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Crawlspace Encapsulation Concerns

cchaser | Posted in Energy Efficiency and Durability on

I have 2 crawlspaces -40 (x20 and 56X31) and recently removed failing batt insulation from the joists.  I have installed dehumidifiers in each and have the RH down to 50% however the Dehumidifiers have run continuously. I have access to TX-1200 for the vapor barrier and RMAX TSX1500 for insulating the exterior walls of the Crawlspace. 

All my reading points me to insulating the walls and redoing the vapor barrier on the floors.  My plan is to run the vapor barrier 1-5 inches up on the insulated walls and all the way up on the interior/in insulated walls. and then insulate the rim joists with rigid board or spray foam. When reading on this site, it gives me pause putting a vapor barrier on the wall. My questions below.

Will installing a vapor barrier on the wall cause mold?

When insulating and using a vapor barrier on the inside of the crawlspace, are there concerns with the block and band joists not being able to allow the element (Air/moisture) through?

Is a soil mitigation system needed to prevent smells in the home?

Are the materials I listed good for the job?


C. Chase

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  1. GregNL | | #1

    A crawlspace is definitely its own ecosystem as there are a lot of factors at play here, changing any of them can affect the final outcome depending on the location/Zone you're living in and you put forward a lot of great yet technical questions. I'm in the process of encapsulating a mobile home crawlspace so I'll do the best I can from what I've read and learned during the process.

    First and foremost what type of dehumidifiers are you using? Are they a task specific like the Aprilaire 1820 crawlspace dehumidifier?
    Is the space currently vented or sealed? Are you dealing with a mix of cement footings and wood framed walls or do the rim and floor joists sit directly on the cement walls?

    In addressing the TX-1200, I wouldn't use it. Perhaps it's different/better than other fiber reinforced bonded film barriers though many delaminate and the binding agent offgass smelling like urea/cat urine. I'd recommend going with a 10-12mil crawlspace specific non-reinforced single layer vapor barrier.

    If the vapor barrier is installed correctly you shouldn't have issues with soil odors. If you do have concerns with that you could always connect the space to HVAC system to provide fresh and possibly conditioned/dehumidified air and remove the stale though you have to be mindful of air temperature to avoid the dew point in the space if the air entering is cold conditioned.

    Prior to insulating your walls there are several things you may need to address. Does the space currently have a weeping tile system and sump pump installed? Is there weeping tile, a WRB and drainage board on the exterior foundation wall? Do you plan to excavate below grade to insulate as well? I couldn't find anything on RMAX TSX-1500, I assume it's polyiso similar to TSX-8510?

    If insulating direct over cement walls with some form of foam board the risk of mold is low if you have reduced moisture issues either from below, inside and/or outside. Depending on the product you select it may act as a vapor barrier as well so there's no point to installing/layering both.

    As for blocking the rim joist, unless you have pre-existing moisture/water ingress issues at the sil or rim/band joist level either due to failure in your cladding or rainscreen then providing a sealed thermal break in this area shouldn't cause any issues, assuming you're using a minimum of R20, as you've blocked moisture and thermal air flow to the inside of the wood. I blocked mine using two layers of 2" R10(R20 total) DOW Cladmate sealed with only DAP Alex Plus Acrylic Caulking if you make your cuts precise enough.

    If I'm allowed an outside plug check out CrawlSpaceNinja as most of what you're wondering about is discussed and addressed there with some slight variances.

    Good luck on your project!

  2. cchaser | | #2


    Thanks for the response. Great info.
    To the questions:
    I have Aprilaire 1820 in each crawlspace

    Crawlspace is cinder block with pressure treated sill plate

    Unvented - sealed the vents before putting the dehu’s in

    I’ve read about the delaminating and seems feedback is mixed - some blame the soil, some blame the liner

    The insulation is 2in rigid board with a vapor barrier

    As for the weeping tile system, I’m uncertain. I do not have a trench and sump in the crawl and believe the house had waterproofing and a French drain around the whole house however the home is 30 yrs old.

    I’ll look into other vapor barriers and have lived in Crawlspace Ninja and Crawlspace Depots site for the past few months.

    Additional thoughts welcomed. BTW, I’m in Maryland.

    C. Chase

  3. GregNL | | #3

    Sounds like you're off to a good start based on what you've done and are using so far.

