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Community and Q&A

Radiant floor heating logistics and insulation materials

TrAKr | Posted in General Questions on

Objective: install ceramic tile above hydronic radiant floor heating on existing basement slab. 

Zone: 4 (Marine)

Existing conditions: 4 inch uninsulated slab over polyethelene barrier installed in 1974, over crushed stone. Floor to bottom of joists is about 7.5 feet. 

6 mm polythelene sheeting or Delta-FL (8 mm) on slab
1/2 inch rigid foam over moisture barrier 
3/4 non-PT plywood ripped into 12 inch segments with 3/4 inch space between pieces to hold the 1/2 inch PEX tubing over rigid foam
1/4 inch substrate over plywood and PEX
Ceramic tile over substrate


1) I am assuming the 1974 poly sheet beneath the slab is either not present or not working out of an abundance of caution. (It is depicted on blueprints.) That motivates me to put down a moisture barrier on top of the slab. Do you see any problems with doing so? 

1a) Any advantage of Delta-FL over (much cheaper) 6mm poly sheeting? 

2) For the 1/2 inch rigid foam, EPS, XPS or Poliso? I’m a DIYer and keep reading different answers. 

3) For the substrate, thoughts on 1/8 inch Ditra mat versus 1/4 inch Hardibacker? (Cost difference is about 50 cents psf, on a 600-800 foot job.)

4) Finally, part of the basement is a utility area. I had planned to leave that area entirely unfinished (walls, floors, ceiling). If I leave the floor uninsulated, is there concern that all moisture in the slab will just move to the uninsulated area and enter the basement there? My question is, should I just finish this part of the basement floor, too, but skip the hydronic heating in the floor area? 

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  1. onslow | | #1


    Just curious as to why all the effort to put in pex? Do you have some form of forced hot water heat now? If so, you will need to know how much capacity the system has before adding more loops. There will be valving and pumps to add as well as checking the loop lengths of any new pex. Others with radiant installation hopefully can chime in.

    Logistically, you are facing a lot of concrete screws to stabilize the 12" plywood panels. This also means lots of holes in any plastic sheet or alternate choices. Moisture migration into the foam from the concrete could be increased. I would use XPS over EPS in this context as it is less vapor transparent. Polyiso would not be a good choice unless you can be absolutely certain the slab is very very dry. And still not a good idea then.

    Putting Ditra down instead of hardiboard would be beyond very risky given the plywood islands on foam not providing the same stiffness as full sheets. Even 1/4" hardiboard is still very dicey given the way the plywood is in patches. 1/2" would decrease the risk, as would setting smaller size tiles in Flexbond or equivalent. Plan on lots of screws to hold the hardi and clearly marking the pex path as you go along. One wrong screw...

    The heating you will get from this plan will likely be very "bandy" and subject to losses through the minimal R value of the foam. Tile does not spread heat like you think. The lack of heat spreaders will be very clear. If you have sufficient space on your electrical panel, I would suggest going with resistance wire or film heaters instead. Maybe putting down 1" foam over plastic film, 1/2"multiply (cabinet grade) plywood, 1/2" hardi, wire mat or heat film and then Flexbond and tile. The total stack up would be close to that planned and the insulation better. One set of screws to set in the floor with no risk of damage to pex.

    This would still NOT be a recommended method and you would definitely be on your own when it comes to how much activity and furniture and weight you are planning for the floor. Sofa legs are point loads and exercise equipment can create shock loads. IF you are planning on using the space for more than yoga mats, then 1" foam, 3/4" ply (again not CDX), 1'/4" hardi, heat layer then tile.

    Tile does not like to flex very much, which is why most tile manufacturers require very stiff sub-flooring. 1"-1 1/2" depending on the tile or stone being set. Putting plywood on top of foam may feel plenty solid, but technically it is very squishy. Hardi doesn't like the squishy much either, so locking the hardi to 3/4" ply properly with thinset and screws will help. Setting small format tile in a flexible thinset will further help you survive a very not standard floor profile. Big (14-24" tiles and planks) will be at greatest risk of cracking if overloaded by furniture legs.

