Introduction: Cozy Low-Energy Shower
Showering is one of the most energy intensive activities in a typical American's day, using energy to heat the water at a rate comparable to driving a mid-size car. Rather than trying to shorten your shower time, or make do with cooler water, you can make your shower more luxurious and make it use less energy.
The main ideas are to make the shower stall fully enclosed, to make it cozy and warm without using as much hot water, and to make it easy to turn the water on and off as needed, with a consistent, comfortable water temperature.
There are actually four specific changes I made in my shower towards this end. For the ultimate shower, I recommend all four, but you can use as many or as few as you want. The total cost is around $100 (depending on many options), but if you want to do it on the cheap, you can also do much of it for under $10.
This project works regardless of whether your goal is to be super-green and have the ultimate low-energy shower (while still being comfortable), or you want to have a super-luxurious shower, while saving a little energy.
Step 1: Four Components
I've changed four things in my shower to make it cozier, and to make it use less energy and water. They can be used alone or in combination--you might start with one or two and start saving right away, and consider doing more later.
The four components are:
A) Enclose the shower stall fully, to keep it cozy and warm inside, even when the water is off. The idea is the same as the Shower Dome, available in New Zealand for about US$300, installed, but costs much less!
B) Pedal-controlled water flow, so it's easy to turn the water on and off, and modulate the flow as needed.
C) With the water fully shut off at the showerhead, either with the pedal control or with a conventional button valve at the showerhead, mismatches in pressure can make the hot water back-feed the cold water or vice-versa. When you turn the water on after it's been off, you get a blast of hot or cold water. If you have this problem, there's a plumbing trick that can solve it.
D) Choose your showerhead carefully. Some low-flow showerheads chill the water by evaporative cooling when they atomize the water into a fine spray. Then you need to use more hot water to stay warm. There are better showerhead choices.
Step 2: A) Enclose the Stall: Why?
A fully enclosed shower stall is a luxury. You will be surrounded by warm steamy air, and will be completely comfortable, rather than having one side warm and one side cold.
You'll also save energy: as the shower stall warms up, you'll be comfortable with lower and lower water temperatures. And you'll be warm and comfortable even if you turn off the water while you soap up.
In addition, an enclosed stall steams up the bathroom less. So you'll be able to use the mirror right away after a shower, and there's less need for an exhaust fan. A dryer bathroom is less prone to mold problems, and towels hung there dry faster.
You can even save on air conditioning: Air conditioners remove moisture from the air, and require a lot of energy to do so. It's much more efficient to avoid putting as much moisture in the air to begin with.
You might think that you'd put just as much moisture in the air regardless of enclosing the stall--if running the shower produces moisture in the air, wouldn't the total moisture output be the same in either case? The reason you put less moisture in the air with an enclosed stall is that the humidity gets to 100% pretty quickly in the stall. The air inside is saturated, and no more moisture goes into the air--it stays in the water and goes down the drain.
You might also think that even though the bathroom stays dry, the stall itself would get so wet that you'd have terrible mold problems there. My experience is that the opposite is true. It seems that if the air in the bathroom is dry, the shower stall dries much more quickly once you open the door to the stall.
For more about how useful this is, and to read customer testimonials, see the New Zealand Shower Dome site.
Non DIY options include buying a new fully enclosed stall, or, if you are in New Zealand, getting Shower Dome to add a lid to your existing shower. Commercially available fully enclosed showers include ones meant for steam showers such as these, or there are showers meant to be built in with a lid. But upgrading what you have is a lot greener than throwing it out and buying all new stuff! And you can make one for a lot cheaper than the ~USD300 it costs to buy one from Shower Dome, even if you are in new Zealand.
Step 3: A) Enclose the Stall: Planning
I enclosed my shower by putting a piece of Plexiglas (clear acrylic plastic sheet) over the top, and hanging clear plastic drop-cloth material to cover the gap in front. The Plexiglas is supported on three sides by sitting on top of the fiberglass shower stall; and by an aluminum channel in front.
