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Challenging Floor Details

HgnJXLgAkS | Posted in Green Building Techniques on

I’ve seen a lot of discussion about wall and roof details with respect to air, vapor, and thermal transmission. What I haven’t seen is discussion of the flooring and how this contributes to thermal and moisture problems. And I have a quite challenging situation that has me a bit stumped.

The home in question is 140 yrs old in Western MA. Foundation is brick on field stone and there is significant water infiltration in the “basement.” There is an addition (built circa 1960, approx. 250 sf) with a poured concrete foundation and dirt crawlspace. For reasons unknown to me, half of the crawlspace was filled to within 18″ of the floor joists with rubble (brick, concrete, field stone, dirt). All of which makes installing a vapor retarder somewhat challenging.

So there are two questions I would appreciate input on.

1) For the primary structure, because of the significant water infiltration (think rivulets of water running through gaps in the foundation) I am considering declaring the basement a cellar and just allowing it to be damp (as if there was any choice). The sill plates (6×8 hickory) are at the level of the floor joists so installing an insulating vapor barrier system (like 2″ foil faced polyiso) to the underside of the joists and filling any penetrations with a 2.0 lb closed cell foam would be feasible. However I’m uncertain about whether there is a better solution.

2) For the addition, similar challenges are present with the crawlspace (instead of rivulets of water coming through the crawlspace walls, water vapor is readily present from the soil) but new flooring and subflooring is being installed so there is opportunity to address moisture issues from either side of the floor joists. It’s worth noting there is 2″ foil faced polyiso installed on the interior of the walls and cathedral ceiling (I’m told all seams were filled with closed cell foam and foil taped).

Any suggestions are greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Corn Walker,
    I disagree with your conclusion that the best approach is "just allowing the basement to be damp."

    The first step is to do everything possible to address your water entry problem, following the usual steps:

    1. Improve exterior grading so that the soil slopes away from the foundation. If necessary, install a drained swale on the uphill side of the house.

    2. Direct roof runoff to gutters and conductor pipes; convey the roof water away from the foundation.

    3. If the above measures fail to correct the problem, consider installing an interior French drain around the perimeter of the basement. The French drain should be filled with a perforated pipe and crushed stone; the pipe should direct water to a sump. Install a sump pump and convey the water away from the foundation.

    4. Improve the cramped crawl space by clearing out the rubble and excess soil. In other words, lower the soil level in the crawl space.

    5. If the homeowner can afford it, cover the walls of the basement with a dimple mat, rubber membrane, and closed-cell spray polyurethane foam. If previous measures have adequately addressed moisture entry, you may be able to spray the foam directly against the stone walls, without any dimple mat or rubber membrane.

  2. HgnJXLgAkS | | #2

    Martin, thanks for your detailed response. I'm open to all ideas within budget.

    As you might expect, the section of wall with the greatest water issues is on the uphill side of the house. The soil has been sloped away from the foundation and roof runoff already goes to conductor pipes leading to a small stream downhill of the structure. I recommended a drained swale but it may be cost-prohibitive (water, sewer and gas utilities plus stone patio are all on this side of the house). Avoiding these items would place it 12-15' away from the foundation wall.

    The interior french drain is something I hadn't considered. I will need to perform some exploratory excavation to see what I might run into. Some of the stones in the foundation are quite large and irregular, and I'm concerned both with running into boulders used as footers and compromising foundation integrity in removing material (I can't imagine this foundation was "engineered" in any sense of the word). There is a floor drain on the low side of the basement floor, which is simply 1-2" of concrete poured over the existing dirt floor, so there have never been issues of standing water.

    I have a similar concern with excavating the crawl space. I am betting that the reason the crawl was filled as it is because the footers for the poured concrete foundation on the uphill side are not at the same depth as the downhill side. I was also remiss in failing to mention that the foundation wall on the gable side only extends half-way (on the downhill side) and does not connect to wall on the uphill side. It's almost as if they ran out of concrete or money for the same.

    At some point I can't help but wonder if the "green" thing to do is deconstruct the house and start over. Air sealing and insulating this old house has been a challenge in every respect.

