Waterproofing foundation
I have a 100 year old triple brick house with a brick foundation. I replaced the foundation on 2 sides with poured concrete. The house is semi-detached so I am attached on one side and the back wall is still brick, but it has been repointed since it was quite small.
The company installed a french drain with sump pump and the joists are embedded in the new poured concrete wall, but they were wrapped with blue skin before so the wood is not in direct contact with the concrete.
On the outside, the put a foundation coating, but no dimple mat. Foundation walls are 12″ thick. I read a lot and it seems dimple mat is best practice, but not always necessary. I’m worried about a crack in the future and water seeping into the basement since I will be finishing the basement later this year.
I will also be insulating the basement with 3″ of spray foam for an R-21 and spraying inside the rim joist cavity. Even though the joists are embedded, I believe the potential ‘rot’ issue is negated by the blue skin and I will be installing a 2×4 walls 2″ in front of the concrete to support the joists and run electrical, etc.
A lot of different points are touched on in different articles on this site. I’ve read them all. I believe interior spray foam with protected joists (wrapped) and a stud wall is correct and while the foundation walls would be better served with a dimple mat, I am loathed to excavate again to install one. Is the coating sufficient for a 12″ thick wall? Should I install an interior dimple mat?
Thanks,
Jason
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Replies
I would probably assume that the exterior coating will handle most of the groundwater and add a dimple mat on the interior before spray-foaming to handle any water that still gets through the wall, leading to an interior perimeter drain that goes to a sump pit with pump, or to open air. If it's only a small amount of water, the tiny spaces between the foam and the foundation will provide enough of a drainage plane, but a dimple mat is more reliable.
Do you mean Henry Blueskin VP100? That is a vapor-permeable product and won't protect your joists from moisture, only liquid water. I believe there are also Blueskin products that are vapor impermeable so whether your assembly is safe really depends on which product you heave.
Thank you for the reply and your comments. I can't really do an interior dimple mat with a drain because I poured a fresh slab with in slab heating so I guess my question boils down to is the coating enough or should I re-excavate and add the dimple mat. I have an exterior drain connected to a sump pump.
I believe the product used was the Blueskin WB Air and Moisture Barrier. Regardless, I will have a stud wall supporting the embedded joists.
Thanks,
Jason
No system is 100% reliable. When it's critically important to keep water out of basements, I include at least two water management systems. In your case, both would have to be on the exterior. In other words, I would consider adding a dimple mat to the exterior.
Thank you!
Add an interior waterproofing layer:
• Apply waterproof paint or a rolled waterproof membrane to the interior concrete walls before spraying foam.
• If you’re particularly concerned about water, consider installing an interior dimple mat to channel water to the sump pump.
source: Block Blast
The international residential code specifiies where you need to waterproof and where you need to dampproof. It sounds like the exterior coating is a dampproof coating which is the norm in most places.
Sections 406.1 & 406.2 specify where you need damp proofing vs waterproofing. If you're not sure about your area then it's best to err on the side of caution. https://codes.iccsafe.org/content/IRC2021P2/chapter-4-foundations#IRC2021P2_Pt03_Ch04_SecR406
Personally if I am going to have underground rooms I'd go with waterproofing unless I'm in a desert or very dry location but if it's already installed it would be tough for me to dig it up and redo it and I would probably just ride it out and see if the damp proofing is sufficient. If you do end up digging it up be sure to use a waterproofing sealant. I'm not a huge fan of the dimple membranes but I suppose they do work.
dimple mat on the exterior relieves hydrostatic pressure on the below grade wall, but it should have a pipe system at the bottom so that the water is not concentrated against the footing. ideally this system should slope to daylight, but often the site doesn't allow since the weep system is so low relative to finish grade.
you might consider installing moisture resistant finishes and materials as much as possible to mitigate damage if ever you get a leak.
Absent a weep system at the bottom, then the drainboard is just protection against damage to the waterproofing during backfill operations.
For a secondary measure OP should consider a ground gutter -- placing a 6' wide impervious layer under the soil to channel surface water away from the house. Fine Homebuilding talks about this on podcasts and likely somewhere in articles or forums.
Interior drain systems are challenging to do correctly since those gutters need to have slope to drain. even at 1/8" per foot, that's a significant amount of fall to accommodate. there's also issues tying the underslab vapor barrier back in. Conceivably you could mount the gutter system on the wall, betting that any leaking would be high.