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I'm trying to prevent moisture from turning into mold and growing on concrete exterior walls and getting into the fiberglass insulation.

I see polystyrene, foil, chemical-based concrete sealing, and spray foam.

What is the suggested way for doing it reliably on the cheap side?


I should mention more detail. The area is fairly dry, but ranges quite a bit in temperatures (Plant zone 3). Grading does not cause a lot of water to pool. The issue is more with condensation. The city code requires "damp proofing" and I'm trying to see what options there are.

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  • access to foundation from the outside is blocked by concrete slabs to about half of the way around. – Daniel Aug 17 '15 at 20:07
  • Failure is the main option, which is why the bias towards waterproofing from outside exists. But if it's really condensation as in @wallyk's second answer, then insulation (outside again being best) will help, even if it's only above-ground level. – Ecnerwal Aug 18 '15 at 3:25
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    Note that condensation is an entirely separate issue from moisture-proofing. Both are important to having a nice finished basement, but are two separate issues. – DA01 Aug 18 '15 at 4:49
  • Also, are you finding that you have mold inside your walls? Is the fiberglass insulation covered in plastic (vapor barrier)? If so, the problem is actually how your walls were built, unfortunately. – DA01 Aug 18 '15 at 4:52
  • I've removed the old setup, which was 2x2 framing along concrete with fiberboard on top of it. (and a coat of paint). This lets through a lot of heat and moisture. After discovering this I've gutted the basement in two rooms, and I'm preparing to insulate, which should reduce the moisture. I'm just not sure if I should leave the 1" space between the wall and the walls or look to a product that I can put in between, like a dimpled membrane, some sort of reflective foil, polystyrene, or a chemical sealant. – Daniel Aug 18 '15 at 18:03
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You can't reliably waterproof a basement from the inside. You can't reliably waterproof a basement 'relatively cheaply'.

The only sure way to waterproof a foundation and at the lowest cost is to properly waterproof it on the exterior before you back fill.

Barring that, you need to accept that fact that most basements may get wet at some point.

Given that, you need to do a few things:

  • make sure you're not fighting a losing battle with a high water table. (if you are, then you need to look at water moving solutions such as perimeter drains and sump pumps (and back up sump pumps)).
  • make sure you remedy all surface water issues. Make sure there is proper slope around the house, all gutters are adequate, working, and have a proper downspout, etc.
  • build proper walls on the interior

Regarding the latter, a proper wall would be, from the outside in:

  • foam insulation. Spray foam is ideal. Polystyrene (expanded or extruded) are more typical
  • studs (I'd strongly recommend steel studs. See here: https://diy.stackexchange.com/a/8644/1209 )
  • sheetrock (I'd strongly recommend going with a paper-less sheetrock product like densarmor.

What you won't see in a proper basement exterior walls are:

  • fiberglass insulation
  • plastic (no vapor barriers!)
  • mold-loving surfaces (wood and paper)

The idea behind the 'proper' wall is that it assumes moisture may be an issue at some point--either from the wall itself, or moist air trying to condense on the wall. A spray foamed wall prevents all air movement and seals tight against the wall...this prevents moisture migration. XPS and EPS can't do that as well, but both can also breath to an extent, so moisture can eventually migrate as needed.

Avoiding wood and paper faced sheetrock also reduces the surfaces mold can attach to.

Note that even after all of that work, you will still want a dehumidifier in most climates down there as well running at all times.

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The new information has radically changed my perception of the actual problem. Preventing moisture movement through the foundation won't help.

Instead, it is cold temperatures outside causing condensation inside: No matter how well sealed the foundation, the problem will persist.

A much less expensive solution may have several parts which can be mixed and matched:

  • A dehumidifier (or other measures to limit humidity like exhaust fans) to maintain a drier environment in the basement. If the wall temperature is 40 °F (4 °C) and the interior temperature is 68 °F (20 °C), the relative humidity would have to be below 36% to prevent condensation. For a wall temperature of 34 °F (1 °C), 28% relative humidity prevents condensation. At freezing and below, condensation is unavoidable this way.
  • Insulation on the interior basement wall, especially above and near the exterior grade. This will keep airborne moisture from contacting the wall and prevent any condensation. The insulation will have to be adequate enough to keep its inside face above the dew point temperature.
  • Wall heating: a little bit of heat applied from the inside to keep the basement wall warmer will prevent condensation. You could do "off label" use of an under carpet floor heater. I wouldn't be surprised if manufacturers of the higher end products have suggestions for satisfying local building and electrical codes. In the DIY realm, this one has 310+ feet of cable which could be looped behind the finished wall several times (more than 3 is probably be overkill). Placing it a little lower than needed allows the heat rising effect to keep the upper wall warm.
  • Exterior insulation. Rigid foam insulation covered by a protective layer will go a long way to warming up the foundation.
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    A dehumidifier is definitely the easiest and most cost-effective place to start. Most humidifiers have a humidity meter so they can be set to only run when necessary. – Hank Aug 18 '15 at 3:32
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    Heating a wall would work in theory, but you're essentially heating the earth, so likely a very impractical solution. – DA01 Aug 18 '15 at 4:43
  • @DA01: Sure, but heating the air in the room heats the earth too. Heating the wall will reduce heat loss so, from an energy viewpoint, it is not all a loss. – wallyk Aug 18 '15 at 6:03
  • @wallyk well, the assumption is that the walls would be insulated (at least based on the OP's current situation). – DA01 Aug 18 '15 at 6:20
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The best solution is to waterproof the foundation from the outside. Dig around the outside deep enough. This can be done inexpensively yourself or by offering local teenagers a few bucks or your buddies a few beers and pizza. Beware of sprinkler plumbing, utilities, etc.

