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Our bathtub has recently started draining very slowly (bathtub fills with water during a normal shower and takes a while to completely empty). This problem has coincided with winter, and I'm wondering whether it is possible that the drain pipe (maybe the bend in the pipe) in our crawlspace could be freezing?

We live in Boise, ID where it is not super cold in the winter, maybe 30s - 40s during the day, and in the 20s at night. The house was built in the 1940s, and the drain pipe is cast iron. It goes down into our crawlspace, which is uninsulated (just dirt covered with plastic lining on bottom of the crawlspace surrounded by the concrete foundation).

Does anyone know if there's a good way to determine this? And how could remedy the situation? Would foam pipe insulation do the trick?

PS - no problems with other things in the house draining (sinks, washer).

If that was the case the bathwater would melt the ice plug in short order (unless it was totally blocked). I suspect a common debris plug.

Also, it usually takes much colder weather to freeze things in a crawl space. Geothermal heat typically prevents it unless you have a lot of ventilation.

Cast iron corrodes and accumulates very hard crud over time. You may get away with a good hard auguring, but you may end up digging up the iron pipe and installing PVC or ABS. It's not usually a difficult job, access permitting.

It seems pretty unlikely that the drainpipe would freeze in your crawlspace. Even if there was a 20 degree wind whipping through it (which there isn't: it's enclosed), the only way that could be is if there was standing water in the pipe to freeze. That would mean a trap in the crawlspace which would be highly unusual. And if you really had a frozen pipe, it wouldn't drain at all.

Most likely you have a hairball in the drain. Try a "drain zipper" and/or a drain auger. When buying a hand auger, it's worth it to pay more for one with a toggle lock as opposed to a thumb screw. It will save you a tone of time and your hands will thank you.

Here are some products that I have used for years and would recommend before spending a lot of money with a plumber.

Glug drain cleaner is a non-acid formula that works really well when you first experience a sluggish drain.

Crystal Heat is actually crystalized Sodium Hydroxide and Sodium Nitrate. You can't use this when there is standing water. Remove the drain stopper and pour about 1/4 cup down the drain and add a small amount of water. This stuff will boil and heat up to a point that it will actually melt grease and soap scum. It will also burn out hair clogs.

Finally, start using an active enzyme drain maintainer such as Tough Guy to help prevent future clogs.

According to the respective manufacturers: Each of these products are safe for all plumbing applications and septic systems.