Hi there,
Ive seen plenty of hydronic installations while building in the Bay Area, but this will be my first actual install.
Ive ripped back 60 years of laminates in my 14'x 17' kitchen to expose 1x12 (not true, 3/4 x 11.5 actual) boards nailed to the 2 x 10 joists 16 oc with a 14' unsupported span. Assuming they are douglas fir then a 80# load (d+l) should be supported L/360 with that span. (EDIT: There is to be a wall installed to finish the basement area (utility and bath) that will run directly under the center joist which will give some support for the granite on the center island that i want to build.) At this point i plan on taking up the origional planks to run t&g on each, then gluing them to the joists and screwing them back down through the origional nailholes. This should stop the squeeking. The next step will be to lay 3/4 ply NOT nailed to the joists but screwed to the boards 8" field , 4" perimeter. Next with a round router bit i will install grooves into the ply layer around the island. I am still sourcing a thin film aluminum sheet to slip into the groove before the 5/8 Pex tubing. ?Mortar and screw 1/4 backer on top of that and finish with tile.

My questions are:
1) is the effort of t&g on the origional boards overkill?
2) will the 3/4" ply on 3/4" boards have sufficient rigidity for the recommended 1 1/4" subfloor for tile if the ply has been routed to accept 5/8 Pex?
3) do you have a recomendation for aluminum film for radient transfer within the grooves?
4) does thin set stick to said aluminum film for the backer board?
5) is it safe for me to assume 80# load for tile, granite, cupboards, subfloor, ect using this dry install method?
6) currently looking at a dual boiler/ on-demand domestic supply for the system but am wonder if that would be similar to combining your toaster with your coffee maker in that if it goes out you loose both. any thoughts?

Thanks for the help...

Welcome,
You calculation for an 80# load at l/360 would only allow a 12' span for Douglas Fir joists.
Not sure why you are using 80# for your calculations. Live load should be 40 and dead load should be 20.

Your plywood idea for routeing the pex tubes would be of no structural benefit to the floor system, not to mention how much extra work that would be. There are subfloor panels already routed for pex which are approx. 1-1/4" thick to begin with. They would be installed directly to the floor joists. I'll post a link at the end for this type of installation.

I wouldn't bother to pull the 1x12s just to T&G them. You sould just pull them up and use the system I described above.

Have you looked into installing an under subfloor type of pex installation?

Normal pex radiant heat tubing installation over a wood subfloor usually goes like this:
- 3/4" subfloor plywood or OSB is attached to the joists by construction adhesive and screws.
- pex tubing is then attached to the subfloor by u-clips at the required spacings
- Tubing is then covered with either a gypcrete or cement based self leveling material to a depth of 1-1/4" high from the subfloor.
- An uncoupling membrane such as Schulter Ditra is then installed
- Tile covering can then be installed as per Schluter's installation specs.

There is probably another 3 or 4 ways to do this type of install, but the above spec is by far the easiest in my opinion.

No matter which method you decide on, if it's going over the subfloor be prepared to deal with excessive transition heights to abutting floor areas.

Here's the link to a prefab structural panel pex installation: http://www.warmboard.com/wp-content/uploads/2007/03/install_guide_102610.pdf

Thanks Randy,

The construction proceedure for the home i bought allowed for the 1x12 foming lumber to be pulled from the basement walls and used for the subfloor after absorbing moisture from the wet concrete. They have dried and seperated from each other and some split down the middle over the past 60 years but are still solid except for the squeeky nails. I guess this is the way it was done way back then.
For my clarification, there is a 1 1/4" subfloor is required for installation of tile over joists normally, but if i try to insert hydronics into that layer, will it loose rigidity with 5/8 grooves routed through it? Wouldnt it then be only as sound as a 7/8" ply? Does the Hardie backer count as part of the 1 1/4" subfloor before the uncoupling membrane? I was looking to do a dry install to avoid the excess concrete mass on the joists with that span, although i understand the need for thermal mass. For that matter will the heat transfer plates, hardie backer, mortar and tile have enough thermal capacity to match the 1 1/4" gypcrete?
I happen to have access to a large wood shop where im building my own cabinents so routing sheets of ply is no problem at all. Do you have any idea what guage aluminum plating is recommended for adequete heat transfer into the hardie backer in the dry install method?
Any thoughts on the dual purpose boiler/domestic hot?

You obviously will compromise any structural integrity of the plywood by routing deep grooves.

Double layer plywood is normally required for a ceramic tile installation if it is being installed/bonded directly to the plywood. Other applications for double layers of plywood in conjunction with an approved tile underlayment are for natural stone installations. Both of these applications must also still have the correct joist and span specifications for the type of tile or stone being installed.

The TCNA (Tile Council of North America) produces an industry guideline handbook for different types of installations. These methods have been tested and approved. I suggest you may want to pick up a copy, either online or by mail.

Your method that you are trying to do does not fall into any recommended TCNA spec.

The example I posted previously is an approved specification.

If you do not choose to use this method or any other approved methods and devise your own system, then all I can say is good luck.

There's no point in discussing the use of Hardibacker for your proposed structure as it also does not meet the approved guidelines for a radiant hydronic installation over a wood subfloor.

As for the type of heating system,I leave that up to my local HVAC contractor. Best to check with them to see what's most efficient and available for your area.

How can you recommend warmboards directly on the joints if there is a 5/8 groove in them? The min subfloor for tile is 1 1/4" and although the overall is that the structural integrity of that product would be only a 7/8 " layer. Anyhow if I use the old boards and then 3/4" ply I will have the min required for tile, just not sure if I should sandwich another layer for hydronics. I still feel that the 1 1/4 layer of create is too much weight for the 14' span. Perhaps under the joints is best although direct contact to the hardie backer would be the most efficient thermal transfer. Any one else have problems with the dual boiler/ domestic hot heaters?

Warmboard is a structural floor sheeting that incorporates the pex tubing into the foam top layer. It works because the minimum required subfloor thickness for a hydronic application is 23/32" exterior rated tongue and groove plywood. There's no requirement for double layer plywood.

The main problem you have is with the existing 1x12 boards. They are not structural enough to support a tile floor when combined with the grooved plywood layer of the pex.

This is NOT an approved method, but if you want to try it your way, then you need to remove the 1x12s and replace them with a 3/4" plywood or OSB T&G glued and screwed to the joists.
Install your additional 3/4" layer of plywood over the subfloor with 1-1/4" screws only at 8" OC. Route your pex runs and install the pex. Make a template of the pex runs so that it can be transfered to the tile underlayment layer to aid in fastener location. Install the CBU tile underlayment as per manufacturer's instructions. Install your porcelain tile.
The above method will NOT work if plan on using stone tile!

I wish you the best of luck and hope it works for you.