Subfloor radiant heating

Hi everyone!

We are shortly going to be performing a ceilingectomy in our dining room to address a number of problems - plumbing, electrical and the poor condition of the ceiling itself. It's right below an upstairs bathroom with a lovely old tile floor. While it's open I would like to see about putting subfloor radiant heat under the bathroom.

Has anyone done this from this approach? I know about the electrical mats you can install under flooring on top of the subfloor, but this would be having access from the bottom going up.

A quick internet search yielded too much information for my short attention span. I was curious if someone has actually done it this way and can offer pointers.

@PeggyC did it and loved it. I really wish we could have afforded to do it in our second flor bathroom but alas. Our bathroom has a sort of separated room where the toilet is for privacy and it is on the outside wall and far away from the steam radiator. Radiant heat from the floor would be awesome.

I would have gone with the electric mat approach but the PEX tubing hydronic systems have a certain appeal, too. If you're considering it make sure you take into account any changes in the height of the floor once it is all finished up.

I don't know about energy usage but these things can be put under the control of their own programmable thermostat.

I assume if we put something in between the joists from below it should not affect the floor height. We love the tile floor that's in there so we can't do the floor mats.

I like the idea of localized heating for the bathroom so we can keep the whole house at a lower temperature in general but get the bath room a little toasty for chilly winter mornings.

The mats would obviously be between the joists and heat up the subfloor and floor tile above. This will make for great supplemental heat and a nice touch for the foot. It is clearly not as efficient as placing the mat directly below the tile, but it will still help.

I regret not putting in the mats when I did the reno on my bathroom floor. I was going to do it as a primary source of heat but bailed when I learned the electric mats could die leaving me without any heat. I went straight radiator which leaves me with a cold floor.

good luck.
ETA: your assumption above is correct.

Your bathroom floor sounds original which means that it is a "mud job" set in several inches of concrete. I doubt that any underfloor heating other than a blow torch would be sufficient to heat your bathroom.


Mats are not applicable here, even in the joist bays. You'd have to do a pex hot water loop with aluminum heat transfer plates screwed into the subfloor from below. You insulate below the plates and pex to help direct the heat upward. The water heats the pex which heats the transfer plate which heats the subfloor which heats the mud job which heats the tile which heats your feet. Water temp might have to be fairly high to make this work, but it will work. You'd have to have a pro do the calculations.

+1, Mr. Murphy. I always wondered about what you'd have to do to the joists to make it work though. Just loop the pex under each one or drill holes? I'd be skeered of drilling holes or making notches.

Thanks for the answers. I imagine once we get the ceiling out then we'll know better what kind of mud job is in there. I'm not sure if the bathroom is original to the 100-year-old house. At some point an extension was put out the back (not too recently judging by the lack of grounded outlets) and I don't know if this bathroom was installed at that time.

If you do the PEX hot water loop does that heat and circulate a local reservoir of water or do you loop the hot water supply for the bathroom back and forth through the joists? I'm guessing the latter option would cool off your hot water supply a little too much before it reached the shower.

Drilling holes is fine - they just need to be in the right places so as not to compromise the structure

https://engineering.purdue.edu/~jliu/courses/CE479/extras/notching_IBC.pdf

heating and domestic water would be handled separately.

PEX is the way to go. What type of heating do you have? Investigated this option about 2 years ago. As I recall it would not work with a steam boiler. With a hot water boiler, would need a cold water tempering valve. I'm a little hazy on the details, but recall it being both more complex and costly than I wanted.

Hot water loops can be run off a steam boiler.

http://www.oldhousejournal.com/hybrid_hydronics/magazine/1349

We have a steam boiler. From what I read in that article this is not something I want enough to commit to the kind of time and money required to pull it off.

I am kicking myself a little bit now though. We had our sunroom floor replaced with a nice looking composite. I could have easily put down a few electric mats on the subfloor but I was in post-house-purchase tightwad mode and couldn't bring myself to do it. With the cold weather we've had lately I would like to send a time travelling email to myself in September that it would be worth it. That room be cold.

Yeah, not a simple or cheap solution. Good luck with the rest of the project.

We did water based radient heat in our kitchen and all bathrooms and its wonderful, i can give you our plumbers contact info if you would like

All of this depends on that tile floor. This isn't really practical if you have a mudjob like bobk described. tagging off of a steam boiler to run lines below a thick tile floor would be expensive and not heat well, i fear. Installing a new tile floor is a whole different story, of course. new207040, I don't think you caught that he is trying to retrofit this into an old floor.

Wasn't sure of thickness of subfloor when we pit them in it was part of a major reno including demo of floors. If his floors are anything like ours there was a ton of concrete. Our tubes went onto of the new subfloor and then they finished with some type of fill or mortar then tiled it was a big job

Has anyone done this in their Maplewood room? Ours has a slate floor and it basically negates any insulating we do with the ceiling/windows/etc.

