I own a house (Built in 1966) on a shallow hillside, around which recent rain has caused complete ground saturation. On my bottom floor, which is on a concrete slab, the three floor vents were recently filled entirely with clear water. We paid a contractor to remove over 700 gallons because I did not own a pump. Yesterday the same thing happened, so this time I purchased my own pump and removed about 500 gallons. Overnight they filled back up, so it seems the water has moved enough soil to find its way in much faster.

So now I am researching ways to stop this from happening altogether, instead of reactive repairs. I have found two possibles:

  • Digging a curtain trench uphill of the house to stop groundwater from penetrating the foundations. Then digging french drains downhill to allow groundwater to flow away from the house faster.

  • Using a product, such as Airseal, to completely seal up the vents.

The first I can probably do myself for under $1,500. The second seems to cost around $2,500 for my size house (2,200 Sq.Ft.).

So I'm looking for advice on where to start. Am I overlooking something? I did purchase some masonry waterproofer to apply around the edges of the house, but have yet to apply it. Is that worth doing?

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    Awesome question; I can't wait to see the answers. When it rains is there a puddle or a stream of water anywhere along the outside of your foundation? Like, do you know where the entry point is? Jul 10, 2015 at 19:41
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    This might be a stupid question as this is out of my area of expertise, but I am curious: Is there a crawl space at all under the concrete slab where you can access the ductwork? This could be a common, but it just seems odd to me (and asking for water trouble) that someone would have thought to put floor vents into a slab foundation which sits directly on the ground. Jul 10, 2015 at 19:52
  • @ZachMierzejewski - Good point that I missed. I suspect my concrete back patio slopes in towards the house, due to standing water there after rain, which is why I purchased the masonry waterproofer. Having said that, I don't think it is the only point of entry, since that's a lot of water I've pulled out.
    – Paul
    Jul 10, 2015 at 20:59
  • @statueuphemism - So it's a split-entry ranch house. The upslope is one-storey, the downslope is two-storey. The downslope part of the house is the concrete foundation. It's essentially basement with a back door that leads outside at ground-level. There is crawlspace under the upslope one-storey area where I can access that ductwork, but not under the concrete area. I agree, it's strange to me too and extremely worrying to be honest.
    – Paul
    Jul 10, 2015 at 21:01

2 Answers 2


Instead of patch repair solutions for the ductwork in the foundation, have you considered looking into quotes to add new ductwork along the ceiling of your walkout basement by branching off the ductwork under the house on top of the hill. You could then add ceiling vents that should never have water issues.

In the short term, this could be a costlier option, but in the long term I think it could be the cheaper option because:

  • A "patch" solution for the existing ductwork seems prone to failure again at some point in the future which will cost more money to repair incrementally.
  • You will waste less money on heating/cooling the ground year-round by having the ductwork inside the conditioned space.
  • Great idea. I had not thought of this, but it seems the best solution given the underground ducts look pretty terrible after I took some pictures with my phone down there. Considering my family is breathing air that has passed through them, I will probably go this route. Thanks!
    – Paul
    Jul 13, 2015 at 17:43

Problem #1: You have under-slab ducts. These are bad because they can flood, harbor mold, and increase the dampeness of the house, and let in creepy-crawlies.

Problem #2: when it rains, water gets under your slab--and floods the ducts! This means you have very poor drainage and a high water table.

The best solution to this problem would be to seal up your slab ducts with poured concrete, build new ducts, and then focus on waterproofing your foundation perimeter. Probably the least costly and most DIY-friendly way to do this is by building in-ground gutters. See more here: http://www.finehomebuilding.com/how-to/qa/inground-gutters-keep-basements-dry.aspx

You can also use this as an opportunity to add insulation foam around the perimeter of your foundation slab, which is a great investment that may pay for itself eventually in lower heating and cooling costs.

  • So the option to add overhead ducts seems obvious now it's been mentioned. It seems in-ground gutters are more for surface fluid, so wouldn't a french drain front and back be a better solution? I already have roof gutters, and I plan to reroute them to the french drain.
    – Paul
    Jul 13, 2015 at 17:47
  • Holes in the ground just wanna flood. You can do an elaborate water diversion project that will never be 100% successful, or you can do an elaborate ducting project that will eliminate the problem, save you money, and reduce the amount of moisture in your house.
    – iLikeDirt
    Jul 13, 2015 at 17:51
  • Absolutely. So yes to be clear, I plan to get a quote on ceiling ducts and sealing off the in-ground ones. But for the outside it would be nice to try and help groundwater flow away from the foundation. So french drain vs. inground guttering, I'll do some research on them.
    – Paul
    Jul 13, 2015 at 17:55
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    This is what you're looking for, I believe: greenbuildingadvisor.com/blogs/dept/green-communities/…
    – iLikeDirt
    Jul 13, 2015 at 18:02

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