I have a porch that is on an existing slab and unfortunately I did not put tubing in during the pour. Now that everything is built and I want to heat the space, I have a problem. The builder put the doors (two sliding patio doors on each side of the porch) on a 2X4. Therefore, I only have about 1.5" to work with to install some type of radiant heat. I would like to use a hydronic system but everything I've read indicates that it would be next to impossible to keep the thickness of the entire job to 1.5". Realize that I would need to install some type of finished flooring (ideally some type of tile). Is there any way I could use a thinner tubing and reduce the thickness of the system? Note that I will have another heating unit in the porch. My plan is to use radiators along one of the long walls which will give me enough heat for the room. I just want to heat the floor enough to keep it from being uncomfortably cold in the winter. I'd like to use hydronic instead of electric just because I will be using hydronic for the radiator. My hope is to replace my electric water heater with a gas boiler/water heater combo and to tie it in to some solar panels. Any suggestions on how I could keep this as thin as possible and use a hydronic system would be greatly appreciated. I know I could do it with electric, but my hope is to reduce my use of electric for heating purposes. Note also that the existing slab is not insulated. Thank you.

If the doors are sliders, don't you have a little bit more depth to work with than you would have with swingers? Schluter Bekotec will require a depth of 1-11/16" PLUS an antifracture membrane and tile which would total about 2-1/4". That will give you a minimal amount of insulation, and a "knob" pattern on which to fix the radiant PEX.

Posted By ICFHybrid on 08 Dec 2013 10:36 AM
If the doors are sliders, don't you have a little bit more depth to work with than you would have with swingers?

Yes, but that space is the frame of the slider. I am concerned about using that space because I expect one day that I will have to replace the door and the extra width will create issues. I'd be more inclined to bevel the new concrete (or whatever) as I get close to the doors - but again, trying to avoid that.

Maybe you could just lay a sacrificial strip near the door that will have to be chiseled out one day for removal. Very difficult to get a quality floor to meet these requirements.

Yes, you could use a thin-slab hydronic approach (e.g., 1/4" diameter pex, 1" poured gypsum underlayment, and 3/8" ceramic tile). Don't exceed 125' in length for the circuit(s) when using 1/4" diameter pex. I trust that the standard slab already has adequate insulation below it, otherwise you will lose much heat.

With your uninsulated slab, I'd use most of the space you have for insulation. Look into:

3/4" polyiso
1/4" Hardibacker cut to leave channels for 1/4" tubing
1/2" for tile (or whatever)

With the limited insulation you may want to put tubing only where people walk or sit. Ie, don't heat any more floor than you have to. Insulate the exterior slab edge well and consider adding "wings" of foam.

Didn't read the OP last sentence...which should have been the first sentence... Don't know the location, but 2" of insulation is typically the minimum required for hydronic radiant floor heating. Without adequate insulation under the standard slab, hydronic radiant floor heating would be a very poor choice. In fact, heating the porch at all is a likely bad choice. A little bit of thought and planning goes a long way to having a successful project.

Jon, there is no antifracture layer and no stability to that sandwich. That floor will crack for sure.

It was meant to be a three season porch, however, I thought heating it would be no problem. The first mistake I made was not researching radiant heat before the slab was poured. The builder I chose wasn't familiar with radiant heat and just went on and did the work like he thought it should be done. I will call him to see if he put any insulation down before the pour, but I doubt that he would have done that and I don't remember seeing any.

I recognize that this is kind of a botched job, but I'm trying to think of ways to make the best use of this space. While three seasons are terrific and the porch works very well for many of the uses we intended, we like the space so much that we'd like to use it in the winter. I'm sure I could just put insulation down and then some type of flooring like a laminate/pergo type of product and it would be acceptable, but then I may as well just get a small gas heater with a direct vent and use that. I was hoping that I could heat the room with radiators which would allow me to have some solar fun (yes, some of this is just about this being interesting to me - I'm a tinkerer). In doing so, I was hopeful I could put some heat on the floor to take some of the chill out of it.