With the foundation exposed:

  • Apply an appropriate exterior foundation sealant but be sure to seek local expert advice because soil conditions, water table, surface runoff, and foundation engineering vary considerably by region.

  • Consider installing drain pipe which is routed to move water away from the foundation and dispose it where it won't be a problem. Local soil conditions may benefit from local knowledge for the type of pipe and how best to bury it.

  • After back filling, be sure to grade the soil carefully to prevent surface water from going near the house while also being careful to keep the appropriate distance between the top of the soil and the non-masonry part of the building. In most jurisdictions this is specified by local code.

You might have luck sealing the interior of the foundation, but any such technique is at a disadvantage as dictated by physics—particularly hydraulics, pressure, capillary action, and chemical bonding.

(Perhaps you meant interior wall?)

  • I did mean Exterior wall. The problem is that large part of the access to the foundation from outside is blocked by concrete slabs, that I'm not looking to remove at this time. What do you mean by "Deep Enough"? – Daniel Aug 17 '15 at 20:11
  • @Daniel: How do you feel about paving the rest of the perimeter of the building? That might be an option if groundwater is not a factor and only surface water causes the problem. – wallyk Aug 17 '15 at 21:07
  • I think that would be difficult, and not really address the concern of mold growth due to condensation. – Daniel Aug 17 '15 at 22:58
  • I would absolutely not recommend hiring teenagers to dig out around your foundation walls. That's a liability disaster. – DA01 Aug 18 '15 at 4:44
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As wallyk mentioned, the best way to seal a basement from moisture is from the outside. Sealing it from the outside is the most foolproof way to fix the problem since it diverts water away from your entire foundation instead of just trying to prevent penetration.

Going from the outside is not always an option for many homeowners. It is extremely costly. Plan to spend at least $10,000 if you plan to do this. It is also very invasive, and destroys landscaping such as plants and shrubs. It might not also be an option because of physical restraints such as proximity to other properties etc.

If you are only experiencing is a damp basement, and aren't subject to flooding, you can try to seal the walls and floors with a concrete sealant. On the walls you can either use a latex or oil based product.

The floors are a lot trickier. The best method would be to get an epoxy based concrete floor paint. The preparation of the floor surface for paint is extremely important.

The floor must be completely free of dirt and oils as well as perfectly dry to the touch. The first step after cleaning the floor would be to etch the floor. There are commercial concrete etcher products on the market. This will dissolve the very top layer of concrete and give the paint something to stick to. After the floor has been etched, you should also apply a coat of bonding primer to ensure that the paint won't peel.

After the walls and floors have been sealed, check to see if there are still moisture issues. The next step would probably to cut a drainage channel into the floor around the perimeter and install a sump pump if sealing does not work.

  • I will not be treating the floor. as the moisture entering from outside does not seem to be a problem. The problem is mold buildup along/above grade, especially in the corners. This is most likely due to the condensation resulting from the heat exchange within the concrete between below and above grade. – Daniel Aug 17 '15 at 20:34
  • @Daniel are you planning on doing anything with the floors? Note that in a lot of climates (northern US/Canada) the floor slab will be the coldest surface especially if you are insulating the walls. This will cause condensation issues if you're not running an adequate dehumidifier at all times. – DA01 Aug 18 '15 at 4:47
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My Dad and I always used this Drylok. It doesn't cover much per gallon but it works pretty well.

  • Drylok will always fail if their is water pressure on the exterior. It will also do nothing to prevent condensation issues. It's mostly a wasted effort. – DA01 Aug 18 '15 at 4:35
  • @DA01 Really nothing prevents those issues. Even concrete will fail if there's enough water pressure from the outside. That's why standard practice now is drain tile & sump pumps. Keeps moisture down. You are correct though Drylok will not "waterproof" the basement it does help significantly in my experience. – Dano0430 Aug 18 '15 at 14:03

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