Interior sealing for damp basement — good idea?
I have a 1912 house in Michigan (zone 5) with an unfinished basement. The basement walls are poured concrete and are often slightly damp, some places worse than others. There is occasional water on the floor in heavy rains, maybe a few times a year. The walls are painted and seem to have some sort of thicker, rougher coating under the paint, either an old Drylok-style coating or a thin skimcoat of something. As I was prepping a corner of the basement for repainting recently, I noticed that some of the old wall paint/coating was coming off and in those places there is often efflorescence and spalling underneath — it’s like little blisters under the paint filled with dust. Some are maybe nickel-sized, others a few square feet or so. At most, the spalling has been maybe 1/2″ – 1″ deep, but most places are much less and there is plenty of the wall paint that remains intact with no evidence of efflorescence or spalling. There’s no cracking in the walls and I’m not concerned about the structural situation currently, although it seems that another 50 or 100 years of this will probably lead to problems. My current project is to remove the existing deteriorated coating where it’s come loose, and then scrub away the efflorescence and any loose material. This will leave me with maybe 80% of the existing wall coating intact.
My question is how to best move forward from there without making things worse, and ideally improving things. I have no plans to finish the basement (!), but a bit drier and cleaner would be nice.
The gutters are intact; the downspouts let out at least a few feet from the house and the grade slopes away from the house. There’s simply a high water table in my neighborhood and many basements are damp. Digging down to the footing from the exterior isn’t feasible due to concrete driveway/porch on two sides and a crawlspace on a third. My understanding is that an interior perimeter drain isn’t feasible because the house likely has no separate footing (unconfirmed, just based on local practices of the era) and the slab is likely at the bottom of the wall, so digging down for a footing drain would undermine the wall.
This leaves me with interior coatings (or omission thereof). Here is what I’m considering: skim coat of lime mortar as a sacrificial parging, followed by Thoroseal to manage water and then a couple coats of paint to manage water vapor and for aesthetics. I’ll only be able to do the parging and Thoroseal on the little islands of bare concrete. Are the Thoroseal and paint a bad idea? I know some would say to leave the wall uncoated so it can dry to the interior, but to me it seems like slowing that evaporative flow of water from exterior to interior would be a good thing and slow down the efflorescence. Is the sacrificial parging necessary if I also Thoroseal and paint?
If the paint starts to bubble off over the years, that’s not a big concern — I just want to avoid additional harm to the concrete.
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