I have a slab-on-grade floor (pre-1900) that sweats enough to where it can get slippery in summer
What can I do to prevent this?
The building is a raised “shotgun” in New Orleans. The lower area (about 7.5 ft. tall ) has been converted to an apartment. This is quite common here. Originally, these areas were used for storage only due to the possibility of flooding.
It is basically an unconditioned space, the hvac provided by window units.
The walls are insulated with Rolux batts, no ceiling insulation, and the old single pane double hung windows are fairly tight.
Is there an elastomeric, or epoxy type product that I can use to mitigate some of this problem?
I understand that back then the slabs were poured directly on the ground without the benefit of a moisture barrier, and the water table is not far below the slab.
Any thoughts, or solutions?
Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Roger,
You are seeing condensation, I imagine. This isn't moisture that is coming up through the slab -- it is moisture from the air, condensing on a cool surface.
To reduce this condensation, there are two possible approaches (or you can adopt both strategies):
1. You can reduce the indoor relative humidity (RH) by operating the air conditioner for more hours per day, or by operating a dehumidifier.
2. You can install a continuous layer of horizontal rigid insulation above the slab, followed by a new plywood or OSB subfloor and new flooring. This new flooring won't be as cool as the slab; by raising the temperature of the flooring, you will reduce the chance of condensation.
-- Martin Holladay
Thank you Martin.
As usual you have all the right answers.
I had a feeling that the RH was the problem.
My client is thinking of a ductless mini split, but given the ceiling height, lack of insulation in the ceiling
( open ceiling/floor joists ) and the fact that it is a double shotgun with a masonry load bearing wall down the middle with few openings, I will advise her to go with a dehumidifier, finish the upstairs, move up there, and use the "basement" as storage.
Thanks again, and Happy Mardi Gras!
Air sealing the building envelope to limit the amount of humid air from entering is probably the first and most important step (and probably the cheapest.) The windows may be tight, but the seam between the bottom plate of the studs and the foundation is often responsible for more air leakage than all window & door crackage combined.
Just about any concrete sealer would reduces the amount of ground moisture wicking contributing to the problem. Sealing any cracks in the slab with the appropriately formulated polyurethane caulk would come ahead of sealing the slab.
As little as 1/2" of rigid EPS between the slab and a subfloor would be sufficient for keeping the flooring above the indoor air's dew point, but if flooding is likely the subfloor would have to be replaced after every flood. If you take that route, glue the sheet foam to the slab with dabs of foam board construction adhesive, and put a sheet of 6-mil polyethylene between the foam & subfloor as both a vapor barrier & slip surface, then TapCon the subfloor to the slab. Don't sweat the small annular-ring holes around the screws that penetrate the vapor barrier- the steel is a vapor barrier, and the total cross sectional area is miniscule, too small to pass enough water vapor to become a problem. Vacuum out the holes to remove the concrete dust before installing the screws.
You may be able to use a cementicious board instead of wood subflooring, and tile the floor to avoid the cost of replacing the floor & subfloor after every flood. Fully suported by the slab there isn't much flex to the foam- far less than 3/4" plywood supported by 16" o.c. joists. Whether cement board or wooden subflooring it's key to overlap the seams of the subfloor with those of the foam by at least 6" (12" is better) to avoid compression of the foam edges in high traffic areas.
Thanks Dana1.
We just might go for the dehumidifier on this one, but your info is great, and this sounds good for other homes with the same conditions that aren't as flood prone.
At the very least, caulk the cracks with polyurethane then seal the top of the slab with Xypex or similar to limit the groundwater fraction of the latent cooling load. A dehumidifier converts latent loads in to sensible heat (the heat of vaporization of the water it is condensing), which isn't very welcome during the long cooling season, and every little bit helps.