Interior French drain — wall detail
We’re installing an interior french drain to address water issues. The grading slopes away from the house and all down spouts are routed 10+ feet away from the house. We have elevated radon and the french drain will be integrated into the existing sub-slab depressurization system. The walls are currently uninsulated, but I plan to insulate the walls with rigid foam in the coming months. The floor will be insulated when the basement is finished in a few years.
What’s the best way to detail the slab/wall intersection? I’ve found a couple of suggestions, and want to make sure that the chosen system can handle minor wall moisture and good air seal for the radon system.
http://www.structuretech1.com/wp-content/uploads/2011/11/BasementInsulation.pdf
Figure 15 suggests dimple mat a few inches up the wall, with rigid foam on the walls. This should allow any minor moisture to migrate down the back of the foam and into the drainage mat. I could then foam between the slab and the foam, in front of the drainage mat to ensure a good air seal.
https://www.greenbuildingadvisor.com/blogs/dept/musings/fixing-wet-basement
The last picture in this post suggests a similar drain mat border, with foam spaced from the wall and placed in front of the drainage mat. This would require some furring the thickness of the drainage mat, but provides a clear path for moisture.
The waterproofing contractor has also suggested a 15 mil polyethylene mat that would be placed on top of the stone before the concrete is patched. It would extend up the walls and be sealed with caulk at the top. I think I like this system the least, because it seems hard to detail well to ensure a proper air seal for the radon system. I think I’ve read on this site that polyethylene is best avoided on basement walls as well.
Would it be overkill to extend the dimple mat to the top of the wall, and air seal at the top of the dimple mat? This is recommended in the Hammer and Hand Best Practice manual. How would I attached rigid foam over that mat without puncturing the mat and compromising the air barrier?
Thanks.
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Replies
Jeffrey,
Q. "How would I attach the rigid foam over that mat without puncturing the mat and compromising the air barrier?"
A. As explained in the GBA article, How to Insulate a Basement Wall, rigid foam is usually fastened to a concrete wall with TapCons with washers or specialty fasteners like Hilti IDP fasteners or Rodenhouse Plasti-Grip PMF fasteners.
If you are worried about maintaining a tight air barrier, you could always install a small rectangle of Siga Wigluv tape over the top over every fastener head.
One general principle to keep in mind: typical water management details often conflict with typical radon mitigation details. In this case, the radon mitigation contractor has ultimate veto power over details. Don't finalize your details without getting the OK from your radon mitigation contractor.
Thanks, Martin. My main concerns are detailing the floor/wall junction and making sure the wall can handle small amounts of moisture. I intend to monitor the walls after installing the perimeter drain to ensure dryness, but want to provide some margin in case of water intrusion after large storms.
Is dimple mat needed on the entire wall, or can XPS applied over furring (scraps of drainage mat perhaps) the thickness of the drainage mat serve the same purpose? This is represented in your post linked above.
Or should the foam be applied directly to the wall, and terminated above drainage mat at the bottom of the wall? This is detailed in the other link.
Jeffrey,
Q. "Is dimple mat needed on the entire wall?"
A. This is a judgment call. If your basement shows signs of water entry, or if your neighbors report episodes of water entry, the dimple mat would be a good idea.
If your basement wall has been dry for years, and your basement is well detailed, then the dimple mat over the wall would be overkill.
I can't think of any reason why a dimple mat on the wall would cause any problems. It just costs money.
Martin,
"One general principle to keep in mind: typical water management details often conflict with typical radon mitigation details. In this case, the radon mitigation contractor has ultimate veto power over details. Don't finalize your details without getting the OK from your radon mitigation contractor."
I just caught this addition. I've been talking to the waterproofer and the radon contractor. The waterproofer suggested the 15 mil poly sheeting against the block, and the radon contractor thought this was a good solution. After reading your article on basement insulation, it seems poly doesn't belong on basement walls. I'm searching for an alternative.
In your article 'Fixing a Wet Basement' the illustration of the slab/wall intersection shows the rigid foam held off the wall the thickness of the dimple mat, with the dimple mat terminating a few inches up the wall. What's the practical difference between this detail (and presumably furring the insulation higher up the wall) , and running dimple mat up the entire wall?
Jeffrey,
Q. "What's the practical difference between this detail (and presumably furring the insulation higher up the wall), and running dimple mat up the entire wall?"
A. If the dimple mat runs up the entire wall, it provides open channels that allow any water entering through wall cracks to flow freely down to the French drain.
If there is no dimple mat, and the foam is installed tight to the wall, it's harder for the water to find the French drain -- and conceivably, water might pop out in unexpected places if the hydrostatic pressure is strong enough.
Thanks, Martin.
I see that the NYSERDA projects used grooved EPS against the foundation walls. This would combine the drainage of a mat with the insulation of foam. The bottom of each piece could even be placed directly on the footing, and the concrete patch placed right up against it.
Is there any disadvantage or advantage to using a slotted foam (like dow Perimate) instead of dimple mat and foam?
Jeffrey,
Again, we're coming down to a judgment call. Clearly, a three-dimensional drainage mat allows more water to flow to the French drain than slotted rigid foam.
Either method will work for moderate amounts of water, but if you are anticipating large amounts of water, then the dimpled drainage mat is the way to go.
These are good / better / best decisions. Choosing between them is up to you. Your decision will be based on your budget and the volume of water that you anticipate.
Time to decide!
Thanks, Martin. I appreciate the dialogue.
Hi Jeff,
I'm curious where you ended up as I have a similar situation. If you went with the EPS directly to your foundation wall, how did you seal the EPS the the drainage mat to avoid air leakage (potential radon)? I have a perimeter drain already installed and the drainage dimple mat runs up the wall 3-4"
Thanks, much appreciated!