    You'd definitely want to seal the sil plate from air drafts, even if it has the sil foam gasket. If there is no gasket accoustical sealant easily works and it's cheap. If you do have a gasket foam tape, products like Siga Wigluv, 3M All Weather Flashing, 475 Pro Clima Contega Solido SL and others will work to bridge the gaps and bond well to both surfaces. After that I'd work on sealing and insulating the rim joist as it's less time you're working and walking on the new vapor barrier.

    Moisture in the home will also move to the dryer cooler spaces (crawspace) so you need to start looking at how well the entire house is sealed from the outside and crawlspace plus the relative humidity of the living space. There could be many reasons the crawlspace dehumidifiers are not cutting out. Cinderblocks have a perm rating between 5-10 if hollow(not filled) meaning that's a large area for moisture to continually move through. Any roller based bitumen foundation sealant, if applied and typically only below grade, has outlived it's useful lifespan after 30 years in many cases. Work on sealing and insulating the interior. After that consider an above grade membrane and thermal break insulation of R8 or higher due to dew point as you're in the same climate zone(6a/6b) as myself. If at some point in the future you're eager to dig up the entire foundation and seal it to today's standards with membranes, drainage boards and full insulation you certainly can.

    In considering a crawlspace weeping system with sump, what is the current soil condition? Does it change when you dig down a foot? Do you have any issues of water logged soil primarily on the north side of the house or where a slope may exist? Is there ever a risk of flooding? Are there any cracks in the foundation? If you don't think a sump and weeping is needed certainly spend the time and money elsewhere.

    Contrary to most opinions on crawlspaces, mobile homes being slightly different as they typically don't have insulated skirting walls or conditioned crawlspaces, I super insulated the floor with R22 Rockwool ComfortBatts capped the bottom with 2" of DOW Cladmate for R32. Essentially I have the bottom sealed from the crawlspace. The majority of plumbing is accessable between the center beams where the panels are removable. The reason in doing this is to isolate or create thermal bridge zones between spaces, a concept I've read on here or elsewhere and it made sense. The crawlspace temperature difference between the ground and outside is different from that and the floor zone where I have 100w "rough service" bulbs installed to maintain floor warmth during the colder seasons as the oil furnace has been replaced with a Fujitsu Mini-Split and of course the living space is warmer yet again. I'm not saying to reinsulate your floor but it could potentially be a viable option if cold floors are a concern, especially with the stack effect of temperature and air pressure differences inside a home.

  4. cchaser | | #4

    Thanks again - great information. I will focus on the rim joist next and report back.

    C. Chase

  5. Eric__S | | #5

    C. Chase - I'm in northern virginia and used the following for my 30x30 crawlspace:

    Rim joist bays sealed with silicone then insulated with 3" polyiso and spray foam around edges.

    Floor covered in dimple mat, taped at seams and sealed at penetrations. Then over top of that, 20 mil polyethylene sheeting, sealed w/ 2 sided tape and mastic.

    Walls have 12mil polyethylene sheeting running the full height of the wall and sealed at the top and to the floor sheeting with mastic. Walls then have 3" polyiso boards attached which are taped (why I did that, I don't really know).

    HVAC supplies (2x) on the far end, and a return with a filter grille on the near end. This return runs into the main return inside the basement.

    Dehumidifier (yet to be purchased) will sit about in the center.

    For radon mitigation planning purposes, I installed a 5 gal bucket upside down in a corner, with 2x3 framework around it, if needed once tests are run later on. The five gallon bucket sits in a penetration through the floor (both the sheeting and dimple mat, which is sealed with tape and mastic. The bucket is located in a corner closest to where I would vent out through the rim joist and up to the roof line (along with my basement radon vent line/fan). Under the dimple mat is 3-5" gravel and an old 6 mil sheet (was there when I started, thankfully). The gravel and dimplemat permits enough airflow to support a radon mitigation system, if we need one.

    No exterior insulation and I'm not sure if there is much/any damp-proofing on the outside.

    Hope this helps.


    1. cchaser | | #7

      Hi Eric,

      Any chance you can post pictures of your work?

      C. Chase

  6. cchaser | | #6


    Sound like an amazing job. How long did it take you? The dimple mat sounds like a great idea. Have you noticed a difference in air quality and moisture?