    This is hardly an exhaustive list of options, others may have examples of success that are less rigorous. Still if you plan on selling the home in the future, not creating a nightmare for the next person will be to your advantage.

  2. TrAKr | | #2

    Hi Roger,

    Thanks for the advice. I'm planning on putting hydronic heating in the joists between the basement and the first floor, so I figured it would make *sense* to use hydronic for the basement slab tile installation too. The electric mats were terribly expensive when I looked, although maybe not so bad considering all of the different components I'm planning on here.

    I currently have a boiler with small tankless heater attached. The allure of a hot water heater with an open direct system was attracting.

    The planned tiles are 9x38, so your concern about big tiles applies in spades.

    Do you have some electric mats to recommend? It is for a weekend house so I didn't plan to heat the slab all around the clock. I'm looking at about 650 sf on the slab (and a separate 2nd floor loft with about the same square footage where I'd probably do the same type of installation).

    I should add, I'm happy to switch from tile to wood, if that makes a difference in terms of recommendations. I'd use the same wood everywhere -- basement, kitchen, bathrooms (maybe?), living spaces, so I assume I would need to be in the narrow width (5 inches or less?) engineered category.

    Finally, should I plan to insulate and tile the storage room that is in the back part of the slab (just without heating)? That way the whole basement floor would have the (perhaps additional) vapor barrier and insulation even though only the front half has heating.

    Thank you, again, for you guidance here. I'm a competent DIYer...just competent enough to do serious damage.

  3. Expert Member
    Akos | | #3

    You can cut a small hole in your slab to see if there is poly. If not, the best bet is to seal the concrete, look at the products listed here for options:

    Poly over concrete is no the best as you can get puddling underneath and mold down the road. Concrete doesn't mind being wet, so as long as you can keep the moisture in there, you'll have no issues.

    I would run your basement through one of the one line load calculators (ie and see what your actually heating loads are. My guess is for a 650sqft insulated basement, you are looking in the 3000 BTU range. This means that you only need to heat a very small portion of the floor (~1/4 of the area), saves a lot on install cost.

    Just put the floor heat where you'll likely be walking or standing, don't worry about heating the entire space. This would be significantly more comfortable as the floor in those areas would actually get warm enough to feel. If you heat the whole surface, the floor will never be warm.

    I have done heated floor with only 3/4" of insulation when tight on space but you will loose a fair bit of heat to your slab. Not ideal but can be made to work. For tight spaces look at Quik Trak, it is only 1/2" thick. For finished surface go for engineered click or laminate. Tile is too much labour and not very comfortable.

    You don't need to do anything in the utility room. Just coat the concrete there as well, no need to finish or heat there.

    I would be very carful with open systems in a place you are not occupying all the time. That is asking for trouble.

  4. onslow | | #4


    Akos has added some very good points that my own myopia failed to address. Particularly the better solution of sealing the floor over using plastic. The new fact of it being a part time house does change the scenario. I took the tile work to be a set condition, but if you are willing to go with engineered wood or pergo-type flooring (even the snap vinyl wood look) then any "squishy"ness is of little concern. My actual Pergo has a foam backing for resilience and sound control.

    Do take time to check if any choices you make are rated for underfloor heat options. Many engineered woods are good with the temp ranges of hydronic floor heat. Very wide choices may be subject to cupping when you kick on the heat if the humidity in the basement is generally higher. Electric wire is much more localized. Do note that wood is insulating relative to tile, so the transmission of heat will be perceived differently even if ultimately the same level of BTUs make it to the room.

    I would recommend a different path given the circumstances. Cork tile and cove heat.

    For the extended reasoning - if TLDR is not in your lexicon.

    Under floor heat is pricey no matter what form it takes and often behaves differently than expected. The "bandiness" I spoke of is similar to the suggestion of localizing the heating areas Akos made. A smaller, more densely populated heating area will warm the flooring to a foot sensible range without over heating the room. That temp being about 80-84F.

