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Please help me decide which option to go with for my encapsulated crawlspace retrofit.

hifiaudio2 | Posted in Energy Efficiency and Durability on

I am in Nashville, TN – zone 4. I just bought a two story house that was built in 1997 and is on a crawlspace. The current crawl is in “good to great” shape according to the various contractors and energy professionals that I have had down there giving me bids. This is even though I have already fixed numerous issues with overflowing gutters or gutters with downspout connections at the ground level that did not correctly meet up with the black pipe to take it away from the foundation. Despite these issues, and despite that the house is 20 years old with a traditional vented crawl with batt insulation in the floor joists, there is very little to almost no hanging insulation or the “normal” look of a vented crawl. No standing moisture and no mold, there is some condensation on the ductwork. BUT… the hardwood floors above did show significant cupping, which we have since fixed with sanding and refinishing them. (The previous owners were just a man and woman and apparently they kept the house REALLY warm… around 80 from the looks of the programmed modes on the thermostat and the resulting cooling and heating bills) We dont want this cupping to come back, and want to do what is best for the house in the long term. The main level of the home is around 3,600 sq feet. I have crawlspace encapsulation bids that range from about $5,400 for a sealed and taped vapor barrier only that includes a “Mega dry cs70” dehumidifier to bids for encapsulating and then foaming the walls (closed cell below rim joist / open cell in rim joist area) and a 70cfm or so exhaust fan along with a transfer grill somewhere in the house. And variations in between. Some want a dehumidifier, some want to cut supply and return, and some want to vent. I have read and read and read and cannot decide which is the best option.

I am also getting bids to put open cell in my rafters of the whole house as part of this project. Some want to do 5″, claiming any additional is a tiny improvement for the money, all the way to a contractor who will simply “fill the rafters”.

And one last wrinkle… I am having low voltage wiring installed, and along with that a chase of a couple of small pipes running from the crawl to the attic. I was wondering if NOT foaming them in and allowing air to be pulled out from the new crawlspace from the foamed attic would help with any off gassing that could occur in the newly foamed attic space. Is that any kind of a good idea?

So… in a nutshell, I like the dehumidifier idea since it seems energy efficient, even when the dehum runs, since I am not “Wasting” my conditioned air by sending it to the outside from the small fan, but I like the small fan exhaust idea since I feel that would get rid of off gassing from the attic or new crawlspace foam, as well as radon, etc. (radon test in this home upon moving in was a very low 1.1).

I know that is a lot, but I wanted to get all the variables and decision points into the question. Thanks so much for any help and advice!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    John,
    In most cases, a properly sealed crawlspace should not need a dehumidifier. For more information on this issue, see Building an Unvented Crawl Space.

    If your climate zone, building codes require a minimum of R-49 of roof insulation. If you want to use open-cell spray foam for this, you'll need about 13.5 inches of spray foam to meet minimum code requirements. (There are cheaper ways to do this, by the way, but I don't recommend skimping on R-value to save money.)

    For more information on this issue, see

    It’s OK to Skimp On Insulation, Icynene Says

    How to Build an Insulated Cathedral Ceiling

    Open-Cell Spray Foam and Damp Roof Sheathing

  2. hifiaudio2 | | #2

    Thanks I will read over those articles again. Since I only have 2x8 rafters, I would not be able to go deeper than ~7.5 inches and be able to ever drywall the interior of the roof to make a room in the future, which is something I would want to be able to do. I guess 7.5" of closed cell would "almost" get me there, but that would be very costly and seemingly not recommended because of the "cant detect a leak" issue...

  3. GBA Editor
    Martin Holladay | | #3

    John,
    As my article (How to Build an Insulated Cathedral Ceiling) explains, it's possible to increase the depth of your cavities if necessary to provide more room for needed insulation. Among the methods used: On the interior side of the rafters, you can install 2x4s (16 inches o.c.) at 90 degrees to the rafters to deepen the cavities. Or you can use plywood gussets to support new 2x4 "rafters" beneath (and in the same plane as) the existing rafters. Or you can install a continuous layer of rigid foam on the interior side of your rafters, followed by 1x4 strapping, 16 inches on center.

  4. hifiaudio2 | | #4

    OK thanks! I will review that... and I am shooting for R49 in my zone? I was thinking r38 was code here....

  5. dsmcn | | #5

    John, I am in Knoxville and the house here had a very wet crawlspace. First I dug a curtain wall: a 4' deep trench (well below the grade of the crawlspace interior), lined with geotectile fabric, perforated solid pipe on the bottom with slight pitch, filled with crushed rock. Put this all across the back, at the foot of the downslope that drained toward the house. Ran solid pipe from the low end to daylight. Connected downspouts to the solid pipe. This significantly reduced moisture below the house.

