Rainwater storage in an IBC, in (barely) freezing conditions
October 29, 2021 10:11 AM   Subscribe

I need some kind of guideline to determine when a mostly-full IBC needs to be emptied before a freeze in order to prevent damage.

Got an IBC this year to store rainwater for the garden and inside plants, and it's worked very well.[1] Because of how well it's working, I want to keep using the water we've stored for as long as possible, but, it's going to freeze in a few days.

If the air temperature goes down to 32F / 0C for 30 seconds, that's technically a freeze, but it's obviously not going to freeze the water in the IBC. If the air temperature goes down to -20F / -29C for a week, that obviously will freeze the IBC. I'm looking for a safe guess about where the line is in between those two extremes. How much predicted cold, over what period of time, is enough that the IBC needs to be drained in order to prevent damage?


• Maximum capacity around 290 gallons (1100 L); probably will be around 250 gallons (950 L) during the period in question
• Container resembles the one in this photo -- thick white plastic with a metal cage. We have made modifications to the spigot, which is more exposed to the air than the rest of the tote.
• On a concrete patio, next to the house
• Wrapped in one layer of plastic house wrap and with a corrugated plastic sheet on top (both more to prevent algal growth than for insulation). The plastic wrap covers the spigot, but maybe not particularly well.

Obviously adding salt or antifreeze to the water to prevent freezing isn't an option, though we could put blankets or other items (other containers of water?) around the base if that would give us a few more days of usefulness. Might be possible to add hot tap water to the container before bed as well, though probably not in useful amounts, given how big the container is.

Our current forecast predicts

4 hours below freezing on Monday night, with an overall low of 30F / -1C, followed by

11 hours below freezing on Tuesday night, with a low point of 29F / -2C, then

6 hours below freezing on Wednesday night, with a low of 30F / -1C, then

6 hours below freezing on Thursday night, with a low of 30F / -1C.

The immediate question is whether we need to drain the container on Monday or Tuesday. Remember that the National Weather Service forecasts aren't super reliable several days in advance.


[1] Reduced home water usage significantly (about 25% lower: I have a LOT of plants); several plants that hadn't done anything in a long time suddenly started offsetting, producing new growth, blooming, etc.
posted by Spathe Cadet to Home & Garden (11 answers total) 1 user marked this as a favorite
Best answer: You could figure this out using some complicated heat transfer calculations. For example, I just googled "freezing time calculation heat transfer" and this came up - it's for a different context but you could work from the equations there.

There might be some ways to estimate, but honestly? The engineering advice would probably be to drain the system sooner (i.e. once the temperature drops to freezing) rather than later, to avoid risking damage. Especially because you'll get freezing in the spigot/drain lines/corners before bulk freezing.

The Canada Mortgage and Housing Corporation has a good guidance document for rainwater systems which covers a lot of considerations for freezing.
posted by Paper rabies at 10:25 AM on October 29, 2021

Best answer: Days where the overnight low is in the high 20s or low 30s (and significantly above that during the day) are certainly not going to meaningfully freeze much of the water in the tank. Freezing water requires the removal of a lot of heat. So, you’d need sustained temperatures well below 32 to freeze the tank in any way that’s going to damage it. Any water in the spigot may freeze a bit, but wrapping the spigot with some modest type of insulation will probably keep it from being damaged.

That said, how much of your water usage is on inside plants? In my experience, I use MUCH more water on my outside garden than I ever do on inside plants. I know you say you have a lot of plants, but is the savings from the extra ~2 weeks of not using municipal water really worth it when you’re going to have to drain it soon enough anyway? Just the cost of the spigot insulation may be enough to put you into the red.
posted by Betelgeuse at 10:28 AM on October 29, 2021

Best answer: I don't think any of those temps are going to even freeze the water in your IBC, especially if you just throw a few blankets over it. Here's a guy whose IBC froze inside a chicken coop with low temperatures of -5F/-20C and daytime highs around the freezing point, and contrary to the headline on his post, he only had a few inches of ice in it and no damage.

When it freezes, it's going to freeze at the top, not the bottom, obviously. So some freeze protection at the top will help. The container will only be in danger of breaking when you get a longer, sustained period of deeper freezing, like days on end in the 20s. (Even then I suspect it would be fine, but you don't want to take chances.) And, even if these predicted freezing temps cause an inch or two of ice at the top, you can still drain it, since it drains from the bottom. If you did that, the layer of ice might fall to the bottom, or remain suspended; either way it wouldn't damage your container.