We have steam heat and really wanted to do radiant heat in our new kitchen and powder room, but the difficulties in the link jim provided ended up being too much for us. Our contractor estimated that it would end up running us about 5k just to get the system going, not counting labor putting it in. And since everything is more difficult than anyone estimates, I assumed it would be more.

jameskpolk, a little space heater in our Maplewood room makes a big difference, though our floors are wood. Some nice thick carpeting on the slate floor + a space heater could help? It'd certainly be cheaper than installing radiant floors, especially if you only need to turn the space heater on when you're in there (I turn ours off after we've been in there for a bit, once it's warm enough).

mrincredible said:

We have a steam boiler. From what I read in that article this is not something I want enough to commit to the kind of time and money required to pull it off.

I am kicking myself a little bit now though. We had our sunroom floor replaced with a nice looking composite. I could have easily put down a few electric mats on the subfloor but I was in post-house-purchase tightwad mode and couldn't bring myself to do it. With the cold weather we've had lately I would like to send a time travelling email to myself in September that it would be worth it. That room be cold.


You are in good company. We decided to skimp on the radiant flooring when we renovated our bathrooms. While we can live without it, it's currently the season that reminds us how cold a bathroom floor can be first thing in the morning.

I just read the article linked to above and I think the fact that it lists all the hardware involved in the installation makes the whole thing a bit fatalistic sounding to the average homeowner. We do this lots of times throughout the year. Don't be a scaredy cat.

The problem with scavenger loops described in the article is that it can be difficult to regulate the water temperature in them. This is not such an issue when using baseboard radiation but radiant tubing is designed to run at only 120 degrees. Any hotter and it would burn your tootsies to walk across the floor.


Chicagonative said:

mrincredible said:

We have a steam boiler. From what I read in that article this is not something I want enough to commit to the kind of time and money required to pull it off.

I am kicking myself a little bit now though. We had our sunroom floor replaced with a nice looking composite. I could have easily put down a few electric mats on the subfloor but I was in post-house-purchase tightwad mode and couldn't bring myself to do it. With the cold weather we've had lately I would like to send a time travelling email to myself in September that it would be worth it. That room be cold.


You are in good company. We decided to skimp on the radiant flooring when we renovated our bathrooms. While we can live without it, it's currently the season that reminds us how cold a bathroom floor can be first thing in the morning.

Well replacing the windows in that room solved the problem.  It was nice and toasty even in the chilliest winter months.  Glad we didn't waste money on that radiant flooring!!!!!

That being said, the kitchen was freezing.


We did love the radiant heat in the floors of our bathroom in the West Orange house, but it was electric mats under only one layer of mud and tile. I doubt it would be any use through more than one layer of flooring plus the subfloor. Too bad, though... it was an interesting idea!

Bikefixed, the funny thing in your 2nd floor bathroom is you seem to have a hot water line for the heat and the sinks going across under the floor, so you end up with partial radiant heat anyway!


Count me in the group that redid a bathroom and didn't do this.  In our case, I had never heard of it until shortly AFTER we did the job.  It was DIY and I'm pretty sure my spouse knew about it but didn't mention it as an option - grrrr!  We have another bathroom in our house as well as kitchen which may someday get redone and I'm really hoping we can do this for those rooms.  But that assumes we actually undertake the project.


I think a lot of people only know about the kind that uses pipes and hot water, which is quite expensive to install. The electric mat ones (like installing an electric blanket under your floor) are very inexpensive to buy and easy to install, from what I saw when we had our bathroom done. I'd do it again in a heartbeat.



PeggyC said:

I think a lot of people only know about the kind that uses pipes and hot water, which is quite expensive to install. The electric mat ones (like installing an electric blanket under your floor) are very inexpensive to buy and easy to install, from what I saw when we had our bathroom done. I'd do it again in a heartbeat.

It would have been SOOOO easy for him to do this when he ripped out the old floor and put down new subflooring and then tile ... sigh!


This Old House usually has an answer: http://www.thisoldhouse.com/toh/article/0,,20163505,00.html





PeggyC said:

I think a lot of people only know about the kind that uses pipes and hot water, which is quite expensive to install. The electric mat ones (like installing an electric blanket under your floor) are very inexpensive to buy and easy to install, from what I saw when we had our bathroom done. I'd do it again in a heartbeat.

 its a trade off-- while the electric is inexpensive to install its much more expensive to operate - we went with hot water based as we just have the expense of the hot water heater ( gas vs electric) and since all of our water rooms (kitchen and baths) have it there is allot of efficiency and little impact on our monthly utility 



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