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What is the best way to manage moisture in a cinder block building in a hot, humid climate?

shnnn | Posted in Green Building Techniques on

Having rebuilt my tiny bargeboard shotgun house in New Orleans, I now wish to turn my attention to the garage. Unlike the house itself, which has a pier foundation and a wood frame, the garage is a cinder block building on a concrete slab.

My overall goal is to make the garage a functional workshop and to add a second floor or at least a mezzanine level to provide a place to store guests. In terms of “raising the rood,” my notion is to build a wooden balloon frame inside the existing cinder block building, rather than trying to reinforce the cinder block to make it able to support 2x the existing wall height. The existing leaky asbestos shingle roof will be replaced with a metal roof.

As y’all know, down here we struggle with things like water, mold, and termites. So as I think about building a wood frame inside my garage–and perhaps cladding that frame with plywood and/or pegboard, I want to be very sure that I am going about it in a way that will decrease my chances of mold and termite damage.

My initial thoughts would be to paint on a waterproof sealant on both the interior and exterior sides of the cinder block shell, then do my internal wood frame like a rainscreen, making sure there’s an airspace with openings at top and bottom for moist air to escape.

Am I way off base yet?

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  1. GBA Editor
    Martin Holladay | | #1

    There are several issues here.

    One is structural. The best way to get good structural advice is to consult an engineer.

    It's quite possible that your existing CMU walls are perfectly capable of supporting a second story.

    If you build a load-bearing 2x4 or 2x6 wall on top of the existing garage slab, the first step is to evaluate the thickness of the slab to see whether it can bear the weight of a load-bearing wood-frame wall.

    The next issue concern termites. If you are worried about termites, your existing CMU wall is a winner compared to your plan to install new wood framing.

    A third issue is the dampness of your slab. It would be good to know whether there is a polyethylene vapor barrier under your concrete slab before deciding how to proceed.

  2. shnnn | | #2

    Understood, and let me clarify that I'm asking in preparation of consulting an engineer, not in lieu of.
    It's been my experience as a woman and a non-professional in the building trades that if I can arm myself with a little knowledge before approaching an engineer, I am less likely to be patronized and more likely to be listened to.
    Of course, it's also true that a little knowledge is a dangerous thing, and I definitely have only a little knowledge ;)

  3. LucyF | | #3


    I know exactly what you are talking about in terms of talking to people (almost always men) in the building trades. I live in the southeast as well where I think this is a much bigger problem than in other parts of the country. One issue is that sometimes you understand building science principles so much better than they do and they think you're crazy because, of course, a house has to breathe.

    I think if you waterproof the shell, you shouldn't need that airspace. Or fill the airspace with mineral wool board insulation. Termites won't like that. I doubt they can get through mineral board insulation easily at any rate, but I don't know that for a fact. Also if your slab is airtight at the cement block intersection, there shouldn't be a way for termites to get in, right? Caulk like crazy before and after framing - tremco acoustical sealant or Prosoco joint and seam sealer ( - use a really long-lasting flexible caulk, not construction adhesive. Use treated material that touches the garage slab. Spray the framing material and sheathing with a borate solution.

    With a cement block building, any moisture generated from the inside, must be exhausted by your HVAC. It can't go anywhere through those walls. I've been intrigued with the Ultra-Aire SD-12 which is both a dehumidifier and provides some cooling. ( and That might do well in New Orleans. You would still need an AC or a heat pump. Do you run the heat much in the winter?

    I volunteered for a couple of years in New Orleans after Katrina and gutted some houses so people could rebuild. There was not a speck of insulation in those homes. I assume if you read GBA, you must have insulated your shotgun house when you rebuilt it.

    Good luck with your garage/workshop/guest storage center.

  4. shnnn | | #4

    Lucy Foxworth,
    Thanks so much for your general well-wishes and your specific advice about sealants and the Ultra-Aire SD-12. I'll look into those!

    With respect to insulation in my Nola house, I fear most GBA readers would cringe at my approach, but so far I'm happy with it. I expect I'll make the garage conversion a tight envelope with lots of insulation and thoughtful venting more in line with current building science best practices.

    tl; dr:

    If I may hijack my own thread, and in case anyone's interested, in the house itself I veered off in a different direction. Basically, I tried to figure out why it was built as it was and then rebuild it so that it would function as originally designed rather than to suit most modern folks' heating and cooling tastes. I had the luxury of doing it just to suit myself, and I am someone who would generally rather have the windows open than the A/C on.