By the way the IBC standards outfit says on their FAQ page that is can survive among other things: "Drop Test: Container is filled to near max capacity, stored to -18°C (0°F), effectively freezing IBC and contents, placing the IBC thermoplastic in stress, which is then dropped from approx. 6ft. with impact on the IBC base’s most vulnerable part, often the discharge valve" So it appears they are actually designed not to break when frozen solid.
posted by beagle at 10:35 AM on October 29, 2021

Best answer: Your current forecast: so long as it is a quarter-full, you don't need to drain at all. So long as the average temperature for a day is above freezing you have essentially no chance of freezing.

For figuring out this in the future, get a thermometer and measure the change day-to-day. You'll have many days warning that is approaching freezing.
posted by flimflam at 10:39 AM on October 29, 2021

Best answer: You can't add salt water, antifreeze, or hot water, but could you drop in an electric stock tank/pond de-icer? They're essentially intended to do exactly what you want here--keeping a relatively large volume of water free of ice outdoors in subfreezing weather. They're widely available at feed stores and places like TSC in rural areas.

You'd need to find one that would fit through whatever aperture you have in the top of the IBC (they tend to be flat discs about 12" in diameter, but there's a lot of diversity), and it would probably be a one-time expense of $30-100, plus electricity.
posted by pullayup at 10:55 AM on October 29, 2021 [3 favorites]

Best answer: In the lake around my boat house in the Adirondacks where the winter temps were near -15F at night and 0 during the day, we used a bubbler around the boat house and docks to keep it from freezing. It mostly worked. I was driving my truck on the ice on other parts of the lake ice.

I don't think your tank is going to freeze solid, but if you are worried about it, a bubbler of some sort and some blankets, will definitely keep it from freezing and doing damage.
posted by AugustWest at 11:22 AM on October 29, 2021 [1 favorite]

Best answer: It seems to me that in addition to nighttime lows, any formula you're looking for would also consider daytime highs--heating the water during the day will slow the freezing at night.

You don't say what color the housewrap is (it's usually white), but if the sun is out, wrapping it in a dark color could help warm it enough each day to prevent it from freezing for some time to come.
posted by hydropsyche at 12:14 PM on October 29, 2021

Best answer: I have 50 gallon barrels of the same HDPE material as an IBC, which were left full of water for 8 years, and are undamaged. At some points they did freeze solid. But no spigot was attached.
posted by joeyh at 2:57 PM on October 29, 2021

Best answer: There is zero chance this is going to significantly freeze while the daytime highs are above 0C. It will take days to freeze this much water as you need to get rid of massive amount of heat to freeze so much water and you can only transport away a relatively small amount of heat in the surrounding air. You definitely don't need to worry about this at all until the temperatures are much colder.

Even then, if the tank does freeze while it is consistently below zero, I doubt it would be damaged by freezing. Water only expands by about 9% when freezing. If you put a water bottle full of water in the freezer, it does not generally explode, even if you leave no airspace for expansion at all because plastic can stretch a little. Your IBC won't stretch that much, but with a little airspace, you should be fine. The only concern might be your valve, but assuming it is plastic, it's very likely going to be fine as well.
posted by ssg at 5:14 PM on October 29, 2021

Best answer: If you’re worried, a stock tank heater is cheap insurance.
posted by MonsieurBon at 10:19 PM on October 29, 2021

Response by poster: Thanks, all.

@Betelgeuse: It started out financially motivated, and then became less so when we calculated how long it would take us to break even (about 5 years), and even less so when it turned out the plants like it so much.[1] So losing money over the cost of insulating the spigot isn't really a concern.

Also, the inside plants use between 6 and 20 gallons of water per day, with an average about 9 gallons; a full IBC can last me about a month.


[1] I'm not good about up-potting plants when they need it, so minerals from our hard tap water build up in the soil. The rainwater can't be dissolving away that much that fast, so maybe the plants' response is more about the change in pH than about the change in mineral content, but either way, it makes a lot more difference than I expected.
posted by Spathe Cadet at 10:49 AM on October 31, 2021

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