Replies

1. Expert Member
   ARMANDO COBO | May 06, 2014 10:22pm | #1

   Generally you get a soil geotechnical report where it’ll give you the actual soil condition for your lot. Base on that report, a Structural Engineer will design the required foundation for you. If you leave in a mountain area or your lot has more than 3’-4’ drop, you may need to have retaining walls and fill to level your building pad. If there is any big rock in your land, that can complicate things. There are just too many unknowns in your question.
   If there are not unusual conditions in your area, and you have a flat, sandy lot, typical of the SWTX, many folks use monolithic slab on grade, because it costs less.

2. 
   VMarshall | May 07, 2014 12:53am | #2

   Thanks Armando! The geotechnical report is a great idea and we will definitely do that. The lot is 20 acres so we should able to pick a reasonably level building site.

   I'm equally concened with what type of foundation would help keep the house cooler in the long and somewhat brutal summer. Would the airflow under the house help with cooling, or would the slab, being in contact with the earth, keep the house cooler? So many questions! :)

3. 
   piggyfinger | May 07, 2014 01:23pm | #3

   Hi Vicky,
   A slab on grade foundation is a very common and cheap option that works well in your area. Assuming you go with a slab on grade, omitting sub-slab insulation (while still insulating the slab edge) and thermally coupling the house to the earth in some parts of the country can have a net heating/cooling benefit due to the deep sub-soil temperatures in the area. Generally, the sub-soil temps are close to the average annual air temperatures of the region. I'm not exactly sure where you are in West Texas, but the average annual air temp for El Paso is about 64 F. This may be a little too cool to give you a net benefit, i'm not really sure. I'd guess that insulating under the slab is not cost effective in your area. Insulate the slab edge and detail for termites.

   The real benefit of a slab in your area is the added thermal mass. West Texas has great diurnal swings, getting down into the 60s at night even in the depths of summer. That extra thermal mass of the concrete will absorb heat all day, and then release it in the cooler evenings. Opening windows at night or operating a whole-house fan will help the process.

   I asked a similar question about my climate last year here, and Dana and Martin helped greatly. There is also a nice discussion about earth coupling here. To learn more about thermal mass, read this.

4. 
   jackofalltrades777 | May 07, 2014 01:54pm | #4

   A soils report will generally cost you around $2k - $5k depending on the area, they are not inexpensive. You still have to pay to an engineer to engineer the home.

   Engineers will usually default to the IBC minimum of 1,500 psf and design off of that baseline. Unless you are in a really poor soils area (sand, swamp land, etc) the soil will usually carry around 2,500 psf average.

5. Expert Member
   ARMANDO COBO | May 07, 2014 11:25pm | #5

   It’s my experience, a geotech report will cost around $1500 and $500-1000 for the engineered foundation in Dallas, Albuquerque or Las Cruses. If you are in an area that is far away from "town", it could cost more, however, if the foundation is not done right, you may end up spending 10-20 to fix settlement issues.
   When it comes to slab insulation, in a high-performing house, a well-sealed and insulated envelope, heat loses can be higher than cooling loses in a hot climate, because we normally do not insulate foundations or slabs in the southwest due to termites. Having said that, there are meshes, insulations and ICFs that are termite resistant that can be used for your perimeter walls. The bottom line, I would be more incline to spend my money and effort to the best building envelope above ground you can afford.

6. 
   davisbaker521 | Mar 19, 2022 11:09am | #6

   Hi Vicky,

   Any chance you can give an update as to what you all went with? We’ve also acquired 20 acres in the Chihuahuan Desert in West Texas and are discussing build options (we’re tired of tent camping on our property!).