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Waterproof coating on the inside of a fieldstone foundation

We recently moved in to a wonderful old house whose basement fieldstone walls are in desperate need of repointing.

Initially I just wanted to find a contractor that would repoint the now sand-like mortar. I have a very reputable contractor that wants to waterproof the walls. Waterproofing is not something I necessarily wanted to do.

After almost half a year in the house, the basement seems generally dry - maybe some dampness occurs in the warmer months.

It seems to me by sealing from the inside that this could trap moisture in the walls and cause issues akin to a spalling brick. I definitely don't want to apply a solution that does not have a problem.

Am I right to be concerned about the stability of a sealed foundation?

Re: Waterproof coating on the inside of a fieldstone foundation

You should have the mortar analyzed for make up.
Than use a matching mortar for repointing.
To use a waterproofing over the existing mortar could do more harm than good.
If walls were constructed before 1864 it is most likely lime based with no portland cement.
If after 1864 it could contain portland.
You could send samples to one or both of the following:
U S Heritage Group, Inc.
Vinginia Lime Works.
I would get two sample reports just as a cropss refference.
Also any good soil consulant should be able to analyze it.
check with the cloest Historical Society for contractors in your area.

Re: Waterproof coating on the inside of a fieldstone foundation

Ask which brand of masonry waterproofing he was fixin to use.

Read the manufacturer's installation instructions. If you have more questions, call your local rep for the product.

Re: Waterproof coating on the inside of a fieldstone foundation

In the northeast basement wall dampness limited to the warmer months is almost always caused by humidity that leaked in from the outside air condensing on the cold basement walls; Not by water in the ground seeping through the wall. In this case waterproofing is no help whatsoever.

Re: Waterproof coating on the inside of a fieldstone foundation

Thank you all for your comments.

You are all confirming what I was concerned about (either I might make things worse or I'm doing something that just doesn't need to be done).

Incidentally, the house was built in the 1930's. And I see no evidence that water has been getting in to the house - so, yes, it looks like the sandy mortar is a humidity issue.

I'll be doing some more research before getting any work done.

Thanks again.

Re: Waterproof coating on the inside of a fieldstone foundation

Stone is not going to deteriorate like a concrete block will. Stones can be exposed to water for centuries or millennia and not deteriorate. Not so for the mortar between them. Since you don't seem to have a water infiltration issue, you either live in a very dry climate or the foundation has been parged, either when it was built or sometime after. That is a process of waterproofing the outside of the foundation.

I think your contractor is right to waterproof the inside surfaces as well. He does need to make sure that is is removing all the deteriorated mortar first, replace it with a compatible mortar and then water proof to keep condensation for deteriorating the new mortar. The stones themselves should not be waterproofed. They should be allowed to breath so they can conduct moisture from the interior of the wall to the basement.

Re: Waterproof coating on the inside of a fieldstone foundation

Just repoint inside and out. If there is a water infiltration issue, excavate outside and water proof there. If this isn't practical explore drainage options. I am not a fan of the idea of inside-only waterproofing coatings. I think a lot of waterproofing companies push this option because it is less labor intensive than excavating outside and patching/waterproofing the wall there, but with a higher profit margin. Serious water infiltration has to be dealt with on the outside first. In my opinion it is less harmful in the long term to actually leave a leaking wall un patched than to coat it in the inside and trap water within the structure. It won't hurt anything overnight but it won't be good in the long term. You're talking about large amounts of water freezing and thawing inside those walls. Not good. If there are serious leaks an interior coating will flake off from the pressure eventually anyway.

The hardest thing is that these foundations were never designed to be 100% dry. They were built when basements were not typically used as living spaces, and intended to let excess moisture pass through and drain into the soil without lingering and causing the structure to rot away. So now when we try to get them 100% water tight like modern basements we're basically asking the structure to do something it wasn't initially designed for.

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