Basement insulating for zone 5
I bought a late 1930’s foreclosure in the Chicagoland area with concrete block basement walls (and a living room addition over crawlspace) and am looking to tackle this area next in my airsealing overhaul. I’m thinking of either rigid foam board cut/fit or closed cell spray foam for rim joist area and 2″ XPS foam board over dimple mat for drainage on walls. There were water issues due to poor drainage and all the prior owner did was install interior drain tile system. I addressed the biggest risk with re-grading the slope of the backyard and sending sump pump to the city storm drain system though want to make sure I protect against water as we are about 500ft from a river flood zone. I also intend to insulate the crawlspace to make it conditioned space. Anything I need to consider with my situation I haven’t mentioned or have a better solution?
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At 2" XPS is only R10. IRC 2012 and newer code minimum for zone 5 basements is R15.
Worse still, as XPS loses it's climate damaging HFC blowing agents over a few decades the performance will drop; to R8.4, the same as 2" EPS.
Instead of 2" XPS use 4" of EPS (blown with pentane, most of which is recovered at the factory, but has a VERY low enviromental impact relative to the HFC134 + additional agents used for XPS), or use 3" of polyisocyanurate (also blown with pentane.) These are both cheaper per labeled-R and greener than XPS. If you use polyiso, keep the cut edge at the bottom off the slab, since it can wick (slowly) ground moisture over time.
To save money and the planet, it's FAR cheaper to use reclaimed or factory seconds rigid foam. There are several reclaimers and salvage outfits trading in reclaimed foam in your area, some advertising here:
https://chicago.craigslist.org/search/sss?query=rigid+insulation
If using reclaimed XPS, for design purposes derate to R4.2/inch, it's fully depleted performance.
If you are installing a studwall on the interior side of the foam, put an inch of EPS or XPS (but NOT polyiso) under the bottom plate of the studwall as a capillary & thermal break. That keeps the framing from wicking ground moisture, and keeps the temperature of the bottom plate above the summertime outdoor dew point temperature, mitigating mold risk.
As the former owner of a very wet 30's block basement in the Chicago area I would advise you to apply some form of water sealing coat on the block walls before pursuing any insulation plan. Anything applied to the exterior side of the block occurred almost 90 years ago and the wall is now behaving more like dimple mat that channels water to you.
I chose Thoroseal when I did my basement 30 years ago as I did not like the alternative materials available. Chemistry advances might make my choice different now, but then I felt the thicker stucco like coating Thoroseal provided, filled the block pores and mortar lines best. I also wire brushed and washed all the walls and replaced many feet of failing mortar before application. It was a good thing I was 30 years younger.
The walls at the start were quite wet and some of the core stacks were filled with water up to three or four blocks above the slab. Mind you this occurred despite my lot rapidly sloping to the back to an elevation at least four feet lower than my basement floor. All downspouts were controlled with drain lines to the back of the lot. I believe the heavy clay soils prevalent in the area make basement holes act like giant flower pots. I would recommend drilling 3/8" holes in any blocks near slab level that appear darker or have efflorescense on them. I got quite a flood out of the wall next to the driveway. I took special care when coating that wall after plugging the holes I made with hydro cement. Thoroseal is meant for water tanks and the like and can be applied to damp concrete. Not weeping.
When I sold the house it was scheduled to be demolished for a McMansion so I took a peek behind my insulation to see if I had issues. GBA was unknown to me at the time I built the walls ( if it existed), so my scheme of building a 2x4 wall about 1" off the block face and insulating with batts worked better than I would now expect. My addition of dado notches in the baseplate might have provided enough air exchange to keep the transpired moisture coming off the wall at manageable levels. I can't recommend Dana's idea of foam under the baseplate. Other more solid plastic like the PVC stock for trim would be far better for stability.
I don't think any waterproofing product will claim it prevents transpiration of moisture just bulk transport of water. This is critical when doing the insulation part of the job as you will need to keep the cold block wall from condensing the atmospheric moisture in lovely humid Chicago.
Current recommendations of isolating the wall surface with continuous EPS behind framing might be be easy or tricky depending on how well the wall was laid and how many pipes are in the way. Others might weigh in on the value of some of the EPS systems that have pre-formed furring slots that allow a overall thinner installation, albiet one with no easy way to true a wall.
You will almost certainly find the floor slab to be moister than it appears, especially if you are near the DesPlaines River or any of the frequently flooding tributaries in the area. Last couple of weeks should have made the risks quite clear. To test the floor for moisture output, place a square of impermeable plastic dropcloth or the like on the floor and then place a bucket or tub with continuous rim contact on the plastic. Weight the bucket or tub down a bit to ensure a seal like a jar lid. In a couple of days disassemble and see how wet the spot under the bucket is. A really bad floor with direct contact to soil under the slab might produce bulk water under the plastic. A properly isolated slab on crushed stone might only get darker. The waffle like panels sold at home centers for basement floors do not address serious amounts of moisture. I would recommend seeking local council if you intend to put down any kind of flooring/carpet.
For very similar moisture reasons I would caution as to how to incorporate the crawlspace into the conditioned envelope. Even if you seal the crawlspace floor with a rat slab or a foundation bonded plastic film similar to a radon installation, the moisture levels will be high and temperatures will be driven by ground temps. Either the rat slab will transpire ground moisture along with condensed atmospheric moisture or the plastic film will act like a cool condensing surface for the air born moisture. The air exchange between the crawl space and the basement (typically through a former basement window opening) will be limited and most likely stagnant in most situations. If the floor of the living room addition is insulated with batts like most are, you will slowly build up a load in the batts and have excess levels in the framing too. Not too sure what to recommend to increase the air exchange in what amounts to a dead pocket. Hopefully, others will make their experiences known.
Last but not least, many houses had the foundation tiles (now illegally) connected to the city sewers. It is good that you have correctly connected to the storm system, but be aware of unpleasant surprises that may lurk yet. My floor drain at an earlier Chicago area home gave me a most unpleasant surprise when a major storm caused the sewer lines to back up into my semi finished basement through a floor drain in the laundry area. A stand pipe solved the future risk, but we lost a lot of stuff to the poo pool. The floor drains you have now may not actually go to the sump pit.
Ain't basements in Chicago fun!
Michael B,
I suggest that you read this GBA article: Fixing a Wet Basement.