Since all of the surfaces inside the enclosure will get very damp during the shower, you don't want to have any unprotected wallboard or plaster inside the enclosure. Normally this means that the Plexiglas lid should go right at the top of the existing tile or fiberglass, low enough that any wallboard is above the lid. If that means you'll bump your head, see step 23 for some other options.
Measure the size needed for the Plexiglas carefully. It needs to be big enough to sit on top of the tile or fiberglass around the edge of the stall without falling in, but small enough to fit between the walls.
If there are three walls where the Plexiglas can sit on top of the stall, the support for the third side can be a simple piece of aluminum angle extrusion. If there are two sides that need added support, it may be necessary to add a support for the corner between them--either a leg going down to the outside of the stall, or a cord hanging it from the ceiling.
In some showers, the showerhead comes out of the wall above where the lid will sit. That's OK--it just means you'll need a hole in the lid for the showerhead to come in.
Step 4: A) Enclose the Stall: Materials & Tools.
1/8 inch Plexiglas (acrylic sheet) cut to a pretty precise size. You can buy Plexiglas at most hardware or home improvement stores; some will cut it to size for you. You can also order it online, cut to size. For example, from Interstate Plastics. From Interstate Plastics, you can specify a tight tolerance on the size (I'd go with +/-0.030") for only about a dollar extra--well worth it to be sure it fits right. You can also cut it yourself. You can scribe it with a utility knife and break it along the scribe line, or you can use a table saw with a fine-tooth carbide blade if you have one.
You could use any light, reasonably rigid material: thin plywood, fiberboard, even cardboard waterproofed with a plastic sheet or paint. But a transparent, or at least translucent, material helps let light in.
Aluminum L extrusion, long enough for the edge that needs support. The exact size and thickness doesn't matter much--mine is 1" x 1" x 1/16" thick. You can buy it at a hardware or home improvement store and cut to to length by hand with a hack saw.
Polyethylene sheeting (drop-cloth material); or other material to cover any gaps. Other materials can be used: You might cut up an old shower curtain, or buy some water-resistant fast-drying cloth such as nylon. I chose polyethylene sheeting because it is less toxic than vinyl shower curtains, because it's clear (you want the shower warm but not dark), and because I had some around. IKEA has a great selection of non-vinyl shower curtains if you want to have some color options without using vinyl.
Binder clips and/or clear packing tape for attaching polyethylene sheeting
Hack saw for cutting aluminum
File to smooth edges of aluminum
Scissors for cutting polyethylene sheeting
Step 5: A) Enclose the Stall: Cut the Lid Pieces
Cut the Plexiglas if you didn't order it cut-to-size, either by scribing with an Xacto knife and breaking it, or using a table saw with a fine-tooth carbide blade.
Cut the aluminum L extrusion to length with the hacksaw. Better to err on the side of making it too long, and have to cut again, than to have it too short. But even better, measure twice and cut once. After cutting, smooth the edges with a file. Check that it fits.
Step 6: A) Enclose the Stall: Assemble the Lid
Have the Plexiglas and the aluminum within reach at the shower stall. Place the Plexiglas in position, and then place the aluminum L extrusion under the front edge. You could glue or screw them together, but I didn't find that to be necessary.
The aluminum piece is necessary: I first tried without it, but found that the Plexiglas wasn't stable--it would flex and fall down every once in a while. (Note the broken corners which I crudely patched--don't let that happen to yours!)
Step 7: A) Enclose the Stall: Cover Other Openings
I simply cut a polyethylene sheet with scissors to drape over the opening above the door, and clipped it in place with binder clips. Since I'm taller than the door, I wanted a flexible material that I wouldn't have to worry about bumping my head on.
When it's really cold outside the stall, and warm inside, you can feel cold air coming in any little air leak, especially near the bottom. To greatly reduce that effect, I cut a one-inch strip of polyethylene sheet and taped it across the bottom of the door as a weatherstripping flap.
