sill plate wet

My sill plate is getting wet. I have a 6 foot overhang so rain is not getting to it. Im trying to figure out how to deal with it. The sill is not pressure treated unfortunately. The foundation is cement blocks and the sill plate doesnt cover the opening of the blocks which i asume is a good thing allowing the space to vent. The problem is only in the corners.

I was thinking of installing foam around the outside of the sill plate overlaping the foundation. The thought is that it would create a moisture barrier and insulate it at the same time. I have access the the sill plate outdide the house if i pull off some wood trim relatively easily.

Does that make sense?

Im worried about installing foam inside since it would trap the moisture against the wood and make it worse?enter image description hereenter image description here

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    I think you are right that foam is not the correct product. You need to stop the water from getting into the wall, and you don't want to do anything that will trap it and prevent drying out. Consider metal flashing to keep the water from getting in. What is the exterior covering of the wall: siding? If you don't have a proper moisture barrier draped over flashing and water is getting into the wall above (maybe through a gap in siding at the corners), then it could drain down to the floor joists. A picture of the outside of this corner would be helpful. Commented Nov 18, 2017 at 0:44
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    Jim that answer is a good start with the flashing, but OP really needs to find where the water is coming from and seal it before coming in (good advice).
    – Ed Beal
    Commented Nov 18, 2017 at 1:38
  • Do you know that this discoloration is water? Can you dry it out with a fan (or a hair dryer held by hand to insure no overheating)? The discoloration is not primarily on the sill, but on the floor joists and appears to come from the right in the picture. What is above and to the right of this corner in the living space? This could be a water supply or drain leak. Commented Nov 18, 2017 at 12:04
  • There are no water sources or pipes near the area. Im begining to wonder if the wetness is coming from condensation? The humidity level in the crawl space is around 65% and about 13 degrees c. I recently encapsulated the exposed soil in the crawlspace after which the humidity was reduced from 80% to about 65%. I was thinking of adding foam around the cinder block walls to bring the humidity down a little bit more do i can then apply heat without worying about potential mold issues.
    – Alex1234
    Commented Nov 19, 2017 at 20:43
  • I added some pictures to clarify the area im concerned about.
    – Alex1234
    Commented Nov 19, 2017 at 20:50

1 Answer 1


Looking at your original picture, I can tell the moisture is coming in near (or at) the top of the floor joist not the bottom, because water won’t “wick” up that floor joist uniformly like it is showing. (In fact, it seems wetter along the top of joist than middle of joist.)

Therefore, I’d concentrate on the joint at the top of the 2x8 trim.

According to your drawing, it looks like you’re relying on caulking at the top of the 2x8 to keep moisture out. That means the caulking has to be perfectly installed and well maintained. Even then you can develop problems. (Look at at knot in the first row of logs and notice that moisture can penetrate into the knot and run down behind the caulking.)

We usually use a piece of counterflashing that laps over the 2x8 about 3” and extends into the wall void about 3” with a 1” hook in the up position. (Make sure the flashing is no more than 10’ long, for expansion, and each piece laps about 6” and is sealed with mastic where it laps.)

Also, I’d install that 2x8 on peel and stick moisture barrier and extend it under the counterflashing and over the subfloor, if possible.

Also, we don’t like caulking at the bottom of the 2x8. We like moisture to run out if it gets behind the 2x8 at the joints, corners, etc.

  • That makes perfect sense! Thanks for the help. Im not sure how i would get the flashind on top of the existing floor though between the logs and the floor. Is there another way to acheive the same goal? If i remove the 2x8 and install a vapor barier over the rim joist and floor would that help?
    – Alex1234
    Commented Nov 19, 2017 at 21:47
  • Yes, and lap over sill plate and onto concrete block about 2” minimum. That would be ideal. We “kerf cut” the flashing into the wall for existing retrofit projects, (like letting-in flashing into brick fireplaces.) Cut s slot about 1/4” high and 2” - 3” deep into the subfloor.
    – Lee Sam
    Commented Nov 19, 2017 at 22:20
  • Oh, and flashing must have 1” hook to keep moisture from getting pushed (blown) into house. The flashing will leak, but the hook keeps it from blowing into the wood flooring system.
    – Lee Sam
    Commented Nov 19, 2017 at 22:28
  • Ok perfect. Thanks again! I will have to tackle this in the spring since winter is setting in here. For now i guese i will apply heat and try to dry the area out until then.
    – Alex1234
    Commented Nov 19, 2017 at 22:31
  • Do you have a reference online i could possibly see a picture of the flashing installation?
    – Alex1234
    Commented Nov 19, 2017 at 22:35

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