    I sealed the crawlspace with reinforced plastic sheeting, taped and caulked for an airtight application. I did not remove the R-13 in the floor joists, and I did not insulate the perimeter walls.

    Upon considering the options, I did not want the moldy basement air communicating with our conditioned space, so I did not punch a passive vent to the house and I did not put a vent from the HVAC into the crawlspace. I considered adding a low-cfm fan to the exterior, but decided to simply dehumidify it as a first step, maybe to add the fan later.

    The third option is to dehumidify only. Instead of getting one of those $1600 dehumidifiers that the crawlspace specialists recommend, I went with a small $150 unit with an energy star rating. Ran the condensate line to the sump.

    Result: the house still smelled moldy—but significantly less so—during the first year. Ever since the house has smelled clean and fresh—and my wife has a real nose for mold. The dehumidifier does not turn on much anymore either. By the way, the HVAC ducts are all in that crawlspace, so you can imagine why the house would smell moldy!

    Finally, I built a fully gasketed access door, and I added one of those toggle clamps to suck the door against the weatherstripping to make it airtight. Works like a charm.

    Personal gripe: when I go on the Parade of Homes tour around here, even the high-end homes have ventilated crawlspaces, with block walls that are below grade and not even waterproofed. When I peek, saturated soil and damp stem walls are common. And again, ductwork in unconditioned space that is destined to become moldy. I wish continuing education was required to keep up a contractor's license.

  6. GBA Editor
    Martin Holladay | | #6

    John,
    The R-49 minimum comes from the 2012 International Residential Code (IRC). If your jurisdiction uses a different code -- and that's entirely possible -- the requirements might be different. If you are uncertain about local code requirements, call up your local building department.

  7. GBA Editor
    Martin Holladay | | #7

    David,
    You wrote, "Personal gripe: when I go on the Parade of Homes tour around here, even the high-end homes have ventilated crawlspaces, with block walls that are below grade and not even waterproofed. When I peek, saturated soil and damp stem walls are common. And again, ductwork in unconditioned space that is destined to become moldy."

    I couldn't agree more. I continue to be astonished that high-end homes, including homes costing between $500,000 and $1,000,000, have the same lousy construction details as $180,000 starter homes. This happens because (a) code requirements aren't enforced, and (b) buyers are unaware of how poorly built these homes are.

  8. hifiaudio2 | | #8

    Thanks for all your help here Martin... if I may, two more questions:

    1. Does my thought of the two wire chase pipes that are now installed in my home from crawlspace to attic being left open allow attic air (and thus any supposed off gassing from the upcoming install of open cell foam) to mix with the leaked house air to be deposited outside the home from the 70CFM fan that would be installed in the sealed and insulated crawlspace?
    2. Before I foam, should I pay a roofing company to remove all of the vents from the roof and "re-shingle" the areas, or just leave them and foam under and around the current vents? My assumption is that the vent areas are just asking for a leak down the road, but my wife thinks we should leave well enough alone and just foam under them.

    Thanks again!

  9. GBA Editor
    Martin Holladay | | #9

    John,
    I'm not a fan of your plan to deliberately leave wiring chases that connect your crawl space with your attic unsealed. Sealing these wiring chases (at the top as well as the bottom) is the best practice, in my opinion, especially since future changes to your house are unpredictable.

    If you plan to install spray foam on the underside of your roof sheathing, thereby creating an unvented conditioned attic, then the air in your attic cannot be considered "leaked house air." It will just be "house air," because your attic will now be indoors.

  10. hifiaudio2 | | #10

    Here is my cheapest option... is installing JUST the vapor barrier and not any foam in the crawl but keeping the existing batt insulation a viable option?

    Here is what the contractor proposed:
    • Remove the existing vapor barrier and debris from crawl space.
    • Install a 12-mil (up to 20 mil available) plastic vapor barrier to cover the entire crawl space floor. (approximately 3,600 SF)
    • Seal off the foundation vents.
    • Extend the vapor barrier up the perimeter foundation walls within 1” of the top of the top block.
    • Wrap the 25 support piers a minimum of 16” above the crawl space floor.
    • Seal all joints in the vapor barrier.
    • Install a dehumidifier and electrical outlet near the center of the crawl space.
    • Install ¾” diameter PVC discharge line to discharge in the positive drain at the rear right corner of the crawl space.
    • Dispose of debris off site.

  11. GBA Editor
    Martin Holladay | | #11

    John,
    I stand by the advice provided in my article, Building an Unvented Crawl Space.

    I don't think that a dehumidifier is needed in most sealed crawl spaces (although sometimes a dehumidifier makes sense on a temporary basis for the first few months in a new construction job).

    I am a strong believer in insulating crawl space walls.

    The other measures mentioned in the article include efforts to reduce water entry into a crawl space (good exterior grading, roof gutters, etc.).

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