Cement Block Moisture

This is a cement block four foot plus deep foundation under a new addition. Notice how it is more moisture on the top cement block. This photo was taken about twenty hours after a one inch rain with 30 MPH winds after a very sunny windy day with temps of about 80 degrees. It is on the north side of the house so not much sunlight till PM. The dirt crawl space under the new addition is dry, as is the sill plate. The original part of the house also has cement block foundation with a full unfinished basement. The original cement block foundation didn’t show any signs of moisture retention like this. I talked to the home owner and she said that she had noticed this before and that it stays like this for two or three days after a rain. I thought perhaps water was getting behind the siding and running down on to the block, but I didn’t think that water would be leaking behind all the siding on the new addition. The addition is about 35 feet in length



I can’t explain it, must be a row of more porous blocks, but would suggest painting might help reduce the absorption of moisture.
Also suggest any dirt crawlspace should have a poly vapor barrier installed. :mrgreen:

John Kogel

Is the crawl space properly vented?

The blocks are not parged, and it’s possible that rainwater splashes onto the blocks and wets them. If they are not parged, I wonder if they are dampproofed. There should be a cement plaster parge coat extending all the way to the footing, and the dampproofing applied OVER that to just below grade.

The crawl space is not vented as it is open to the heated basement, there is rigid insulation on the interior foundation walls, the insulation extends into the ground in the crawl space.

Yes, I agree, there should be a vapor barrier installed, although the dirt floor is as hard as a rock. The crawl space is only about two feet high and my knees feel like I was crawling around on concrete.

Steve; CMU is a very permeable product, and when not coated with a moisture resistant product will absorb much water and moisture which will eventually disintegrate its mattrix of components.

I would suggest that the blockwork be coated with a product like Thoroseal gray or ConProSeal to prevent any further moisture intrusion that may cause the CMU to spall and disintegrate due to freeze and thaw cycles.
Continuous wetting could and possibly contribute to algae growth at the interior of the crawl space as well as the exterior.

Hope this helps.

Marcel :slight_smile: :smiley:

Thanks for all your help…

Here’s some water damaged CMU. :smiley: Baby scupper at top of wall didn’t have a downspout. :shock:



Do you have a moisture meter? I recommend a GE Protimeter Surveymaster. It really works well. Visual appearances is not enough data (for me). I would use a meter on that wall.

The problem is that appearances of moisture and hygric capacities are deceiving. Something dark doesn’t mean damp. Something that is dark may actually be just a stain.

I agree - the top row appears abnormal in relation to the rest, but without a moisture meter measurement - who knows? It’s just guessing without one.