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Straightgrain 03-05-2009 10:24 AM

Basement floor over concrete slab
Hello - New member, frequent reader! Great stuff here!:thumbsup:

I'm finishing my basement and have a brand new exterior/interior drainage and sump system, much to the dismay of my bank account. So I feel my basement (short of a 500 year rainfall) is fairly protected against invading water. However, I've read alot about moisture wicking thru concrete floors via underground moisture, humidity etc. Therefore, I'm trying to determine the best and most affordable choice for flooring. My wife would prefer carpet but don't want to chance mildew/mold issues over time from moisture absorption. We're open to laminate flooring. Any suggestions?

Also, a key question is how can I warm/moisture proof the flooring? Has anyone heard of placing polystyrene foam or other barrier right over the concrete slab and then laying the laminate floor (or other flooring) on top of that without a plywood layer? I'm trying to minimize the height increase of any materials used.

I will be doing the work myself and I'm fairly skilled...have framed houses and laid laminate floors before above grade...first time laying a floor in a basement.

Any advice would be most helpful and appreciated!


Bob Mariani 03-05-2009 11:25 AM

placing the a thin foam board (less then 3/4") will provide a capillary break and balance the pressure difference between the basement and the soil below the slab and stop moisture transfer. I would place a poly over the foam then the laminate flooring.

Straightgrain 03-05-2009 11:36 AM

Thanks Bob - So if I'm understanding correctly, essentially in this order: concrete slab (already existing), then 3/4" or less polystyrene foam board, then 6 mil plastic for example, then laminate?

Bob Mariani 03-05-2009 12:16 PM

Let's start at the beginning since the fix must match your conditions.

1) Do you know if the 6 mil pol was installed as it should have been under this slab?

2) Do you notice any moisture on the slab? Best to actually test this, but need a pro for that. A DIY method, although not 100% fool proof will be to tape a 4' X 4' piece of poly to the floor and let it sit. (summer is best time to be sure and maybe during rainy days) Any moisture will show up as condensation droplets under the poly. I would try to determine your conditions prior do determining how best to address it.

3) One way to address your slab is placing something like Dri-Core panels Also if no salts are present seal first with dry-loc (two coats) Poly is not needed unless no poly was installed under the slab. Newer homes will have this.

4) Foam board will provide the insulating value you are looking for. But it is also compressive and may not work well with most laminates. In this case you need to add the plywood over the foam board. Carpet will work well and will be as safe to use as the laminate floor.

Straightgrain 03-05-2009 01:33 PM

1) not sure about 6 mil poly under the slab.

2) I will try the moisture test again...did this last year and didn't notice any moisture.

3) I'll try the dryloc and do the poly film just to play it safe

4)how thick do you think I need to go on the plywood that will sit on the foam board?

Bob Mariani 03-05-2009 02:54 PM

price out the Dri-Core first. It safely combines all steps you need. The dimpled plastic bottom is what makes these systems work. this air space will insulate and stop water transmission as well has handle unexpected wet conditions. You need to match up the foam to the plywood and the laminate. Call the manufacturer of the laminate you select. It is the only way to be sure you are okay with this method. I used 6 mil poly, 2" foam and 3/4 advantec, but placed carpet over this. But I had a know wet condition, since the house is on a mountain and everything is on granite ledge.

Straightgrain 03-05-2009 07:35 PM

Bob - Thanks so much for your've given me some guidance. I will check out the dry-core. I'm also looking into something called Delta-FL...similar to drycore from what I've read so far.

Thanks again!

KristyKlean 03-09-2009 10:59 AM

I have Flexi-Tile in my basement, which does still get a bit of water in it, aside from my continual efforts to stop it.. It doesn't mildew or anything and when I've had water issues, since it is loose lay, I can take the tiles up if I want to let the wet areas dry.

concretemasonry 03-09-2009 11:46 AM

If you have a new drainage system, make sure you give it a year to find out if or how well it works, if you can. the guarantee will certainly ocever the cost of replacing any finishing, if anything.

You did not say what kind of system you have, but a lot of the interior systems are not waterproofing systems, but just surface applied, patented, collection systems sold by slick salesmen where the guarantee means little.

The only reliable interior (after construction) system is a traditional drain tile system properly installed with the pipe invert below the level of the bottom of the footing. - Messy installation, but reliable and gives a lot of protection from moisture (even from under the slab) and some important structural benefits.


Straightgrain 03-09-2009 12:38 PM

Kristy Klean - the flex tiles...not sure my wife would go for those...she's wanting something a bit more plush.

Dick - One exterior drain tile sump/system was installed at the foundation level when the house was built. We found it that wasn't enough after 6 inches of rainfall in the spring of 2008. As a result, we had a concrete expert who builds/installs basements and drainage systems in new and existing houses (he came highly recommended from my neighbor who happens to own a plumbing business and did the plumbing/sump systems for several of the houses in our neighborhood, not ours unfortunately...those houses that he did didn't have the water issues we had). Anyway, this concrete/drainage expert came in a recommended an additional exterior sump and a new interior drain tile system, complete with several tons of pea gravel, be installed. They cut a 12' path around the interior perimeter of our basement and 3 paths across the basement (basically dissecting the basement in 4 parts) from wall to wallabout every 12 feet....YES, YOU ARE CORRECT, IT WAS MESSY AND NOT CHEAP, but his quote was reasonable based upon other quotes we got. The new interior system has two sump tanks/pumps of its own. So, now I have one exterior drain tile with 2 tanks/pumps and one interior drain tile system (a total of 360 linear feet of interior drain tile) also with 2 tanks/pumps at opposite ends of the basement, which drain into a retention pond. Yes, quite an elaborate system and one that we hope we withstand most rainfalls/snowmelts.

You make a good point, because the issue we were having was water (pressure really) coming up thru small cracks in the slab. The concrete guy said the main objective was to relieve the water pressure created from the water table rising to the point of the slab and needing "a place to go, i.e. a lower pressure area)...the interior system created a place for the water to go...i.e. channel it to the tanks and pump it out! We never have had any issues with water coming thru the walls, evidently that was insulated/protected quite well. But it is my belief that no membrance, poly, or anything was put under the slab by the builder before they poured it. They didn't find anything when they broke up the slab during the install of the interior system...much to my dismay.

So far so good....the system has held out to heavy rainfalls so far (one last fall and one just this past weekend...we live in the great lakes area)

Now I just need to find something to protect the flooring that I put down, from vapor and minimal leakage (just in case) under the slab.

Thanks for the recommendations.

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