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Sealed Crawlspace

J H | Posted in Green Building Techniques on

Location: Asheville NC
Crawlspace: Dirt Floor, Block Walls (semi-buried as I am on a hillside), Wood Floor Joists above. Exterior walls above block walls are Masonite.
Size: 30×30 with about 10×30 usable with 7 foot ceilings – the other 20×30 is about 4 ft high.

Environment: Water heater and central air pump are located down here. I think the builder left large gaps where exterior siding meets the foundation walls which allowed way more ventilation than I ever wanted (or perhaps this is just poor carpentry). I installed a dehumidifier when I moved in 5 years ago but it has been a constant battle to keep it below 50%. Recently, I have already sealed the vents, electrical plugs to exterior walls, and holes for plumbing, electric, cable, phones into the interior walls overhead. It is mid Feb and temps are rather warm in the 50s but the temp in basement is about 60. Much improved over the low 40s temps I was getting. I have 40% or lower humidity readings now. The goal is to get the humidity readings down to 30%. I intend to use this as a workshop and pantry area so having it semi-conditioned is the goal.

Radon: Tested for Radon 3 years ago and will do so again after this plan below is finished.

Please provide critique/feedback on my plan. I want to make sure I am not making mistakes or missing other opportunities. I have been researching here and other places like and came up with this approach. I also have a few questions below.

Floor: Cover dirt floor with 6 mil black poly. Seal the seams with 3M duct tape. I will stake the poly to the floor with 6 inch landscape stakes and cover those places with duct tape. I will cover part of the floor that is usable with a 10’x30′ treated wood floor built on concrete stone supports. This will have black poly underneath as well.

Walls: 6 mil Black poly with 3 inches expose at top of the wall for termite inspection. Will paint that 3 inches with white sealant paint for easy termite inspection and to provide vapor retarder. All seams on the black poly sealed with 3M duct tape.

Foundation Wall to Floor Joists area: Where the floor frame sits on the wall there is of course a small < 24″ area where the exterior side is mounted to particle board. This is either un-insulated in some places or at the least under-insulated. I plan to cut and piece in foam board there and put some additional backed insulation pieces. This should cut down on the cold floors the house seems to have. Ideas here are particularly appreciated. Questions: Opinions on the above plan? What should I use to glue the black poly to the walls? Double sided Tape/DAP adhesive/other product?? Should I attach foam board to the black poly walls to add additional insulation to the space to add to the conditioning of that space? Thanks JW

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  1. Danny Kelly | | #1

    I would consider using a 12mil poly - although the 6mil and 12mil will perform the same as far as being a vapor barrier - the 6mil poly is so flimsy it is very difficult to tape the seams properly. A 12 mil (or greater) poly will lay much flatter and will enable you to do a better job. We typically use mastic to attached the poly to the walls. Installing thermax or some other rigid foam that meets the proper flame spread requirements is a good idea and will help condition that space - I would recommend 2".

    How are you currently providing ventilation to that space - if the crawl vents are sealed - you must either have an HVAC supply in the crawl or an exhaust fan vented to the exterior.

  2. GBA Editor
    Martin Holladay | | #2

    I don't think that 3M duct tape is the right tape for polyethylene. Here's more information on taping poly:

    Air-Sealing Tapes and Gaskets.

    Here's a good resource with information on creating a sealed crawl space:

  3. J H | | #3

    Danny, Thanks for the feedback. I will look into the rigid foam recommendation. About your question of current ventilation - I am not using any ventilation. I have no combustibles down there and it passed a Radon test already. I will test it again for Radon after the sealing work is done but I dont expect it to be any different. Venting or no venting seems to be a big topic of discussion. I am going with the advice from and not doing any venting. I do work down there almost every day and I leave the door open to allow some fresh air in unless it is too cold or too humid.

    Martin, Thanks for the input. I guess I didnt study that site as well as I should have. I will take the advice in the article you wrote. Thanks again!!

  4. Danny Kelly | | #4

    JH - we may be talking about different "venting". I am referring to mechanical ventilation. If you seal up your crawl vents (not venting), you are creating what the code refers to as a "Closed Crawlspace". Per NC Building Code - A closed crawlspace must have mechanical ventilation - your options are supply only (HVAC supply duct), exhaust only (a continuous running exhaust fan (my preference)), or a balanced system (ERV or supply and return in the crawl). Energy typically installs a supply duct from the HVAC system in their closed crawlspaces from what I remember from their research projects. You cannot just have a "dead space" in your crawl completely sealed from your house as well as the exterior.

  5. J H | | #5

    I think you are correct. Not being even remotely experienced with HVAC, I didn’t realize I needed to have any ventilation at all. So I poked around and I found this page that explains the details about supply vents.

    But I am having a hard time finding a “how to” on exactly how to install an exhaust fan or a supply like. Do you have any advice on where I can get that kind of detail? Thanks again for the help.

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