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Is there a reason people do not use hydraulic cement to parge an entire stone basement?

pfields | Posted in Energy Efficiency and Durability on

I have a stone basement in Wisconsin. It’s depositing sand along the edges of the walls in various spots. Not bad, and it only leaks a small drizzle of water if there is a huge downpour, but I want a clean water tight basement.

Is there a legitimate reason to not use hydraulic cement throughout the entire basement after clearing all the loose sand in between the stones and vacuuming out any gaps and then apply a waterproof paint or coating to the walls to seal it? I want to make the space usable beyond laundry and mechanicals because the ceilings are over 8 ft., for clean storage and work space.

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  1. ethant | | #1

    Your first step will to be to walk around your house and make sure that all drainage is AWAY from the house and that downspouts are sending water about 10' minimum from the walls. If water wants to get through the wall, it most likely will, so you are best off sending water away. The underground roof is an interesting idea too:

  2. GBA Editor
    Martin Holladay | | #2

    P. Fields,
    Ethan is correct: If your basement has a water-entry problem, the first step is to check exterior grading and the configuration of any roof gutters and downspouts. For more information on this issue, see Fixing a Wet Basement.

    If the hydrostatic pressure on the exterior side of your basement wall is high enough, the water will enter your basement, regardless of the type of cement you use for parging. So step one is always the same: "Keep the water away from your foundation."

    I haven't ever used hydraulic cement. But I can't think of any reason why you can't use it for pointing and parging -- as long as you don't have unrealistic expectations.

  3. pfields | | #3

    Thank you, We are also working on exterior drainage and regrading as well. I just want to make sure hydraulic cement won't damage the stone wall somehow. I've tried to research it online and there is really nothing I can find that says it's not ok to use, other than it being cost prohibitive because it's a bit more expensive than regular cement, but I thought I would ask some expert advice before I proceed.

  4. charlie_sullivan | | #4

    Are you proposing the waterproofing coating before or after the hydraulic cement? I would be concerned about doing it before the cement, because it might degrade the adhesion. But even without it, the adhesion might be a weakness in the plan.

  5. Jon_R | | #5

    If directing external water away doesn't work, I'd do something to drain away the water after it comes through the wall.

  6. KeithH | | #6

    First, I'm a DIYer not a pro but I've been researching this for a family home.

    Are you proposing using hydraulic cement as your mortar? I don't think that will work or is even advisable. Hydraulic cement expands slightly as it cures, cures very rigid, and has generally poor surface adhesion. Set time is very fast and it is pretty expensive.

    If you have deteriorating mortar, I'd fix your mortar first with a traditional mortar product. And fix your exterior water management. Then I'd look into a potassium silicate agent (paint isn't quite the same word) and vet its application to your stones and mortar with the manufacturer. Any latex waterproofing will fail and will trap moisture that could deteriorate your mortar.

    I found this good old discussion on Fine Home Building that pretty much addresses your question exactly.

  7. GBA Editor
    Martin Holladay | | #7

    Thanks for your answer. It's clear that you know more about hydraulic cement than I do.

  8. pfields | | #8

    So I've attached a picture of the walls. As you can see there is some type of cement applied on the wall already but it literally can just be pulled off by hand.

  9. pfields | | #9

    Most spots are still hard but others are loose

  10. GBA Editor
    Martin Holladay | | #10

    P. Fields,
    If we leave aside the question of hydraulic cement -- because I have no experience with hydraulic cement -- we can discuss how to address your stone-and-mortar wall.

    The first step to restoring this type of wall is to remove all of the loose mortar, using your hands or a stone chisel and hammer. Then you need to point the wall with new mortar.

    Older foundation walls often used high-lime mortars instead of Portland cement mortars. Some were deliberately parged with a lime mortar to act as a so-called sacrificial layer. This is a way to handle efflorescence. Every 20 or 30 years, the sacrificial layer is removed and renewed.

    If you plan to insulate the wall on the interior, you probably won't be taking the "sacrificial parging layer" approach. Instead, you'll probably want to point the wall with a modern mortar mix, and then you'll want to insulate the wall on the interior with closed-cell spray polyurethane foam.

    If you are worried about water entry, you should install a layer of dimple mat between the stone wall and the closed-cell spray foam. The bottom of the dimple mat should connect with an interior French drain at the base of the wall. The French drain should connect to a sump.

  11. jkopec | | #11

    I am in a similar situation with an almost 200 year old farmhouse that I just purchased. Is there any concern with pointing a stone foundation, then covering that with closed cell spray foam? Water will still get to and eventually deteriorate the new mortar from the back side, but you will not be able to see it, or access it for repairs from the inside any longer. So much water comes through my foundation (no gutters, we are fixing that!) that there is absolutely no mortar left any longer, and the structural engineer said pointing was not necessary, so maybe it doesn't matter?

  12. GBA Editor
    Martin Holladay | | #12

    If large quantities of water are coming through your foundation, you have to take this situation seriously. No cutting corners to save money.

    First, read this article: Fixing a Wet Basement.

    In your case, you absolutely need an interior French drain connected to a sump. The sump should drain to daylight or be equipped with a sump pump.

    On the question of the dimple mat, there are two schools of thought. School #1 says that you absolutely want a dimple mat, to make sure there is a clear path for the water to get to the French drain. Once the dimple mat is installed, you can install closed-cell spray foam on the interior side of the dimple mat.

    School #2 says to install closed-cell spray foam directly against the stones, because the spray foam fills the gaps and helps reinforce the loose stone work. The foam strengthens the wall. If you are a believer in this approach -- I'm not sure about it -- you tell people that these old stone walls have so many paths for water to trickle through, that the water will find the French drain, even without the dimple mat.

    1. kenmarcou | | #15

      As I understand it, the dimple mat should go all the way up the wall. Should it connect with the sill plate? Or seal to whatever the sill/rim joist air control and thermal control layer is? Not sure where to terminate the dimple mat. Although maybe it doesn’t matter that much exactly how far up it goes since it’s going to be covered by rigid or closed cell spray foam which will have to be continuous up to the rim/sill anyway?

  13. jkopec | | #13

    Thank you for the reply. We are installing an interior french drain to a sump pit and possibly daylight. I believe adding gutters will significantly reduce the water coming through the foundation, as the gable end walls do not leak. I am not sure whether to do a dimple mat, 20mil cord reinforced poly against the foundation and under the sill then closed cell foam over that, or just do the spray foam against the stone for the added structural reinforcement. I guess the question is can I go wrong with any of these 3 methods? I would install a capillary break under the sill in all 3 scenarios. I had originally considered putting XPS against the stone and covering that with the 20mil cord reinforced poly to the curtain drain, but I haven't found anyone else that has done that so I am certain it is a bad idea!

  14. GBA Editor
    Martin Holladay | | #14

    I've shared what I know. The only person who can make this decision is you. Good luck.

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