Step 8: A) Enclose the Stall: Using It
Enjoy the warmth!
If you notice any cold air leaks, cover them with polyethylene flaps, taped on, as needed.
If it gets too hot and steamy inside, just turn the water temperature down, and think of all the money you are saving and greenhouse gas emissions you are avoiding.
When you are done, if it's cold outside the stall, you might want to quickly open the door, grab a towel, and dry off inside the stall with the door closed, to stay warm. After you've dried yourself, the cold air outside won't chill you as fast.
Make sure you leave the door open to let the enclosure dry out--which will happen quickly with the rest of the bathroom still dry!
Step 9: B) Pedal Control: Introduction
For many years I've had showerheads with a little button valve to shut off the flow, on the theory that I could shut off the flow while I soaped up. But that was just the theory. I rarely shut off the water in practice. In part because I got cold, but even once I had the stall enclosed, it was still just a little inconvenient to reach up and shut it off, and my early morning motivation level is not what I'd like it to be.
My solution is a pedal controlled valve. To push myself on the motivation factor a little further, it's a momentary valve--the water only turns on while I'm pushing the pedal. That might seem to make it hard to turn around and such. Turning around does require turning off the water momentarily, but operating the pedal from different angles is no problem, and it quickly became second nature, even before my morning coffee. The pedal makes it very easy to turn the flow on and off as needed.
Falls in showers are a common cause of injuries, so if you or some of the people using the shower are at risk for falls, consider whether you want to include this feature. If it makes sense for some users and not for others, the pedal can easily be hung out of the way for people who don't use it. Installing a non-slip mat or grip tape on the floor and grip handles on the wall are good ideas in any case.
Step 10: B) Pedal Control: Materials and Tools
The valve I used is a spring-loaded valve that is actually intended for showers such as poolside or beach showers. I ordered it from Kings Supply. $38 sounds kind of expensive but it's nicely made and finished, and sells for nearly twice as much elsewhere.
For the arrangement I used, the other plumbing parts I needed were two 1/2 inch "street elbows" (a 90 degree bend with male threads at one end and female threads at the other end), and one short threaded piece of pipe (male threads at both ends, sometimes called a "nipple"). I used the length of the valve to move the showerhead up vertically because I'm tall and wanted more headroom under the showerhead, but if you don't want that, or there isn't room, you can also turn left or right, or come straight out, instead of turning upward. I used standard zinc-plated steel fittings, which looks kind of grungy. If you want to dress it up, you can use chrome fittings. One other part that might be useful in some situations is this adjustable shower arm.
The pedal is connected to the valve with a polyester cord. The cord needs to be something that doesn't stretch too much, so that stepping on the pedal opens the valve rather than just stretching the cord. The less stretchy it is, the better control you have. Polyester (Dacron) is much less stretchy than nylon. I used some polyester cord that I ordered from REI, but they don't seem to stock it anymore. You can often find polyester cord locally at hardware stores or boating stores; you can also get it from "The Rope Guy" on ebay . Manilla (hemp) rope is pretty good at not stretching too, but the rustic brown color might look a little dingy in a shower, and it might be more prone to rotting. A chain is another option, but a steel chain would be prone to rusting, and chains might be hard to clean.
The pedal itself can be a thin plank of wood or a sheet of Plexiglas. Plexiglas is easier, since you don't need a waterproof finish, but I used wood since I had some nice oak scraps from another project. The exact dimensions aren't critical, but mine is 5/16 inch thick by 3 inches wide by 13 inches long.
Other materials needed are Teflon tape for threaded pipe connections and a waterproof wood finish if you use a wood pedal. (I used pure tung oil and dewaxed shellac, because I like non-toxic natural materials, but you could also use any marine varnish. See step 25 for an appendix on the tung-oil/shellac finishing process.)