    Due to my former house being designated "dozer food" long before we sold, I had the option of experimenting a bit while trying to keep the house livable. A temporary kitchen do-over, presented me with a chance to test wire heating under tile.

    I purchased a 70 s.f. kit and tried three wire spacing layouts. The goal was to see how it felt under foot as I poked at design options on our future retirement home. The 3" spacing between wires was most like advertised, with the tiles generally cozy to bare feet. 4" spacing created detectable banding and 6" was like feeling warm worms. The under 3/8" tile did not spread the heat at all well. Conductivity is highly localized. Engineered wood may not respond favorably to localized heat.

    After experiencing one thermostat failure and one under tile thermal sensor failure ( I knew to put in two thanks to GBA) I declared under floor wire heating not for me. The additional fact of raw cost of materials ranging between $8-17 a s.f. finished the question. Add the cost of tile and installation of both pushes in floor heat cost to $20-40 s.f. easily. Only orphan tile and my own back made the test practical. I will never reveal how I put the tile down.

    I do not have enough experience with hydronic floor heating to judge much beyond not wanting the risk of water damage. I did install tubing in my basement and garage slabs in case I needed the heat, but they are unused to date. Management of the supply reservoir for the loops is nothing I have messed with.

    After all the commentary of how to deal with what you asked for help with, I suggest one heating answer and one flooring answer that might serve you well in a moderate climate zone and intermittent occupation.

    Cork flooring is readily available in sealed form, which makes maintenance much easier than the one I lived with. The feel under foot is almost like a warm floor since the insulation properties of cork prevent huge losses from your feet, unlike unheated tile. If you have time to obtain a cork tile or plank, place it in the basement and come back later to test with a bare foot. If you have existing leftovers from the wood flooring upstairs, compare the two.

    For heating, a cove heater may appeal to you. For other TLDR reasons, I have chosen to heat my entire home with them in climate zone 6B. Yes they will eat Kwh when running, but your intermittent presence and milder climate could be a good match. Low entry cost and no water surprises. Plus you can devote more of the height loss to insulation under the plywood. Not placing your heat source under the flooring will also limit losses to the slab as the DeltaT will be much lower.

    The heating response is indeed slow from a cold start, but leaving the thermostat at 62 or so should allow for you to bump up on arrival and be close to desired range in a few hours. I don't think a hydronic system would create a much faster rise if you are setting back while elsewhere. At least the baseboard heat I lived with in the past took some time to get things warm if set back.

  5. Expert Member
    Dana Dorsett | | #5

    >" For the 1/2 inch rigid foam, EPS, XPS or Poliso? I’m a DIYer and keep reading different answers. "

    XPS is the least green insulation material in common use today, with 8-10x the CO2-equivalent green house gas damage as EPS or polyiso. As the high global warming potential HFC blowing agents diffuse out over a few decades the performance drops to that of EPS of the same density. The R5/inch instead of R4.2/inch for EPS is solely due to the HFCs, and they don't stay put. (HFO blown XPS has similar performance benefits a longer half-life and is comparable to EPS environmentally, but SFAIK isn't yet commercially available in the US, probably due to the high expense of HFO blowing agents. Until the US signs up for the Kigali Amendment (that bans use of HFC for these applications) to the Montreal Protocol the industry will likely stay with HFCs for cost reasons.

    Polyiso (unlike polystyrene) is somewhat hygroscopic, and somewhat prone to taking on moisture when in soil contact. It may be OK to use it above the slab if the polyethylene is between the slab and foam, but never under a slab. But if the floor were ever flooded by even an inch of water it could take a decade to dry. Akos has it right- putting the polyethylene on top of the foam is more protective of the wood than under the foam, and putting poly over the foam rules out polyiso.

    So really EPS is the best choice of materials here, but it needs to be at LEAST an inch (2"-3" would be better) to no be excessively lossy under a radiant floor. If you can't tolerate that much headroom, go with 3/8" PEX and half-inch substrate, or a low-voltage mesh electric solution. When supported fully by foam (of any compressive strength) over a slab there is no need for 3/4" plywood- even 3/8" isn't going to flex enough to dent the foam or crack the ceramic.

