GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Waterproofing and Insulating a Basement

lukievan | Posted in General Questions on
Hi all. I’ve been deep-diving into this wonderful resource and figured it’s time to go-ahead and ask the questions that I’ve been searching for answers to. Apologies for the long post – I figure it’s better to be as specific as possible up-front.

I bought a house in upstate NY (Zone 5b/6a) with some substantial “water in the basement” issues. The house – built in the early ’50s – is on a hill; the unfinished basement starts below-grade and ends up at ground-level on the other side. The concrete block basement walls are completely uninsulated on the interior and exterior I’m fairly certain there’s nothing but dirt under the concrete slab. The walls on the below-grade half were wet and crumbling in some places, especially down near the floor, with visible water trickling through after heavy rain. The work to fix these issues is already underway but could really use some feedback on final steps in the waterproofing/insulation process.

I don’t really need a properly “finished” basement but something dry and not freezing cold in winter would be nice. I had the exterior perimeter excavated and after draining the standing water we found sections of unattached, mud-packed drain at the footing. Proper drainage (perforated pipe, gravel, etc.) has been put in and the most egregiously deteriorated sections of the basement foundation wall were rebuilt by a skilled mason. Other sections where the block was not substantially damaged and crumbling were treated and reinforced with 2 layers of cement over a wire lath.

Once we’re all done, the excavator/drainage guy says he’ll backfill with “2 inch minus” gravel, which I’m all for. In the meantime, I have a giant moat around my house and need to make some decisions about any additional layers of waterproofing/insulation to put in before backfill.

The mason recommended Guy #1 who does spray-on waterproofing with “Tuff-N-Dri H8”. Someone else recommended Guy #2 who uses “Blue Max”. Seems like similar products and unless I hear a compelling case against it, I’ll have one of them come asap to spray/brush one of those directly on to the concrete block foundation exterior.

Guy #1 says he doesn’t recommend doing any additional exterior insulation or waterproofing besides the Tuff-N-Dri. He says that if it were him, he’d rather spend that money on insulating the basement interior. Ok, I hadn;t really thought too much about the interior as i don’t intend to spend much time there but this would buy me time and expense, as I could buy interior insulation and do the job myself. Guy #2 recommends 2″ XPS over the waterproofing and before backfill, and my friend – let’s call him Guy #3 – recommends I do a layer of at least 2″ EPS . After diving down the internet rabbit hole, it seems like adding an exterior layer of rigid foam is not unreasonable advice. But I’m having a pretty strong aversion to the thought of attaching underground foam panels next to my house. I know they’re stronger than a gas-station styrofoam cooler but I just can’t believe they are sturdy and stalwart enough to hold up for decades of shifting earth, freezing and thawing cycles, water coming from every which way, etc. Not to mention that it’s not exactly an earth-friendly material to make or to break down. I guess I just hate foam. Sorry, foam.

I’m leaning more towards rigid mineral wool panels between the waterproof layer and the gravel backfill. My thinking is that this will give the waterproof layer some physical protection from the backfill, as well as creating a final drainage channel before the waterproofed block. I like that mineral wool is fire and insect resistant (we’ve had issues with carpenter ants) and won’t deteriorate from water saturation. The downside is that it has a slightly lower R-value than the equivalent thickness of EPS and it’s more expensive and most guys up here are not as familiar with it as they are with rigid foam. So if I were to go that route, I’d have to find someone who knows how to install it, or someone who I am confident will take the time to follow the specs and instructions and do it right. The product I was looking at is “Roxul Comfortboard 80” because that’s what pops up on my searches. But if there are other brands or versions of below-grade mineral wool that I should be looking at, I’m all ears.

Do I need the additional layer? If so, what material, and how thick should it be? How best to attach it to the waterproofed block? Is mineral wool strong enough to be an effective layer of physical protection for the waterproofing on the block? Is there some simpler/cheaper/more eco-friendly option I’m missing? Ok, I’ll wrap it up. Thanks for reading and for any insights or critiques coming my way!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    NICK KEENAN | | #1

    The insulation layer needs to be continuous. It's easier to do that on the interior.

    If you go with exterior it has to be covered with something. It has to extend up to where the interior insulation starts, typically the rim joist. That's a tricky detail on an existing house.

    1. lukievan | | #2

      Thank you for responding. Do you mean that the mineral wool board would need to be covered above-grade? Or above and below? Covered with what, exactly? I know that effective insulation layers should be contiguous and extend up to the existing insulation/siding. The primary purpose of the exterior layers I mentioned would be to keep water out of the basement rather than as insulation. That said, it makes sense to maximize the effectiveness of the barrier as an insulation layer if there's a relatively simple and cost-effective way to do it.

      1. Expert Member
        NICK KEENAN | | #3

        If you want to keep water out then use waterproofing.

        The above-grade insulation needs to be covered with something to protect it from damage and from critters. Some sort of siding. The usual problem is that if the house is already built the insulation sticks out further than the wall above, which creates an entry point for rain that is hard to flash.

        The purpose of insulating the basement is not to make the basement comfortable, it's to prevent heat loss from the rest of the house. Otherwise the basement is like an open door. A secondary benefit of keeping the basement warm is it makes it less likely you'll get condensation.

  2. onslow | | #4

    You could well have been describing my previous home though mine was built in the 30's. I never did fix the external drain pipe problems because I knew the house would not ever be saleable in a modern market. After tuck pointing the failing mortar and coating the walls with Thoroseal I did insulate inside to make a tolerable workshop. Search the GBA threads under "insulating basements" and sit back for a very long read. What I did 35 years ago would not float now. On the inside, if that is where the mesh and parge coat was applied, I would add either Thoroseal or UGL type coating once the parge cure point is past.

