Under Floor Radiant Heating

Crissie

Recently, a contractor told me he put radiant heat several areas of a home using the ceiling below the floor, so putting the heating elements on the underneath side of the subfloor for the rooms being heated. The advantage of this is you don't have to tear out and replace the floors. Has anyone successfully done this? Any feedback or insight?

EdTheHeaterMan

Radiant floor heating is great. Staple up is the term we use for this type of job. You can also add aluminum plates to spread the heat a little better. I have done both and don't notice any difference.

Most important is the amount of insulation you place below the tubing. There are literally hundreds of discussions on this website about this topic.

What are your concerns or questions about this, are you thinking of installing something like this?

EdTheHeaterMan

Here is a diagram using the aluminum plates.
I would use thicker insulation below the tubing. The rule of thumb I was taught is 4 times the R value of the floor above. So if there is a subfloor 5/8" and a finish floor of 1/2" hardwood you might have R-2 insulation value. If someone adds a carpet with padding then you have maybe R4.5 insulation value. This means that you would want a minimum of R-18 insulation below the tubing before you install the ceiling on that room below, just in case someone wants to have a carpet in that room.

Crissie

EdTheHeaterMan said:

Radiant floor heating is great. Staple up is the term we use for this type of job. You can also add aluminum plates to spread the heat a little better. I have done both and don't notice any difference.

Most important is the amount of insulation you place below the tubing. There are literally hundreds of discussions on this website about this topic.

What are your concerns or questions about this, are you thinking of installing something like this?

I have 750 sq ft open concept room with full unfinished basement below it. Option is to either tear out and redo flooring or use the staple method below the subfloor. I have 1.25" subfloor and 3/4" hardwood on top of it, so it sound like I would need a ton of insulation on the ceiling. Certainly would be easier than tearing out the floor though. Would it also heat my basement? Thanks

EdTheHeaterMan

I would definitely do the staple up!

If you don't install the insulation, the basement will get plenty of heat
I had a customer that installed his own tubing to save $$$ on the job. I told him to be sure to insulate below the tubing. He did insulate half of the basement before finishing off that area. But he never got around to the insulation where the boiler was or the laundry equipment was. It was like working in a sauna when ever I needed to work on that boiler in the winter. The thermostat was up in the room above and the heat all went into the basement so the basement was overheated and the thermostat never shut off.

Every year his wife complained that the rooms above the laundry and utility area never got warm enough. This went on for 5 years and each year when I did the oil heater maintenance they said the same thing and I said to finish the insulation. When they filed for divorce, she kept the house but she would not sign the papers until all those unfinished projects were done. That included insulation he basement ceiling.

Amazingly the heat started to work upstairs and the basement was much cooler. Go Figure!

If you want to heat the basement, then put heat emitters (Radiators, radiant ceiling tubing, blowers etc.) in the basement with a dedicated thermostat for the basement.

EdTheHeaterMan

I was just guessing at the R value of the wood floor material. Here is a website that will give you accurate R-Values https://www.heatizon.com/installation-manuals/r-values-building-flooring-subfloors
Plywood subfloor 1.25" x 1.1= 1.37 R value and if you have oak floors .75 X .85 = .6375 for a total of about R-2. Your actual floor may vary. But I would still put a minimum of R-20 foam.

I actually had an insulation company do Spray Foam in my basement. Sprayed right over the tubing and aluminum plates

HomerJSmith

Using staple up with the heat transfer plates, I put talcum power on the grove that the tube fits in. This acts like a lubricant so the tube slide easily in the groove. The tube expands when hot water circulates thru it and it stops the binding of the tube and clicking nose associated with the binding.

I also don't place the insulation tight against the plates and subfloor, but leave 1-1 1/2" below the plates. That allows the heat to spread thru out the joist bay.

The finished floor covering also can affect the heat transfer into the room. Tile is the best as the R-value is so low. Wool carpet with a conventional pad is the worst.

Keep the total loop length under 300' for 1/2" tube, which is the easiest to work with.

Crissie

EdTheHeaterMan What type of insulation did the divorced couple use in the ceiling? Would fiberglass be okay? Is the installation of the staple up much easier than the below the finished floor application. Thanks so much. I only need 5000 BTU's of heating in the basement.

EdTheHeaterMan

Fiberglass is what the divorced guy did.

In your case I would install the staple up, then insulate, then add another loop (or loops) below the insulation with it's own thermostat in the basement to control that loop(s). Valve actuators are available to send vthe heat in the different loops as needed. Your installer should know how to pipe and wire it.

Will you be installing a basement ceiling to cover the pipes? A drop ceiling? You don't want the insulating type of ceiling tiles.

hot_rod

Did you run a load calc for the room to see what kind of output you need from the floor?  R/2 of wood to drive through may take some high supply temperatures, if loads are high.

As for insulation, fill the entire depth of the joist. If you have 10” joists for example, use 10” batts. Really no reason to scrimp on insulation, especially if you plan on covering it up.

No need for an air space with plates, the r-value is more important, push the batts right up against the tube, the heat transfer is conduction.