Tools needed include pliers, a rag, scissors or a knife, a saw (unless you buy Plexiglas for the pedal cut to size), a drill, and supplies for wood finishing if you use a wood pedal.
Step 11: B) Pedal Control: Do the Plumbing
Unscrew the showerhead. If you need to use pliers, wrap the rag around it first to avoid scratching it. Now you have a pipe sticking out of the wall--the "shower arm". Unscrew it too, again using the rag and pliers if necessary.
Now assemble the elbows, pipes and valve. Wrap each set of threads with Teflon tape before threading it in. It's easiest to tighten all the pieces together before connecting to the wall, but you might thread them into the wall loosely just to be sure everything fits together the way you want. Don't forget the trim ring. (You can use the old one.) Tighten well--not only to prevent leaks, but also to make sure that it will stay together when you pull on the valve with the pedal.
When finally tighten the assembly into the wall, it might get tight at a different final angle than you want. If that happens, you can adjust by adding more Teflon tape so that it gets tight sooner.
You can check that it works now, but without a pedal, it's not very convenient for showering!
Step 12: B) Pedal Control: the Pedal
Cut the pedal to size. Since it's small, and the dimensions aren't critical, you can use pretty much whatever kind of hand saw or power saw you are familiar and comfortable with. Sand or file any sharp or rough corners or edges. Drill a hole in the center of one end, just a little bigger than the cord you will use, about 1/2 inch from the edge.
If you are using wood, sand it smooth and finish it. To get a truly waterproof finish using non-toxic materials, I used tung oil and de-waxed shellac, as described in step 25. If you use a different finish, follow the instructions that come with it.
Cut a piece of cord about a foot longer than the distance from the floor to the valve. Put one end through the hole in the pedal and tie a knot in the cord so it can't pull through the hole. Then tie the other end to the ring. It might take a few tries to get the length adjusted just right so that the valve opens all the way when the pedal is pulled to the floor. Be sure it's not too short--you don't want to pull the pipes out of the wall if you step on it hard!
Step 13: Pedal Control: Use
I like to adjust the temperature while I'm out of the shower, reaching in to hold the valve on. Then I turn off the water, step in, and step on the pedal.
As you are showering, you can move the pedal around on the floor as convenient for use with either foot, and you can face towards the pedal, using it with your toe, or away using it with your heel.
When you are done, don't count on the new pedal valve to shut off the water, but make sure to turn off the regular valve(s)-they might drip otherwise.
Hanging the pedal up after a shower can help it dry--that probably only matters for a wood pedal.
Step 14: C) Temperature Stabilization: Introduction
If you shut off the water at the showerhead, either with the foot pedal or with the more common "button" type valve on a low-flow showerhead, in some cases it will come back on much hotter or much colder than when you turned it off. If that happens, it could be because the hot and cold water supply don't have exactly the same pressure. If they are different, one can back feed into the other when water is turned off at the showerhead.
For example, in my system, the pressure in the hot water supply is higher. With the valve at the shower head turned off, the hot water feeds into the cold water pipe. When I turn the valve back on, I get scalded by 100% hot water, until the hot water has come back out of the cold water pipe. See the technical notes at the end (step 22) for more on why this happens, but more important is how to solve the problem: by adding a check valve (one-way valve) to prevent the back-flow. The following pages describe how to determine what you need and install it.
There are other possible solutions: a simple one is to only turn the flow way down, not completely off. For small pressure differences, that can work OK; but it still wastes some hot water, and the temperature stability may still be poor. Another option is to put in a thermostatic mixing valve to automatically maintain a constant temperature, but that's more expensive and more work to install. The check valve is a good compromise--reasonably easy to install, and works well to solve the problem.
Step 15: C) Temperature Stabilization: Identify the Problem
The first step is to check whether you have a problem, and if so, whether the hot is back-feeding the cold, or vice versa.
