New concrete foundation is damp and basement is humid
Hello, I am asking this question for a family member who is building a house right now. They hired a concrete contractor to pour the foundation. The foundation has had a damp wet look to it since being poured. Everyone involved in the process is extremely confused by what is going on and no one has been able to provide an answer, including the company where the concrete came from.
I highly doubt the issue is due to building tightness after researching the forums on this website. Over the past couple weeks the basement has been extremely humid, registering 90% humidity levels. A local building official contacted a couple engineers that he knows and one suggested possible hydrostatic pressure pushing the water up into the concrete.
One actually suggested maybe it is because the building is too tight, which seems a little ridiculous. The house has OSB (half inch) sheathing with all penetrations and cracks/seams well sealed with caulks and spray foam. R-21 walls. R-49 attic.
We were also thinking it could be a failed waterproofing membrane that was put on the exterior of the foundation prior to backfill. Although this may not be a green question, I was hoping with the level of experience on this site someone may be able to shed some light on this problem.
The house is located in eastern Connecticut, the home is on a sloping lot with one side of the basement having a knee wall and full-size windows. It is not a walkout basement though. Also, the footing drains appear to be functioning okay. Thanks for the help.
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Replies
New construction often needs dehumidification until the structure dries out. While you are trying to figure this out, it would be prudent to install an energy star rated dehumidifier.
Also, do the house have a ventilation strategy? And are the ends of the perimeter drain open to daylight?
So you say there is a waterproof membrane on the exterior of the wall but is there a capillary break between the footing and the wall? Also under the slab?
Is there an insulation plan for the basement or is the concrete wall the finished surface? What was the waterproofing done on the exterior of the foundation? Is there mechanical ventilation in the house? Was there a blower door test?
I'm not sure I would have an answer for you either way, but these are some questions that may help things along.
There are two possibilities:
1. This is construction moisture (the water added at the concrete batch plant) that hasn't dried out yet. That's possible, even after "a couple of weeks." To reduce indoor humidity during and immediately after construction, you have several options: ventilation, space heating, or dehumidification (or a combination of all three).
2. This is ground moisture -- that is, a sign of a high water table or very damp soil adjacent to the foundation. To limit the entry of ground moisture into your home, you need to include a capillary break between the footing and the basement wall. Did your contractor do that? For more information on this topic, see Capillary Breaks Above Footings.
If the grading around your foundation fails to direct rain away from your house, the grading error can lead to damp soil. For more about grading and other factors that contribute to a wet basement, see this article: Fixing a Wet Basement.
-- Martin Holladay