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Community and Q&A

DIY Crawlspace Encapsulation

mcp1234 | Posted in General Questions on

Hello all,

I have a few questions regarding a homeowner crawlspace encapsulation, there are quite a few things at play here but I’ll try to keep it brief while giving as much info as possible. Last year I bought a 1970s ranch in Ontario, Canada, 6A. I am a carpenter by trade and knew when buying the house that the crawlspace was not sealed and that someday it would have to be done, I figured I’d tackle it this summer (hopefully by myself, but I’m certainly not opposed to hiring professionals if necessary). The top of the footings are pretty much at grade, not sure how deep they are, bedrock isnt far, looks like there has been some movement but as far as I can tell no bulk water coming in from the CMU walls or footings. Walls are about 36″ high, ~800sqft of floor space, no signs of any standing water, slight mildew on some joists but nothing alarming. The house is heated with baseboard and wood, no a/c unit. The previous home owner wrapped the entire house with 3″ of XPS right down to grade, so it is ‘insulated’ down there, also there are additions on two sides of the house, meaning that only two walls of the crawlspace are actually ‘exterior walls’. The vents were sealed by the previous home owner, the craftpaper fiberglass was removed from the rim joist bays. There is no sump pump and probably no weeping tile on the outside of the footing, but I doubt that is a problem seeing as it is ‘dry’ down there. My plan was to clean out the debris down there, staple 12 mil poly to the sill beam, covering the dirt and taping seams, add a standard 2500 cuft dehumidifier and run the hose to the kitchen sink drain which drains into a small gravel pit in the corner of the crawlspace. Having been down there for two days cleaning an unforeseen and truly absurd amount of garbage (anyone need an old exhaust manifold?), and being confronted with a massive decommissioned oil furnace, I started having second thoughts. Here are the concerns:

1) In order to get rid of every small bit of debris I would literally have to dig out the dirt over the entire surface of the crawl. There are probably thousands of little pieces of wood/plywood/osb/fiberglass amongst and buried in the dirt. If the VB operates as it should, water will condense on the dirt side of the poly. Will that promote mold and rotting underneath the VB? Concerns that it will affect the well water quality (we are on a shallowish well)? Should I care?

2) Will the upfacing side of the VB become a condensing surface in the winter, as the warm air from the house meets the cold dirt?

3) Will the underside of the sill beam rot if I staple the VB to it? To me it is ideal as then I don’t have to hammer drill and fasten to the CMU.

4) What the heck do I do with the old oil furnace and how to I detail around it if I decide to leave it? Is there a concern with leaving it? The crawlspace opening is only 2ft x 2ft, so it would need to be removed in pieces. The oil tank is gone, it looks to have been kept outside, however there is a valve and copper pipe with an oil filter that runs from the CMU wall, so far the copper pipe looks to be in tact but will rot out some day.

5) Any concerns with connecting a dehumidifier to the kitchen drain, which just runs into the gravel pit in the crawlspace?

Thank you for reading and thanks in advance for any advice. I am a long time reader and first time poster.


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  1. walta100 | | #1

    First let’s decide on a plan, are you committed to conditioning the air in the crawlspace to more or less the same temp and humidity as the rest of your home? If so, then start calling it a conditioned crawlspace. If not and you want to keep using the language if the huckster salesmen go ahead and call it an “encapsulation crawlspace”

    Seems to me the crawlspace has found a balance point for moisture as it is. If you chouse to change things it is possible you could mess the balance no matter how well intended your changes are their maybe unintended reactions.

    "Any concerns with connecting a dehumidifier to the kitchen drain, which just runs into the gravel pit in the crawlspace? "

    Did you really say the kitchen sink is draining into the crawlspace? Seems unlikely to be code compliant. Are you connected to a public sewer system?


    1. mcp1234 | | #2

      Hi Walta,

      Thanks for the reply. My plan was to deal with some musty smells in the house when we are away for the day, particularly in the winter. I was told the likely source of the issue is the crawl, so I figured i'd start there. Also, I want to install some hardwood flooring and figured it would be a bad idea unless the crawlspace had been completely sealed, maybe an unwarranted concern. I could perhaps start conditioning the air down there...the only thing is there is no functioning HVAC system in the house, so the only way to do that would be to run a chase or vent from the crawl to the house which I am hesitant to do.

      I agree with your point about equilibrium...being down there these past few days has made me question the validity of my solution long-term. Hence the question to you guys.

      Yes, really, the kitchen sink drains into the ground in the crawlspace. Into a gravel pit to be precise. It doesn't seem to create any standing water, the thing drains okay. The previous homeowner was a DIY die-hard, many strange decisions...seems to have done it to keep the greywater out of the septic? Who knows. Thanks again!

  2. walta100 | | #3

    Draining gray water to a drywell has always been commonly done to reduce the size of a septic systems. What seems strange to me is locating the drywell under the house.

    The first step in drying out the crawlspace is to remove every source liquid water entering the space including this drywell.

    Since you said septic and not sewer, I would first relocate the drywell at least 15 feet away from the house.


    1. mcp1234 | | #4

      Thank you, I'll look into a suitable relocation spot. Otherwise, do you think a dehumidifier down there without a VB will do anything?

  3. walta100 | | #5

    Dehumidifier should not be a problem so long as the water is somehow drained outside of the crawlspace and you think of it as a heater as almost 100% of the large amount of energy it will use is converted into heat that will remain in the crawlspace and the crawlspace could get quite warm.


    1. mcp1234 | | #6

      Hi Walta, thanks for this. So the plan thus far is just to leave as is, except for installing a dehumidifier and draining both it and the kitchen drain outside

  4. walta100 | | #7

    I like the idea of getting rid of the moisture sources.

    I see a dehumidifier as a Band-Aid but not a real fix.

    The dehumidifier is going to be expensive to run depending on your cost per kWh.

    Most dehumidifiers made today tend to fail just after the warranty expires so you will be changing it every few years.

    Are you committed to conditioning the air in the crawlspace to more or less the same temp and humidity as the rest of your home?

    This is the first step to fixing the problem. The next step is to stop putting liquid water in the crawlspace. If it is still to damp you will need to prevent the water vaper from the ground from entering the crawlspace with a vapor barrier.

    Should you get tired if trying to condition the air in the crawlspace separately with the dehumidifier you can connect the crawlspace to the rest of the house so you HVAC system will condition the air in the crawlspace.


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