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.
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.
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:
- Behr Semi-Transparent Concrete Stain: “Weather-resistant”
- Behr 1-part Epoxy Paint Concrete Coating: “Stain, chemical and weather resistant”
- 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.
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.
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.
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.
Table: Water Absorption
|Behr 1-part epoxy
|Weight gained after turkey baster wetting (lbs)
|Crate + rigging tare
|1-block dry weight
|2-block dry weight
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.
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:
- BSI-047: Thick as a Brick
- BIA Tech Brief 6A – Colorless Coatings for Brick Masonry
- 1.3.2 Recommended Approaches to the Retrofit of Masonry Wall Assemblies: Final Expert Meeting Report – Oct 2011
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 here. Photos courtesy of the author.
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