Radiant Tubing: Are Heat Transfer Plates Necessary?
skinnymaplesco
| Posted in Energy, Heating & Insulation on
I’m installing radiat tubing in the floor joist cavites of my first and second floor addition.
I’ve been advised that I need to use aluminum transfer plates with the tubing for maximum support and heating efficiency. I’ve also had people tell me I only need to attach the tubing to the underside of the subfloor by whatever method, then insulate the top two or three inches of the cavity to insure that the warm air there moves up through the floor, and does not fill the cavity and the ceiling of the room below, wasting heat.
What’s the best practice? How much heating efficiency am I gaining or losing either way?
Confused. Thanks.
Replies
It is a matter of efficiency
The plates with a slight air gap and then insulation below will be the most efficient at transfering heat to the floor.
A plate with no insulation will transfer heat to both above and below.
The tube, with an air gap, and then a good insulation will transfer more heat to the floor, than the uninsulated plate. The insulated plate will transfer more heat to the floor than the tube with an air gap and then insualtion.
Jigs and fixtures said it well. No plates, no direction for the heat.
previous posters are not quite correct. INSULATION provides the direction for heat. Plates provide increased output to the wood, but if you are not insulated below you'll increase downward output even with plates as well.
plateless pipe is just a very weak output method. practically speaking you shouldn't plan on much more than 15 BTU/sq ft unless you are using very high water temps and I would never consider plateless installs without a serious room by room analysis to ensure it will keep up unless you are just adding some "floor conditioning" to an already full capacity emitter like an existing baseboard or radiator.
Plates are required in most full heat situations unless you use very high water temps or have very low heating loads. You can crank up to 30/BTUs/sq ft with the heavy guage plates.
when you insulate, don't bother with reflective foil. I like friction fit rigid foam, with good air sealing at the ends of the joists and, if it's cold below, around the cracks.
Question about foil
Why don't you like the use of reflective foil?
No personal expirence - just curious.
Terry
it's dramatically oversold for radiant applications and in an upward facing application it gets dirty and loses efficacy over time.
Just to clarify, can you address the accuracy of my understanding of this issue?
Plates, with air gap, provide resistance to radiated heat--I suppose they could also reduce somewhat the effect of convection transfer.
Foam, or other insulation provides resistance to all types of heat transfer (radiant, convective, conductive). but its effectiveness is mostly dependent on its thickness, usually expressed in R-values.
It seems to me that, in order to compare the effectiveness of plates alone vs. insulation alone, one needs to know how thick (or, R-value) the foam needs to be in order to acheive better resistance to heat transfer than plates alone.
Do you have information to nail this down?
you're on the wrong track. conduction to the surface dwarfs most other concerns once you are appropriately insulated. you can't compensate for lack of plates with insulation at all. lack of insulation can cause ANY radiant method to underperform, including a plated system, as whatever you have for downward loss reduces your upward heat transfer. that doesn't matter if you're plated or not, that's a simple heat loss equation. it's worse with a plateless system because its output is already low, that's all.
heat emission varies with conductivity from water to room floor surface. plates lower this average number significantly compared to staple up naked pipe, so you actually emit more heat. You can put R-10,000 under a staple up system and never ever get any closer to an R-20 under a plated system in output... the heat simply will not be removed from the pipe and it will return to the heat source.
this is because the plate contacts a LOT more of the pipe than naked staple up does, and conducts that heat to a much wider area of floor. but, this depends on your plate having good contact to the pipe as well, which is why lightweight plates are not nearly as good as the heavy gauge firm grip plates are.
Thanks--my mistake is in thinking that the reason for plates was to reflect radiant heat, but their real function is to unload the heat from the pipes and help tranfter that heat--right? So you want the plates to be in tight contact with the pipe, if that's the case.
Insulation below helps the heat move upward.
insulation doesn't help the heat move upward... it prevents it from moving downward. it's a perhaps subtle distinction but it's important.
correct that plates are not reflectors. they are conductive fins.
insulation doesn't help the heat move upward... it prevents it from moving downward. it's a perhaps subtle distinction but it's important.
Picky, picky :-)
I suppose you really get worked up when people say heat rises. (I used to, but finally gave it up.)
that's an industry wide pet peeve, yeah. I'm past the worked up part though ;)
Thanks Rob,
I think you're the first poster rd has bothered to listen to.