Use plates with the tight drive fit, the squeak noise is from tube sliding in the groove. The ping and pop noise is expansion of the thin material type plates, avoid them.

Dave Carpentier

Wait.. sometimes you gotta suddenly do all of your to-do list during a divorce ?
Wow, Im not getting divorced !

Crissie

EdTheHeaterMan and hot_rod I am not planning on finishing the basement, will leave ceiling exposed. Prefer fiberglass because i think it is less toxic if left exposed. The open concept area I want to heat is 750 sq ft. Did a manual J, load is 19,000 BTU's. Hoping it should be fairly straightforward.

EdTheHeaterMan

As long as you have separate thermostats for the basement and the room above you should be fine. Don't try to get "Some leftover heat" from upstairs by not insulating a section of the upstairs radiant tubing. The thermostat upstairs will not react properly to the different rate of heat loss that a basement has compared to a room with windows and walls exposed to the outdoor temperature.
This is an illustration to help understand the concept.

hot_rod

25 btu/ square foot is getting on the high side of what a wood floor can do. Don’t put any throw rugs down:)

Crissie

hot_rod said:

25 btu/ square foot is getting on the high side of what a wood floor can do. Don’t put any throw rugs down:)

Wow! That is good to know. I didn't realize there was a BTU limit / sq ft on the floor radiant heating.

TAG

Crissie said:

hot_rod said:

25 btu/ square foot is getting on the high side of what a wood floor can do. Don’t put any throw rugs down:)

Wow! That is good to know. I didn't realize there was a BTU limit / sq ft on the floor radiant heating.

PEX in a slab can run hotter as the slab does not care .... at some point a wood floor is not going to like the high heat. Also -- a carpet and pad will insulate the floor ... so even pumping hot water through the pipe will not get enough of that heat to the room.

The plates spread out the heat -- the AL that the plates are made from is very conductive. With the plates I have always pushed the insulation up to the floor since the plates will spread the heat to the floor. With simple staple up the contact with the floor is very small .... just that small spot on top of the PEX .... you have to leave an air gap so the heat from the PEX fill the area with heat. I have used various plates .... depends on the BTU's needed and the water temp you are going to use.

hot_rod

Two issues with trying to get higher btu output
First, the surface temperature becomes uncomfortable to bare feet. Around 82-83F the floor becomes uncomfortably warm, feet begin to sweat
Second is the temperature the wood build up requires to get the surface temperature warm enough to cover the loads. It can exceed the maximum temperature the wood fibers can deal with

some suggest at around 23-24 btu/ ft it is time to add some supplemental heat for design days

Commercial buildings can run higher floor surface as folks generally have shoes on, and ambient temperature may be cooler then residential applications 

Crissie

Very helpful, thanks!

HomerJSmith

What I should have added was that if you are running two Pex tubes with heat transfer plates in a single 14-1/2" joist bay place the insulation against the heat transfer plates;

My earlier comment wast for a single Pex tube in a bay.

hot_rod

Radiant ceilings and walls are other retrofit options. The wall will give you the highest output, maybe 32 btu/ ft as you can run them much warmer than floors. Unless you walk on walls?

Ceilings will be a bit less, around 28 btu, due to some stratification influence.

Walls are sometimes used as a supplemental to floor heat if the loads are high. I've done them in a 3' tall wainscoat, heating just that lower section of the wall, staying away from wall hangings.

TAG

hot_rod said:

Radiant ceilings and walls are other retrofit options. The wall will give you the highest output, maybe 32 btu/ ft as you can run them much warmer than floors. Unless you walk on walls?

Ceilings will be a bit less, around 28 btu, due to some stratification influence.

Walls are sometimes used as a supplemental to floor heat if the loads are high. I've done them in a 3' tall wainscoat, heating just that lower section of the wall, staying away from wall hangings.

Did you do the wall in a retrofit situation ?

I did walls in a lower bathroom and in a bar area ---in a basement where I did not have room for anything else .. both on a rehab when the walls were gutted

ethicalpaul

I haven't gotten around to insulating mine yet, it seems to be doing great regardless. The plates, being in direct contact with the wood floor, are transferring a lot of heat to that floor, vs the air in the basement. Air is a much worse conductor than the aluminum plates and hard wood of the floor it seems.

And there is very little air circulation down there, so the air that does get warmed tends to stay up there against the bottom of the floor.

Probably also helps that all I have is the 3/4" heart pine floorboards, there's no subfloor for the heat to have to get through. The R-value is tiny.

hot_rod

Retrofit and new. I liked to put in in the wall of large tiled showers in new home, a space that always feels cold as the tile pulls heat from your body. Get them up around 80 and the space feels much better

TAG

hot_rod said:

Retrofit and new. I liked to put in in the wall of large tiled showers in new home, a space that always feels cold as the tile pulls heat from your body. Get them up around 80 and the space feels much better

Agree ..... My latest project has one bathroom on a heated slab and the other on Warmboard. The showers are each around 5x5 ... just looped the return into every other bay and held up the top of the PEX -- insulation in the walls. I was worried about it becoming too hot. I did loop every bay on the outside wall .. It's nice