Basement walls: Encapsulate with plastic sheet, tied to interior French drain?
Hello brain-trust,
Wondering everyone’s thoughts on this approach.
I’ll be installing a full interior french drain in the basement (1960 house), to fix seepage around the cove joint. wondering if i should install “wall encapsulation” at the same time. Attached image shows the idea. basically, a sheet of poly is glued/fastened near the top of the foundation wall, and is run all the way down the wall, into the newly installed drainage system. in theory, and moisture behind this sheet should run down into the french drain–always staying out of the basement.
Then, if i wanted to insulate, layer(s) of foam could be added on top of this plastic.
Not sure if I’ll fully finish the basement in the future.
The concrete walls were painted with what i assume is DryLok, many years ago. it’s flaking off in spots, pretty beat up. Already scraped off the loose stuff. So, the plastic would encapsulate this paint.
I suppose the other approach is to skip the plastic on the walls, and just glue and seal the foam directly to the wall (https://www.greenbuildingadvisor.com/blogs/dept/musings/how-insulate-basement-wall).
But I’m thinking that the advantage of the plastic on the walls is that, when (not if, i dare say) a crack in the wall starts to leak, all that leakage should run down directly to the drain, instead of finding its way into the basement.
appreciate any thoughts!
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Replies
Corry,
The product to use for the purpose you have in mind is dimple mat, not polyethylene. In homes with an interior French drain, the use of dimple mat on the interior of a basement wall is fairly common. For more information, see "Using a Dimple Mat to Keep a Basement Wall Dry."
Martin,
Thanks for the reply. Dimple map looks interesting. But seems like it would be more difficult to seal the top edge, and any seams.
I've talked to ~6 waterproofing companies in the chicagoland area, and none of them use dimple. all use some flavor a fiber reinforced plastic, typically 15mil thick.
Hey Corry,
I am currently in the midst of doing the same type of project. I too have done days of research and have gone down the rabbit hole of different opinions of water proofing and insulating a basement...
I am now currently in the same situation that you left off at. But I do plan on finishing off my basement by installing a Foamular 250 insulation, stud wall and sheetrock.
I had a waterproofing company come into my basement and install a pressure relief drainage system that drains to (2) sump pumps on either end of the house. They installed (13) Steel beams to correct and prevent any more bowing/stair-step cracking from Hydro static pressure I had on the foundation wall. I have also re-routed all gutter drains on the outside of the home to get the ground water away from the home. The waterproofing company did install a plastic membrane that runs to the top of the foundation to the floor Joists. The theory is that any moisture that builds up to the interior wall will run down the plastic into the drainage system...
I am wondering which route you decided to take and if by leaving the plastic up I will do more harm then good...
>I do plan on finishing off my basement by installing a Foamular 250 insulation, stud wall and sheetrock.
Can I convince you to change the spec on the foam?
While labeled R5/inch, that spec is multi-year average. The initial years' performance is somewhat higher, but the 20 year warranty is only for R4.5/inch (90% of labeled R-value), and over the decades it eventually drops to about R4.2/inch. The initially higher performance is due to the HFC-soup blowing agents used, the predominant component of which is HFC134a. All HFCs are extremely powerful greenhouse gases (HFC134a runs about 1400x CO2 @ 100 years), and as they slowly diffuse out of the foam the performance drops as the environmental damage takes off.
In Europe (where Kigali Amendment signatory countries ban the use of HFCs in this application) XPS is blown with CO2 (which runs exactly 1x CO2 @ 100 years), and at density of 1.5lb per cubic foot or higher runs about R4.2/inch, with no degradation in performance over time.
By contrast EPS (same polymer, different process) is blown with a mixture of variants of pentane (all running about 7x CO2@ 100 years), and at the same density delivers the same fully-depleted performance of XPS, which is about R4.2/inch at densities of 1.5lbs per cubic foot or higher. Better still, most of the blowing agent leaves the foam while still at the factory, where it is recaptured rather than vented to the atmosphere. Like European XPS, the performance of EPS is stable over many decades.
Similarly, polyisocyanurate is blown with a low-impact hydrocarbon soup, has a somewhat less impactful polymer chemistry, and a higher R/inch.
At the bottom line HFC-blown XPS is BY FAR the least green insulation material in common use today, delivering about three times the damage of even HFC-blown spray polyurethane (which also isn't great compared to HFO-blown polyurethane or EPS/polyiso):
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png
It's fine to leave the plastic sheeting in place.
Dana - very informative, thank you for this. I will absolutely re-consider using XPS Foamular 250, I had no idea how much of a negative impact the Owens Corning Foam boards had on the environment.
I am open to suggestions to an EPS product that is comparable to the Foamular 250... Any suggestions?
In fairness, it’s an issue with XPS, not with Dow Corning. There are other manufacturers of XPS out there, all using the same blowing agents.
All you need is 2” thick EPS, which is around R8.4 or so. Type II EPS is easier to work with, and more expensive (similar cost to XPS), but you'll usually need to order it unless you’re working with an insulation distributor with a large inventory.
I’d suggest looking into reclaimed insulation too. Reclaimed insulation is usually both the cheapest and the greenest option, so a win-win. Reclaimed insulation has pretty much no downside if you’re using it in a hidden application.
Bill
>"I am open to suggestions to an EPS product that is comparable to the Foamular 250... Any suggestions?"
SilverBoard/ NeoPor or other graphite loaded EPS runs about R5/inch (forever). Generic (pick any manufacturer) Type-II (1.5lbs per cubic foot nominal density) runs about R4.2/inch (forever). So to hit R15 (IRC code min for zones 5 and higher) takes about 3.5" instead of 3" for graphite loaded EPS. Cheap Type 1 EPS (1lbs density, often comes with plastic or aluminum facers, which is fine), runs about R3.9/inch, which would take 4".
Standard (no graphite) Type-II EPS is usually 25-30% cheaper per labeled R than XPS. Type-I EPS is cheaper still, but is pretty flimsy, easy to damage in handling. They use facers on Type-I to mitigate against breaking off corners, etc. The facers make it easier to air-seal with tapes though (housewrap tapes are fine.)
If you're open to using fiberglass or rock wool in the studwall, a couple inches of EPS (any density) and R13-R15 batts would hit the same or higher thermal performance point with low/no risk of wintertime moisture accumulation in the wall cavities.
>”Standard (no graphite) Type-II EPS is usually 25-30% cheaper per labeled R than XPS.”
Was that “cheaper” when bought in quantity? My price has usually been pretty close to XPS, maybe $2 cheaper per sheet for type II EPS. I’d *love* to find a source for cheaper type II EPS that doesn’t need big quantity lots to get good prices since I usually only order around 5 sheets or so.
Bill