  3. Riversong | | #3

    Corn Walker,

    If water is entering from the uphill side of the cellar and grading and gutters are already accomplished, then it's almost certainly sub-soil water (ground water) from strata on the hill. You might consider an exterior French drain to intercept this flow on the far side of the stone patio, outletted to daylight or to the stream. How deep a drain? You'll know when you excavate and see the seepage. Make sure to wrap both perforated drain pipe and surrounding washed stone in a burrito of filter fabric to prevent fines from washing in.

    It sounds like the crawlspace is already vented (incomplete wall?). I would install 2" foil-faced polyiso under the joists, tape and/or the joints, and make sure the crawl has cross ventilation at a minimum of 1 sf per 150 sf floor and that rainwater cannot run into it.

    If the cellar has enough headroom, you can pour a new concrete floor (min. 3" with 6" welded wire mesh) over existing rat slab, 2" crushed stone and Tu-Tuf vapor barrier. The stone layer will be a subslab drain leading to existing floor drain. Finish cellar walls by either of Martin's methods, depending on mitigated flow volume, before pouring new slab so water can drain to stone layer.

  4. David Meiland | | #4

    Although it might challenge the budget, there is always the possibility of retrofitting a poured concrete crawl space or basement under the house. I've done two projects where existing crawl space foundations were removed (consisting of fieldstones, stumps, and other goodies) and poured concrete was installed. It probably makes more sense in earthquake country, but it's quite possible once you get past the idea that a house is too heavy. You could completely solve any water issues in the process.

  5. Riversong | | #5

    Corn Walker,

    Check out the recent Building Science Corp report (with Joe's usual tongue-in-cheek style) on sealing and insulating old stone cellars and crawlspaces.

  6. HgnJXLgAkS | | #6


    Thank you for the link. I had searched his site but failed to come across this article. It sounds like his situation is nearly exactly like the one I've encountered, except I do have a bit more clearance above grade (min 18" on the uphill side, 45" on the downhill).

    Headroom is an issue so it makes sense to either break up the existing "slab" or use the method detailed in Figure 2. While I've lifted a foundation or two before, I am concerned about lifting this foundation to install the capillary break - who knows what rot that will uncover. I'll need to make sure I have some timbers on hand to replace any rotted sills and posts.

  7. Riversong | | #7

    Corn Walker,

    Where in W MA are you (I lived in Greenfield for many years) and is this your house or are you the contractor?

    Yes, I was struck with the similarity between your situation and Joe's. If you're going to do a thorough basement renovation, that's the time to replace or repair rotted sills and framing.

    Beware, however, that if the sill beams are rotted, you can't use them to jack the house, and you can't replace them if they're being jacked and supporting the rest of the house. There are several ways to jack a house, depending on what needs replacing. Make sure you have someone with experience and the right tools do this.

    I typically remove some siding and lag a ledger beam into the studs near the top of the lower wall and use angled jack posts on screw jacks (hydraulic jacks are wonderful, except when a seal blows and the house drops). This takes the weight off the sills and allows replacement or repair of both sills and the joist ends which are typically also rotted. Angling the jack posts also moves the jacks away from the foundation, leaving room to work.

  8. HgnJXLgAkS | | #8


    I'm in Hatfield and own the property in question. I used to work in construction but I'm no longer a contractor. Fortunately I have relatives who are still in the business, so permits are never a problem.

    There is only one sill that is questionable, the other three are incredibly solid (as in, 12d nails bend when trying to attach sheathing solid). Fortunately the only weight bearing on this sill is the uphill wall. The timber frame construction is such that the floor joists run from the gable walls to a center beam that bears on posts inside of the foundation. Still, the house is old enough and there is enough deferred maintenance to warrant a pragmatic approach.

    I agree with your sentiments on jacking. I've never trusted the hydraulic jacks and have always preferred screw jacks. What lags are you using to attach your ledger beam? I did some quick load calculations and it looks like this could be a concern lifting in this manner.

  9. HgnJXLgAkS | | #9


    A poured concrete foundation would be easier to deal with, certainly, but is probably not in the cards. Too much hassle hand-digging because of water, sewer, and gas.

  10. Riversong | | #10

    Corn Walker,

    I've typically used ½" lag bolts, two per stud, and 2x12 PT ledgers for the strength of yellow pine. If the bolts are tight, the friction between the wood members (ledger and sheathing) spreads the load. I've never had a problem with this jacking system. Posts and jacks, obviously, have to be spaced for the load (no more than 8' oc) and all posts x-braced to each other.

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