Turn on the shower, let the hot water warm up, and adjust for a moderate temperature. Now shut off the flow at the showerhead and go away for a few minutes. Come back and turn it on. If the water quickly becomes scalding hot, your hot water is back-feeding the cold water pipe. If the water is ice cold, the cold water is back-feeding the hot.
If it came out hot (hot water back-feeding the cold pipe), you need a check valve in the cold.
If it came out cold (cold water back-feeding the hot pipe), you need a check valve in the hot.
Now you need to find a location to add the valve. If the pipes are accessible near the shower, that's ideal. If not, you could add the check valve to a main hot or cold water feeding the house...but then it won't always work as well if other plumbing in the house is being used. So you may need to open up the drywall behind the shower or in the ceiling above it in order to get access to a pipe that is just feeding that shower. Once you've found the spot, measure the pipe diameter--you'll need that to choose the right fittings.
Step 16: C) Temperature Stabilization: Materials and Tools
All these materials are things you can find at a good hardware store.
-Check valve. If you are installing it in the hot water line, make sure it is one that is OK with hot water.
-Adapters to connect the fittings on the check valve to the pipe. There are lots of ways to do this, but I got adapters from the threaded fittings on the valve to solderable copper fittings, and got four solderable copper street elbows.
-Plumbing solder and flux. Even you don't plan to drink from the shower, somebody else might, or you might be wrong about where the pipe goes, so don't use (toxic) lead solder.
Wire brush to clean the inside of solder fittings prior to soldering.
Torch for soldering
Step 17: C) Temperature Stabilization: Do the Plumbing
First turn off the water at the main valve where it enters the house. If possible, empty the pipe by turning on a faucet (cold or hot--whichever you are installing the valve in) at a level below the shower, and turning on the shower. Then, with a bucket underneath the pipe, go ahead and saw through with a hack saw, and saw out the correct length of pipe needed to put the valve in with the fittings you are using.
Clean the outside of the pipe with fine sandpaper so that it will solder well. Unless the fittings are very new and shiny, clean the surfaces of them that you'll solder too, using sandpaper for the outside and a wire brush (or sandpaper) for the inside.
As you can see below, I'm not a pro at soldering plumbing. So rather than taking my advice on soldering, read the instructions that come with the solder and the torch. But even though my job is ugly, it doesn't leak, so don't be afraid of trying this is you aren't a pro either.
Solder one pair of elbows to the adapters that will thread onto the valve. Do this before attaching them to the valve, to avoid overheating the valve. Then assemble these assemblies onto the valve, using Teflon tape to ensure a good seal. Next assemble this onto the pipe using the other two elbows. First solder the elbows to the pipe, making sure the spacing is right to fit the valve assembly.
Before the final soldering pause and make sure you've got the flow direction right! (You want flow towards the shower.) Now solder the last two connections, taking care not to overheat and damage the valve.
Turn the water back on and check for leaks.
Step 18: C) Temperature Stabilization: Testing and Using
If you are sure none of the joints in the valve assembly are leaking, test the hot and cold water individually at the shower. Next try mixing a moderate temperature, and turning off the flow at the shower head. Go away for a few minutes, and come back and turn the flow back on. It should start up right at the same temperature, and stay close to that temperature.
One further step you can take to help maintain a more constant temperature is to insulate the hot water pipes wherever they are accessible. That will save additional energy as well as keeping the temperature more stable.
Step 19: D) Showerheads: Intro
It's well known that low-flow showerheads can save water and energy. But not only is it tricky to find one that you like the feel of--it's also tricky to find one that doesn't incur a hidden extra energy cost.
Low-flow showerheads typically aerate the water and deliver it in a fine spray. The fine droplets cool very quickly as they fly through the air. You might not notice this, because you can simply turn up the temperature, and still have the water be hot when it hits you. But turning up the temperature means extra energy to heat the water. The first person to note this effect was perhaps Alan Meier in a 1985 Home Energy Magazine article.
