Staple-Up Method of Radiant Floor Heating
Hi:
Not sure if my first post worked so I’m trying again:
I have an existing 2.5″ thick concrete floor on open web steel joists (approx. 3,000 sq.ft above a conditioned basement). I’m doing a full reno and would like to add radiant floor heating, however I am reluctant to add to much thickness to the top of the floor (I have floor to ceiling windows along 80% of the exterior walls and only 1.5″ between existing floor and glass).
Can I install hydronic heating from the underside of the floor and heat the steel decking (i.e. I think it’s called the “staple-up” method but I’ve only seen examples for wood subfloors using aluminum plates)?
Would this be sufficient to heat upper surface of the floor through the concrete? Would I have to worry about cracking or different expansion rates between the steel deck and the concrete? I’m planning to put either tiles or a new surface coat of concrete on the top of the floor. I don’t mind if the system heats the basement as well, since my bedroom will be down there (kids will be upstairs on main floor).
Any thoughts or advice would help.
Thanks!
-DK
(Toronto, Canada)
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Replies
DK,
I don't know much about radiant floors, but some posters here do who hopefully will be able to answer your primary question.
Just wanted to address the worries about cracking. There shouldn't be any problem as both steel and concrete have the same coefficient of expansion.
Do you have a plywood or OSB subfloor, or corrugated steel decking under the concrete?
I'm aware of two types of in-floor heating installed from below. One is staple-up, where you install the tubing first and then thin aluminum shields are installed below the tubing. The other is thick aluminum plates that you install first and press the tubing into. I recall reading that there is not a huge difference in efficacy between the two. The important thing is to insulate below the tubing, preferably with a small air space and a reflective surface facing up, to force the heat to go in the direction you want it to. I might be wrong on the reflective surface; I'm going off memory... I would expect very slow response from changing the thermostat but I believe it would be an effective system.
Thanks Michael. It's steel decking.
I guess I would have to use some sort of adhesive to stick the aluminum shields to it?
Some thoughts:
Concrete is a lousy floor for residences, hard ceramic is lousy for anything other than bathrooms. If it were my house, I'd put down a 3/4" subfloor over the concrete and some sort of more traditional flooring.
With traditional flooring you can go with more traditional floor heating. I like the approach of WarmBoard, although I'm loath to recommend it because it's so expensive. But you want something with high conductivity and low heat capacity to give even floor temperatures and good responsiveness to the thermostat.
For rooms other than bathrooms, you want floor temperatures to be barely noticeable, in the mid to high 70's. Any warmer than that and your feet get uncomfortably hot. My first step would be to do a Manual J or similar heat loss analysis and make sure that a floor temperature in that range can satisfy the heating needs of the room.
The concrete floor between the basement and the first floor is going to create issues, it's a good air barrier so you won't have mixing of air. A well-insulated basement -- and if you're doing a renovation I'd expect the basement to be well-insulated -- will almost always have lower heat loads than an above-ground floor. The concrete in the floor is a good conductor of heat, so in-floor heating will go both up and down. There's a good likelihood that you find that the level of heat that makes the first floor comfortable overheats the basement. The solution would be to put some insulation between the heat and the basement. The most practical place would probably be along the underside of the ceiling.
Thanks DC_C.
We actually like concrete floors, besides, our dogs destroyed the wooden floors in our old house anyways! We also don't have a lot of room to add subfloor to the top - hence the question about affixing the heating underneath. But I do appreciate the feedback on the risk of overheating the basement.
This house is literally a glass box - 200 linear feet of floor to ceiling glass and only 36 feet of wall, plus it was built 50 years ago - so heating it with forced air costs a fortune! We are hoping the radiant heat helps lower the cost.
I will look into the Manual J.
There's really no reason to believe that radiant heat will lower the cost of heating.
Pushing water instead of air would save a few kilowatts and there is the theory that heated floors are so comfortable that you can keep the space at a lower temperature. But it would take a loooong time to recover the cost of a whole new system with heated floors.
It's probably been twenty years since I've heard someone seriously propose the theory that heated floors save energy because you get the same comfort at a lower set point. That claim has been pretty much debunked by actual experience.
I think what confuses people is that infra-red heaters -- which transmit heat primarily through radiation -- do have a use in environments where it's not possible to control air infiltration. With an infra-red heater it's possible to warm a person to the point where they experience thermal equilibrium, even though the air around them isn't warmed. They work best when people are bundled up anyway, prolonged exposure to infra-red isn't good for the skin.
Of course an infra-red heater operates typically around 1000F. Radiant heat -- and radiators -- are mis-named, they transmit heat primarily through convection.
Reply to DC #9: that's why I wrote "the theory" that heated floors save energy. I hear it from at least half of my clients and a lot of professionals as well. And I agree that in-floor heat isn't really radiant heat; I call it in-floor heat.
That sounds like a poor application for radiant floor heat. What you've described sounds like something that's going to need a lot of heat. The limiting factor of floor heat is how hot you can make the floor before it starts being uncomfortable, and how much heat you can get out at that temperature.
I'm intrigued by the idea of using the radiant as floor heat for the floor above AND as a radiant ceiling for the level below.
It's not my area of expertise, but frankly I don't see why the aluminum would be needed, from a heat distribution standpoint. The steel deck would probably conduct nearly as well. It's just a question of getting decent contact between the piping and the steel, which is the one thing a form-fitted aluminum plate would do better.
When you get this all installed I'd like to hear your impressions of the final results.
The problem is there's no way to regulate how much heat goes up and how much goes down.
Imagine a cold, cloudless day. The upstairs, with all that glass, might be comfortable with no heat at all, but the basement still needs heat. Then imagine the following night, when it gets cold and that glass isn't keeping the heat in, but the well-insulated basement doesn't need that much heating. There's no way to have a single heating element serve both and maintain comfort.
Here's the problem with concrete slabs as radiators.
The rule of thumb is you get 2 BTU/hr per square foot for every degree difference between slab temperature and room temperature. So if your room is at 70F and the slab is at 85F -- which is close to the limit of comfort -- you're getting 30 BTU/hr/sf. If you have a 3" thick slab, it weighs about 25 pounds per square foot, and raising the temperature of the slab takes about 12 BTU per degree per square foot.
So when it's time to turn the heat on, to get that slab up to 85F takes 15*12= 180 BTU/sf. But the heating system you installed is designed to provide 30 BTU/sf, so the first six hours of output go into heating the slab. And when the sun comes out and it warms up and you don't need heat, it takes six hours for the slab to cool down and stop heating the room.
This is actually a simplification, as the slab warms it does start emitting heat, so some of the incoming heat goes into heating the room and some into heating the slab. The upshot is it takes far longer than six hours for the slab to get to full heat. And when heat is no longer needed the slab continues to heat the space, making it uncomfortably warm.
But the problem is real. There's a reason systems like WarmBoard emphasize "high responsiveness" in their marketing. You want a system that starts and stops producing heat as quickly as possible when conditions change.