I have a split level house with a crawlspace. I've pulled the insulation out and discovered some minor mold issues on the wood framing and concrete walls. I removed the framing to replace any moldy pieces. After removing the base plate from the framing i've discovered a channel that runs underneath the framing base plate along the outside wall perimeter. Inside of the channel was damp with a 2x4 wrapped in plastic along the channel. I've attached some pictures of the channel. Can anyone tell me what the channel is for and why the 2x4 is there?

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4 Answers 4


You asked specifically what is the purpose of the channel. If it is connected to a sump, then it is an open drain to give a controlled flow path to the sump for water that somehow passes from the outside to the inside. Your walls look like there is no leaking, so probably the water is entering between the wall and foundation which likely were poured separately.

If the channel is not connected to a sump or to a drain that leads to a sump, then it is likely just an expansion gap (or joint) between the floor and walls.

The problem you have, though, is that the trough is damp/wet which means there is water coming in from the outside. This should not happen - ever. If the drainage around the house is functioning properly, any water that comes down the outside of the wall would be captured in an exterior drainage system and taken away from the house (or alternately flow through a drainage pipe that comes through the wall and ends inside the sump). Additionally, all ground around the house should be sloped away from the house to keep water from flowing down the outside wall in the first place.

If there is no standing water in the trough, then you are experiencing seepage into the trough (probably coming through the joint between the wall and foundation) which keeps it wet most or all of the time. To address this situation, start by removing all the wood from inside the trough. Spray a Clorox solution over all the mold to kill it. Using fans circulate the air to dry up the troughs. Once they are dry, spray with a mold preventer (available at hardware stores). Give the mold preventer a day to dry and then paint the troughs with a water stopping paint.

Address any exterior drainage around the house that would prevent rain water from flowing away from the house. Also make sure all your gutters are functioning properly and not overflowing. If necessary seal leaking gutter joints, reset the slope of gutters and/or add splash plates to keep water inside the gutters. If your downspouts are routed to underground drain lines, be sure none of these lines are partially or fully plugged and causing the flow to back up through the downspout-drain line transition piece.

Finally, any framing done on a concrete slab MUST have a baseplate of pressure treated lumber, whether or not it is wrapped in plastic. That is code that should not be ignored.

I probably gave you more info than you were looking for, but this is how I would approach the issue if asked by one of my home improvement customers.

  • Can you give a cite for the PT baseplate? (I haven't seen that anywhere in the IRC, so I'm wondering if it's a local amendment where you're at?) Nov 3, 2017 at 11:43
  • ConcreteNetwork.com. // SAMPLE CODE REQUIREMENTS FOR FINISHED CONCRETE BASEMENTS. // . Wall Construction. // Studs may be utility grade or better. Stud spacing shall be per TABLE 1, shall have a pressure treated bottom plate and can have a single or double top plate. ||| Also, in local code (Dublin, OH Building Standards) decay resistant [i.e. treated] wood is required for bottom plate in basement walls. Nov 4, 2017 at 23:46
  • Found it! IRC R317.1 calls out decay resistant wood (either treated or naturally decay-resistant) for lumber (incl. bottom plates) in contact with on slabs unless there's a capillary break between the concrete and the wood. However, even a PT bottom plate won't stop moisture from penetrating it and getting into the end grain of the studs, which makes the capillary break the better bet. Nov 5, 2017 at 0:00

It appears to be a drain channel as part of a groundwater mitigation system. Most water enters a basement through the area where the floor and walls meet, and this would carry that away, presumably to a sump pail. I can't tell you what brand yours is, but a search reveals many similar products.

The bottom plate of the wall is wrapped to minimize moisture absorption and transmission. Whether the channel was installed at the time of initial home construction or later is unclear. I can imagine a solution where channel is installed on top of an existing slab and a thin layer of cementitious material is poured inside it to raise the slab level accordingly.


The channel is probably there because of the order they installed the materials. First you pour the foundation walls, then you insulate the foundation walls with rigid insulation, then you pour the concrete slab that you're walking on. If they removed the foam and framed a wall there, then you would be left with a channel the thickness of the insulation.


Contractors don’t pour the concrete floor with the walls and footings. Generally the footings and walls are poured first, followed by the slab.

When the slab is poured, often a piece of pressure treated wood is laid against the wall and the slab is poured up to it. This piece of wood is called an expansion joint. Actually, this is mis-named, because the slab doesn’t “expand” it shrinks. (Often a 1/2” wide fiber strip is used...with the same result.)

So, the slab is not sealed to the wall and moisture has come up through the joint between the slab and wall. Over the years, moisture has deteriorated the wood and caused mold to grow on the back of the insulation.

You’ll need to remove the wood strip (or at least dig enough of the strip out) to seal the slab to the wall.

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