EDIT: My apologies, I confused you rd with another poster that really doesn't listen when radiant and how it works (at least from a few) discussions take place. If I could, I'd insert one of those smiley things up there now . Really................., sorry.
Sorry to give that impression--the fact is, I "listen" to your posts with interest and nearly always learn something from them.
I don't always reply because there's nothing useful I could add and nothing about them that I see a need to question.
rd
See my post above, I retract every damn wrong thought, wrong idea and wrong guy.
sorry.
CLARIFICATION
Quote from Cavin:
"I think you're the first poster rd has bothered to listen to.
EDIT: My apologies, I confused you rd with another poster that really doesn't listen when radiant and how it works (at least from a few) discussions take place. If I could, I'd insert one of those smiley things up there now . Really................., sorry.
A Great Place for Information, Comraderie, and a Sucker Punch.
Remodeling Contractor just outside the Glass City."
Calvin,
You are probably thinking of me and what I said about radiant heat.
Yup, I'm thinking that's the case.
Now I'll just have to mind my P's and Q's..............
or
R's and K's.
Radiation Revisted
No problem Calvin, but if you disagree with what I said about radiant floor heat, I would be happy to discuss it and listen to all views on the subject. As far as I can tell, I never said anything that disagrees with what NRTRob has said in this thread about the subject.
What I was talking about in that other thread was heating both the ground floor space and the basement as one zone by placing fin tube in a distribution loop about a foot below the open-joist basement ceiling. This a compact 24’ X 28’ basement and first floor, and there is an open stairway connecting the two spaces. The fin tube warms the air and the warm air stratifies at the highest temperature near the basement ceiling. Some of the warm air rises up the stairwell to enter the ground floor space.
The warm air layer under the basement ceiling conducts heat to the ground floor, including joists, subfloor, and finish wood floor. The warmed ground floor/basement ceiling radiates heat upward into the ground floor space and downward into the basement. It also heats the air film contacting the ground floor by conduction, and that heated air then rises and circulates in a convective flow in the ground floor space.
As I mentioned, I have really high insulation values with minimum R-40 in all walls, including the basement walls. With that much insulation, the heat loss is slowed down to the point where the heat has time to diffuse through the house and distribute evenly. So it reduces the need to focus and concentrate the heat distribution to exactly where it is most needed inside the building envelope.
never said anything that disagrees with what NRTRob
Then I guess stand corrected.
But, and we did discus this, the idea that the ceiling heat is going to radiate down and warm the non heated but insulated concrete mass I don't think will work. Reason being, I have a perimeter isolated and insulated radiant slab, but use a contraflow heater to do the job in the spring, fall, and as added heat source in the winter.
But w/o the water flowing through the slab, it is cool, uncomfortably cool. That's certainly not as pleasing a feeling as walking w/o shoes on a radiant slab .
So, if your basement is a basement-no problem.
But, if it's a living area as ours, radiant in the slab is necessary in my mind.
Further, once the slab is poured, it's done.
As far as the envelope- We have an open floor plan. Not powered movement of air except for the great room ceiling fan.
The walls downstairs-batt insulated 9-1/4"deep, 1" foam to exterior-furring-siding, 5/8" drywall
Foundation stepped-2" rigid fibreglass exteror, blocks filled with vermiculite, interior 3/4"foam, 5/8" drywall.
No ledge or foundation to the exterior, flush sheetrock floor to ceiling interior (which is pertinent to this thread).
The R figures to close to your 40, so similar.
Now, salvage and $ limited the 2nd floor to 2x6 @24" oc. Exterior still flush. R in high 20's, when I figured it, I'm remember near 30.
Have 22 yrs in this house and unless there's some differences, I'm thinking that your slab will not be warm (to comfort) while not being the cold of ambient ground temperature-low 50's, like not too cold beer.
So while this isn't the thread for it, there's gotta be something appropriate to insert it here.
So,
nice weekend?
Slab temperature
Calvin,
It was indeed a nice weekend here. Minnesota has been exceptionally warm this winter with hardly any snow. This last week has gotten down to more like normal temperature with highs in the +10 to +25 range. But it has been in the 35-50 degree range much of the winter so far.
I understand what you are saying about the basement slab. While it does receive some heat by radiant transfer from the ceiling, it will never heat up like it would if it had hot water running through it. Even the ground floor will not heat up to that extent, but it heats up more than the basement slab.
