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New (first-time) home owner here (4 months in). I recently discovered what at first seemed like an un/disconnected supply-side duct in our vented (open) crawl space. Photo attached. After several hours on the internet educating myself, it appears this might've been put there intentionally to keep the plumbing in the crawl space from freezing (although I did notice tape residue near the perimeter of the opening, which might suggest someone tried to close this off at one point).

We live in eastern WA (Spokane-CDA), which is Zone 1 for radon and sees several days of 32F (or below) weather, but nothing close to, say, the mid-Atlantic. I have a thermo/hygrometer down in the crawl space that was putting out 59F when this "vent" was open, and ~ 57F with this temporarily shut off with the foundation vents open (it's about 32F outside right now - humidity in the crawl space has stayed below 50% RH). The fact that the crawl space is 10F below the house setpoint is probably an indication of a larger leaky ductwork and subfloor air leak issue, however I was curious what everyone thought about leaving this particular port closed off? Note there is no mesh or damper on this right now.

I'm generally curious to know what is considered the "minimum" ambient temperature inside a vented crawlspace at which the likelihood of pipes freezing becomes likely/imminent? Am I ok to have the ambient temperature pretty much drop down to 32-35F before freezing becomes likely? There is a delta T of about 25F between the inside of the crawlspace and the outside right now (foundation vents open), so I imagine this would be the scenario where the outside temperature drops to ... 10F? Unlikely where I am, but I'd like to know where my goal posts are.

Thanks for reading, and I'd appreciate any thoughts/comments on this.

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    W/o it your floor will be colder and the crawl space wetter. Passively vented crawl spaces are yuck. I'd leave it if you're not going to change the venting scheme of the crawl space. Freezing is a tertiary concern, but cannot be ruled out.
    – Mazura
    Dec 27, 2020 at 6:06
  • Thanks for the input, @Mazura. For the time being, I'm planning to install a mesh screen + damper or something similar so I'm not wasting supply air necessarily (there are other issues leaving this wide open re: not having enough air upstairs). This leaves me the option of bleeding hot air into the crawlspace if the temp/humidity warrants it (extremely cold days, etc.). Longer term, I think I'll probably be overhauling how I regulate venting, temperature and humidity down there.
    – rokmeamadeus
    Dec 27, 2020 at 23:46
  • 6" White Round Ceiling Vent w/ Damper – installerstore.com. If you close it all the way it will probably still leak enough to be the best of both worlds. Available at just about every hardware store on the planet.
    – Mazura
    Dec 27, 2020 at 23:58
  • "The fact that the crawl space is 10F below the house setpoint is probably an indication of a larger leaky ductwork and subfloor air leak issue" I disagree. Warm air rises, and floors over crawlspaces typically aren't airtight. Not to mention your crawlspace is vented and has one air duct (presumably) for the entire space. Your house, on the other hand, is not vented to the outside air directly, and has at least one, probably multiple, air duct openings per room. It makes perfect sense that your house is warmer than the crawlspace, especially in the wintertime.
    – TylerH
    Mar 31, 2021 at 16:22
  • Personally I have not heard of having air from your HVAC system blow directly into your crawlspace when the crawlspace is vented... it seems like you're just wasting that air for the most part. If the crawlspace were sealed, it would make sense, but you would also need an air return vent somewhere as well so that used conditioned air could be returned to the HVAC system. However, I'm in the Southeast, so I know our climate is different from the PNW. Maybe this kind of thing is more normal there.
    – TylerH
    Mar 31, 2021 at 16:24

2 Answers 2

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Without addressing the moderately unanswerable "do I need this duct" since that depends on a lot of factors specific to your house/insulation/comfort/budget/etc.

Moderately standard freeze protection thermostat setting is 45°F/7°C and "freeze alarms" at 41°F/5°C - running much closer to freezing than that risks freezing in spots that might be colder than where you are measuring the temperature, or in the case of the alarms does not give you time to take action.

The level of engineering you need to "not freeze pipes" while running them as low as 33°F/0.5°C is considerable and rarely engaged in. i.e. standard "heat trace" (the quality electric pipe heating tape that is the thermostat) starts to heat at 50°F/10°C, and (along with an insulation overwrap) is the standard (quality) solution to below-freezing crawlspace pipes down to sub-zero °F temperatures. The cheap heat tape with an external thermostat is much more prone to fail, but is a common approach by those who like to make their own mistakes rather than learning from the mistakes of others.

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  • Perfect - thanks for sharing. The 45°F/41°F value is what I was looking for. I suppose I should've looked up freeze alarms as a first step. Just getting down to 55°F makes me uneasy, so I definitely wouldn't want to be anywhere near 32°F.
    – rokmeamadeus
    Dec 27, 2020 at 23:39
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At least a screen is definitely a good idea. The open duct provides the perfect highway for rodents and other critters to enter your home.

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    If a rodent or critter is going to get anywhere on this highway, it's already in the house, since this open duct goes from the crawl space into the duct work. Granted, you don't want a kitty crawling through the ducts and heading toward the furnace because it's nice and warm in there (kitties are annoying like that), but the cat will already be in the house by then...
    – FreeMan
    Mar 31, 2021 at 11:57

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