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Community and Q&A

Sealing rim joists in WI home

suect | Posted in General Questions on

Last April foam insulation was sprayed along the corners of each rim joist. 

Could this raise the moisture in the exterior wall cavities and attic?

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  1. user-2310254 | | #1


    I'm assuming this was closed cell foam, and it was installed to cover the rim joist all the way around the foundation. Please confirm.

    What is on the exterior of the rim joist? How elevated is the rim joist from the ground? Are there foundation plants? Do you have gutters that channel the water away from the foundation?

    1. suect | | #2

      Yes, closed cell foam was used in March of this year, but just to box out each cavity, sealing each corner then fiberglass in each cavity.

      James Hardie planks are on the opposite (exterior side). In some areas the bottom of the rim joist is 2 inches above the soil, other areas about 6 inches.

      Gutters are draining about 4-6 feet from the home. I do need to find a way to create a swale to move water away from the house as there is only a slight pitch.

      There are foundation plants but currently sparse.

      I had attic penetrations sealed 2 years ago and had added insulation.

      I noticed shadowing in corners of walls and ceilings that are exterior walls this past summer.

  2. user-2310254 | | #3


    The experts may chime in with a better answer but sealing the building envelope often increases interior moisture levels. That's why so many posts harp on monitoring humidity levels. If you don't already have one, consider buying an inexpensive humidistat to get a better handle on your indoor conditions. If the levels are elevated in your basement or living space, you may need an Energy Star dehumidifier.

    I'd also consider racking back the soil in the areas where it is just a few inches from the rim. The more space you can create between the ground and mud sill, the better.

    1. suect | | #4

      I will definitely work on the soil outside when the weather allows.

      Indoor humidity is low. Currently the living space is 27 % with the cold weather here in WI. During the summer it hovers around 35-40% in the living space.

      I run a dehumidifier at 45% in the basement which turns on automatically from June to September. The only time the home seems more humid is July and August.

      This surprises me to see the shadows on the drywall when the home is dry for the most part. The only thought I had is could the moisture be between the walls to cause this?

      Unfortunately an energy audit by the utility company advised sealing up the house. Wish I had the information you discussed before taking steps.

      1. Andy_ | | #5

        "Shadows" on drywall are typically the result of dust clinging to moisture that is typically the result of condensation on a cold surface. It usually takes quite a while to accumulate to a point to be noticed.
        How well is the rest of the house insulated? 2x4 walls? Forced air heat?
        I don't think we have enough info about your house to really draw any accurate conclusions. For all we know prior to sealing the rim, your house may have been so poorly insulated and drafty that the amount of heat needed to keep it comfortable was warming your walls all the way to the outside. Not saying this was definitely the case, but just that it's all speculation at this point.
        Air sealing can increase interior humidity levels vs living under a pergola, but there's a lot more to it. A house is a system and turning any one dial too far can change the performance, but leaving all the dials at essentially zero has its own issues too, like a huge energy bill and low comfort level.

  3. walta100 | | #6

    Let’s think about the amount of setback you have programmed into your thermostat. A few degrees are great but if you go overboard and programmed in 10° or more you are asking for problems. When the heat goes off the weak points in your insulation will cool off faster and are likely to fall below the dew point of the air in the home. Those damp spots will attract dust and can grow mold. It seems possible that is what you are seeing.


  4. suect | | #7

    We leave the temperature constant at 71 degrees day and night during the winter. Summer is 75 degrees.

    Our home has 2x6’s for the exterior walls, forced gas heat (2 stage, I have been told). There is a 6 inch fresh air intake. During the past winter’s humidity could get as low as 16% causing wood floors, door panels and drywall to shrink. This is a concern as there may possibly be outdoor air filtration. The humidity is now in the high 20’s. I am monitoring the attic humidity as well. It is in the low 50’s during the day and high 50’s at night (lower than outdoor humidity).

    We have 2 gas fireplaces with ceramic logs. I do not see any damper, it is controlled with the wall switch.

    Would negative air cause moisture in the corners? We have had new windows, and air sealing in the attic. Just wondering now if we are in the negative now with all the improvements.

    Would an air exchanger possibly be a remedy?

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