    1. Eric__S | | #8

      Yes, dimple mat was well worth it, in my opinion. Saved my knees big time, but also acted as a great primary air barrier when sealed w/ tape and backed up by the floor sheeting sealed w/ tape and mastic.

      Hard to tell total time since I took it really slow. I wanted to get all the details right, so I have technically been working on it since like February and it still isn't completely done done, but it is close.

      I'll post some pics.

    2. Eric__S | | #9

      Pictures attached

    3. Eric__S | | #10

      One more picture

  7. cchaser | | #11

    WOW!!!!! Very professional install. What is the device is the the first pic? Is it pulling air out? Pic 2 is that rockwool on top of the vapor barrier - is that an interior wall?

    Thanks for the pics. BTW, do you plan on covering the dimple board with the vapor barrier?

    C. Chase

  8. Eric__S | | #12

    Thanks C. Chase. The device in the first pic is the HVAC air return. I installed a filter grille on it so that as the supply air washes across the crawlspace, it has to get a "prefilter" before it gets back into my main return and mingles with the rest of the house air. This is my attempt at preventing any mold that is existing in the crawlspace (there is some on the joists from before I sealed it all up), from getting into the living space.

    On the wall, our county code specifies that you have to leave a 2 inch termite inspection gap above the rigid foam insulation, so my polyiso boards only come up to within 2 inches of the top of the wall, and then I stuffed some mineral wool in there which can be removed if needed for termite inspection.

    I do plan to cover the dimple mat with 20 mil polyethylene sheeting, taped and sealed with mastic at seams and penetrations. I have the poly sheeting, I just haven't rolled it out yet.

    The blue 5 gal bucket is the one I was talking about as being for the radon mitigation system in the future if I need it. not pictured is the 2"x3" framework that goes around the 5 gal bucket which will get wrapped with the 20 mil floor sheeting.

    I owe Dana D., Bill (Zephyr), and Martin H. greatly for their sage recommendations and for the original Unvented Crawlspaces article (by Martin). My kids are going to learn from me being a DIYer, that it is important to build better than people built in the past. Thanks for teaching me that Martin, Dana, Bill.


  9. Expert Member
    BILL WICHERS | | #13

    Nice install! Looks like it is a pretty low crawlspace, so that must have been... fun... Glad it wasn't me crawling around in there :-) My crawlspace is about 4' clear so it's easy (relatively) to move around.

    I like your bucket idea for a potential future radon system, that's clever. If you have to drill that bucket, be careful: I've had them split on me in the past. A hole saw at slow speed and slow feed is probably the safest way to get a clean hole in one of those.

    I'm guessing you put in dimple mad as part of that potential future radon system, but I'm curious why you want to put an additional layer of poly over it? The dimple mat itself should be enough of a vapor barrier, but are you thinking the poly will help to keep things clean? I could see a dimple mat being hard to keep clean since dirt would get into all the dimples and you wouldn't be able to just sweep/wipe it clean.

    I see a few cables draped over a drain line. Next time you're down there, I'd get those properly secured with some cable staples. Technically, you're not supposed to use pluming to support wires. In the commercial world, I've seen buildings fail final inspection because a wire (often a low voltage network or security cable), was hanging down enough to touch a fire sprinkler pipe. The inspector would city "cannot use pipe as cable support" and red tag it. Yep, a fussy inspector, but technically also correct.

    We're all here to learn about building things better. Our payoff is in increased energy savings and more comfortable structures. It's always worth doing a quality job too.


    1. Eric__S | | #15

      Thanks Bill,

      I put the dimple mat down first honestly to try to protect the poly sheeting from the gravel below. I'm a belt & suspenders type. I didn't trust that even 20 mil would be able to avoid getting punctured by the gravel as I crawled over it 5 million times. Turns out the dimple mat itself took puncture damage here and there from particularly sharp rocks! I taped over a few holes in the dimple mat. Once the poly sheeting is down, it will really be air tight and it will have a clean look and nice feel under the knees.

      As you said Bill, dimple mat also helps with the future radon system if needed.

      I'll take care of those wires =) Thanks.

  10. Andrew_C | | #14

    Eric - that looks beautiful. Like the idea of dimple mat under your primary vapor barrier. Good job.

    1. Eric__S | | #16

      Thanks Andrew!

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