What is the best foundation system for a new house near a coastal river?
My plan is to use a high-density cross-laminated polyethylene sheet under the poured concrete footers for the house. This layer will be wide enough to be placed against the waterproofing and rubber cover against sprayed hot asphalt to the sides of the concrete block foundation which will be at least one block above what is required by code. Then I hope to place rigid insulation against the foundation up to ground level where the laminate sheet should be closed off. I am also considering venting attic air down through the basement to keep it drier.
Building in areas with high water tables and exposure to water at the foundation level can be a tricky proposition.
You’re off to a great start with the poly under the foundation. There are many caveats and tricks to getting this right the first time to avoid a second or third attempt -- which can be catastrophic.
- When speaking about foundation specifications, the first place to look would be to Building Science Corp’s builder guides.
- Choose the correct guide for your climate -- this guide is full of specifications for different types of foundations.
- These guides will provide you with specific guidance of not only what to do but also what is not recommended.
Get data on the building conditions at your site
Next, I would recommend prior to completing your construction documents that you ascertain the water table on your site.
- This information will provide you with knowledge on where to expect water during the wettest parts of the year.
- It may help determine when excavation and foundation installation can be best accomplished (typically during dry summer months with high water tables).
- The local building inspector, other builders, and civil engineers may know what is typical for your area.
If this is not available, I recommend getting a soil sample/analysis completed. While this may seem costly, a soil analysis will provide recommendations for foundation depth and bearing conditions, which is extremely important when building a durable home.
Superior Walls foundation systems
Now that you know your building conditions, we can get to the heart of your question.
I have quite a fondness for Superior Walls foundations. Their Xi system comes pre-insulated and this pre-cast system is installed with a proprietary waterproofing system that does not typically require waterproofing.
The steel structural support members conveniently make up stud bays for the basement/lower level of your home that can be used to run electricity and plumbing. The best part is that once you have completed the rough installation of these items, you can insulate and drywall right to the foundation wall without furring out a wall system. For more information on this system, have a look at their website here.
The Superior Wall system is set on a footing of crushed stone. Regardless of your choice of foundation system, I would recommend installing crushed stone in 4” lifts following local code requirements for compacting and installation.
Stoning a hole provides a significant boon to drainage under your foundation. I’ve gone as deep as 30 inches to provide excellent drainage away from the foundation and under the slab.
The most important specification for this foundation will be drainage.
- I would recommend inside and outside drains with a sump pump under the slab inside and perimeter drains on the outside that drain to daylight or a holding tank that drains to daylight.
- Install a sealable sump pump box into your slab as protection against radon exposure. In areas of high radon exposure, be sure to install a radon mitigation pipe that can be vented through the roof.
EPA’s Indoor Air Plus program
Lastly, a great resource for constructing dry basements in your local area is the EPA’s Indoor Air Plus program (formerly Energy Star Indoor Air Package).
This program is focused on achieving superior air quality in your home. The first step in achieving indoor air quality is a dry basement. For more information on Indoor Air Plus, consult their website here.
- If you need expertise with these specifications, I would consult a verifier on the Indoor Air Plus list for your local area.
- Strongly consider pursuing this certification to ensure it has been constructed in accordance with Indoor Air Plus specifications.
Best of luck with your project!
For more information:
Read "We’re building a new home. Should we insulate both sides of the foundation?" a Q&A answered by John Messerschmidt.
Also, read "We are building a new "green" home in Utah. Can a foundation drain contribute to home health?" a Q&A answered by Greg Fitzpatrick.