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Upgrading a vented crawlspace on a hillside

JacquesGT | Posted in Energy Efficiency and Durability on

I’m getting ready to retrofit a house in San Francisco (Climate Zone 3c).

The house is on a steep hill with over 25% grade. There’s a concrete foundation with vented crawlspaces. Because of the slope of the property, there are actually two crawlspaces, one at the same level as the bottom floor of the house/under the portion of the top floor that doesn’t have bottom floor below, and another crawlspace below the bottom floor of the house. In places the concrete foundation is mostly above grade, in other places it’s mostly below grade. The cripple walls range from non-existent to 10′ high.

There’s no insulation or vapor barrier in the crawlspace. There is a concrete rat trap.

There’s plumbing and HVAC ductwork running through the bottom crawlspace.

There are a bunch of water issues in the house, including around the windows, where the cantilevered deck protrudes from the wall, and where the garage wall meets the slab over the crawlspace (the garage is above the upper crawlspace). That’s led to rot in the cedar shingle siding in places, and in localized parts of the framing.

The crawlspaces gets very humid. I just have a few days of humidity monitoring, but it’s ranged from 65 to 73% RH. That humidity carries over into the house itself, with the bottom floor at basically the same humidity and the top floor more like 60-65% RH. That’s with no HVAC running. In a lot of my reading crawlspace humidity comes from warm moist air condensing on the cool crawlspace floor/foundation, but in my case. I think it’s often just a matter of 100% RH fog enveloping the house most mornings/evenings.

The bottom floor of the house often smells damp/musty, especially after rain. There have been termites, and a lot of the hardware in the crawlspace has rusted. There are also some dry rot issues, but those look localized to bulk water intrusions. 

I’ve been talking to contractors and architects about solutions and gotten a lot of different answers, especially about the crawlspace. Some suggested adding a fan (not sure that’ll help when the foggy air is at 100% RH). When I talked to one about insulating, he said there was no need because the ground isn’t insulated.

My general thinking is that since I need to replace the siding, repair the decks, and replace the windows anyway, I should take the opportunity to re-insulate and air-seal the house. That means peel-and-stick or fluid applied WRB, adding rigid insulation above the portion of the roof that doesn’t have attic, and then sealing the crawlspace.

Exactly what to do about the crawlspace feels kind of tricky though. Here’s where I’m at:

– It seems obvious that with the high exterior humidity I need to seal/encapsulate the crawlspace. I might not be getting condensation like in humid climates, but 100% RH fog coming into the building can’t be good.

– The above means closing up the vents, extending the WRB down to the foundation, and using something along the lines of Siga Fentrim to air/water seal the wall/foundation interface if I’m not just going with a fluid-applied WRB.

– I assume I should also insulate the cripple walls enclosing the crawlspace, say with EPS board or mineral wool batts.

– I also assume I need a vapor barrier on the crawlspace floor. Is there any problem installing it over the rat trap?

– It’s not clear to me if I really need insulation on the foundation walls or not. The ground is fairly warm in San Francisco (groundwater is 57°F and soil temperature ranges from 40 to 60 °F in the coldest month). But, if I need insulation in the cripple walls, then I guess I would need it on the foundation walls as well. I’m having an especially hard time figuring out if the usual warm moist air contacting the cool foundation walls and condensing scenario applies, because the highest RH usually occurs on cooler days, leading to fog.

– One contractor said I should add insulation to the floor above the crawlspace because I’m not insulating the ground in the crawlspace. Everything I’ve read says that’s unnecessary or even counterproductive. Because of the unevenness of the hillside the crawlspace isn’t always dug deep, if at all, but the ground doesn’t get super cold in San Francisco anyway. I’m not totally sure what to think here.

– Given how humid it can get outside and that air-sealing the crawlspace will be much more challenging than the rest of the house, it feels like I’ll probably need a humidifier in the crawlspace.

I realize this got pretty long. Does my approach sound reasonable? Anything else I should think about? The thing I’m really trying to figure out is how to find a building professional who knows how to come up with the right approach.

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