With respect to being a good outcome, part of me wonders if it wouldn't be better to have the lower level of insulation and some heat on the floor than having none at all. Maybe it would be better to use as much of the space I have for insulation and not worry about the temperature of the floor (keep your shoes on in the winter). I don't know, but I'm exploring my options and am thankful for everyone's input.

I would leave the floor unheated and just go with hydronic radiant wall heating as you originally indicated. Zone it such that you can easily turn it ON when you use the porch and turn it OFF when you don't use it to minimize heat loss and heating expense. Be sure to protect the system from freezing by using propylene glycol. Get some comfy house shoes and you will be just fine...most folks live that way and don't even know what they are missing

Posted By sailawayrb on 08 Dec 2013 08:20 PM
I would leave the floor unheated and just go with hydronic radiant wall heating as you originally indicated. Zone it such that you turn it ON when you use the porch and OFF when you do not use it. Be sure to protect the system from freezing by using propylene glycol. Get some comfy house shoes and you will be just fine...most folks live that way and don't even know what they are missing

That's probably what I will do, but one thing I already learned is that the room needs to be heated to a certain level at all times during the winter. Otherwise, when you heat it up to use the room, you end up with condensation problems depending on the temp and humidity that day. So I was thinking of heating the room to around 55 to 60 degrees in the winter.

I've stayed back quietly watching to see if there was any way you could do it with hydronics. I don't see an answer. If after all these responses you decide to go with electric, I can get you all the answers you need.

Appreciate the offer but the more I look into this, the more concerned I am about my belief that there is no insulation under the slab. I guess I can waste a lot of money trying to heat an uninsulated slab but that's not too appealing to me. I think the room will just have to remain a three season room. I may put a heater in it to make it a little more useful in the winter, but I don't really see a path to making it a comfortable room in the winter. If you think different, I can give you my e-mail address and we can discuss further. At this point, I think I need to accept that I screwed up and live with it. Unfortunately, not the first time I've done something like this. Maybe some day I'll learn.

Hey, you weren't the first and you won't be the last. We've all screwed up at some point in our lives. "Nobody expected the Spanish inquisition!" I agree, you would loose some of the heat downward. I wouldn't expect to design a primary heating system that didn't include an insulator of some kind. Though, in cases where the choice was to have electric in-floor or baseboard, then I would say you'd be happier paying for the in-floor. Good luck and don't dwell. It's not worth it. Look at the finished project and be happy with the progress! A warm and happy Christmas to you and yours... (credits to the Python for the quote)

Posted By warmsmeallup on 22 Dec 2013 08:33 PM
Hey, you weren't the first and you won't be the last. We've all screwed at some point in our lives. "Nobody expected the Spanish inquisition!" I agree, you would loose some of the heat downward. I wouldn't expect to design a primary heating system that didn't include an insulator of some kind. Though, in cases where the choice was to have electric in-floor or baseboard, then I would say you'd be happier paying for the in-floor. Good luck and don't dwell. It's not worth it. Look at the finished project and be happy with the progress! A warm and happy Christmas to you and yours... (credits to the Python for the quote)

On a passive house design, would be a good addition to add the radiant electric heat in the slab or would it be overkill?

How "passive" are we talking? If you're looking for a backup to passive solar, it would require a heat loss calculation to know what you would need and if it's cost effective for you. If we're just talking floor warming, it would not "save" you anything if it's in addition to the heat source you already have.

Posted By warmsmeallup on 23 Dec 2013 09:03 AM
How "passive" are we talking? If you're looking for a backup to passive solar, it would require a heat loss calculation to know what you would need and if it's cost effective for you. If we're just talking floor warming, it would not "save" you anything if it's in addition to the heat source you already have.

The home will have 2 ductless mini-splits and one ducted mini-split.