Wow - just discovered this site and my head is swimming.  I'm in the midst of converting my forced-air heating system over to radiant heat as part of a state program to help home owners improve home energy efficiency.  As money is really tight, I'm doing as much of the work myself wherever possible.  About ready to install staple-up tubing between floor joists in crawl space (insulated) and came across issues regarding thin vs. thick heat transfer plates.  Have already purchased a slew of thin plates and now wondering if I've wasted my money.  Tubing water temp. will be 180 degrees.  Joist spacing is 16" o.c.  Also, I'm not clear on type of insulation needed under this?  How would 2" of rigid foam board work?  Oh yes...will be using 3/8 Viega tubing.  I live in Kenai, Alaska where it commonly gets 20 degrees below zero at times duing the winter and where summer temps average in the high 50's to low 60's.  I would be so appreciative of any help....

180 degree water in a staple up is very high indeed. I would definitely use heavy plates and a reset control strategy however, the question is why 180? Did someone calculate that, or is that just what your boiler is set to?

Over a cold but insulated crawlspaces, in alaska? R19 minimum, and I would vastly prefer to hit R30. that's 5 or 6" of rigid foam if you're going that route.

Thanks Rob for your reply!  Hmmmm....about the water temp at 180....the plumber who installed the new boiler said I would probably need it that hot.  Easy enough to dial the temp down though - once all is installed should be easy to determine if a lower temp would be adequate?  I guess I should research how to do a heat load calculation...

Regarding the heat transfer plates...now that I've already got the thin ones...how much improvement do the thick ones provide and would it seriously be worth it to purchase even in the event that I can't get my money back from the thin ones?

Insulation - I like your suggestion of pushing to R30 between the joists.  That's pretty expensive to do with rigid foam, so fiberglass might be the way to go?  And what are your thoughts about creating a 1 to 2 inch air space between the insulation and the tubing?  Seems like I read somewhere in this forum as well about the effectiveness of a reflective barrier before installing the insulation (or fiberglass batts with foil paper already installed) - does the foil really improve performance?

Another question regarding the loop design for this room....if I run two lines per joist, it'll require about 500 linear feet of tubing.  That would make 3 loops of about 167 feet or 2 loops of 250 feet.  I understand that with 3/8 tubing I should keep it under 200 - is that right?  This is, however, not considering where the manifold will go.  Figuring that one out - what to do with the manifold - is giving me a headache.  I just read another site describing a method of using what they call "headers" for the supply side and the return side, with each loop essentially just tapping into the header right where the loop starts.  (Rather than having all of the loops begin and end at the manifold which is located in one location - which can add additional footage.)

I hope I make some sense.

Thanks for any thoughts you may have.

I vastly prefer having a manifold with loop shutoffs in a central location. saves a ton of copper work.

airspaces are not required when you are using plates.

I don't know if it's "worth it" to eat the price of the lightweights, but without a load calc, you don't even know if the light plates will work, never mind if they will work well. Heavy plates are a very significant improvement for sure.

I'm not a fan of reflective insulation products in upward facing applications, others disagree. they definitely work at first but I would NEVER reduce a real R-value product with reflective foil, only supplement.

do 3 loops.

Excellent - thanks for the info Rob.  I'll check around here locally to see what I can find for heavy plates - and I'll talk to my plumber to see what he can get them for.  Do you know of an excellent online vendor that I might consider as I shop around? 

About the earlier issue regarding water temp. - what do you think of the rationale of putting up 3 lines per joist (these are 16 o.c.) (heck, even 4 lines if there's room) as a means of generating more btu's with a lower water temp?  I realize that at some point the cost of the additional tubing, plates, etc. will exceed the savings from running cooler water.  What's your experience with this?

Thanks for your advice, Rob - you're really helping me clarify what I need to do.

Hi Rob - I forgot to say....floor covering in this room is wood laminate with, of course, a foam membrane underneath.  Maybe that's why the plumber suggested 180 for water temp - just guessing?  I'll talk with him this week and I'll ask...

foam membranes under laminate are extremely thin and of little concern from a heat transfer standpoint.

and more pipe in joist bays beyond the standard 2 per joist do not appreciably help your situation.

Hi Rob - Well, after checking on the price for extruded aluminum heat transfer plates, I'm definitely NOT going that route.  Unless you have access to some incredible deals on the thicker plates, I don't know how anyone can justify a 250% increase in cost (and in my case even more as I'd have to eat the cost of the thin plates that I've already purchased).  And from what I've read (and it was hard for me to find any hard data on this), the thicker plates provide less than a 10% gain in heating efficiency over the thin plates.  Am I way off base on this?  Do you have a source that would contradict this? 

I'm wondering if this is an example of a situation where we go to all lengths possible to maximize efficiency while losing sight of the bigger picture? 

Remember, I am talking about whether or not your system will WORK, not just about efficiency. You're in alaska. You need heat. Serious heat. Lightweight plates may not give you the heat you need. without a heat load you're guessing, and you're guessing on a product that is close to the edge in your case.

if it does work, that kind of water temperature drop can mean big differences in efficiency especially during shoulder seasons. It can mean the difference between being able to use a condensing heat source to its full effect or not. It can mean being able to use lower temperature heat sources or not. and on any heat source, it will translate into an efficiency gain.

I have the entire heating industry to contradict the few, out of date or wildly misguided sources that claim that heat output isn't much better. When we use heavy plates, water temperatures drop 20 to 30 degrees compared to light plates in joist systems assuming normal heat loads... more at higher loads/floor resistance and less at lower loads/floor resistance.

But that is not "heating efficiency". I don't know what you mean by that specifically and I suspect you're not sure either ;) if it DID increase your heating system efficiency by a full 10%, that would likely have a very quick payback period, in your case, unless you are building a thermos bottle with no windows. And if you can drop a cast iron boiler 30 degrees from 180, the "old rule of thumb" is 1% efficiency per 3 degree drop. Dunno what your yearly heating costs are but I bet if you did the math you'd find it worthwhile.

What is the best system to heat 2 floor house , need gas furnance to supply baseboard heat, hot water abd supply for 600 sq ft in-joist radiant heat under wood floor on 1st floor, what is better, tankless on demand , a combo heater unit, approx cost for furnance and radiant heat system thanks mitch