The worst in this regard is, ironically, the classic small-head low-flow showerhead often promoted in water and energy saving programs. Whether or not you like the prickly feel of the high-velocity fine spray it delivers, the cooling effect negates some of the energy savings the low flow provides.
Fortunately, a few manufacturers have noticed this problem and introduced showerheads specifically designed to deliver a forceful spray with larger droplet and no aeration to maintain the heat in the water. Or, knowing about this consideration, you can find simple, inexpensive showerheads that work well.
Step 20: D) Showerheads: Choices
Some showerhead options:
The most intensive effort to develop a low-flow showerhead that offers an intense spray without too much cooling seems to be the work done by [www.bowlesfluidics.com Bowles Fluidics] on the Delta "H2O-Kinetic" system, which Building Green named one of the "Top 10 Green Building Products for 2006". It's available in an adjustable 1.6 to 2.5 gpm version, model 75152, from Amazon for $30, or in a fixed 1.6 gpm version, model RP46384 for about $35 from several sources, including Faucet Depot. I bought one and found that it does seem to maintain the temperature well, and it delivers a really intense spray--I'd call it more like a blast. If that's what you are looking for, it's a great choice.
Personally I prefer a somewhat less forceful spray, so I went back to a basic plastic 2.5 gpm showerhead, which doesn't aerate much delivers the water in coarse streams that also don't cool the water much, even though a lot less engineering went into it than into the H2O-Kinetic system. The one I have (second picture) is labeled Masco, which is the name of a company that owns lots of plumbing brands but as far as I can tell they don't make this one anymore. But it's nothing special. Similar ones are available for a dollar or two.
Some other options worth checking out:
The Niagara non-aerating showerhead advertises similar temperature advantages, at 2 gpm, and is adjustable from a "rain-like" spray to a more intense massage. It's available from EFI.org in white
for only $5 or chrome for $6.
Holiday Inn Express did extensive research on consumer showerhead preferences (according to this article) and is so proud of their chosen Kohler showerhead that they sell it in their online store for $80. It has three spray settings, one of which is aerated. Its 2.5 gpm rating isn't super low, but the endorsement of 7,000 testers and the flexibility of several settings to find ones that you and others using your shower like might make it worth $57 from Amazon (It's the Kohler Forte K-10284).
If you have a system like the foot pedal control to shut off the flow easily, it's not as important
to have a low-flow showerhead. But it does still matter that it doesn't chill the water too much, and that you like it!
Step 21: D) Showerheads: Intstallation
To install a showerhead, you'll need, in addition to the new showerhead,
-Teflon tape for the pipe threads (find it in a hardware store with plumbing supplies).
Before gripping the old showerhead with pliers to unscrew it, wrap a rag around where you are going to grip it, to avoid scratching it. Then unscrew it (counterclockwise).
Wrap several layers of Teflon tape on the threads, and then screw the new showerhead on. Finger tight is almost good enough, but you might snug it a big with the pliers (again, use the rag to avoid scratching).
That's all there is to it!
Step 22: Enjoy It All Together
With a fully enclosed stall, easy instantaneous control of the water flow, and a showerhead you like, you can have a relaxed, comfortable shower while using very little water and energy, without steaming up the bathroom.
When you are done with the shower, turn off the water at the main valve to prevent dripping, and open the door to allow it to dry.
The project is done, but you can read on to learn more about how the enclosure saves energy, about why the water pressure mismatch arises, about other options for enclosing the shower, and about nontoxic waterproof wood finishing.
Step 23: Appendix: Other Enclosure Options
If the height of the tile or fiberglass shower enclosure is not right for the level of the lid, you can't just rest the Plexiglas on the rim like I did. Here are some other options:
1) If the tile goes all the way up to the ceiling, you can hang the Plexiglas lid from the ceiling. For example, drill holes about an inch in from the corners of the Plexiglas, and hang it with synthetic cord. Keeping it at least a few inches below the ceiling will help prevent moisture from accumulating between it and the ceiling. The lower you hang it, the warmer you'll be since you need to heat up less space.
