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Vapor-reducing sealer for concrete / brick?

bennettg | Posted in General Questions on

My house is in zone 3, marine/coastal, on the water, so really humid much of the year. The house is on pilings with the ground level a concrete slab with a single wythe of brick infill in between the pilings. I have a ~500’^2 workshop area I’d like make reasonbly dry for tools, etc.

What would be the appropriate sealer to apply to the concrete floor and brick to help reduce the water vapor transmission up through the concrete floor and from the outside air/rain through the brick?

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  1. GBA Editor
    Martin Holladay | | #1

    There are various sealers available (including epoxy coatings for slabs), but I don't really recommend them.

    If the slab and brick wall are uninsulated, the best approach would be to install a continuous layer of rigid foam above the slab and on the interior side of the brick wall. The rigid foam will stop moisture transmission and will also provide some insulation.

    Before you install rigid foam above an existing slab, it's always a good idea to install a layer of 6-mil polyethylene. After the rigid foam is installed, you can install a plywood subfloor, fastened through the foam to the slab below with TapCon fasteners.

  2. bennettg | | #2

    Thanks, Martin. Due to the chances of flooding, I'm hesitant to add foam & finish as you describe, particularly the floor, which is slab-on-grade 6' above sea level. Do silane/siloxane masonry sealers retard vapor transmission? If not, is there a penetrating sealer that would? A film coating would work for the floor. Painting the rough interior of the brick wyeth would take a huge amount of coating to fill, similar to painting virgin cinder block. I did a fair amount of searching for penetrating sealers, but I didn't find anything on their water vapor retarding characteristics.

  3. GBA Editor
    Martin Holladay | | #3

    I now realize that I can't quite visualize this building's foundation.

    You wrote, "The house is on pilings with the ground level a concrete slab with a single wythe of brick infill in between the pilings."


    1. Are these wooden pilings or brick piers?

    2. Is there a framed wood floor above the pilings, or a suspended concrete slab above the pilings?

    3. Is the room you are talking about at grade level (with a slab on grade), or above the pilings?

    4. What do the walls look like when you are standing indoors on the slab? Are you looking at round wooden pilings with brick infill between the wooden pilings?

  4. bennettg | | #4

    The two-story living area of the house is up on 7' treated wood 8x8" pilings to avoid a level of storm-surge flooding. This design is prevalent along the low-lying south atlantic and gulf coastal areas. There is a slab poured at ground level. This one has a utility area at the ground level enclosed partially by a single wyeth of brick between the pilings (which is unusual) and the rest with conventional framing/siding. I've attached a pic of the interior.

    So to your Q:
    1) treated wooden pilings
    2) framed wood floor, boxed in and insulated
    3) room is at grade level, slab on grade.
    4) square wood pilings with brick infill on most of 3 walls, framing on 1+.

    I'm possibly overthinking this. It is a utility room that I will be drying out with a dehumidifier . I'm looking to slow the inward movement of water vapor to reduce the load on the dehumidifier. I don't think I'll need to thermally condition the space. We'll see.

  5. user-2310254 | | #5

    DIY. Check out FEMA's technical data sheet no. 27. I assume you have flood insurance on your property.

  6. bennettg | | #6

    How about I go with a simpler question: Is there a penetrating masonry sealer product suitable for application to concrete flooring and rough, exposed brick that will reduce water vapor transmission?

  7. Expert Member
    Dana Dorsett | | #7

    If you're air conditioning &/or heating the shop space insulating the walls is going to be worth it.

    For single-wythe brick walls in zone 3 you'd be better off installing rigid insulation on the EXTERIOR, which would allow the brick & pilings to dry toward the interior. As little as 2" of EPS or 1.5" of foil faced polyiso on the exterior would bring it fully up to code-min, taking advantage of the thermal mass of the brick. From a exterior finish point of view it's probably cheaper to go with EPS foam + EIFS, but with furring through-screwed to the masonry you can use many types of siding if EIFS would destroy the curb appeal.

    To hit code-min with the foam on the interior would take 2.5" of polyiso (2" of foil faced polyiso if you created a 3/4" air gap behind the gypsum using furring) or 3.5" of EPS (3" if foil faced w/gap), and it would raise rather than lower the average moisture content of the pilings.

    Insulating the slab floor of a work shop space in zone 3 is only worth it in new construction, and even then only ~R4-R5 under the perimeter or at the slab-edge. IRC code doesn't even require slab-edge insulation that climate. A skim coat of Xypex followed up with a garage-floor paint would probably be sufficient for ground moisture migration. Be sure to seal-up any cracks with mortar or portland cement before the Xypex or the paint will likely blister & peel in the crack zones.

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