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Advantech sub-floor with tile and radiant heat

James Stufano | Posted in General Questions on

New construction – I installed 1 inch Advantech and debating which way to heat SF one floor home and plumber suggested pex radiant heat under sub-floor with the alluminum heat shields.  When he suggested it, I dismissed the idea as were thinking using 3/4 inch TG  oak floors with the logic of  how would heat go thru 1 3/4 inches in this Zone 5 house. But thinking we may put down large 24×24 tile which has a nice cement gray type of appearance, that maybe being 3/8 to 1/2 in thick, that the plumbers idea may work and use ductless to cool and backup heat.  Have a 1000 gallon propane burried tank and will have some type of boiler.  

I would also like to edit this to add spray foam and maybe rockwool insulaton
will cover the pex and there is a full walk out basement below that is not finished. Someone suggested that even though the basement slab is not insulated, I should still insulated the basement walls

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Replies

  1. DCContrarian | | #1

    Radiant floors are tricky because there's a pretty narrow band of temperatures that work, too hot or too cold and it's not comfortable. The ways to adjust the temperature of the finished floor are the design and the input water temperature. So once the floor is installed the only tool available is the water temperature and flow rate, which is tough unless the floor is on its own loop, and that can get expensive.

    The aluminum plates seem an odd choice to me. Aluminum is used because it has high thermal conductivity, but tile has very good conductivity, as does mortar. Here's how I've done it: Take up the subfloor, and nail a 2x4 to each side of each joist about 2" below the top of the joist. Cut 13 1/2" strips of subfloor and nail them to the 2x4's. Staple pex tubing to the subfloor, and then fill the joist bay with mortar to the top of the joist. Since drilling close to the edge of the joist isn't recommended, where the tubing goes from bay to bay you have to go down, over and up. Put a layer of tile substrate over the joists, then tile.

    I don't remember how I did the calcs when I did this, but my calculation was that it would take a very low water flow to get the tile temperature I wanted. So I used 3/8" PEX, which was easier to work with. I also put fiberglass batting between the subfloor and the ceiling below.

  2. Expert Member
    Akos | | #2

    Wood is R1/inch, so your subfloor is not adding all that much insulation. There is absolutely no reason that you can't heat either tile or hardwood. In new construction, hardwood is not an issue as you'll probably never get close to the temperature limits of the floor. My own home has this and even on the coldest of days, the floor never needs to get above 80F.

    The setup that DC proposes works great but it is a lot of work and materials. A simple staple up with either AL plates or Ultra-fins works almost as well and much cheaper to install (I prefer the ultra-fins as it is quicker and you need less pipe).

    Aluminum heat plates are a good thing as they reduce striping and also reduce the temperature of the water needed for heating. Lower temperature means better efficiency on a condensing boiler.

    If you go with heated floors, you will need some insulation bellow otherwise you'll overheat the basement. The cheapest batts you can get in there work great, you don't need much more than R8. Even with the batts in the floor, you STILL need to insulate the walls of the basement (see my earlier post).

    Hydronic floor heat adds about $25k to your build, it is a very expensive way to heat, better places to spend the money.

    1. Nils Bird | | #5

      In Quebec there are radiant floor heating kits available for less than $5000 cdn. When I first heard of it radiant flooring was available using 3/8 inch aluminized plastic tubing meant to be stapled to the underside of the subfloor. It seemed to be a good way to heat the basement especially if you had two inches of solid wood on your floor. Now I get told by purveyors of underfloor heating that the only way to go is with two inches minimum cement with tubing embedded. For my retirement home I plan to put half inch tubing on top of reflective paper laid on top of the subfloor. In between the tubing, every eight to twelve inches will be furring, which is exactly the same thickness as the half inch pex. Hardwood flooring goes on top of the furring. The water in the pipes will provide some thermal mass and still be able to heat up the room above quickly, giving the luxury of a heated hardwood floor without the weight and expense of adding cement.

      1. Expert Member
        Akos | | #7

        Nils,

        Material wise a hydronic heated floor is not that expensive, the BOM cost should be even less than $5k. The problem is with labour. If you can DIY a decent setup than by all means go for it, if you need to pay somebody to put it in, the cost will quickly skyrocket. Heated floors is considered "premium" so expect to pay accordingly.

        DIYing a well performing system is also not easy.

