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Conditioning converted under house space/ averages 70% RH

carlhaber | Posted in General Questions on

I’ll give a little info before question. I’m working on a house in Cal. Bay Area. Crawl space was partially dug out creating a 12×30 room into hillside with another 300 sq’ of crawl space 5′ in height down to 2′. Crawl was originally vented via foundation vents. House has sub-drainage system of separate solid and perforated to manage roof and foundation bulk water. Unfortunately crawl rat slabs were poured on grade with no vapor barriers. Room slab was poured 5″ on 6″ of gravel with no vapor barrier. Concrete mix was batched with Xypex. Area averages 70% RH. No mechanicals.Room is planned to function as a gym space. My thought is to install an inline 300cfm fan w/humidistat and perhaps a passive return connected to living space. Insulating space with rigid on foundation walls and perhaps sprayfoam on exterior stud walls. Is the this plan sound? How about the lack of vapor barriers under concrete. I will add, cardboard storage boxes left on both slabs for years haven’t shown any real deterioration.

Many thanks,


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  1. GBA Editor
    Martin Holladay | | #1

    You didn't mention whether there are any problems or questions. Perhaps your (unstated) question is,
    "What is the best way to condition this room?"

    Or maybe your question is, "What is the best way to insulate the walls and floor of this room?"

    Or maybe your question is, "Will this room have any problems with elevated indoor humidity?"

    You didn't really describe the walls of this room, except to mention that there are "exterior stud walls."

    Here are my questions:

    Are all of the walls of this room above grade? Or are some of the walls below grade?

    Are the below-grade walls made of poured concrete, or some other type of wall system?

    What type of foundation is there below the stud walls?

    Finally, please state a question.

  2. charlie_sullivan | | #2

    The Bay area has a lot of micro-climates. In a lot of them, 70% humidity is not surprising and not an indication of moisture coming through the slab. The cardboard box experiment shows that you don't have a severe moisture problem--probably the concrete additive plus the drainage is working out OK despite the lack of a vapor barrier. You could add a vapor barrier on top of the slab under your flooring just to be sure, if you put in some flooring above the slab, but I'm not sure whether you want that.

    Your proposal to condition the space by connecting it to other conditioned space could work if you don't mind it being a little less precisely controlled than the other space, and if the HVAC system for the other space has the capacity. What is the HVAC system for the other space?

    Adding rigid foam (on the interior, polyiso would be a good choice in your climate, on the exterior, EPS) to foundation walls or stud walls could be a good idea. I don't see a reason from what we know to get into the expense, risk, and environmental impact of spray foam.

  3. carlhaber | | #3


    I value your expertise and thanks for your insightful internet offerings. You've given me plenty of building science to try to understand.

    Hopefully answering your question...Zone 3 house sits above grade on a typical concrete T foundation on a down sloping lot. At lot's lower level the rear of house floor joists at were originally 7 feet off grade at their highest point with crawl space narrowing as it climbed the up slope. The excavated room sits within the home's perimeter concrete foundation. The stem walls grow in height as the room backs into the hillside. A concrete wall retains the area in between the two exterior walls and is parallel to the rear of house. So walls consist of exposed concrete below and above grade and the perimeter 2x framing. As stated above the rest of the under house area is the narrowing crawl space. Rat slab on grade. This whole area had fresh air exchange through foundation vents above the mudsill along the sides of the house. Those vents have been closed with rigid and canned foam. I've been running dehumidifier set a 60%. The other details I included above should fill in the rest.

    Being in Oakland (right off the foggy bay) how much attention should I pay to observed humidity readings above 70%? Is my worry of compromising the home's envelope by sealing the space justified? Or should I just do what most do and not care about long term impacts?

    Again, appreciate the time and energy you provide to help educate.

  4. carlhaber | | #4


    Thanks for your reply. Here in Oakland we have micro-climates within micro-climates. Before the 1860's this was quite a place. In an area maybe 2sq miles some of the largest redwoods on earth once existed. Perfect mix of fog and sun I guess.

    You asked about HVAC. Slightly oversized 100k BTU modulating forced air furnace. There is a cold air return a couple feet from where I could passively connect the spaces. The furnace has no direct easy route to space. A warm air 3x12 sq. duct cut down through a wall plate may be possible. Mechanical room sits on slab.

    Insulating the walls rather than floor joists makes sense to me. Using a DIY spray foam kit makes sense from an air sealing standpoint but I get the environmental impact. You would recommend just hitting the right areas with my foam gun and rigid between the studs? Cost wise, would Roxul batts kept proud of mudsill with foam be appropriate?

    Gym rubber mat is planned on top of slab. Would a good 10 mil low perm barrier taped to the stem walls with rigid on top be overkill? There's still the rat slab and it's potential vapor issues. The rat slab area is used as storage and will continue. Topping the slabs with barrier and 2 inches of concrete isn't a reality. Something like Moistop ultra 15 on ratslab is possible but being on up grade it'd be slippery moving on it and still not great for long term traffic.

    You say 70% RH isn't surprising. Yeah we average about 65% How should I look at humidity and when should I be concerned?


  5. GBA Editor
    Martin Holladay | | #5

    Q. "How much attention should I pay to observed humidity readings above 70%?"

    A. The answer depends on the temperature of the room when the 70% RH was measured. For example, if this room (which is now a crawl space) was at 45°F when the RH was measured, you have to ask yourself, "What would the RH be if I warmed that air up to 70°F?" The answer is, the RH of that air would only by about 28% at 70°F -- which clearly is not so concerning.

    If these calculations are new to you, you can learn more by reading this article: How to Use the Psychrometric Chart.

    I agree with Charlie that "the cardboard box experiment shows that you don't have a severe moisture problem."

  6. carlhaber | | #6


    Thanks for the article, I'll spend some time with it.

    Question. Considering costs and limited use; what conditioning methods seem most appropriate?

  7. GBA Editor
    Martin Holladay | | #7

    I don't know what type of conditioning system is needed. Possibilities include a forced-air supply and a forced-air return; an electric-resistance space heater; or a window-mounted air conditioner. It depends in part on whether the homeowners expect space heat, air conditioning, or both.

    It also depends on their budget.

  8. carlhaber | | #8


    With hope this may be my final question to you. Under what conditions would you favor the use of a humidstat controlled vent fan directed outside? Or would you dissuade the use as a conditioning device altogether.

  9. GBA Editor
    Martin Holladay | | #9

    If a basement room has high humidity levels, installing an exhaust fan is not recommended.

    I would suggest a long series of remedies, most of which have to do with reducing the rate of water entry. These remedies are listed in this article: Fixing a Wet Basement.

    If all of these remedies fail, or if the homeowner can't afford these remedies, I would suggest the installation of a stand-alone dehumidifier.

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