I am considering radiant wall heat as the sole heat in some areas of new vacation home in cold climate (eg lowest -30 typically -15 or so).  Planning to use extruded aluminum heat spreader plates (eg Radiant Engineering?) 8" OC running vertically on crosswise strapping/sleepers between joists for the bottom 4' of exterior walls, directly contacting on the back side of 1/2" wallboard.  This leaves the vertical stud lines free for nailing/screwing the wallboard etc.  The applicable wall area would be eg 50 or 60% of the floor square footage of the room.  I'm guessing I could get 25 to 35 BTU/hr/sqft (or more?) on 120 to 140 degree water, based on Radiant Engineering and ClimateTrak data.  That might not give me enough total BTUs?  Don't know the heat loss data yet, but the walls and ceiling will be well insulated, and I'd aim to provide some isolation of the heat plates from the 2x6 studs.  Would it help to spray paint the wallboard-facing surface of the alu plates flat black to improve radiation to the wallboard in any areas with faulty/gappy contact?  How about running hotter water?  Can I actually expect more BTUs of output?  Does this all sound workable?  Why don't I hear much about radiant wall heat?  In the wall you're not limited to the max 85deg surface temp limit as with a floor, the wallboard gives much less R than most finish flooring, wall systems might also be useful for radiant cooling in summer, etc.

Any experience or advice regarding in-wall radiant heat would be very much appreciated.
Thank you.
F.F.

you can't run hotter than 120 with gypsum wallboard products, but that can easily get you 30 BTUs/sq ft even with lightweight plates. Wall can be "blocked" by furniture and is a puncture risk (minimized if you keep it low like you are) and usually impeded by window and door placement, so if "not floor" is the goal, the ceiling is often the best bet.

Thanks NRT.Rob.  I am learning as I go.

Can't the gypsum take more heat or does the wall just get too hot for comfort?  Is some other wall finish better than wallboard for the radiant?  I once stayed in a condo with electric radiant ceilings and got a hot head and cold feet....  is that typical for radiant ceiling?   The goal (sort of) is to figure out something that might be as quick/responsive as Warmboard but with a lower price tag.  I am considering Warmboard for the great room/living room, kitchen, dining, but some other system for the bedrooms and bathrooms.

FF

it's a problem with gypsum, and also comfort. there is little reason to run hotter and if you are running warmboard you won't want to run that hot anyway.

hot head and cold feet is a symptom of un-modulated radiant ceiling. Low temp ceilings, preferably with reset, don't have that issue typically.

Walter, if there is one place that I'd most want radiant heat in a house it is in the bathroom. You'll really appreciate the warm floors in the rooms you're more likely to use barefoot.

Radiant walls (and even ceilings) are good - complete control of all the surface temperatures maximizes comfort.  Plus, more radiating area allows the use of lower supply temperatures, which increases heat pump efficiency.

Better (more efficient and cheaper) wall and ceiling radiant systems are needed.  Can't someone build a panel similar to the HDPE solar panels for swimming pools that would be paintable and would serve as a wall panel?  Probably with aluminum foil on the backside and well marked places where nails should go through.  All water connections should be on the back side.   Ie, I want to replace sheets of drywall with a plastic radiating panel.

I have seen this done with Warmboard placed in the walls and on the ceiling. Ceiling is not so good for heating and was designed for cooling. The problem with radiant cooling is condensation, behind your walls or ceiling this can be catastrophic. Chilled beams are an option but you still need to address condesation if cooling with any radiant system. I am attempting to heat and cool a home through a slab-on-grade system. I know cooling from the floor up isn't the best option but ceiling fans will be employed to assist with moving the coolth upward. I also would not have tried this except the concrete slab is also being used as the finish floor. We installed a dehumidification system to minimize the risk of condesation, but if it does occure no big deal. No finish over it to get wet and moldy. With the slab it may get wet, but it will be visible and can be addressed without any damage to the building. Stay tuned cooling season #1 coming up this year. Another reason I am attempting this is, we have a water to water geothermal heat pump that will provide heating and cooling through one distribution system at an incredibly efficient rate. Minimizing costs of installation as well as continued energy savings for years to come.