        The solution you are proposing does mostly work. You would still need some metal sheet under the pex to spread out the heat otherwise you'll get a lot of striping plus warped flooring.

        You also have to watch that the flooring edges only fall onto furring strips, you don't want the ends overhanging too much otherwise you can break the T&G.

        Radiant barrier in this case is not doing anything. Since it is not facing an air gap, it provides no insulation and the coating on it is too thin to conduct heat. 28 gauge AL sheet should work fine though. I think Uponor sells a thermally conductive caulk to for bedding in the pex.

        There is really no need for all the concrete. It does work but there are many other ways.

        1. Nils Bird | | #29

          Doesn't heat radiate through solids as well as air? In the late fifties my grandfather (chief electrical engineer for a major corporation) tried to convince me that silver paper above the gyprock in the ceiling was all that was needed for insulation. Recently I read a book called Your Engineered House ( or Home) published 1954 that stated this as a fact. Why do we bother with foil faced polyiso if this is not somewhat true? And wouldn't foil faced paper under heated pex reflect more heat upwards than would otherwise be the case?

          1. Expert Member
            Dana Dorsett | | #31

            >"Doesn't heat radiate through solids as well as air?"

            Nope absolutely not.

            Almost all solids are opaque in the deep infra-red region, the part of the spectrum where most of the heat radiation is at these temperatures.

            Low density fiberglass batts are modestly translucent in the IR, and let a surprising amount of heat to pass through via radiation (which is a good reason not to use them for wall & roof assemblies, but fine here). But manufacturers began adding materials to the glass fibers to make them more IR-opaque, and high density fiberglass behaves pretty much like a solid , at least for these purposes.

            >" In the late fifties my grandfather (chief electrical engineer for a major corporation) tried to convince me that silver paper above the gyprock in the ceiling was all that was needed for insulation"

            Let's just say your grandfather was sorely mistaken, at least at current energy costs and performance requirements. Air tightness ( or air retardency) is also critical for home efficiency (particularly during the heating season), and a single layer of the shiny stuff in a horizontal layer performs nowhere near good enough.

            >"Why do we bother with foil faced polyiso if this is not somewhat true? "

            The facers are there primarily to low the vapor permeance (foil is a true vapor barrier) and for ease of air sealing. Since polyiso is hygroscopic (it hangs on to water under some conditions) it's performance in some stackups can degrade seasonally if the facers are too vapor open.

            >"And wouldn't foil faced paper under heated pex reflect more heat upwards than would otherwise be the case?"

            Sure, but an open air channel is a thermal bypass allowing convection currents to easily rob performance, and without the channel the foil's thermal performance is nil. For the money you're almost always better off using cheap fiber insulation and snugging it up to your heat transfer plates or stapled on tubing. Even though cheap R13s or R19s are somewhat translucent to IR, the temperatures are low enough that it hardly matters- the blackbody radiation is all deep infra red.

          2. Nils Bird | | #34

            Dana, I apprciate the info. The floor in question will be surrounded on 5 sides with insulation. The 5/8inch air space between the furrings and the pex would benefit from heat radiated from the pex and reflected to the flooring above and convection would just further the diffusion of the heat. All to the good n'estce pas?

  3. James Stufano | | #3

    Thank-you. The 1 inch Advantect is screwed and glued to floor truss. Very solid and I simply could not remove it. When I mentioned in my initial post aluminum, I was referring to the Aluminum groved heat emisson fins that would go under the sub-floor and the pex tube would run through. Akos - when you mentioned Hydronic floor heat is an expensive way to heat and there are better ways to spend money, I was hoping to ask you for more thoughts. Are you saying I should skip thinking about radiant floor head and put the 25K into another solution?
    I figure I am going to need a boiler anyways to heat water and there are a lot of DYI solutions.
    I am curious what you would do. I hear all the time how people do radiant in the floor so I am surprised it's not a good solution. I want to do ducted Mitsubishi in the attic for air and supplemental heat and fresh air. Maybe you are thinking the 25K would be better just going to the mini-split system

    1. Expert Member
      Akos | | #9

      My suggestion would be to put in electric radiant where you want the warm toes feel and heat the rest of the house with a ducted mini split. Your dollars are better spent on better walls and windows.

