My home is just over a year old. I am starting to finish the basement and am sealing off the air leaks in the sill plate area. When I pulled the insulation away from the first floor joint that sits on the sill plate, I found moisture and a little mold starting. I have engineered floor joists that look like I-beams and the moisture and mold is concentrated mostly in the webbing area of the joists. In some spots the insulation was stuck to the floor joist and as I pulled it away hairs of fiberglass clung to the joist like it had frozen there at some time. It's like this in only certain areas and not everywhere along the basement perimeter. Also, in some areas, I can see the vinyl siding through the cracks between the sill plate and the floor joist that sits on top of it.

The outside grading was done last year and we tarred a little higher up the outside wall and added dirt in order to have a better slope away from the house. There is approximately 6 inches between the ground and the bottom of the vinyl siding. There is hardwood mulch around 3/4 of the perimeter of the house and dirt around the rest. The moisture inside is in both mulch and dirt areas.

Is it condensation or water ingress? How do I stop and seal it so it won't happen again?

Please help.

Thank you very much.

Phil McCarthy
philmelissa@yahoo.com

 

Or if you prefer capillary action. The ends of cut I-beams are more susceptible to absorbing moisture than other parts of the wood. The area that you are describing is known as the rim joist area and is the thinnest part of the structure. If you combine this with the heat from the first floor, the apparent gaps leading to the outside in that area and only being 6 inches above grade, the probability for moisture to be present in that area is quite high.

The moisture problem probably would not occur if the end grain of the I-beam was not exposed. This is because the moisture is being easily absorbed and held by the end grain. What I would try first is let the area dry out. Then either caulk or foam as much as I could between the rim joist and the I-beam. If I used foam, let it dry and then spray paint it to minimize shrinkage. Then re-install the insulation.

 

Resercon,

Thanks very much for the information!

I'm not sure if I got all your text or if the beginning was cut off.

I don't know if I explained myself very well but the ends of the
I-beam joists are not exposed to the outside. The entire face of the I-beam is to the ouside. The floor joist ends butt up to and are nailed to the engineered I-beam rim joists at a 90° angle. Along the adjacent walls, the rim joist is running parallel to the floor joists. The floor joist and rim joists are the same material and it's the rim joist webbing that is damp. I can foam/caulk as you suggested on the inside and I can see how this will insulate but how does this affect moisture flow? Can the moisture get trapped between the webbing and the foam?

I was able to pull back the vinyl in one spot outside and noticed that the exterior sheeting starts on the top edge of the rim joist. (I don't know it that is typical or not). Would it help to cut some foam sheets to fit in the webbing area on the outside between the vinyl? How would this affect moisture flow.

I sure appreciate you feedback.

- Phil McCarthy

 

The idea here is to seal the ends of the cut I-beams because they absorb moisture easily and retain it. The other portions of the I-beam do not absorb moisture so easily nor do they retain it as much. The moisture flow associated with heat flow will be unaffected by the sealing of the ends of the I-beams. The wood members in this area will still absorb and expel moisture. However, the amount of moisture would not be sufficient to promote mold and mildew growth.

 

Resercon,

You've been really helpful. Let me ask just a couple more questions.

Would it be worthwhile to pull back the vinyl siding a little and hit the entire rim joist, sill plate and exposed concrete area with some waterproof paint? Any recommendations on paint?

Also, for the inside, what kind of foam and where can I get the quantity I need to do the entire perimerter area of the rim joist. (I don't want to have to buy 50 cans of Great Stuff!).

Thanks.

- Phil Mcc

 

I am assuming the moisture problem is only located on the webing of the I-beams near the ends of it. It is usually this portion of the I-beam that is susceptible to retaining moisture. Also where portions of the webing has been hammered or damage is where you will get some moisture build up. In most cases any type of spray paint will seal these areas. Usually this will reduce the amount of moisture it will absorb and retain. Caulking and/or foaming gaps in this area will also reduce the probability of moisture problems. You don't have to foam the entire area. Then re-install the insulation.