Siding Checking insulation behind mopboard

Published on September 6th, 2016 | by Henri


Frost forms above mop board

Q.  Our house is 12 yrs old. The inside lower portion above the mop board, where the south wall meets the west wall on the ground floor, always frosts up during the winter months. We have changed aluminum siding to vinyl siding.  When the siding was replaced, nothing was pointed out that there might be a gap underneath the siding to cause this. Of course, maybe they weren’t looking for such things.

How can we alleviate the problem? Will it cause mold? Will it deteriorate the dry wall in time?

We also have a problem in the spring/summer with bugs entering the family room along the south and west wall.  It doesn’t happen to the upper front room which is east and north wall also ground level. What can be done besides continual spraying? Is there a gap to the outside under the siding?

A.  Yes, molds can develop, and drywall may become damaged. It does sound as if there is not only a gap that lets in cold air and bugs, but also a problem with the insulation at the base of the wall. In normal construction, a sill sealer is set on top of the foundation and all subsequent floor platforms, and the bottom plates of the walls are set on the sill sealer.

The purpose of the sealer is to prevent air infiltration between the various elements of the construction. If, for some reason, it was omitted in the problem area, there is a gap. It may be necessary to remove the bottom piece of vinyl siding to see if the area can be accessed and the gap caulked.

The fact that frost forms above the mop board (as opposed to its base) tells me that there may be a problem with the insulation between the wall studs. This will be more difficult to correct.

I suggest that you deal with the assumed gap, and wait and see if that solves the frosting. If it does not, you have two choices:

  1. Talk to someone who does infrared thermography. Ask him or her if they would come and just check that area, and what would the cost be. This needs to be done in cold weather.
  2. Have several siding boards removed and open the sheathing to check if this is the case. This can be done at the same time as the caulking of the gap.
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