GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Controlling Moisture in Crawlspace and Basement

[email protected] | Posted in General Questions on

Hey everyone

First post here on GBA.   I also posted this similar question in FHB, My wife and I recently bought a 1880 farm house in the highlands region of Maine(climate zone 6) and we are just looking for advice.

We located several rotten floor joists during the inspection and were able to negotiate a decent amount of money off the asking price so we proceeded with the purchase. We have a plan in place to sister all the joists and work on leveling up the floor but my major question relates to the moisture issue.

I should note the home has a dirt “full basement” plus two additional crawl spaces   . The interior perimeter walls all have spray foam, just no floor coverings.  The electric water heater is placed down in the main basement as well as a force hot air furnace.

We recently had a basement systems company give us a quote for their recommendation. This recommendation included six mill plastic encapsulation in the floors of two crawlspaces sealed up to the spray foam, a 3” slab foundation in the main basement portion, trenching the perimeter of the main basement, a 1/3hp sub pump and a dehumidifier. this quote rolled in at around $18,000. They claimed and showed on their software that the slab was actually $1000 less then doing the 6mill barrier in main basement. He also showed me his humidity monitor and observed 92% humidity in the main portion of our basement.  There is no question this is an issue we must deal with.

I will also add we intend on replacing the hot water heater with a heat pump hot water heater to assist in efficiency as well as deal with some dehumidification.

although I believe the companies proposal would be the best solution, $18,000 seems like a lot of money to spend.

Do you folks think I would achieve a similar end goal if the put 6mil or greater down myself, brought down another dehumidifier along with the heat pump water heater? What could I use for tape to seal the plastic to the already existing spray foam? I realize these folks are professional at what they do just trying to weigh out all my options.



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
    Michael Maines | | #1

    You should have a vapor retarder under the slab portion or the soil will be pushing moisture into the basement forever. Basement specialists always seem expensive but they have systems and know what they're doing (usually), so $18K in part buys peace of mind. You would undoubtedly save money by doing it yourself but if you haven't done that kind of work before you'll be surprised how long it takes and how many questions you'll have.

    If you don' t have drains that extend to open air, a sump pit with pump is an important part of encapsulating a basement.

    A portable dehumidifier can work well, if sized properly, but the less you spend on one, typically the shorter its life span.

    Siga Fentrim is my go-to tape for difficult situations, such as sealing poly sheeting to foam.

    6-mil is the thinnest poly allowed by code, and unless reinforced it's not particularly durable. I spec 10-mil, and know others who prefer 15-mil or more. (Everyone in Maine is now required to follow the state building code. Enforcement (code officials) are not required in smaller towns.)

    1. [email protected] | | #2

      Thanks for the reply Mike. They recommended some sort of drain material or bed but I'm not confident that would constitute a vapor retarder. What would you recommend for a vapor retarder?

      1. Expert Member
        Michael Maines | | #3

        They may have suggested a dimple mat or drain field, which can be good if you have a lot of water moving through the basement. I typically spec 10-mil class-A, sometimes thicker if it will be walked on. These are two reputable companies:

        1. [email protected] | | #5

          Yes I believe you are correct, it’s some type of dimple Mat. Its a little disappointing they wouldn’t have recommended any vapor retarder under the slab. I certainly will be clarifying with them once I get the final proposal.

          1. Expert Member
            PETER Engle | | #7

            Many (most) dimple mat products are also vapor barriers. They are molded from a single sheet of plastic, therefore have no holes for water or vapor permeability. The mat does double-duty providing both drainage space and vapor protection. These mats would be nearly useless under the slab, but they have lots of value on walls. They are also useful if you are placing a new slab on top of an old one. Placed in between the slabs, they help to move moisture towards the edges where the drains are (hopefully) left open to the mat.

          2. Expert Member
            Michael Maines | | #8

            Peter they also help with radon mitigation if that's an issue.

          3. [email protected] | | #9

            This is the msds data sheet that the basement systems has provided me. They stated the matting also is a vapor barrier and that the perimeter trench coupled with the sump will be sufficient.

          4. [email protected] | | #10

            Other photos of the proposed matting.

  2. Expert Member
    NICK KEENAN | | #4

    Read this article:

    Although it's mostly concerned with cooling dominant climates a lot of it still applies.

    It's incredibly common in houses of that vintage for cellars and attics to be not really part of the inside of the house, not really outside either,, but sort of in-between. With modern insulation you have to pick a side. It sounds like the recommendation is to make the crawl and basement part of the conditioned part of the house, which means it needs to be air-sealed, vapor sealed, insulated and weather-sealed against the outside.

    The 6-mil sheeting you get at Home Depot or wherever will work as a vapor barrier, it just won't be as durable as a thicker material. I like the HP water heater, especially in tandem with a dehumidifier. Your goal should be to get the basement so well sealed against air and vapor that the dehu only has to run when you open a door and let in outside air.

    1. [email protected] | | #6

      Conditioning the space is exactly what they intend. And use for the marketing verbiage while doing the estimate, not that it’s a bad thing. The article you sent is great. Makes even more sense. Thank you for sending that along.

      It’s seeming like letting the professionals do the job may be best case scenario and let me do the other remodels around the house.

  3. GBA Editor
    Kiley Jacques | | #11

    You might find this article of interest:
    Crawlspaces That Work.

    1. [email protected] | | #12

      Thanks Kiley, that article was extremely helpful.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |