Use Thoroseal in basement –Yes or No?
When I moved in, one year ago, I installed new gutters. Then found mold on some shelving and items in basement and had a large dehumidifier installed. There are french drains all around that seemed dry at first. Now, after 2 mo of dehumidifier, I have a lot moisture filaments on interior concrete block walls and water in the drain. It almost seems that the dehumidifier is pulling moisture through the walls now. I didn’t have the filament growth before the dehumidifier. To avoid the expense of excavating outside, it was recommended to coat walls with Thoroseal AND to drill “weep” holes along the interior and drain all towards the sump pump. But, it appears that sand has been poured around some of my foundation and so I think the weep holes will just bring in sand constantly. I have read NOT to seal interior walls…as they need to breathe. I don’t know who to trust. Also I’m 71, and though I want to take care of my house, I don’t need to make it last forever! Thanks for any HELP!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Masonry walls don't need to "breathe". Concrete is highly tolerant of moisture, which is why they build submerged bridge foundations out of the stuff.
Sealing the foundation is fine- the moisture content of the CMU (blocks) will rise, and if there isn't sufficient exterior above grade exposure to dry toward it can also increase the moisture content of the foundation sill plates unless there is a decent capillary break material (EPDM sheeting, metal flashing) between the CMU and wood. With decent roof overhangs and exterior grading/drainage a foot of exposure would be enough for most houses.
There are multiple ways of sealing the wall from the interior. You may want to read this:
https://www.greenbuildingadvisor.com/article/installing-basement-waterproofing-from-the-negative-side
Judith,
Here's a link to another relevant article that has advice on products like Thoroseal: "Fixing a Wet Basement."
Thanks for the reply. From what I can tell, in general, Thoroseal is ok...if it bonds. It seems the wall would have to be very dry before application...but that's pretty unlikely. I read there are "crystalline" agents that bond better.
Do you have anything to say about the "weep" holes being drilled? Or the sand that is along one exterior wall of foundation? That side of house is definitely down grade from the house next door...there's a 3 ft slope about 15' away from that side of my house. Which is probably why someone put the sand there. What about planting some bushes along that side to absorb water? THANK YOU!
Weep holes to the interior aren't a solution to anything. If the bulk water incursions are substantial enough to require weep hole relief it's time to install a perimeter drain under the slab (yes, that's a big PITA project.)
Plant's aren't going to buy you anything for moisture relief, but a properly set up exterior surface drain (aka "French drain") can do a lot.
Thank you. The French Drain I have is interior about 4 " deep. When I first moved in it appeared dry, but over the year it is damp and the base of the wall has stained dark (wet) between 2-10" up the wall. There is a section of the french drain that filled with about 2" of water. And there's another spot that I can see water "moving" so it is coming in there. The one guy who came to look said the blocks are probably filled and need to drain, and the weep holes would do that interior, but would not address the problem itself. (as you say) I do need to address but not to over-do....I just want 20 yrs out of my house....which was built 1962. I have a second consultation tomorrow. Any other pointers are appreciated.
A hole on the interior side of a concrete block will allow the core space to drain, reducing the amount of moisture passing through the block. This is OK if you have a place for the draining water to go (eg, an interior perimeter drain). Such a hole won't extend to the exterior where sand could enter.
Consider adding foam insulation to the walls - this will reduce heat loss, moisture flow through the block and dehumidification cost.
Jon, That is very helpful explanation. I was picturing the sand getting in, but it won't if the holes just go into the block from inside. And YES I have a full French Drain around the inside that can be channeled to my sump pump. One person did suggest I do foam insulation and I don't know which is better, insulation or ThoroSeal. Which would you recommend, or both?I'm concerned about the moisture filaments thing that has appeared quite a bit since I got the dehumidifier. I can get that cleaned off, but then need to do some new treatment on walls.
Both the Thoroseal AND the insulation are a good idea, for a complex set of interacting reasons. An inch of closed cell foam or rigid foam board is enough for controlling the amount of moisture coming through the wall, but a wall with a history of bulk water incursions is better off having a dimple mat between the wall & foam rather than foam sprayed directly on the CMU.
The total amount and method of insulation needed varies a bit by climate. What is your IRC/DOE climate zone?
I understand efflorescence but not "moisture filaments". Perhaps you have a picture.
I’m in 12203
Albany is in US climate zone 5A. The IRC code minimum is R15 continuous insulation, which would be an expensive 3" if using spray polyurethane, cheaper if using 3" reclaimed polyisocyanurate roofing foam board strapped to the wall with 1x furring through-screwed to the CMU wall with 5" masonry screws, mounting the wallboard to the furring.
A 1" layer of foil-faced polyisocyanurate trapped to the wall with a 2x4 studwall with unfaced R13-R15 batts also hits code minimum performance level, and is easier to run electrical wiring, etc than a 3" foam + furring solution. In climate zone 5 foam board needs to be at least R5 to avoid condensation issues in the above grade part of the fiber-insulated wall, so if using polystyrene (XPS or EPS) it needs to be at least 1.25" thick. With your dampness issues it's worth putting 1" EPS (not polyiso) under the bottom plate of the studwall to keep it from wicking moisture from the slab. The polyiso should also rest on the EPS, not the slab.
If there is a history of actual flooding rather than just dampness, stop the polyiso above the high-tide mark and continue downward with EPS. When submerged or on chronically damp slabs polyiso will wick up and retain moisture, whereas EPS will dry out relatively quickly after a flood.