    Even without the radiant floor at least 3/4" EPS would be a good idea for protecting the plywood if the local climate sees summertime outdoor air dew points much north of 55F on a regular basis, to reliably keep the temp on the cool side of wood above that dew point temp so that it's moisture content stays low. And even without the radiant floor there is still a benefit that is still rational to make up 1.5" EPS cost rational on a full lifecycle energy use/cost basis (though it's decades long, to be sure.)

    Akos' recommendation for running a proper heat load calculation before doing anything is spot-on too, a necessary first step for designing any heating solution.

  6. TrAKr | | #6

    Thanks Akos, Roger and Dana for the continued discussion. I'm learning a ton.

    I'm more than happy to change my plans entirely. Based on the comments here, ceramic tile appears out.

    What would you think about using Luxury Vinyl Planks instead? I could seal concrete, then lay 1 inch XPS, then 3/4 inch plywood ripped into 12 inch segments to install the pex. Presumably then 1/4 inch of .42 inch hardiebacker upon which I could do floating (click) LVT planks.

    Do you see any problems with that install? I assume some sort of material is necessary above the 3/4 inch plywood for the vinyl tile. I'm just throwing out hardibacker for this, so happy to go a different direction.

    I'm learning about cove heaters now, per the suggested option above. Do they really work well for heating a room compared to underfloor heating. (No one but me in my house believes in wearing socks in the winter...)

    Finally, a random question. Supposing I'm installing underfloor pex in the joists for the first floor, can I also lay pex in the same joist space for radiant ceilings? I assume I'd put good insulation between the two pex installs, leaving some space for recessed puck lights. Is this something worth thinking about? It would allow me to do underfloor for the first floor and ceiling radiant for the basement, instead of underfloor for the basement too.

    I really appreciate all the guidance here. I'm a beekeeper and happy to send a jar of honey out as thanks for the advice.

  7. Expert Member
    Akos | | #7

    Pipe gaps will telegraph through the LVT, so you definitely want to cover with something. If you want to avoid striping, probably the simplest is a layer of thin aluminum sheet. You can just glue it over the plywood.

    If it was my basement, I would just go with an LVT with a thick backing (some even come with draining channels but hard to find) directly over the concrete. Getting rigid under does increase comfort a bit, but it is a lot of extra work/cost/height.

    For heat, I would first figure out what your heat loads are for your main floor and basement. From here you can figure out how much of the main floor area you need to heat (newish construction, should be about 1/2 the area). Without any insulation, this floor heat will generally overheat the basement bellow. You can than work out how much of the ceiling you need to insulate to match your basement heat loss. I would than add in a couple of small cove/baseboard/panel heaters in the basement for a bit of control.

  8. darusiob | | #8

    Hey, I'm really excited about this discussion on radiant floor heating logistics and insulation materials. It's a topic that combines my passion for DIY projects with my interest in home improvement.

    Firstly, I appreciate the detailed plan provided. It seems well thought out and designed to ensure efficient heating and insulation. Regarding the moisture barrier, I think it's a smart move to add a new one on top of the existing slab, considering the age of the original poly sheet. Better safe than sorry, right?

    As for the choice between Delta-FL and 6mm poly sheeting, I'm curious to hear from others who have used either option. Cost is definitely a factor, so it would be helpful to know if there are any significant advantages of one over the other.

    Now, the decision between EPS, XPS, or Poliso for the rigid foam insulation is a common dilemma. Each material has its pros and cons, so I would suggest considering factors like R-value, moisture resistance, and ease of installation before making a final choice.

    Lastly, regarding the utility area and leaving it uninsulated, I share the concern about moisture migration. It might be worth considering finishing that part of the floor as well, even without the hydronic heating. Proper insulation throughout the basement can help maintain a consistent environment and minimize moisture-related issues.

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