Preparing floor for radiant

jelly

From all my research online and talking to plumbers, I have come up with this plan. I wanted to run this by the experts before I begin my project. Please let me know if you see any issues or have any concerns... I am sure there will be plenty as I am an amateur DIYer.

My small 5' x 7' bathroom is old, outdated and the subfloor floor around the tub and toilet is rotten. I am gutting it down to the studs and floor joists which are 16" oc. The bathroom is located on the first floor and can be accessed from the unfinished basement below. I am going to install 3/8 pex 6" oc without using heat plates to a 3/4" plywood subfloor, lay Ditra with modified thinset and 12" floor tiles with unmodified thinset. The pex, which ties directly into the copper baseboard lines, will be regulated with a simple valve.

1) I read that finding unmodified thinset can be hard to find. Is there a common thinset I can use for applying both the Ditra and tiles?
2) After installing my shower, vanity and toilet, I've calculated that I will need about 25' of pex tubing. Can I buy this by the foot from any major home improvement center?
3) After installing the pex tubing, can I simply cover them with R30 insulation?
4) Am I going overboard by installing the pex? I love radiant floor heat but the bathoom already has baseboard and it's such a small area that it my not be worth the investment.

Daniel Holzman

1. I installed tile in my kitchen using Ditra, and installed electric radiant floor heat under the tile. I had not trouble obtaining unmodified premium thinset at a big box store.

2. You need PEX rated for heat, NOT potable water PEX. I strongly recommend you go to an actual plumbing supply store, at big box stores they may not know the difference. The fittings may be different than potable water fittings, I am not sure because I have only used potable water PEX, which I purchased by the roll (a standard roll is 100 feet).

3. I don't understand why or how you would cover the tubing with insulation. Maybe I totally misunderstand what you are doing, but the goal would seem to be to get the heat from the PEX tubing to warm the tiles above them, so why would you insulate above the tubing?

4. I love the warm floor in the kitchen, and if I were doing a bathroom tile project, I certainly would plan on installing radiant floor heat, but it might be electric. So I don't think you are going overboard. But I have no idea if tapping an existing hot water line with an inline valve is going to supply an appropriate amount of flow to heat the room, that is a question for one of the plumbing experts on this forum.

jelly

Just saying that after I staple up the PEX, I would simply put R30 in the bays. to keep the heat in. Do they make insulation with a reflective material that I should use?

I tapped in to the existing zone for the upstairs bathroom and it works great. Because all I need is a valve to regulate the hot water flow from the copper into the pex, I think it will be much cheaper since I don't have to purchase a control box. However, since it's only 15 sq ft of floor space, I am thinking that it may not be worth the effort. In that case, I would just keep the baseboard.