    You know your block wall best, but the advanced damage points to water coming down the hill both at the surface and below grade. I also suspect that the block cores were not filled or re-barred anywhere. Likely the cores filled with water after the pipe plugged, accelerating the damage. I know my uphill side blocks certainly were full of water once I poked the joints. I would recommend coating with your choice of exterior water seal on the wall all the way to the footing top. If one product is more flexible after drying, I would tend to favor that feature. Regardless of the foam - no foam question I would get dimple mat between the sealer coat and backfill to further enhance drainage. I would not use No.2 minus as back fill if it is what I think it is. Sounds like road base, which has rocks and fines in it. Can you get a more precise description of it? You will need to compact backfill in lifts to avoid settling and making a catch trench right next to the house.

    Depending on what is uphill from the house, you should plan on grading in a swale to catch surface run off and direct it around the house as code and neighbors will accept. A french drain in the bottom of the swale if surface runoff is really bad. Look carefully at the open cut around the house to see if you have odd soil layering that is piping in subsurface water from further away. A mid level drain line might be in order. Now is the time to do so if the soil layers are part of your problems.

    Hopefully, the yet to be installed footing drain is below the edge of the footing to keep water levels below the slab top. The sleeved drain pipe should be bedded in washed rock that is inside a full wrap of filter fabric to protect against future silting infiltration. The pipes you excavated might have worked at one time, but were soon plugged as ground water carried fine material into them. At a bare minimum there should be filter fabric over the top of the washed stone bedding though full wrap will be better.

    FWIW, I would still use XPS over EPS or Rockwool. It can be found used though your timeline probably won't allow for that. The new low GWP version should be available in the east. Termite/ant treatment may also be available. Much arguing over water retention between the two choices. I would observe that both EPS and Rockwool have small interstitial spaces and the capillarity strength of water concerns me. I know carpenter ants can't nest in Rockwool, but can in EPS. Easy to chew wet wood is their preferred habit, so picking out the little dots and having a drink close at hand probably seems like a good second choice.

    Insulating on the outside is tricky, as any fastener used will violate the water barrier you just put on. I have not tried Fuze-It on foam or waterproof coatings, but the label says most plastics. I would secure the dimple sheet with adhesive only for the same reason. Might see what they use. Another problem with after the fact foam is the beltline of the house probably won't allow for a tidy transition between siding and the thickness of foam plus protective layer.

    There are pre-bonded vinyl/foam panels available from Progressive Foam going by the name Foundation Pro. It is Graphite EPS, but protected against water and hopefully lazy ants. Simpler than what I did on my new home. I have 6+ iches of exterior foam overhang, so the three inches of reclaimed XPS plus cement board looks fine. If the lay of the land leaves large areas of basement wall exposed to ambient temperatures it could be argued that the exterior foam would be of greater benefit than interior. A zero degree wall behind interior foam is a very cold condensing surface. Exterior foam only does allow for a more fail safe moisture escape in the basement. Again, read up on GBA prior posts.

  3. plumb_bob | | #5

    Yes, watch what is used for backfill. My preferred method is to use washed 2" rock from the bottom of the footing all the way up to the top, any water that hits this will head straight down to your new, continuous perforated drainage pipes. Ensure downspouts are tied into a different pipe that ties in down stream of your foundation drain pipes. A dimple mat used between your damp proofing and washed rock will protect the damp proofing and add another layer of drainage. A neighbor friendly swale on the uphill side is also a great suggestion.

  4. lukievan | | #6

    Thank you so much for all the informative responses!

    Just to follow up on a few points: 1) The footing drain is below the footing and is surrounded by rock and wrapped in filter fabric. 2) Uphill from the house is the driveway and then a 4' rise that leads to the neighbor's yard. I don't think there's much room for a swale per-se but you could excavate a narrow trench between the driveway and the rise and lay a drainage pipe there. Or maybe that's what a swale is and it's just that it rhymes with "whale" which makes it sound much bigger than it is. I guess I'd like to see how the current solutions work before tackling that. 3) I hadn't heard of the pre-bonded vinyl/foam panels but will take a look.

    I consulted with an experienced builder I know – who has a deep understanding of energy efficiency and green building practices – and he suggested dispensing with exterior basement/foundation insulation and to just waterproof the block, add a dimple mat, and then backfill. His reasoning: because the unfinished basement with uninsulated slab will not be used as living space, and time and money are limited resources here, it should be decoupled from the house's thermal envelope via insulation of the floor and rim joists between the basement and 1st floor, as well as insulating the basement walls from the ceiling to 12" or so below exterior grade. I was a bit surprised by this, as I thought he would encourage me to go full-hog with exterior insulation but he took a more practical-minded approach. I also thought he would be more enthusiastic about my idea to use mineral wool (I know he's a fan) as an exterior layer but again he was more realistic and said it was expensive and required more time and skill to install compared to a simpler barrier between the wall and backfill such as a dimple mat. He also warned me against using the 2 inch minus as backfill which contains fines that could settle and compact.

    I asked the excavator/drainage guy what other options we had for backfill and he responded:
    "The only thing I would use other than the 2 inch minus is 3/4”-1,1/2”crushed stone. Which is probably a good option it would cost about $800 more than the 2 inch minus. This is based on three or four -20 ton loads. So in short I think that’s the better way to go."

    I checked in with one of the the waterproofing guys (aka Guy #1) and he doubled down on "you don't need anything between the waterproofed wall and then backfill". Even though I'm inexperienced and don't know anything more than I've read on the internet, this seems unwise. I figure it's inevitable the backfill would end up damaging the waterproofing at some point down the line.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |