Self-leveling compound & radiant heat: what I learned...
I'm going to post my experience with self-leveling compound here in case these tips might help others. (I found a dearth of good info on line, myself!)
I have just finished pouring self-leveling compound in my second bathroom. In both cases, the main purpose was to cover electric radiant heat wires prior to laying tile.
I used loose wire from warmingsystems.com.
I used LevelQuik RS from Home Depot. (I would have prefered LevelQuik ES -extended set as opposed to rapid set- but it was not available locally.)
First, the old subfloor was replaced. The new plywood was then primed with the LevelQuik primer per the directions (diluted over plywood.) After the primer is dry, you need to dam up any crevice, gap, or hole where the SLC could run away. In the first bathroom, I did not really understand how important this was!!! I'd heard to go around the edges of the room with Sill Seal (that pink foam strip you use under a house sill) which would create a nice expansion gap with some give. So I taped it around the edges, and around plumbing holes. This was NOT adequate! When we poured the SLC, you could see it moving towards the gaps and trickling down. We even had to do a second pour since we lost so much down into the crawl space below: ouch! In the more recent bathroom we didn't lose any. Here's what we did:
Fill large gaps at the room perimeters with foam insulation backer rod and Great Stuff spray foam (gap and crack formula).
Stuff foam rod around toilet drain coming up through the floor.
To make a dam at doorways and other straight edges, install a 2x4 with the nails not pounded all the way in.
Go around with a cheapie tube of painters caulk and caulk EVERYTHING!!!! Caulk both inside and outside edges of the 2x4s, and anywhere else there's even a remote change of a tiny hole or gap.
Next, lay the heating wire. (Chisel down a bit to create trenches for the factory splice and thermostat probe. WE install two probes, and leave the second's wires free behind the thermostat, in case the first one fails at some point in the future.) Our wire came with aluminum tape to stick it down with. I used only that tape in the first bathroom, but the water in the SLC caused to to let go so a lot of the wires floated to the surface :( In the second bathroom, I first laid out my wire with painters' tape, then secured it with a hot glue gun. Brilliant! It worked really well, no floating wires. (I removed the painters' tape after the glue was set.)
Then get ready to pour. Figure out how many bags you're going to need. This took some heavy math for me, since my second bathroom's floor sloped about 5/8" over 5' and I assumed the SLC would level that out, so I had to figure out the cubic feet needed using calculations for figuring swimming pool volume, then covert to the square footage at 1/8" coverage stated by the SLC manufacturer.
Have your buckets ready: we assumed we'd need 3 bags and had two buckets already measured with the correct amount of cool water, and then another batch of water pre-measured and ready to pour into the first bucket when it was empty.
I read that cool water extends the working time -or at least the pot life- of the stuff, so we made sure our water was cold. (We were mixing outdoors in Maine in November, so that wasn't too difficult!) The pot life is supposed to be 30 minutes, and the working time is only about 5 minutes once poured (10 for feathering) so we wanted to make sure everything was set to go.
Last time I poured, I used a floor squeegee as a spreader. It didn't really work all that well. The instructions call for a gauged trowel (notched) but we didn't want to use a metal implement for fear of hurting the wires. I saw an EXCELLENT how-to on line that recommended cutting a rubber squeegee into notches. I bought an 18" black rubber squeegee head, cut 1" notches out of it with a utility knife, and screwed it onto a broom handle. This allowed me to spread the SLC around in such a way that it was still able to self-level rather than being pulled up into hills like the straight spreader caused.
OK- so mix the SLC with a heavy duty mixing paddle (I used the round type for paint mixing, about 4 or 5" around) and a 1/2" electric drill. With the water in the 5 gallon pail, have one person running the drill while the other dumps the powder in fairly quickly. We mixed about 1 minute 45 seconds after the powder was all in, then did the next bag, then gave the first bucket another 5-second mix and brought it inside. You'll see how at about 1.5 minutes, the stuff is smooth and a consistency of cake or pancake batter: that's what you want.
My DH person poured, I spread the SLC with the notched rubber spreader. DH went out and gave the second bucket another quick mix, then brought it in and poured. If we'd needed a third bucket, he'd have gone and quickly mixed that one. It turns out we did not. It looked like it was leveling out beautifully. You can tell how the stuff is flowing by white contour lines on the surface, so we knew it wasn't dripping down any unseen holes. (We later checked in the basement and this was correct, yay!) The mix had been perfect consistency, smooth, flowed well. It all looked beautiful- as compared to the first time when the level kept going down, the tape let go and the wires floated to the surface: yuck!)
After a few hours I put a long level on it. The surface is beautifully flat, no hills or valleys -so the large-format tile I'm laying should go on well- BUT.... I was surprised to see that the floor still slants in the same direction. it's maybe 1/8" less pronounced, but the slant is still there. It's funny to me that the SLC would level itself out so beautifully in relation to the floor beneath it, but not in relation to gravity as a whole. I'm sure someone can explain why this is... something about surface tension maybe? It's OK- it's not going to be a problem in this bathroom, but it's just curious! I would have expected the floor to be perfectly level now, wouldn't you?
Anyway- I hope this helps someone!