itself. You may wish to check the info at Building Science Corporation. The info presented here is based on multiple sources, and completing energy audits in over 250 homes.

    Reply

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