Topside Insulation Basement Slab Q’s – Old Home
Hi everyone. I’m partially finishing my basement and would really appreciate the community’s advice. House built in 1930, no exterior vapor barrier or exterior insulation, but I did recently add an interior perimeter drain with sump pump. Climate Zone 5 (north of NYC). I’ve read through as many GBA and Lstiburek articles as I could find and have also posted a few questions here as well – I have these outstanding questions. Please see the detail sketch and corresponding questions below. Much appreciated!
1. Currently I have a plastic dimple drainage mat (part of the interior perimeter drain system) that wraps up the wall about 3″-4″. In the article below Lstiburek advises to bring the dimple mat all the way up the wall and seal at the top to prevent any radon etc. from coming into the basement. However since I can’t do this everywhere in my basement due to some rock outcroppings, does it make sense to do it where I can in the finished portion of the basement? If so, does anyone have a specific air sealant recommendation?
https://www.buildingscience.com/documents/building-science-insights/bsi-110-keeping-water-out-basements
2. Is it necessary to have the vertical layer of EPS? Can I just do the layer of EPS on the slab and not up the foundation walls to save on cost? Or is this a building science no-no?
3. The 6mm polyethylene I’ll put over the concrete slab and under the EPS – does this need to fastened down in anyway? Any concern for noise and is a filter fabric recommended for noise (I’ve seen others recommend this for Delta FL)?
4. I am only partially finishing the basement, leaving the boiler/hot water heater room unfinished with the exposed concrete slab. Does anyone have any recommendation how to terminate the finish floor assembly here? Since it’s the water heater, perhaps some metal continuous angle to prevent any potential water from getting into the floor assembly? Any sealant needed here to seal off the floor assembly?
5. Air sealant between the dimple drainage mat and the EPS at the floor?
Thanks so much!
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Replies
To question 2: insulating part of a structure is only slightly better than none.
As Clint Eastwood said, "A man has to know his limitations." In particular, when insulating you have to know where the boundaries of the conditioned space is. Old buildings often make this difficult, attics and basements were sort-of inside the conditioned space and sort-of not. If you've been reading Lstiburek you know that every house needs four layers -- rain control, air barrier, vapor barrier and insulation -- and those four layers need to continuously surround the conditioned space and separate it from the outside. His famous line is you should be able to take a plan of the building and draw any one of those four layers around the entire plan without lifting your pencil off of the paper.
I would say the dimple mat is serving as the rain barrier. In a basement the EPS can serve as air barrier, vapor barrier and insulation if the seams are taped. It does have to be covered for fire safety.
Thanks DCContrarian, much appreciated. It's going to be about a half-finished basement - unfinished in the boiler/hot water heater area. Do you think it makes sense to extend the dimple mat up the full height of the wall and have EPS on the walls, only around the finished area of the basement? And the unfinished boiler / hot water area will remain with uninsulated foundation walls, and dimple mat that only wraps up the foundation wall 3"-4"?
What I'm trying to persuade you to do is have a good building envelope even in the unfinished half of the basement. The building science is that if you have half an area at R20 and the other half at R1, it doesn't average out to R10.5. It averages out to R1.9 -- the insulation on the insulated half does almost nothing. That's the insulation layer. It's even worse with the air barrier layer and the vapor barrier layer, not having them on one half makes them completely useless on the other.
Gotcha, thanks DCContrarian, much appreciated.
Does water seep out of the basement wall?
If you run the dimple mat up only 3-4", then seal the top, your interior drain is effectively cut off from flows higher up the wall.
Another option might be to set the foam off the wall with some (non-wood) spacer or thick bead of adhesive creating a free-draining space that connects with your drain.
No need to fasten poly to the floor, but perhaps fasten the edges to the vertical EPS.
If your water heater leaks, where will the water go? Can it make it to the french drain? A metal dam might be a good idea, but will only hold out so long.
Air sealing to the dimple board seems the wrong location, but it depends on other details. If you run vertical EPS, that should form a continuous corner with the horizontal floor EPS. And the top of the vertical EPS should be sealed somehow to the wall up higher.
Aside to doba1400: nice sketch. A lot of people think they have to have computer generated drawings to show other people, but a sketch is usually sufficient, especially if you’re just communicating ideas. In these Q&A’s, where no one else is familiar with your particular circumstances, a simple sketch can tell a story more quickly than a bunch of prose. (Though good prose is always an enjoyable surprise.)
Some people sketch better than others, but I think there’s a fair amount of truth to the saying that if you can’t draw it, you probably can’t build it.
Thanks for including a sketch.
Thanks Tyler. Water does not leak out of the walls from what we've observed in the 6 months of owning the house. I know, not a lot of time to get to know our home, but that what we've got. This overlaps with another question I've asked separately, but I wonder if it makes sense to try to seal this space between the dimple mat / EPS and the concrete wall at all, because part of my basement has rock outcroppings where sealing wouldn't be possible at all. In this case, just use a radon mitigation pipe connected to the interior drain to deal with the radon, instead of trying to air seal it off?
Andrew C, thanks for the sketch comment!
doba, I saw your other post as well.
Are you planning to install a radon stack? It seems like that would be the next step in radon mitigating (even just a passive stack).
There doesn't seem to be a consensus on sealing the dimple mat. Best practice would be to run it all the way up the wall to catch any and all water seepage. If you're 100% sure water only seeps in on the lower 3", then you're probably safe to go ahead and seal it where it is. They make termination bars for this (used in conjunction with a sealant). CC spray foam may also work.
If the 3" height is of concern (or in the case of your protruding rocks) it seems like you need to extent your drainage plane higher, either by extending the dimple mat, or with something else like poly. In any case, if your adding a radon mitigation system, sealing that termination (whether it's 3" off the floor, or higher) would help prevent the system from drawing air from the living space. Even a passive stack can draw air. Which is of course what you want.
You may be interested in this thread (note that Dana seems to disagree with what I just said in that he seems fine with having the dimple mat left unsealed—it's mostly an energy penalty, but it seems unideal in my opinion on all fronts).
https://www.greenbuildingadvisor.com/question/french-drain-w-radon-concerns
It's a trick because you have two somewhat conflicting goals:
1) to allow WATER communication between any interior foundation wall leaks and the drain tile, and
2) to disallow AIR communication between the interior living space and drain tile/radon system.
The only surefire way (I can think of) to satisfy 1 and 2 is for the air seal to be made above the leakage points.
Oh and note that the sump lid should be sealed if you're adding a radon system for the same reasons as wanting to seal the dimple mat.