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Old 09-10-2010, 11:33 PM   #1
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insulating slab and drainage


Ok guys rock my world...

I have a 1940s cottage with a 1991 addition all slab construction.Northeast

All my homes have had basements and I find a higher level of dampness with the slab. There are portions of the new slab that the concrete is like compacted dust. In fact when I lifted the carpet the floor leveler stuck to the carpet back.

The neighboring cottage lot is a higher elevation with a poor eaves trough run off. The other lot to the right is vacant. It should be of lower elevation has been landscaped and is higher.

I wanted to excavate around the slab; insulate with rigid foam and add a weeping tile. How should I go about this?

How far down and away from the slab to place the weeping tile?
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Old 09-11-2010, 05:45 AM   #2
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You described conditions but not the goal. What are you trying to fix and why?
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Old 09-11-2010, 12:25 PM   #3
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since you asked...


I have moisture in the slab due to run off from the other properties.

I have diverted my downspouts at least 6 ft from the perimeter.

I want to ensure I have a good line of defense from the neighboring properties. I would like to achieve lower moisture in the slab.

All I need is the proper guidance.
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Old 09-11-2010, 12:43 PM   #4
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You say that the downspouts are diverted at least six feet away from the slab. Presumably they drain to a lower elevation than the slab. The lots adjacent are higher than your lot, so presumably the water drains towards your lot from the other lots, but must run off around your house based on your statement that the downspouts are six feet away from your house, and presumably lower than the slab.

While there are numerous threads on this site that discuss in exhaustive detail how to install an exterior perimeter drain (your plan), an interior perimeter drain, a sump pump, and a drywell drainage system, I am not certain you are going to achieve much with any of these approaches. All of these techniques are designed to reduce groundwater level around the house to approximately the level of the bottom of the perimeter drain pipe.

If the groundwater level near your house is at the surface, then the concrete will wick moisture upwards, creating a damp slab feel. However, more likely the groundwater is at least a foot or two lower than the slab, and you are not going to lower it by putting in a typical drain system, which is generally installed 6 - 12 inches below the slab. So you first need to start by measuring the groundwater elevation in your soil near the slab, which is done by digging a pit until you hit groundwater. That will tell you whether you have a groundwater problem, or simply moisture wicking upwards through the slab due to normal moisture in the soil, NOT groundwater. Perimeter drains are not going to help with an upward wicking or normal soil moisture, you need other techniques for that, but at least start by measuring groundwater level.
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Old 09-11-2010, 01:10 PM   #5
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Daniel thanks for the reply;

I am not at the cottage now so I am unable to do any digging.

I did have to dig a hole for a hammock post and since we are north I was digging deep due to frost. I did manage to bore down 3.5 feet before I hit rocks and was unable to go deeper. There was no sign of water.

I realize that due to the elevation of the neighboring properties I am absorbing some the water from my downspouts despite my efforts.

I ripped the carpets out to the bare slab and there are rooms with a greater degree of dampness. These are the rooms on the sides of the house.

Hence my plan for rigid foam, weeping tile and swale

I had found "how to"s on foundations but not slab.
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Old 09-11-2010, 02:59 PM   #6
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Just some more cheer to throw your way.... first winter in my old Michigan slab house I hit the furnace switch, and humidty shot up to 68% instead of going down like normal. Turns out I had standing water in the duct work under my slab. You may want to keep an eye out for that, given your drainage issues.
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Old 09-11-2010, 04:32 PM   #7
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Quote:
Originally Posted by steveel View Post
Just some more cheer to throw your way.... first winter in my old Michigan slab house I hit the furnace switch, and humidty shot up to 68% instead of going down like normal. Turns out I had standing water in the duct work under my slab. You may want to keep an eye out for that, given your drainage issues.

No ducts to worry about, it is electric heat. I installed a wood burning stove.
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Old 09-11-2010, 06:06 PM   #8
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"I realize that due to the elevation of the neighboring properties I am absorbing some the water from my downspouts despite my efforts."

Why do you think that the elevation of your neighbors house is causing you to absorb water from your downspouts? If the downspout water runs at least six feet from the house, and the soil outside your slab grades downhill, the downspout water may not have anything to do with your damp slab.

I have a garage outside the house that is always damp, despite the fact that the groundwater is usually four feet below the slab elevation. The problem is that there is no vapor retarder (plastic sheet) underneath my slab, hence ground moisture wicks upward through the slab, and it is always damp. Its just a garage, so I don't care, but your problem may be lack of a vapor barrier between the slab and the ground, and this problem cannot be solved by installing a perimeter drain. You may need a vapor barrier underneath the slab, which is pretty hard to install after the fact.
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Old 09-11-2010, 06:55 PM   #9
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Quote:
Originally Posted by Daniel Holzman View Post

Why do you think that the elevation of your neighbors house is causing you to absorb water from your downspouts? If the downspout water runs at least six feet from the house, and the soil outside your slab grades downhill, the downspout water may not have anything to do with your damp slab.
If the water exits my downspout towards the neighbors and their property is 18- 24" higher does the water not repel back to my lower elevation pushed
by the downward grade and run off of their property like a serve and volley.

We seem to get rains with torrential downpours with large amounts 1-2 inches in short periods of time

I know there is no vapor barrier under slab as a ground hog tunneled under the rear corner of the slab.

WOuld installing a vapor barrier on top of slab be wise or would there just be an abundance of condensation forming?

thanks again for your time & sharing your knowledge.
Tina
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Old 09-11-2010, 07:20 PM   #10
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Can you grade your yard? Soil should be at least six inches below top of slab and slope away from house... what is it, 1"/ft?

That wasn't possible at my Michigan slab house, I was the low spot among neighbors on either side. I was thinking about doing what you're doing, but tried some other answers first which fixed it for me.

First, I wanted to make major surface water stay away from the house. So I chose to move the drain away from the slab. What I did was to dig a french drain in the back yard, and then down one side yard, and tied my downspouts (after going through a homemade filter) into the drain pipe. This pipe picked up the pond water in the yards around me, and also my gutters water. Once past the house, I took the drain to edge of road ROW where I dug a dry well with bubbler where it could find its way into the roadway ditch. The reason for trenching to the ROW was because my city officials were in the loop and agreed to let me tie the drain into the storm sewer (under the street) if that was what it took.

Anyway, bulk water moved away and that made a huge difference for my slab, without excavating the slab itself. Still it wasn't enough. I followed with good HOUSE insulation to reduce stack effect, because that can suck moisture laden air into the house, and that gave my relatively cool slab a condensation problem.

Since you have electric heat, I bet condensation is a significant part of your problem and much easier to fix than adding a vapor barrier. Do you run any fans to keep the air stirred up? Do you have zones where cold air might sit on the floor without moving around? If so, then bingo, you probably get condensation in those spots.

Insulating your slab will certainly save you some on heating. Not sure it will help you lose the damp feel though.

Well, then again, I suppose if the outter edges of the slab are a bit warmer they'll not condense as much. Still its much easier to first tackle airborne moisture and then see if you want to take the next step to dig up the slab perimeter

Best luck
SteveEl

Last edited by steveel; 09-11-2010 at 07:23 PM.
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