I had posted a message in the Residential forum and was directed here for additional help. I am doing a gut and reno on a 5000 sq ft 60's bank ranch house in NC with a walk-out basement. The living space in the basement is about 1800 sq ft, and I am planning to install hydro radiant heating in it. The building envelop has been sealed and insulated mostly following Building Sciences guidelines, or as much as possible given the existing structure. A part of the slab was jacked for plumbing, and I know that the layers are: clay soil, 4" gravel, ~6 mil (or perhaps thicker) poly, and 4" slab - this should be throughout the two slabs, but I can't be 100% sure. I selected hydro over electric radiant heating given the large area. My plan was to put down 3/4" XPS, 3/4" sleepers, lay the 3/4" PEX between the sleepers and fill with dry sand, then 1/2" plywood for a total of 2" before hardwood or carpet. But many DIY sites suggest a vapor barrier on top of the slab, either using a poly sheet or faced XPS/bubble/EPS/etc. If I do this, my slab would be sandwiched between two vapor barriers, and I would be concerned about trapped moisture. Is this a non-factor or a valid concern, i.e. is it safe to just put down the poly even if it is unnecessary? Also, I would like some feedback on my plan including suggestions for improvements. A reply by Dana1 to my original post in Residential suggested substituting 3/4 XPS for sand to raise R, and use spreaders to warm the subfloor. I have ~3" to play with in one zone and 6" in a second zone. I am now thinking 1" XPS + 3/4" "sleeper" XPS between 3/4" PEX with spreaders at 16" O.C. + 1/2" plywood, leaving 3/4" for hardwood. Thank you for your help. PS Sorry for the long paragraph, but I can't seem to break it into multiple paragraphs.

You are correct about the vapor barrier - the least of your worries.

Have you done this before?

What is the heat load for the rooms being heated?

Why 3/4" PEX? Is is barrier PEX?

Why electric boiler?

Why carpet?

Why sand?

Why bubble foil?

You sound like a professional but the plan is strictly DIY.

Perhaps a supplier with design help or and independent designer.

I concur with all of morgans questions (except i dont see where you mentioned "bubble foil"... not sure where that came from). to expand on his questions:

-Sand is a very poor medium. aluminum plates are much better. I assume it's too late to get tubing in the slab? the low profile, easy, high material cost method is Roth Panels, but you can site build a "sandwich" install if you don't mind a lot of carpentry.

-3/4" PEX is a waste of money and effort. You will never need or want bigger than 1/2" in this case.

-Electric boiler is an odd choice for your area I believe. What is your cost for gas vs electricity?

Nothing should be installed before a load calc is done. a real one, with real math, no "energy factors" or other fudge methods.

To answer the questions:

I have not done this before and am learning as I plan.

I have not done a load calculation because factors are still changing.  For example, there are still areas where substantial new insulation is being added and exterior doors being upgraded.  I was hoping to defer it until I need to replace the 3 now oversized heat pumps, which are nearing the end of their lives.  I am considering a single geothermal to replace two of them.  There are no comfort issues as far as ambient temperature and humidity are concerned.  Our primary intent is to use radiant heating to warm the floors to a comfortable level (we find the cool floors uncomfortable regardless of the right ambient temperature), and secondarily as a more energy efficient source of heat which might let us downsize the heat pumps even more.  Do I really need an accurate load calc, especially since the heat pumps will later be downsized to account for the radiant floor?

PEX will be oxygen barrier, and cost-wise, 3/4" at 16" OC seemed to be about the same as 1/2" at 8" OC.  If 1/2" is a better choice, then I can do that.


I'm not sure where the electric boiler came from...I have a on-demand NG tankless water heater.  Eventually, I would like to tie in solar water heating as primary with the tankless as supplemental, but this is down the road.  My comment "hydro over electric" meant that I chose a hydronic radiant system over an electric mesh radiant system thinking that hydronic would be superior for my application.

Carpet was just a personal preference to soften up the floor in one part of the basement (playroom).  We can put down hardwood and lay area rugs if the carpet would affect the performance of radiant heating.

Both sand and bubble foil are suggested in some radiant heating sites.  Sand seemed more plausible while bubble foil seemed like a bad idea.  With so many different sites touting different methodologies, I am trying to filter out the good from the bad, and posting here is for that reason.

Yes, this is a DIY in the sense that I am pushing my general contractor to use green and scientifically proven methodologies (e.g. Building Sciences) that he is unaccustomed to, but he has been very accommodating and helpful.  I am just an informed homeowner looking to go as green as I can afford.

I cannot jack and redo the original slab.  It is possible, however, for me to lay a thin floating slab on top of it if everything (including the finished floor) will fit within 3", maybe 4".  I did find Roth Panels and similar material, but they seemed costlier even accounting for the extra carpentry labor.

A heat load analysis is where you start.

Without it, you can't size the heat pump or the radiant floor heat source or the radiant floor panel for that matter.

You need an experienced designer to sort out all the hype from the real science.

Those who promote sand, bubble foil and yes tank-less water heaters in conjunction with radiant floor heating should be ignored.

If you "warm" the floors, you will "heat" the house.

