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Product Guide

Wingnut Testing: Above-Grade Foundation Wall Dampproofing

There are many coatings we can apply to the exposed band of foundation wall around our homes, but which ones work best?

Water-resistance testing, Wingnut-style, using a turkey baster to dribble water down the face of the two-block column of CMU.

It has always seemed odd to me that quite frequently, from both aesthetic and moisture management perspectives, the above-grade portion of foundation walls are simply ignored.

It’s not hard to find poured concrete or concrete masonry unit (CMU) walls that are left bare above grade. Not only do they look unfinished, but more importantly, they act unfinished, soaking up quite a bit of splashback when it rains or ponded water when melting snow is piled against them.

This is the newest house in our neighborhood, about 8 years old, with a bare-above-grade concrete foundation.

Above grade portions of foundation walls are a grey area (pardon the double-entendre) as far as the building code goes. Here is what section R406 of the 2018 International Residential Code (IRC) has to say about this part of the house: “…foundation walls…shall be dampproofed [except where Section R406.2 requires waterproofing]…to the finished grade.” I can’t find anything in the code about the remainder of the foundation wall, the portion above-grade. Unfortunately, water knows no code.

Some clients needed a spec or recommendation

On our own home, we had some eroded split-faced architectural concrete blocks in the courses closest to finish grade re-worked by our mason a bit ago. When he was done, I asked him what he would recommend to make the blocks more water-resistant. He recommended H&C Decorative Concrete Products, particularly their Colortop water-based solid color concrete stain, which can be found at Sherwin-Williams stores.

This corner of our house is just below the northwest roof valley and there used to be a concrete sidewalk where the white stone is. Our mason masterfully re-built about six concrete blocks in this corner.

After having this work done on our house, I had two clients with existing homes and bare, above-grade CMU foundations. Since they were both insulating and air sealing their homes extensively, and including the basement block walls from the interior, we wanted something on the exterior that would decrease wetting of the block. What would I recommend or spec for their homes? I asked building professionals involved in these projects and came up with two additional readily-available coatings I thought I should evaluate.

Here is how product literature characterizes moisture protection for each of the three:

  1. Behr Semi-Transparent Concrete Stain: “Weather-resistant”
  2. Behr 1-part Epoxy Paint Concrete Coating: “Stain, chemical and weather resistant”
  3. H&C Colortop Concrete Stain: “Deep penetrating protection and long-lasting beauty”

Time to develop a Wingnut water test

I could not find any waterproofing or water-resistance testing for off-the-shelf concrete coatings. So I figured I would try my hand at it. I had a bare concrete block column leftover from previous negative side waterproofing Wingnut testing I had done. I used this as a single CMU for the Wingnut testing.

The three test coatings and the 2-block column set up for driveway testing.

I coated three faces of the block column with each coating and left one face bare. I decided that two coats of each coating ensured thorough coverage. I carefully wrapped the coatings around the bottom margins as well as inside the block so that water running down the block could not turn the corner and wet the bare bottom and interior faces.

The single test block showing the three coatings (brown for the BEHR semi-transparent stain; blue-gray for the BEHR 1-part epoxy; and regular gray for the H&C Colortop concrete coating) and the bare side of the block. Note how the coatings are applied to the bottom and into the first 2 or so inches of the block interior.

I did not want to spray water on the coatings; I wanted to mimic water running the face of the block. And I wanted to be able to measure how much water I was exposing the block to. I settled on a turkey baster as the applicator. When full, it contained 2 cups of water. I figured I would dribble the water down the face of the block 100 times, for a total of 200 cups of water. It was maybe more than I bargained for since that takes about 25 minutes.

Water is running down the face of the block column. You can see that the block is tipped slightly on the metal bars to make it less than vertical and to allow the water to turn along the bottom edge and largely run off the test assembly.

I thought I could just dribble water down the outside face of the coated block and evaluate how much water gets through by inspecting the block on the interior (it’s very easy to see the darker wet mortar or block). Turns out that in a trial run, I could not see any water making it past the coating, at least by inspecting the interior of the blocks. So, I decided to be quantitative. I measured the weight of the two concrete block columns dry and then measured their weight after the turkey baster testing.

The RUNCL hanging scale with the crate and rigging and a measured amount of water in the plastic container to the left in the crate. The relatively small amount of water was to test the accuracy of the weighing scale in the 30 – 50 pound range of our test block columns. Accurate to within a tenth of a pound or so.

Table: Water Absorption

Substrate Colortop H&C Behr Stain Behr 1-part epoxy “bare” face
Weight gained after turkey baster wetting (lbs)
1-block 0.0 0.28 0.0 0.46
2-block 0.0 0.78 0.0 1.16
Crate + rigging tare 3.02 lbs.
1-block dry weight 29.3 lbs
2-block dry weight 40.42 lbs
1-block saturated 1.68 lbs
2-block saturated 2.40 lbs

NOTE: The weighing scale is a RUNCL digital hanging scale, maximum weight 110 lbs. There is no information on the accuracy of the scale but I tested it in the range of the concrete block weights with known small quantity of water and it was within 0.10 lbs of the actual weight.

