Hi,

Is there anyone here who has ever used sheetrock as a thermal mass/conductor material when installing under the floor radiant tubing?  I was thinking about using Georgia Pacific's DensArmor paperless wallboard to avoid moisture/bug issues.  Seems like it would be more effective as well as cheaper than aluminum panels.  2 5/8" board layers would give me 1 1/4" of thermal mass.  Any thoughts?

Thanks!

Mass is not relevant to heat transfer improvement. It just stores heat. Unless you need buffer capacity, storing heat doesn't help you.

Also, how are you going to conduct heat from the pipe into the sheetrock in the first place?

All you're doing in this case is reducing the effectiveness of a naked staple up by adding resistance you don't need, unless you are improving the pipe to sheetrock contact area in some way. If you do improve tubing to sheetrock contact, it might be slightly better than nothing (and is an interesting idea), but it is not more effective than aluminum... aluminum is more conductive.

what about moisture though? Cement board is one think. Sheetrock doesn't hold up to getting real wet very well though, and people spill things on floors...

Hey!  Thanks for responding.

I wonder though... if mass is not needed, then why are slabs and gypcrete pours considered the way to go?  I would think slow conductivity with thermal storage capacity would be a good thing.  Then it could radiate up slowly.  Contact between the tubing and sheetrock seems like it would be easy... either with forming a channel with some type of mortar before installation, or mortaring the tubing in it's channel after it has been placed in the channel.

I don't see how my spilled wine would be an issue(except that it is less wine for me to drink!).  It's the moisture from below that would be a concern to me.... and the bugs.  I think this type of installation would HAVE to be done with a well sealed radiant barrier underneath as well.

in some cases the buffer capacity can be useful. it's better to have variable water temperatures though, IMHO. Given two systems with "bang bang" control, the mass system might be more comfortable, if the water temperature is not so far off as to cause temperature swings. Given two systems with variable water temperature control, the low mass system will be just as comfortable and possibly more effective at heat transfer if a highly conductive medium... such as aluminum... is used.

What exactly is it that you think "slow conductivity" does to help? I know a lot of people say this, but ultimately it's a question of conductivity. 'crete is fairly conductive and does a fairly good job. But it's not "more effective" than an aluminum system.

Sheetrock and mortar? Mortar is wet. to use enough to mash the pipe in would seem to compromise the sheetrock..? Even if not, how are you going to channel sheetrock to house tubing that will most likely be a minimum of 1/2" OD if you use 3/8" pipe?

It would be an interesting experiment, but messy and difficult, and dubious in long term durability, in my opinion. and it's a vapor barrier you want under this. do not rely on a radiant barrier alone, though if you want to use it in conjunction with real R value, and double as a vapor barrier, that is acceptable.

Best of luck in any case!

I guess what I mean by "slow conductivity" is that it doesn't need to be as fast given that there is thermal mass.  I would think that an aluminum system(or one without plates altogether even more so) would have to cycle on more often and also require warmer water... a system with more thermal mass would be slower to respond, but radiate out more evenly... thus more comfort and efficiency.  That is my understanding and what makes sense to me.  It's never happened before, but I guess I could be wrong. :}

I could prebuild "panels", for example, ... 1 layer of sheetrock, then the second layer having 2 pieces, leaving a channel that is as deep as the tubing.  I could either preform a "mortar" bed in the channel ahead of time to get good contact, or do it the fun way and goop the stuff in after installation.  This seems at least better than a straight staple up with no panels at all.

To get 4" or 8" of heat spreading aluminum, it costs $6.50 or $13 for a 4 foot section.  That's a big motivator for me to seek cheaper, but at least as effective, systems.  I can see how contractors love the ease of installation that those aluminum panels give.  It sure looks appealing to me as well.  But I'm the homeowner and will be the one forking up the bucks.

Another thing that kinda confuses me about aluminum.  It's a great conductor of conductive heat AND a great reflector of radiant heat.  If I had a radiant barrier below aluminum panels, wouldn't the panels sort of reflect the radiant heat right back down?  I guess it could bounce back and forth and spread the heat around the joist cavity even more or something.  That would be good.  Dunno.

Given an inappropriate water temperature, the aluminum system would cycle more often. Or, a minimal load situation where the bufferring capacity of the concrete might be of benefit. The aluminum would typically... provided all other factors are equal, such as upward resistance.. require LOWER water temperatures, if the aluminum is of suitable thickness and coverage. This is true because aluminum is more conductive, and conductive power is what determines the water temperature requirement, not mass or buffer capacity.

Imagine your water temperature is exactly right to push exactly your heat loss through the floor continually (that is, it creates the exactly perfect temperature differential between water and room to cause heat to flow at exactly the same rate as the room is losing heat). In such a situation, it would not matter at all what your emitter was in terms of mass. This is the kind of effect that "reset" water temperatures has. Mass is a band aid for some poorly designed systems where the buffer capacity can help even out poorly matched water temperatures.

Regardless, you are not going to get enough mass from two layers of sheetrock to really consider it a "mass system".

You are right that as long as you are improving the pipe contact to panel, and not just increasing your upward resistance, that it would be better than with no plate. But it would not be better than with heavy aluminum plates. It probably wouldn't even be close.

The plates aren't all that shiny, so they don't reflect all that well. Reflection is "optically obvious".. that is, if you see a reflection, it's reflective, and a clearer reflection is more than a dull one. Personally, I am not an advocate of radiant barriers under slabs or in upwards facing applications due to the dust factor over time, regardless of the plate presence above, or the lack of it.