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Community and Q&A

Another foundation drainage question: inside vs. outside

steveoneil | Posted in General Questions on

I know there is a recent article on foundation drainage with good discussion, but I don’t think this particular question was touched on.

I’m having a full basement constructed under the existing house. I’m located in Massachusetts. The foundation is being built for obvious structural reasons, but while I’m at it I figured I would take steps now that would make the basement high quality living space (some day). On the exterior is spray-on waterproofing membrane on the walls and the top of the footer. On top of that will be 2 layers of 1″ roxul drainboard. I also had several egress-size windows installed.

The egress windows are quite large, so there is the potential for water intrusion there, although I intend to use a clear cover on the wells. But my thinking is that these large wells necessitate a drainage system on the exterior of the foundation, as is typically done in new construction.

However, bot the GC as well as the drainage contractor want to install the drainage system on the interior of the basement, with a sump on the interior that will pump water back out of the house. Because this is a brand new foundation, and all excavation is still exposed, I was surprised at this recommendation.

For the window wells, the recommendation is to use solid PVC pipe to drain the window wells and pitch it back to the interior sump pump. This would be a safeguard in the unlikely event the natural drainage was insufficient. I say unlikely because this seems to be a pretty dry site. Puddles drain quickly from the surface, and even 10 feet down in the trenches, the large puddles from the recent heavy rain disappeared in a few hours. But this plan does seem to be different from what I’ve been reading about. It seems that interior perimeter drains is a retrofit measure to solving water problems, while exterior is a new construction method.

So I’m interested in hearing other’s opinions: Is an interior perimeter drain as effective as an exterior perimeter drain in addressing both ground water and water intrusion from the surface? (measures such as proper grading and downspout placement will be taken of course).

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  1. GBA Editor
    Martin Holladay | | #1

    There is no reason not to install perforated pipe on both sides of the footings. It's easier to do it now than in 10 years. The two systems can be tied together by a pipe that penetrates the footing, and both can direct any water to an interior sump.

  2. steveoneil | | #2

    Martin, that's my thinking as well. But the GC who is building the foundation was only going to do the interior one. So I brought in a contractor that specializes in foundation waterproofing. He told me the same thing--just do the interior.
    I'm all for redundancy and I'll even pay for it now for peace of mind, but they don't think it's necessary:
    The GC doesn't like the idea of water being brought back into the house to be pumped out again, so even getting him on board with any window well drainage is difficult (unless there is an exterior holding tank).
    The drainage contractor thinks the interior system will suffice, and the non-perforated pipe as backup window well drainage has the advantage of little chance of failure, due to no filter fabric getting clogged.
    I suppose I could insist that hey do both, but i'm wondering if I could only choose one, is an interior system just as effective as an exterior system in keeping a basement dry?

  3. GBA Editor
    Martin Holladay | | #3

    You might tell your contractor that most building codes require that a drain be installed on the exterior side of the footings. Below is a screen grab showing IRC requirements.

    I also found the same provisions at this code site -- but I can't tell which code this is.

    "1805A.4.2 Foundation drain. A drain shall be placed around the perimeter of a foundation that consists of gravel or crushed stone containing not more than 10-percent material that passes through a No. 4 (4.75 mm) sieve. The drain shall extend a minimum of 12 inches (305 mm) beyond the outside edge of the footing. The thickness shall be such that the bottom of the drain is not higher than the bottom of the base under the floor, and that the top of the drain is not less than 6 inches (152 mm) above the top of the footing. The top of the drain shall be covered with an approved filter membrane material. Where a drain tile or perforated pipe is used, the invert of the pipe or tile shall not be higher than the floor elevation. The top of joints or the top of perforations shall be protected with an approved filter membrane material. The pipe or tile shall be placed on not less than 2 inches (51 mm) of gravel or crushed stone complying with Section 1805A.4.1, and shall be covered with not less than 6 inches (152 mm) of the same material."


  4. steveoneil | | #4

    Yep, did that. I think he was insulted by it. That's part of the reason I brought in a contractor that specializes in basement water management.
    The code does allow for an exception if the soil is well drained, but this site has not been tested for that specifically. It just seems to drain well.
    Anyway, let's pretend code allows for either interior or exterior perimeter drains, and that they are both placed at the same depth relative to the footer and slab. Would one be better at keeping the basement dry? In terms of handling ground water from seasonal variation in water table, i would think they would perform equivalently. For water seeping toward the walls due to saturation by storms or other poor water management at the surface, it seems like the exterior system would be better. But even if there was no exterior system, wouldn't the interior catch any water before it became a problem? I dunno. That's why I'm confused. One advantage of the interior system I can see is that it would be easier to access clean outs, relative to something buried 10 feet under the ground. Sounds good in theory, but also feels like I'm going against conventional wisdom.

  5. user-2310254 | | #5

    I suspect you are getting the recommendation for an interior drain and sump because you hired a contractor in specializes in installing those systems. If you have high ground water, you may need those extra layers of protection. But it also could be an unnecessary expense. Do you have neighbors with full basements? Are their basements dry or well?

  6. steveoneil | | #6

    You are probably right, he seems to do a lot of repair/retrofit on existing basements, and therefore prob most of his experience is with the interior system. Even with that bias, I'm assuming his argument is that if the interior solves all the problems, is an exterior one necessary?

    Yes, all neighbors have full basements, but I do not know for sure if they've ever had problems. I think in one or two cases, there has been water, but it was more due to severe rain coupled with pour grading. The street is elevated relative to the surrounding neighborhoods.

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