Basement foundation drainage

[MattF]

I recently bought a house in a high water table, near a lake. Recently we had a storm that knocked out power, the sump pump stopped working, and we had a flood. After pulling back vapor barrier and insulation along the basement walls, I noticed something strange about the way the foundation walls and slab are constructed.

The slab does not meet the foundation wall. Instead the slab is framed in 2x2s all the way around the perimeter of the basement, with a gap of 2" or so between this frame and the foundation wall. The basement walls are wrapped in Tyvex on the inside that extends into this gap. When we had the flood, the water filled this trough around the perimeter. I have never seen anything like this in the houses I have lived in, but I have never lived in a high water table either. It's a small, independent builder, and the home was custom built for the prior occupants.

We contacted the builder, and he came to the house to explain. He said he builds all his houses in this manner. It is so when cracks occur in the foundation wall, any water will travel down along the Tyvex and into this trough, eventually making it's way to the sump pit. I have never heard of this before--I was wondering if any of you folks could give your opinions on this. Would this not be an entry point for Radon gas?

Thanks in advance for any insight you can provide.

 

[MicEd69]

Yes, that is a system to keep the basement dry. Get a battery backup pump or system for your sump pump.

Get a radon test kit and see if you have a problem.

 

[RS]

That system if fairly common in areas where the water table is high, one is called the Beaver system. Like M said, get a battery backup or a generator. If power outages are very common I would get a generator, then you can keep your food from spoiling and have a hot cup of coffee! I would get a radon test kit and check the house, we found out our last house had high radon when we sold it, and now my wife and I both have CLL, but hers is W&W, and mine is treatable.

 

[MattF]

Yes, that is a system to keep the basement dry. Get a battery backup pump or system for your sump pump.

Get a radon test kit and see if you have a problem.

Thanks very much for the reply. Good to know this system has been seen before. I've ordered a Radon test kit, so we'll see what that says.

 

[MattF]

That system if fairly common in areas where the water table is high, one is called the Beaver system. Like M said, get a battery backup or a generator. If power outages are very common I would get a generator, then you can keep your food from spoiling and have a hot cup of coffee! I would get a radon test kit and check the house, we found out our last house had high radon when we sold it, and now my wife and I both have CLL, but hers is W&W, and mine is treatable.

Thanks for the reply, and the information; I appreciate it. That's scary about the CLL--I've got a Radon detector on its way, so we'll see what that says. Hope you both are doing better. Take care.

 

[Mitchell-DIY-Guy]

Interesting.

The Beaver System is but one interior basement drain system, generally a DIY project. It isn't exactly how you describe, but rather is a means to allow water into the basement and then provide a place for it to drain to by using a system of weep holes drilled into the basement walls and an on-slab drain system. Here's a photo of one being installed: installingsystem.jpg

Generally this system, as well as others are installed AFTER a basement's exterior drain system has failed, or wasn't built properly to begin with. The more conventional drain system these days is external, and is drain pipe along the footer/foundation wall, with through-pipes to the under slab drain system, to a sump with pump. There are a number of ways to do this with various geotextiles, backfill material, external waterproofing, and exterior drain membranes such as this: DELTA®-MS - Below-grade Dimpled Membrane | Dörken Systems Inc. - DELTA® Maybe the home's builder used both internal and external!

Radon test kits are available at home centers and online. North Carolina provides test kits free to residents. Radon is exceptionally site specific; in my Michigan neighborhood all homes were built with basements, and all homes were tested after completion by the builders. Some homes tested essentially zero, others into the hundreds, just blocks apart. After the first year or two, when some houses tested high, the builders were placing the vent system w/o the fan in place during the build, for a nicer look if needed. If, after testing, the results indicated an actionable level (>4 pc/L of radon) they simply added the fan. The mitigation in these houses was quite simple: a 4" hole was cut into the sump cover, the vent was placed in the hole, and everything was sealed up with silicone. The fan was in the garage and sucked air up from the foundation drain system. My fan was installed in 1992 and was running essentially non-stop until I moved out of the home in 2019. In slab homes, they drill a 4" hole in the slab and vent from that. My home in Michigan was tested in the basement at 19 pc/L. After the mitigation it was undetectable.

As a home inspector I always recommend doing a first pass test with a set of cheap test kits. You place them in the LOWEST LEVEL of living space. Thus, if you are on a slab or crawl space, it's your first floor. If you have a home with a basement, in the basement. Radon is heavy and settles low; it doesn't generally go up. My own home here in NC has an encapsulated (sealed) crawl space, and the test level was 2 pc/L. My first floor was undetectable. the crawl space isn't living space thus we don't need to worry about that.

Biggest risk of radon is lung cancer. Health Risk of Radon | US EPA after long term exposure. Meaning, do the testing, assess the results and plan the mitigation if needed. No reason to pay double time on Sunday!

 

[hukre]

I recently bought a house in a high water table, near a lake. Recently we had a storm that knocked out power, the sump pump stopped working, and we had a flood. After pulling back vapor barrier and insulation along the basement walls, I noticed something strange about the way the foundation walls and slab are constructed.

The slab does not meet the foundation wall. Instead the slab is framed in 2x2s all the way around the perimeter of the basement, with a gap of 2" or so between this frame and the foundation wall. The basement walls are wrapped in Tyvex on the inside that extends into this gap. When we had the flood, the water filled this trough around the perimeter. I have never seen anything like this in the houses I have lived in, but I have never lived in a high water table either. It's a small, independent builder, and the home was custom built for the prior occupants.

We contacted the builder, and he came to the house to explain. He said he builds all his houses in this manner. It is so when cracks occur in the foundation wall, any water will travel down along the Tyvex and into this trough, eventually making it's way to the sump pit. I have never heard of this before--I was wondering if any of you folks could give your opinions on this. Would this not be an entry point for Radon gas?

Thanks in advance for any insight you can provide.

In our are the builders put 2x4s along the bottom of the foundation to match the level of the finished slab after it is poured. Once the concrete is hardened, they take the 2x4s out which leaves a 1½" wide channel to run water to the pump sump in case of a flood in the basement. It is important to remove the 2x4's (or 2x2's) otherwise they rot out and are a source of moisture and black mildew.

Yes, that is a system to keep the basement dry. Get a battery backup pump or system for your sump pump.

Get a radon test kit and see if you have a problem.