2) If the tile or enclosure ends too low and you'd bump your head on the Plexiglas, you can either tile more of the wall to be able to put it higher; or you make a tent-like top from plastic sheeting. You can raise the center by hanging it from the ceiling; then tape the edges to the tile or fiberglass enclosure walls using clear packing tape. To attach a cord to the center of the plastic, bunch the plastic around a marble-sized rock (or marble, or anything else about that size) wrap a cord around it, and tie it off. Then hang that from a hook in the ceiling.
Option 2 is also good for a quick, cheap, and dirty option even if the height isn't a problem.
Step 24: Appendix: Technical Notes
Below are some optional technical notes, in case you are interested in learning more about why the backflow problem can happen, and about some more energy advantages of an enclosed shower.
What makes hot water backflow into the cold water pipe or vice versa? It makes sense that if
the hot water pressure is higher than the cold, it would go into the cold water pipe. But
why would the pressure be different?
In a closed system, like a hydronic heating system's hot water loop, increasing the temperature builds pressure as the water expands. But the hot water supplying the shower comes from the same pressure source as the cold water (city water or
a pump from a well), and both should be at the same pressure. At the water heater, they are at the same pressure, but if the shower is a floor or two above the water heater, the difference in elevation changes the water pressure, due to the weight of the water. The cold water density is higher than the hot water density, so its pressure is reduced more--the net result being that the hot water pressure is higher than the cold water pressure at the shower (if it's above the hot water heater).
The enclosed stall has many energy advantages. The simplest ones are that in a warm stall
you don't need the water as hot to stay warm, and that it's more comfortable to turn off the water flow if
the stall is warm.
The high humidity inside the stall is also an advantage: the water coming
out of the showerhead doesn't cool as fast by evaporation when
it's in such humid air, so you don't need to use as high a water temperature. And evaporative cooling doesn't just cool water--it cools wet people too, so the humidity helps keep you warm while your skin is wet.
Reducing the humidity released by the shower into the house is clearly an advantage in the summer when the air conditioning has to work hard to remove humidity. Many houses get too dry in the winter, in which case adding humdity might seem to be a good idea. But excessive dryness in the winter can be a sign that a house has many air leaks. It's better to fix those
leaks, following the excellent instructions
on the Energy Star web site]. You'll save heating energy as well as reducing dryness.
Step 25: Appendix: Non-toxic Waterproof Wood Finishing
Tung oil is a traditional way to waterproof wood. It's oil extracted from tung nuts that cures when exposed to air. You spread it on the surface and allow it to soak in, and then wipe off the excess. It will still be damp, but if you leave it in a warm place for a few days it will cure and partially seal the wood. You repeat this process two or more times. More instructions, and the oil itself, are available from the Real Milk Paint Company. Note that most instructions are for using it for fine wood finishing. For just trying to make something waterproof, you can be less careful and put on thicker coats. Note that it cures faster if you leave it in a warm spot--I use the top of a radiator in the winter.
If you go with tung oil, note that there are lots of "tung oil finishes" that contain little or no tung oil, and use the name simply to mean it is a wipe-on finish, which may include many toxic ingredients. So be sure you are buying the real thing.
For an added layer of protection, you can add a few coats of shellac, which is a bug secretion applied like a paint by dissolving it in alcohol. Normal shellac doesn't do well in water--it turns white. But de-waxed shellac does much better in water. You can buy flakes of de-waxed shellac and dissolve them in denatured alcohol that you can buy at your local hardware store. Shellac.net sells a wide variety of shellac flakes and has further information about shellac. Unlike many other finishes, shellac sticks well on top of tung oil, at least if the tung oil is fully cured first (which might take a week or two).
I doubt the tung-oil/shellac combination is as waterproof as a good marine varnish, but it's kept my shower pedal looking good through daily showers for at least six months now.