Ive tried posting a couple of times and researched everywhere i can on the internet but cannot find any info whatsoever on installing radiant heat over spancrete. Has anyone ever done this or know how to do it? Cant believe its not more popular.

Im going to be installing the tube in a couple of weeks, the spancrete supplier says i need some kind of membrane between the spancrete and topping. My basement underneath will also be heated with radiant tube laid over 2" high density insulation on grade set in a 4" concrete slab, that bits no problem and i already have half of it done. Should i have insulation under my topping on the main floor? im thinking not as the space underneath (the basement) is heated and i want to take effect of the thermal mass of the spancrete right?

Viega snap panels are the only method i've found other than rebar mesh and zip ties to lay out and secure my pipework. I was quoted $5,000 for 1700sf of gypcrete so im pouring regular concrete myself, 3/8U agg. instead.

My main floor is about 1700sf as is the basement, there is also an attached 400sf garage, all ICF.

Thanks in desperation.

Peter

I have worked on two house projects with spancrete floors. i think we specified 2" xps over the planks with tubing & mesh over that with a 2" concrete topping. depending on the crown of the planks, the topping could end up much thicker. i believe you need the insulation to control the heat flow, so that your basement does not end up with a radiant floor and radiant ceiling and your first floor is lacking for responsive heat. i would definitely go with a regular concrete as opposed to the gypcrete, for one more dead load will keep your floors stiffer, also it's cheaper (as you've found). pouring the topping before framing interior partitions will save a lot of headaches as well. i'll be interested to hear other opinions.

Thanks for the response, for ease of pouring im keeping the thickness of the topping constant at 2" the deflection on the concrete planks is very very  minimal.  So far ive received pretty much equal advice on installing and not installing insulation.

Ive been advised it is necessary to prevent heat loss down into the spancrete regardless if the level below is heated too and that i should use either 1" eps or silver reflective bubble.

I've also been told its not necessary because once at temp, the floor will stay pretty constant, the larger thermal mass of the spancrete will help prevent fluctuations in temp. A btu is a btu, the heat is going to be emitted into conditioned space either way.

So i cant decide, im supposed to be pouring at the end of next week, i think im going to use viega snap panels to act as a membrane and to hold the tube in place, the decision now is do i also install reflective bubble insulation under the viega snap panels.

the reflective insulation is a waste of time. don't use it.

You need downward insulation either between your pours or underneath the floor itself. if you put it on top of the floor you get faster response times and less chance of "swing", and less thermal mass which only really matters if you're doing a passive solar design.

put it underneath and your floor will react up and down slower. and you'll have more mass available if you are doing passive solar design.

Rob, I can see that makes sense but with a tight ICF house and a (hopefully) geothermal heat pump and high thermal mas of the floor, wouldnt it be more effective to let the system run constantly so once the slab without insulation is at temp, its just a case of maintaining, yes the underside of the slab would be warm but if anything wouldnt that just add downward radiation to the lower level and keep my bald head as warm as my bare feet?

By the way the underside of the spancrete will not be finished or covered up, hence my above thinking. I can see it would be a different story if i were putting a ceiling in.

If i did insulate the topside of the spancrete before pouring my topping, which might not be a bad idea incase i ever decided to put a ceiling in downstairs, what would you reccomend? id be limited to 3/4" or at the most 1" how about foil faced EP insulation? wouldnt that give the best of both worlds, reflective and thermal? it wouls also serve as a means of securing the radiant tubes while the concrete is poured instead of the viega panels.

Thanks for your advice so far.

downward radiation is output lost to the 1st floor. this means to get the same amount of heat to the 1st floor as you need, you need a higher water temperature to drive more heat out since some is being wicked downward, and with GEO you want the minimum water temp possible, so this is counterproductive.

also, the "underside being warm" is a radiant ceiling for the floor below. but it's a completely uncontrolled one from the point of view of the lower floor and it will also shut down whatever emitters are down there more. You can easily overheat the lower level with no way to shut it off, or if you have radiant down there its effectiveness would be greatly diminished and your feet would be cold.

reflective insulation requires an airspace to be effective. I would much rather fill that space with R-value.

whether you use constant circ or not, mass takes time to change temperature and if you have any solar gain or window wall areas, more mass can make control harder. Not impossible, but harder... better thermostats and reset water temps are needed. with geo you should be using reset anyway, so I'm not advocating against mass, I'm just giving the comparison between insulation between and insulation under.

but you need insulation somewhere below your radiant tubes, and not a foot away from the bottom of the subfloor either. at the subfloor, or between the layers.

Solar gain is going to be limited so im not too worried about swings from that perspective. I see the point in the underfloor radiation being uncontrolled, i think i see the point in not wanting the main floor heat to be drawn into the slab forcing higher water temps to acheive the desired first floor temp, ....i think.
So it all leads me to beleive im better insulating between my spancrete and topping.
Any suggestions on what type of insulation, i'm limited to an absolute maximum of about 1" thick which if i remember correctly is about R5.

So if heat transfer is either condution, convection or radiation, whatever thermal insulation i use will slow down the conduction, there is no convection because there is no airspace, what i dont get is that reflective coatings wouldnt have any effect on radiation through a solid surface, wont there still be some radiant transfer?

I have a thermal imaging camera at work, i wonder if there is some way i could set up a test with some regular insulation and foil faced to see if there was much of a difference?

Thanks again.

radiant heat transfer is energy through space without air movement. a solid object in contact with anything is conduction, not radiation.

You don't need to do any experiments, it's been proven a thousand times, reflective insulation under concrete is junk. Use an R5 insulation if that is all you can do. if you have 1" you could use something like crete-heat to make your life easy, or you can use rigid foam and mesh and tie to that.

Thanks Rob for again trying to set the record straight! As a distributor of Reflective foil insulation, we spend a lot to time trying to explain under concrete is not the place for radiant barriers. We carry excellent products that are made for that as you suggested like the Crete-Heat and the Barrier.

crete-heat would have been ideal for by basement...had i known about it! too thick for my main floor though. Barrier insulation looks like it would work but with an r-value of 1.7 im better off with the regular pink 1" EPS with an R value of 5 - 6 and at about $0.30 SF its very cost effective.

I see now that any kind of foil when sandwiched between concrete would be a waste and that its conductive heat flow im trying to reduce.

Thanks for the advice!

The BarrierX 5 is 1 /14 thick and will give an R value of 5.3 It is made from recycled EPS and has the vapor barrier already built in. Glad you now understand about not using foil. Good luck Green Insulation Products.

there is no need for a vapor barrier in this case, so straight EPS would definitely be the most cost effective, or crete-heat if you are doing radiant over it can make life easy. that has a 1" thick version, plus the knobs for tubing.

make sure if you use EPS that it's got the compressive strength you need. there is flimsy EPS and rugged EPS. I don't know what you need but I'm sure the low PSI stuff won't be adequate under concrete.