Tom Pittsley
ecobuilder@aol.com
www.eebt.org

There is a supplier here in the NW that designs to slab cooling, key is to stay 5 degrees above the temp dew point. I would like to provide the same but to much liability if set up wrong. The mold issue is huge.
Regarding wall heat we have had some customers use our 12 inch heat transfer plates horizontally from floor to about 4 foot level. System is 3/4 sleepers with plates parallel to floor, pipe snaps in to plates, after 4 foot level sleepers go vertical. logic is years down the road picture hangers will not spear pipe below the 4 foot line. Naturally rockers have to be tuned to the system.
Dan

I think you can do some amount of radiant cooling if you have low mass radiators and a control system that measures dew point and the circulating water never goes below that. But imagine what happens when you have a big concrete slab at a cool temperature and then the occupants decide to turn off the AC and open the windows? Condensation.....

Using walls & ceilings for radiant COOLING can use far higher water temps than slabs since it induces convection, whereas cooled slabs generated an insulating boundary layer of cooled air at the slab itself. Except in very humid climates or when using undersized coolling panels, getting the heat out won't require running it near the dew point. (If the indoor dew point is over 60F you'll be needing to use mechanical de-humidification to keep the mold potential down anyway. A RH of 60% @ 75F corresponds to a dew-point of ~64F, and you need to keep it under 60% for comfort & mold issues anyway. Consult your friendly psychrometric chart for verification: http://truetex.com/psychrometric_chart.gif ) If you're using the same amount of vertical or ceiling surface area as you would for radiant heat @ 120F heating water the condensation hazard will be quite low.

Back to one of the original questions: "Would it help to spray paint the wallboard-facing surface of the alu plates flat black to improve radiation to the wallboard in any areas with faulty/gappy contact?" There will not be a measurable impact on heat transfer- the emmisivity of the paper backing just isn't that different from any paint, and the insulation band created by 0.05" of air in any gap between the aluminum plate and the wallboard is already far less than the insulation value of the paper backing & gypsum itself.

How about running hotter water?

Regarding this - yes, hotter water will deliver more BTUs. You will lose efficiency if you are using a heat pump though.

Also, if you are using external walls/ceilings as radiators, you will substantially increase heat loss through the insulation. Ie, if your ceiling is 140F, expect many times the heat loss if it were 70F. I think interior walls make better radiators.

radiant ceiling works great (in response to an earlier post), and while interior walls have less "backloss', they will be less comfortable as well.

I agree with that. Studies show that a nice even mean radiant temperature (MRT) is important to thermal comfort. I have seen pictures of radiant systems that hang panels from the ceiling. Then you get more radiant surface area without the extra loss from conduction through the exterior surface.

Walter, are radiating panels that stand off from your walls possible?  I hate to see your heat loss increased because of hot walls.

Thanks for all the info. The walls will be pretty well insulated, 1 inch (R6.5) Tuff-R outside and probably 3 (or 4?) inches sprayed-in foam in 2x6 stud cavities, probably plus reflective barrier on the inner surface of the foam. The heat plates and the wallboard will be somewhat isolated (e.g. 1/2 inch foam board strips) from the studs. Maybe that's enough to minimize back losses? I figure the exterior walls are the ones that otherwise lose heat from the room and get cold, leading to feeling radiant cold in the room, so if you can keep the inward surface of the exterior walls just slightly warm with the in-wall heat plates, you've essentially created "active insulation". So I'm intending to put plates in the wall behind the wallboard rather than exposed radiant panels on the walls. Or, is anyone experienced with true radiators (baseboard) such as Runtal?

I always pick ceiling first, for reasons mentioned earlier. If not possible, i typically favor radiators over radiant wall to prevent blockage and dodge the puncture risk.

however, low temps are easier to achieve with more wall coverage.... easier meaning cheaper. Extra loss I think would be fairly small, but I've never seen testing. You are radiating into the room of course, that only leaves so much heat energy to leak out in backloss, so it's not like you made your wall the same temp as your water. I'd be interested in seeing a study on it some day...

If you're giving it a 1/2" gap between the plates & exterior wall, this is a stackup where reflective insulation placed at the exterior side of the gap will make a measureable difference (~R5-effective). Rolls of aluminized-fabric radiant barrier goes up quickly, is puncture-resistant, and will likely outlast the aluminized bubble-pack stuff. With foam insulated walls you can use either the perforated or unperforated material. (IIRC in Canada code still requires an interior vapor barrier if it's half-pound foam, in which cast unperforated radiant barrier with taped/mastic-sealed seams should get you there.)

Low-profile radiators are pretty much the standard in Europe. (It probably goes without saying but...) Get the full specs and be sure you can get the BTUs/hr needed with 120F water, not just the 180F number often listed. You can get a lot more out of 'em per square foot than with radiant-wall, since there is high convective heat transfer taking place as well. And if it turns out you NEED to bump up the water temp over 120F you can do so without messing up the wall.