The 2-block column after completion of the wetting test on the face of the BEHR 1-part epoxy coating (the blue-gray). Note how the interior of the block is light gray (dry) and the mortar joint is light gray and dry as well.
This is the 2-stack column after water testing the BEHR semi-transparent stain (brown). Note how the interior of the block on the brown side is dark gray (wet) and parts of the protruding mortar at the block joint is dark gray and wet as well.
This is the 2-block column after water-testing the bare face of the block. You can see that the interior face on the right hand side is dark gray (wet) and the water has wicked most of the way across the interior of the regular gray H&C Colortop face of the block.

Did the blocks absorb a lot of water?

The results show that both the Behr 1-part epoxy and the Colortop acrylic solid stain were water-resistant. The Behr semi-transparent concrete stain was much less water-resistant. And of course, the bare face block testing resulted in between 50% to 60% more moisture absorption than the Behr semi-transparent stain. Did the Behr stain and the bare face block absorb “a lot” of water?

I decided to see just how much water each of the two concrete block columns would absorb when saturated (see the last two rows of the table above). I submerged each block column in water and then let free-water drain out before weighing them. I was frankly surprised to see that at saturation, the block columns only absorbed 5.7% and 5.9% of their dry weight. Not sure why, but I was thinking more along the lines of 10%, about twice the actual. But what this comparison does mean is that the porous nature of the CMU block can mean significant absorption of bulk water if it can get past the face of the block. A continuous, water-resistant coating is key to keeping the block dry.

What about vapor permeability?

While we want to reduce liquid wetting, we may want to maintain drying potential to the exterior as well. I could find no information on vapor permeance for the Behr semi-transparent stain (but I bet it is vapor open, given it’s 100% acrylic) and no information on the Behr 1-part epoxy (but I bet it is a Class I or II vapor retarder because it’s an epoxy). The H&C Colortop does provide ASTM D1653 perm rating results: 11.2 perms (+/- 0.3), making it vapor open and with good drying potential to the exterior.

NOTE: For those familiar with ASTM water vapor transmission testing, you may be surprised to not see the more familiar ASTM E96. D1653 is for coatings and films, a more complicated process since the material is a liquid requiring a substrate. 

Given the water testing results and the information on vapor permeability, looks as though I got a bit lucky selecting the H&C Colortop coating with its good water resistance and good drying potential to the exterior.

What about poured concrete and brick?

My bet is that these three water-resistant coatings will work as well if not better on poured concrete, with its more homogeneous composition, lower porosity/higher density and lower moisture absorption by weight. And each of these coatings are targeted to and named for concrete, not CMU.

Brick is a whole other animal, because the brick behaves differently than the mortar, and the interface/bond between the brick and mortar is a huge factor in water-resistance. For more information on brick water-resistance, see these resources:

If you have used a fluid-applied sealer for above-grade foundation walls, I’d love to hear your feedback in the comments below.


-Peter Yost is GBA’s technical director. He is also the founder of a consulting company in Brattleboro, Vermont, called Building-Wright. He routinely consults on the design and construction of both new homes and retrofit projects. He has been building, researching, teaching, writing, and consulting on high-performance homes for more than twenty years, and he’s been recognized as NAHB Educator of the Year. Do you have a building science puzzle? Contact Pete herePhotos courtesy of the author.

6 Comments

  1. Charlie Sullivan | | #1

    Great information! Thanks.

    Worth noting that many instances of exposed concrete foundation walls are also huge heat leaks, uninsulated inside and out. Before painting it, I recommend thinking through whether you want to add insulation. The best location for insulation might be on the interior, in which case applying one of these products to the exterior makes sense. Or it might be that you want to apply exterior insulation and then think about finishes to apply over that.

  2. Patrick McFarlane | | #2

    Great article. Up where I live in Ontario, Canada, it is common practice to parge the foundation to give it a finished look but the parging does little to improve the water resistance of the concrete plus it's high maintenance, often needing to be redone every 5-10 years due to cracking and delamination.

    On my house, we have some above grade cracks that move a bit seasonally, which shortens the lifespan of parging in those areas even further. So we were looking for something we could coat it with that would be more durable. Turns out a lot of companies that specialize in concrete products make elastomeric (stretchy) coatings that are waterproof but also have rated vapour permeance. These can be tinted different colours and also come in different textures as well to give the surface a more natural concrete look. We went with Masterprotect EL 750, which is rated to 12 perms. But Sika makes a good product as well.

  3. Granular | | #3

    Commercial/Industrial/Public Works projects have used Xypex for many decades - creates a crystalline lattice structure inside the concrete/masonry to block moisture/leaks. Applied as a thin slurry and can be tinted.

    1. Patrick McFarlane | | #4

      The moving cracks I referred to above are also getting treated with Xypex products (dry packed concentrate, then FCM-80) before top coating with the Masterprotect.

      A slurry coat of Xypex probably would have worked well for us if not for the seasonal movement in the cracks.

    2. Malcolm Taylor | | #5

      I've had great luck with Xypex in a variety of challenging situations.

  4. Bill Bush | | #6

    Hmm, i"m considering a ICF foundation, that will be covered with a membrane, and the membrane will be covered by a dimple mat. I was thinking of using Corten steel to cover the exposed ICF from the bottom of the siding to slightly below ground level. Obviously cost would be an issue, but what other factors should I consider?

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