And you are right to say that, as a living space, the basement slab may be too cool, although I am not sure if it is. I am down there all the time in my bare feet, and sense no issue about the slab temperature. I can definitely feel the radiant heat from above, and sense that it also heats all the objects in the space.
But there is heat loss through the slab, and its mass makes it slow to gain heat and slow to lose it. I shut the temperature down at night, so the slab ceases to gain heat. If I maintained full heat all day and night, it would be easier to see where the balance is between the loss through the slab and the gain from the hot ceiling.
For the ground floor, the heat coming through the floor warms it perfectly adequate. The floor, walls, and ceiling of the ground floor seem to be generally identical temperature, and the floor is a little warmer in the vicinity of the fin tube underneath it.
like I mentioned
We live in the basement, it's walkout and there's even passive solar to add to the equation.
But, with the open living area and well opened to the second floor and a circular movement of air from down to up and back again, works like a charm.
The beauty of the radiant slab and the contra flow heater with it's slow releasing mass-warm down and cooler yet not uncomfortable in the sleeping rooms. Have tube under plywood/tile in the upstairs baths and installed 2-4' water basebds in the two remote bedrooms - for the ability to close the doors and still have comfort.
Living with a couple women, you get to know the meaning of comfort.
I live in MN too, and we have a "split entry", with the bottom floor (where our bedroom is) 3-4 feet below grade. Concrete slab. We have carpets down there, and we don't (with dual zone heating) notice any significant difference between the temps down vs up, except that the downstairs bath tile floor does get a bit colder than upstairs. But that's what bath rugs are for.
Maybe it's "nice" to have a heated slab, but its far from necessary for reasonable comfort.
It's nice
and since it's our real whole house heat source, it's location in the lower level is a necessity. But this discussion isn't about forced air comparison, if that's your system.
Since you mentioned "subtle
Since you mentioned "subtle distinctions" in another post:
Unless terminology has changed since I took that course in Heat Transfer way back in engineering school, heat transfer does not occur solely via conductivity from the hot water to the floor surface. Rather, it is a combination of convection and conduction. (heat is convected from the hot water to the side of the pex, then conduction through the pex to its outside surface, then convection again to whatever surface is touching the pex or to air, etc. There is also a radiation component in the whole mess but that would be very negligible.
I know what you meant, as well as many others do. But I've observed these terms being misused over and over again in these sorts of discussion topics. I think it best to keep terminology accurate so as to not perpetuate misuse and misunderstanding.
Convection occurs in a fluid (and air is a fluid). You don't get convection to "whatever surface is touching the pex" -- that's conduction, by definition.
"conduction is king"
conduction is by far the strongest method of heat transfer. convection and radiation work... in fact, in suspended tube systems in joists those are the only two methods working for you. but they are weak.
the reason we harp on conduction is because without it, you have a low strength emitter. Better conduction and conductivity is what translates into high performance in the case of a radiant emitter.
Home > Radiant Tubing: Are Heat Transfer Plates Necessary?
Rob:
Thanks a lot for the thoughtful responses. Great information, served in bite-sized pieces. I'm convinced now to spend the extra money and install the transfer plates. This project happens to be my own house, and as always, we're constrained by limits on our available cash, but we are also trying to think about long-term benefits and efficiency in everything we do.
At our firm we design a fair number of houses each year, and I'm using my project as a kind of learning lab to rethink all elements of how we detail houses for construction efficiency, aesthetics, and energy efficiency. I want to be able to look clients in the eye and tell them that I have hands-on experience with what I'm detailing for their house, and be able to explain how and why things work.
Any recommendations on sources, wbsites, and/or minimum aluminum thickness? If I'm going to do this, I want to get it right.
Thanks in advance for any further help.
skinnymaplesco
it depends, but typically I'd say either your load is so low you don't need plates, or it's not and you should use the heavy gauge "C" fins out there. We deal in Thermofin, but there are other brands as well. the lightweight plates out there are suitable for some applications but typically I would use them only in otherwise lower-R assembly situations like radiant ceilings or over the subfloor sandwiches... in a joist you just need more oomph to make a big difference.
so step one is always a room by room load calculation.
If you do multiple houses and are looking for cost effective radiant... consider radiant ceiling as a go-to option. it has some big benefits and it's only slightly a step down from floor for heating comfort. does require a flat ceiling though. it's cheaper, high output, invisible, no flooring risk, and if done properly... it can do cooling (there are other considerations of course...)
I love planting seeds. muah ha ha!!
whew..........
that was a close one.