Basement ceiling + wall insulation in a hot climate (CZ 2)?
I have a house in Climate Zone 2 with a basement (rare in this neck of the woods). The basement ceiling has fiberglass batts, and parts of the walls have rigid insulation (polystyrene), some interior, some exterior (though I don’t think there’s a sandwich anywhere).
What I think I should do is:
(1) Run a dehumidifier to keep the RH around 45%. I started that, and the basement was very damp before. With the unit set to 45%, it doesn’t have to run often to maintain the 45%.
(2) Remove the ceiling batt insulation, to help the basement cool the rest of the house. I realize I might need fans to circulate air between the basement and the 1F to make this really work.
(3) Remove the wall insulation that’s below grade, to let any heat in the basement easily escape through the walls to the ground.
(4) Keep the wall insulation that’s above grade, to prevent hot ambient air from heating up the basement.
(5) Don’t caulk the rim joists, because I’m not trying to prevent air exchange between floors.
Is all this correct?
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My intuition is that removing the insulation will do very little to cool the house above, but will make the house colder in the winter. I think the cooler basement air will just stay below like an insulated beverage mug/cup.
Non-professional answers:
Basement ceiling is often the wrong place to put insulation. And batt insulation is usually not great in horizontal applications, although it sounds like it was supported by ceiling panels. And horizontal insulation that's been in a basement is probably full of (mice) crap, so you should probably pull it out anyway. Use a mask.
Feel better now?
Standard advice for basements is clean them up and then air seal, focusing on sealing all the holes and seams in your rim joists. After you do that, you can decide if you want to insulate the WALLS with air impermeable insulation (not batt, not fibrous). There are a lot of articles on GBA on this topic. If you get really ambitious, you could then insulate the floor, but that's a lot of work to do correctly. A dehumidifier is a good idea.
Good luck.
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Unless you actively exchange air with the basement, I don't think you'll get much effect from the cooler lower level. I live in a 2 level house built into the hillside, so downstairs is a half basement with the exterior walls insulated and it runs usually 10-15 degrees cooler than upstairs. We have no AC, turn on the heat maybe 1 week a year, and the house always overheats in summer, the downstairs has almost no affect on upstairs. I get some barely noticable affect by recirculating air with downstairs. The main benefit is we can retreat to downstairs when the upstairs is unbearable.
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I'm no expert but from what I gather it's more about air circulation then insulation when it comes to keeping it cooler upstairs. I read something about keeping your furnace fan on, temporarily blocking any vents you might have coming from the basement to stop the air from getting sucked down in there and using fans to blow the air around the first and second floor/keep the basement door shut. Focusing on the attic if you have one is helpful too... Like I said no expert just some stuff I've come across.