When I start a design, I ask about the location, the fuel sources available; the age, size and condition of the home, its windows and insulation.

AutoCAD is preferred, but a computer generated heat load analysis (free, on-line heat loads notwithstanding) is the first and indispensable step.

There is no merit in using 3/4" PEX for residential radiant floor heating, except in distribution piping or for snow melting.

Domestic hot water is usually best done with the boiler. Solar space heating is a generation away unless you live in the southwest. Domestic hot water is a part-time proposition also.

It has been a challenge to find any contractor who has been proven to be experienced in green or non-tradtional building methods, so I'm not optimistic about finding a good designer in radiant floor heating in our area.  But I will try.

I will also see about getting an estimate on the heat load at this point.  However, I don't understand why the heat load would have any bearing on the installation method of the PEX tubing itself.  Isn't there a common best practice for doing this on an existing slab?  Some elaboration about this would be very informative.

Of course, I agree completely that any heat source that is actually installed must be sized properly after a real load analysis, and I intend to do exactly that when selecting and installing the heat source for the radiant floor along with the heat pumps.  But for various reasons, I need to finish out the basement first and want to lay the PEX correctly while doing so.  If you are suggesting that I need a heat load calculation in order to do this, then some additional clarification would be very helpful.  Thank you.

heat load calculations determine everything from water temperature requirements to flow rates and loop lengths. if you ever plan to use geothermal with the heating, water temperatures are a critical concern. You may find something like Roth panel would be a much better choice in that case, for example, as it would require much lower water temperatures than most other methods you might use here. it unlocks all the questions and provides nearly all the answers, if done properly.

If you were to use a "sandwich" or sleeper type install, it would definitely involve aluminum plates, not sand.

thin pour of concrete over an insulated slab would be thermal break (1/2" foam, perhaps), 1.5" concrete, finish floor.

There are many ways to transmit heat to a space.

The "emitters" are often times not seen but the floors walls and/or ceilings become in fact the "radiant panels". Each engineered system, (sand, foil and staple-up generally don't qualify) has a rated output determined by an inside design temperature, delivered design water temperature, floor coverings and back insulation.

All must be determined before installation or the outcome is simply guesswork.

Both Rob and I are Radiant Panel Association (RPA) certified designers. You needn't have a local certified designer or installer (though it doesn't hurt) since the design is based on readily available factors the locals often ignore or are unaware of.

As you so aptly pointed out, accommodating and helpful is a real asset.

Carpet is fine if not too thick, not too much wool and a thin pad. A proper heat load for each room would determine the flooring options.

Posted By BadgerBoilerMN on 26 Oct 2010 04:02 PM

Those who promote sand, bubble foil and yes tank-less water heaters in conjunction with radiant floor heating should be ignored.

Sand is worse than worthless- in this situation- it's an insulator, between the tubing and sub-floor.

Bubble-pack radiant barrier is an expensive "solution" looking for the right problem- and this ain't it.  Best use I've seen for it so far is for binding the 1" of EPS around stacked cases of wine on pallets being shipped cross country in unrefrigerated trucks.  I'm sure it moderates the heat uptake while crossing the sun-drenched basins that might otherwise penetrate the EPS.   Price/performance-wise in a staple up you're better off spending the same money on low density fiberglass batts.  In or under a slab it's COMPLETELY worthless.  Snake oil might work better (or was that mouse-milk?)

Most tankless manufacturers will void the warranty if used for space heating.  Takagi won't, but with them you're pretty much locked into low-80s for efficiency (best case, with a decent system design- which ISN'T a cookie-cutter design from a net-sales-droid, to be sure.)

With low-temp systems like Roth or WarmBoard a condensing burner beats the cheapo 0.82EF tankless by a lot.  Sure, they're cheap, they're modulating and will often beat a cast-iron beasite, where there's a significant heat load or  running expensive fuels like propane it's hard to make a financial case for a tankless over a true heating appliance.  Some of the Navien condensing tankless heaters put to combi-use can give good performance in the right setup, but price-wise they're not dramatically different from a mod-con boiler.  Their new purpose-specific combi boiler based on their tankless designs looks pretty good, but has no track record to speak of. 

The only installation I've seen using a condensing Navien in conjunction with active solar is in UT, but they're only getting about 86-90% efficiency out of it, (based on operating temp requirements and how they're using it-  radiant slab it ain't.)  But the solar array brings their total annual gas use down to the 180-200 therms/year range- less than what they had been using just for hot water.  When the load is low enough, the absolute efficiency of the system becomes less relevant.  I hate to think how much they spent on the solar- they should have spent at least half the money on insulation & air-sealing IMHO.  (It's at least one real-world example of a Rube-Goldberg contraption weirder than mine though! :-) )

With a low to moderate heat load and expensive electricity (and can net-meter at retail) a Marathon cogenerator combined with big buffer tank combi might be a better choice than a mod-con. That design would be WAY beyond the typical DIY installation but truly competent heating designers could handle it. (It's not all that cheap either, but in the land of 20cents/kwh the payback is quite good.)  In the land of 7cent/kwh it's a wash- less if fueled by propane.  It's probably where I'm going if/when the Takagi craps out.