radiant in basement ceiling, under subfloor

Riley12 · April 2022

I am trying to keep this remodel as simple as I can, currently the basement ceiling is unfinished I was thinking to add tubing in basement ceiling stapled under subfloor to heat both main floor and basement I was thinking to leave basement ceiling unfinished and just paint, Is there a design that would work?

Ironman · April 2022

Probably not too well. In order to have proper heat transfer to the floor, extruded aluminum plates should be used. If no insulation is put under them, SOME heat will go downward, but it may not be sufficient to heat the basement.

Then there’s the matter of trying to balance the two different zones which should be on separate thermostats, but you can’t do that if they’re heated by the same tubing.

EdTheHeaterMan · April 2022

you can have radiant floor heat for the first floor. Then place insulation in the basement ceiling. Add a second zone of tubing below the insulation for basement heat on a separate zone, control and thermostat.

Unfortunately your single zone for both will not work. The basement probably needs less heat than the first floor above. However the exposed tubing will release more heat to the basement and less heat the first floor. I know this from experience.

A DIY kind of Customer had a plumber install radiant floor heat in his one story home. To save money, the homeowner would install the insulation required himself. Only 1/2 of the basement ceiling was insulated. For five years, every time I did the tune up On his oil burner, the basement was over 85°. The customer also complained that the rooms above did not have sufficient heat. Every year I would tell him that he must finish the insulation in order to make the heat work in the upper rooms. This would also cool down the basement.

Year number six and the customer was getting a divorce. Part of the divorce was that the wife gets the house and all those unfinished projects needed to be completed. So the insulation was installed and low and behold, the bedrooms above got warm and the basement temperature dropped to a comfortable 64°.

After five years of telling him to put the insulation up, and being ignored, my customer finally said to me “I can’t believe it, you were right, all I had to do was insulate the floor and the system would work!”

EdTheHeaterMan · April 2022

In theory you could try insulating every third joist bay, or every fourth joust bay until you find a balance that you might be able to live with. The problem is … to get the best heat transfer to the rooms above, you want to seal the insulation in the joist bay to form a 1”-1.5”plenum between the insulation and the floor above that has no air currents to convect the heat away from the floor. If you place the insulation between the floor joist in a temporary fashion for possibly making adjustments over the winter until you get it just right l, you won’t have the efficiency required for the first floor to work as good as it should

hot_rod · April 2022

Do you have a boiler system now? What type of heat emitters?
A heat load calc for the rooms you want to heat would be your starting point.
If loads are low, below 20 btu/ sq ft, or if you are supplementing heat emitters in the room, take a look at the Ultra Fin product. Super simple way to add to high temperature systems without adding temperature mixing.

With Eds method you could allow some heat to bleed downward, leave some bays uninsulated, I suppose.

Riley12 · April 2022

I was thinking of using the htp ph76-50, still in the planning stage though. Its only about 1000 sq ft per floor Older brick house so not to efficient. Sounds like a few options one insulate down to a inch from bottom of floor joist (2x10)then run another zone near bottom of joist for basement. Or leave insulation out every other 3rd bay or so, If I chose just the right insulation like a r-9 or something to allow just enough heat transfer down could possibly work also but more risky. I guess end goal is get the two floors within a few degrees of each other probably just do one zone or two for whole house. Was thinking to cut rigid insulation between joist friction fit and caulk or foam edges. Thank You for your wonderful advice. I could also supplement with forced air.

Ironman · April 2022

The Phoenix water heater should not be used to combine a radiant floor and domestic. That would create a huge legionella breeding machine as well as cause numerous problems associated with an open loop system. A tankless water heater is also the wrong tool for the job.

I know that the literature shows a close coupled AHU, but that requires some special controls and has been outlawed in many jurisdictions.

You also don’t wanna use non O2 barrier plumbing PEX in the system.

Properly designing a radiant floor is the key and foundation to getting it right and most plumbers and HVAC people lack that know how.

@Steve Minnich is one who offers design services.

It may seem like an unnecessary expense to pay someone for designing it, but EVERY radiant floor disaster starts with wrong or no design. I see it regularly and it comes on here almost weekly.

There are also several men on here who will gladly comment and offer guidance if you ask.

radiant in basement ceiling, under subfloor

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