Unique tidal basement
I have a one of a kind basement insulation question. The house is a beachfront Cape Cod built in the 1950’s situated about 20ft from the water. Foundation is CMU block and appears stable. The basement has over 6ft of clear head space but the floor slab is porous and allows water into the basement at high tide. Our sump pump removes any standing water and the floor dries up nicely between tidal cycles. We have a well-insulated house (3 inches of closed cell spray foam) but the issue I’m having is how to treat the basement. The floor of the house above is currently un-insulated and I am wondering whether I should insulate the floor joists with rigid foam insulation to separate the conditioned living space or simply continue to treat the basement as part of the house?
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Your "basement" sounds more like a glorified crawl space. Is it being used for storage, laundry, or HVAC mechanicals, or is it completely empty?
For an uninsulated basement such as yours the IRC prescribes under-floor insulation. See the "FLOOR R-VALUE" column in TABLE N1102.1.2, about half way down this page:
http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%2011.html
Open cell foam outperforms closed cell at any given R-value when it is thermally bridged by framing/joists, making it a better value, provided there is sufficient joist depth to get to code-min. Doing it all as rigid foam ends up being pretty thick, but if you did a full cavity fill with fiber insulation with rigid foam below sufficient for dew point control (varies by climate) the thermal break over the framing provided by the rigid foam gives it a significant performance boost.
Dana,
You wondered whether Steve uses his basement for storage. Since Steve told us that the porous floor "allows water into the basement at high tide," and since high tide occurs twice a day, I doubt if he is storing much down there -- except, perhaps, live lobsters.
-- Martin Holladay
Yes it's totally a glorified crawl space. No laundry but it does house the boiler which keeps the basement warm when it runs. We use the area to store some kayaks and that's about all. I have a bunch of salvaged foam insulation from a commercial roof rip off that I can cut to fit between floor joists. I have also been trying to seal as many air gaps as possible at sill plates and rim joists and I am starting to see a big improvement but I am slightly nervous that adding any insulation will trap moisture in the floor joists and sub-flooring by not allowing them exposure to air.
I've been amazed at just how much people will still put in basements that flood regularly, protected only by a $39 sump pump. I don't assume anything anymore.
Funny Martin! We could probably keep some lobsters in the sump quite happily. The boiler is well elevated (at least three foot off the floor) and the kayaks are on saw horses
Cutting foam to fit between joists is a waste of high R/inch foam, since it's thermally bridged by R1/inch timber. It's better to use full-sheets and tape the seams. But in your case you probably don't want to insulate the floor:
Since the boiler is in the crawlspace, it's better to insulate at the foundation walls, despite the tidal ebbs & flows. If you insulate between the boiler and the fully conditioned space above you guarantee that the standby and distribution losses are truly lost, and it may even INCREASE your annual fuel usage.
If your salvaged foam is polyiso it can still be used on the foundation walls, but the bottom edge has to be well above the historical high water mark. If it's polystyrene (EPS or XPS) it's fine to let it get wet. It's fine to do the upper half in polyiso, and the bottom half in polystyrene. The basement wall-R is also spelled out in TABLE N1102.1.2.
The wall foam needs a thermal barrier against ignition to meet code. For the portion that will get wet you can use a cement board through-screwed to the foundation. For higher up the wall you can use half-inch wallboard on furring through-screwed to the foundation.
My first thought, is this a good situation to use Xypex?
I wonder if you can rig up the sump pump as a generator and generate electricity as the tide flows in and out.
More seriously, I would think that something like xypex would reduce the amount of water, although I would not expect it to stop water flow. That would reduce your evaporative cooling rate and reduce your sump pump energy use.
Steve,
Dana's solution gets my vote: recycled rigid foam attached to the CMU walls, covered with cement board.
As I'm sure you know, your house faces a limited future. It's only a matter of time before the real estate panic sets in, and the value of oceanside homes drops precipitously. The New York Times did a good story on what's likely to happen: Perils of Climate Change Could Swamp Coastal Real Estate.
The tipping point in real estate values is likely to happen suddenly, in the wake of the next big storm.
-- Martin Holladay
Thanks Guys the Xypex recommendation would work if the slab weren't in such poor shape. I fully intend to pour a new 5-inch slab over the old one when funding allows. (I'll waterproof first). First order of business will be to insulate rim joists.
Martin, it' already happening out there. The neighbors are raising their houses thru state incentives. I grew up on the coast but have been living in land for the last 20 years and I am just happy to be by the sea
Flood insurance article in today's Wall Street Journal: https://www.wsj.com/articles/congress-set-for-heated-debate-on-flood-insurance-program-1489055403
I had missed the NYT story that Martin referred to, it's quite good.