Radiant floor retrofit - adding extra mass? advice?

I used Thermofin C and 1/2 inch Pex in my retrofit. 3/4 inch osb subfloor, hardwood and tile finish floor.
I used an angle grinder to cut all the hardwood floor nails off flush with the underside of the subfloor.
I used an inexpensive 1/4 inch impact driver and self piercing #8 x3/4 screws to secure the Thermofin.
Drilled holes near the joist end, pulled pex, banged in place in Thermofin with a rubber mallet.
4 runs, 350' (equalized) each. Constant circulation, outdoor reset.

The floor is not cold, but doesn't really feel warm until it is -10 F outside. It delivers 100% of the heat needed on the first floor with an indoor design temperature of 68 F and an outdoor design temperature of -20 F, a maximum water temperature of 145 F, and a maximum calculated floor surface temperature of 85 F. It kept up fine last year with temperatures colder than that.

I am glad I spent the money.

I would caution you regarding PAP over pex. I am absolutely certain I would not have had success pulling 300 foot loops through holes in the joists, bending over and around existing plumbing, wiring, reaching the joist end, going back, and doing it again (and again and again ...) without kinks every five feet. I did not have a problem with pex kinking, except for twice, both of which I was not paying attention to what I was doing.

The Uponor Complete Design Assistance Manual is an excellent reference, available in PDF download for free.
Zurn and Viega have good reference manuals as well.
Start there with a heat loss calculation, then figure your btu per square foot delivery with different underfloor methods and different water temperatures.

Uponor / Wirsbo have Joist Trak, similar to Thermofin
There are also lots of light gauge plates available.




ETA, I have heavy aluminum plates, but additional (lack of) mass has not mattered in my case. The outdoor reset varies the water temperature, and the floor is responsive. We don't overheat when a clear 20F fall night turns into a nice sunny 50F fall day.

Panel radiators were mentioned below, and replacing some or all of the baseboard may be easier and possibly less expensive than the floor retrofit. I installed Buderus panel rads on my second floor, one in each room, sized to provide all the heat needed with the same temperature water as the floor retrofit. A thermostatic radiator valve on each, a Grundfos Alpha pump, and nice even heat all the time.

Before you do anything, do the heat loss calculation, figure out how many additional BTU per square foot you need to supply in your different areas, look at your floor R values, and see if that energy can be delivered with your desired water temperature if you have heavy, thin, or no heat transfer plates.

I would also advise not to use PAP as it's a bear to pull through lotsa joist holes. Use a quality PEX-a such as Wirsbo/Uponor hepex, or Mr.Pex O2 barrier. PEX-a is softer (read more pliable) and makes this type of job easier. Calculate your loop length and start in the middle of the run so you only end up pulling about 150' each way back to manifold. The plates are the best dry system thermo-transfer.

Radiant systems, lowest temp to highest:

1. Radiant slab (tubing embedded in thick concrete)
2. Thin slab/gypcrete pour-over (tubing in slab poured-over wood subfloor)
3. Below wood subfloor with thick transfer plates, insulation below plates
4. Stapled up tubing directly to subfloor with insulation below
5. Stapled up system with no insulation below

EDIT: There are also products that sandwich the tubing on top of the subfloor with flooring directly on top of that such as Warmboard etc.

TS

An alternate plan would be to use wall hung radiant panels.
As mentioned, staple up systems require higher water temperatures, so as it gets colder out, your storage temps will have to be higher and/or you are going to be doing more firings to keep temps up.
It is worthwhile to do a heat load calculation and then compare that against panel or floor performance to see what the lowest temperatures are that you can keep some reasonable comfort.
Pencil and paper are free.

Yep, do the heatloss calculation first and foremost. Any well designed heating system starts here.

Your on the right track with the low supply temps to maximize storage. It's a beautiful thing. With staple up I don't see making this happen without extruded plates-$$$. Panel rads with TRVs, sized for low temps, could start looking pretty good.

Another vote for constant circulation and ODR, whether panel rads or staple up. It's awesome.
I designed for 120 deg F supply at design temp and at avg temps I am around 105deg F. I fire every other day in avg temps. The convenience is worth the price of admittance for me.

Of course you won't know what to expect without that load calc. Figuring your areas and assigning the correct R value will take more time than crunching the btu/hr numbers.

Noah

I made my own hybrid system over my concrete floor. The basement had baseboard heat. I developed a leak somewhere in the floor and decided this was easier then to start cutting concrete looking for it. I layed foil down under the plywood and tapcon the wood to the concrete. Then i routed all the groves keeping each loop less then 250ft i did 6" or 8" centers. Then wrapped it loop in a u shape of foil and put engineered floating floor over top. I did this 5 or 6 years ago. The system works great it was a lot of work but worth it in my option. It is not as good as concrete based in floor but with the built in dressers i could not raise the floor height with concrete.

A good controller, with outdoor reset, running a mixing valve to vary the temperature to the pex circuits according to demand would cost a lot less than a thin slab and probably work better.

Just my intuitive take on it.

Given that you already have 1,000 gallons of thermal storage (mass), you likely won't see a lot of benefit from adding additional thermal mass to the floor. But you will experience some of the disadvantages of a high-mass floor such as the slow response time and the difficulty in controlling temperature as the demand changes (say the sun comes out and warms the space, or you start cooking in the kitchen which quickly adds a bunch of heat to the space).

You already have the piping for the baseboards throughout the house - I'd suggest swapping out the baseboards for panels sized to deliver the necessary BTUs at a relatively low temp (<140) so you can more fully extract the available BTUs in your thermal storage. That will likely cost less, you can implement it a lot faster and easier, and you'll have the ability to fine-tune temperatures on a room-by-room basis.

As I recall, the commercial staple-up floor systems have a channel that the pex pops into, so a large percentage of the tubing is in contact with the aluminum. I wonder if it would make sense to wrap foil all the way around your tubing before stapling it up in order to suck more heat out of the tubing by conduction? Then proceed with the rest of your plan, which looks like a winner to me.