    In gutting the house, it was very interesting to me to see that the materials that were added post-original construction made a nasty moldy mess when flooded, whereas the original materials degraded more gracefully. The original interior wood paneling and the vertical bargeboard walls were definitely damaged but largely salvageable. Suffice it to say that the experience soured me on sheetrock and tar paper, so I used very little of it in the rebuilding.

    I don't have an "HVAC system" per se. For cooling I have a 15k window A/C unit and some great Westinghouse industrial ceiling fans (total cooling system cost under $500). During the real dog days, I keep my windows closed, my shutters closed on the sunny side of the house, and the A/C set at 79 and am pretty comfortable. Sometimes I wish for another 5k A/C unit right by my bed, but I'm not willing to lose the view from my window to get it, nor spend $1000+ for a through-the-wall mini split. Most other times, like today when it's "only" 83, I just open the windows and enjoy the breeze coming in off the river. My house is only about 900 square feet of mostly wide open space, has amazing cross-ventilation since it's only one room wide and has some open space on either side, has great front-to-back ventilation from high-mounted vents, and is open all the way to the rafters. So the house can take pretty good care of cooling itself without much interference from me. Unlike my neighbors with asphalt shingle roofs, my metal roof doesn't soak up a lot of heat, but my only roof "insulation" is a radiant barrier over the purlins, which of course isn't insulation at all.

    All that open airspace makes it tougher to keep warm in the winter, but I do okay with two 30k BTU gas radiant room-vented wall-mounted units given that I only need heat during parts of December, January and February (total heating system cost under $500). Most people in New Orleans of limited means use this type of heater, which I know is controversial elsewhere. We have ODS sensors and smoke and carbon monoxide detectors, and our houses tend to be very leaky, so they seem to work fairly well down here.

    We get a combined electricity and gas bill that averaged about $100/month last winter and about $60 last summer. I would love to decrease those numbers through solar panels and a rocket mass thermal heater as time, money, and energy for wrestling with the well-meaning but unimaginative folks at city hall allow.

    I live in an historic district, and replacing my beautiful leaky old sash windows with "historically appropriate" but efficient alternatives would've cost more than I paid for the house itself. So I resolved to put on an extra sweater in the winter. I did get a great deal on a bunch of 1.5" rigid foam polyisocyanurate ($3/sheet!), so I insulated the floors according to some guidelines put out by the LSU Dept of Ag. Insulation doesn't create warmth, but, if nothing else, I am enjoying the placebo effect of no longer being able to see the ground through the cracks between the wonderful old cypress and heart pine floorboards.
    And with the money I saved, I was able to spend three months in India this year, soaking up how they deal with their own hot, humid climate.

    So...on the off chance anybody's still with me...just another word of thanks to the members of the GBA community for sharing their knowledge even with semi-heretical newbies like me.


  5. LucyF | | #5

    I had to look up "bargeboard house". It sounds amazing. Joe Lstiburek of Building Science wrote an article about mold and how we take perfectly good wood timber and peel it, and flake it, and cook it and glue it together to make plywood and OSB and worse, particle board, which makes very good mold food. A bargeboard house is the antithesis of these modern materials as you found out during your renovation. It also sounds like you manage very well with the climate in New Orleans. There's nothing greener than opening the window for the cooling effect of breezes or putting on a sweater when it's chilly.

    I've been thinking about your renovation. Have you talked to an engineer about the the cement block walls? If they can support the weight of the addition, that's the way to go. You totally solve the termite risk that way as well. If not, your idea of framing within the walls will work, but I think a gap between the walls would be likely lead to rotten framing because of condensation when it is much cooler inside than out. Cool air will leak out and condense in the space between the cement block walls and the framing and lead to mold and rot. You pretty much have to provide some insulation between the two to stop condensation on a cold surface. In New Orleans, it needs to be a termite resistant material - mineral wool insulation, I think.

    Again, assuming your block walls can support the addition, you can insulate on the exterior which would bring the cinder block into your thermal envelope and provide thermal mass, moderating the temperature.

    Let us know what you're doing with your project. It's always good to hear about projects in the warmer parts of the US.

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