Radiant Heat Install Under Hardwoods w/o Subfloor

FFJASON1

Hello all, I am working on installation of a radiant floor heating system in my two story 1940's colonial to replace a failed one pipe steam system. After demoing the ceiling and exposing the underside of the second floor joist space I found that the second floor doesn't have any subfloor. The flooring consists of the oak tongue and groove flooring nailed direct to the joists.

My plan was to staple the PEX and transfer plates to the subfloor but not I am not sure this is the best idea for my second floor situation by stapling direct to the underside of the hardwoods. I considered attaching furring strips to the side of the joists and then stapling the transfer plates to the furring strips. Of course this would create a small air gap between the transfer plate/PEX and the flooring.

My first floor flooring has a tongue and groove sub floor under the oak flooring. So that floor shouldn't be an issue for the install. Thanks in advance for any recommendations and wisdom you can provide.

hot_rod

I have done that method, at my own place in fact. It was a barn remodel into a home, and the upper lever, a former hay loft had 1X4 T&G pine, installed diagonally as the flooring. In the bathroom, the Radiant Engineering plates are fastened directly under that T&G.

It will not be fun for someone, someday that tries to replace that flooring, but it works just fine.

Keep the temperature to the loop as lows possible, tight 6 or 8" spacing for a nice consistent floor surface temperature.

FFJASON1

Hot rod, thanks for the response. I was worried that to much heat would be passed direct to the T&G flooring and cause problems since the sub floor is missing. I planned to run at or less than 120 degrees anyway.

What temp do you recommend below that? Or would it be adjusted based on in use temp and comfort?

Did you staple both sides of the plate or just one side and let the other hang free?

hot_rod

If you think about it this is how Warmboard works also, the finished floor goes right on top of the aluminum. Really any of the dry type panel systems have finished wood flooring installed directly onto them.

The required temperature really has more to do with the heat load, but below 120 would be a good goal. At some point supplemental heat is better than cranking the floor temperature to cover the load.

I'd use the quality, heavy gauge extruded plated like the ThermoFin. Fasten all sides, you want the best possible transfer.

That one side staple method was for the crappy flashing thickness type plates, avoid them! They would pop, crinkle and make all sorts of noise. The goal here with transfer plates is conduction transfer, leaving the plate dangling on one side really defied logic. Good marketing spin however.

For low load areas I have had good luck with UltraFin. They don't claim to be conduction transfer plates, but a step up from the "dangled tube" method promoted by some radiant companies. They are a nice add on to a high temperature baseboard fin tube application, no mix down required. Although they can work at much lower temperatures also, seen here.

Some IR of UltraFin at work on a job I did 10 or so years ago. Amazing how much "leakage" around doors and bottom plates on a brand new home!

stevemikel

I have a similar question. My 80 yr old farmhouse has shiplap subfloor and another layer of fir? wood flooring. So using good staple up plates would be adequate for this type install?

stevemikel

I have a similar question. My 80 yr old farmhouse has shiplap subfloor and another layer of fir? wood flooring. So using good staple up plates would be adequate for this type install?

hot_rod

Are you looking to heat the space or just provide some floor warming?

If you hope to heat the room, or home with retrofit radiant it is wise to perform a load calculation, room by room, first.

It will indicate the amount of energy you need to transfer into the room. the floor may, or may not be able to cover the entire load, so an addition method may be needed to supplement the floor.

Take the room load number and divide by the amount of open square footage in the room. BTU/ sq. foot is what you look at to see if the floor is able to deliver. I suggest mid 20 BTU/ ft as the most a comfortable residential floor can provide.

generally speaking the extruded heat transfer plates are the most powerful transfer link to the floor or walls.