      A reasonably air tight new build even to code min insulation has such low heat load that a full surface floor heat will never get warm enough to be felt. Mostly you'll end up with is the floor not feeling cold. At home, the floor hovers a couple of degrees above house temperature, very farm from "warm toes" feel.

      If you have a large enough unfinished basement (~700sqft), go for a heat pump water heater. The operating cost on it would be less than propane.

  4. Keith Gustafson | | #4

    If you have the inch, put the radiant on top of the advantech

    tubes under the floor are the worst radiant

    1. Expert Member
      Akos | | #8

      I've done most permutations of heated floors. All setups have their advantages and drawbacks.

      The "worst" is a badly designed and installed setup. The bad ones that don't work are the staple up with bubble wrap for insulation.

    2. Nils Bird | | #36

      The subfloor under the radiant tubing will be a triple layer of OSB subfloor (instead of a cement slab) which will be on top of eight inches of EPS. I don't see the heat going anywhere but up, which is where I want it. I'm going for Passive House insulation and airtightness and don't expect that the demand for heat will be very high.

  5. Stanfo3 | | #6

    A suggestion from someone without much any experience but a mind that’s always running.....use a backer board thick enough, cut it into 12” (or whatever works best) strips to allow pex to be placed between seams. Once pex is secured use thin set to fill in gaps and then tile on top of that. Insulate directly below the advantech. MGO board might be a good product to use as a tile backer.

  6. James Stufano | | #10

    I have to say I like Stanfo3's idea - most floor backer board is 1/4 inch and the backer board for walls is 1/2 inch. I don't know if people have used 1/4 PEX in a floor. The Advantect 1 inch is over 26 foot long floor trusses 16 oc - some areas are 12 oc since the truss company gave us 10 more I used them all. Was told not to put 1 1/2 inch cement with Pex to have concrete floors by floor trusses company since the trusses flex and it would be a disaster with cracked floors per truss company. The floor is so sturdy with 1 inch Advantech and strong backs nailed in the opposite direction to lock the trusses in place so I was puzzled by the flex problem, but not worth taking the chance. I have to make sure I don't go over 3/4 inch thickness what ever I do in relation to cabinets and doors and windows. . Windows are all double pane Energy Star rated. I would prefer the Pex on top of the sub-floor especially with 12 foot ceilings, I'm thinking the PEX floor heat would add to lower room level warmth. I would have both heat methods on zoned thermastats so I can adjust comfort. The one thing I know is I have to decide to PEX floor or not as I will not have the opportunity again once wood or tile is down., unless I go under later.

    1. Josh Durston | | #11

      The heat spreaders really do help, and the extruded aluminum ones are the best performing. You want the whole flow to be heated, not just a 2 inch wide strip above each pipe. The plates make better use of the surface area and will let you bring the temperature down, while delivering more heat per sqft.
      Ideally you want to get the temperature down to a point where you have future flexibility in heat sources (sub 120F if possible). (eventually it may make sense to use a air source heat pump).

      Regarding 1/4" or 3/8" tubing, you need to keep pressure drop, temperature delta, and max velocity in mind when designing your distribution. You will require more, but shorter circuits with smaller tubing. You won't be able to go 250' and have any heat left in the fluid with small tubing. It could work, but I suspect 3/8" would be the smallest viable size. At some point the extra circuits are going to add substantial extra cost in larger manifolds, and fittings, and labor.

      The guys on heatinghelp.com like to dig into this sort thing, I would cross post there. Your going to get better big picture advise here since it's a generaly building site. But they have some hydronics experts that will help you avoid pitfalls.

    2. Deleted | | #19

      Deleted

    3. Stanfo3 | | #20

      I’m not sure if this is rated for flooring use but here is 1/2” backer board.

      1. Expert Member
        Malcolm Taylor | | #21

        The outside diameter of nominal 1/2" pex is about 0.7", so you would need 3/4" underlay strips to make it work. That also assumes continuous pex runs without fittings under the floor.

        1. Trevor Lambert | | #27

          OD of 1/2" PEX is 0.625". If 3/8" PEX would work, that is 0.5" OD.

          1. Nils Bird | | #30

            What is the interior diameter of half inch, and of 3/4inch pex? If I know the mass of water that needs to be heated I could figure out how many BTU,s to heat the water. Conversely, the Delta T reqired to cool the water to room temp.

        2. Nils Bird | | #37

          Furrings sold here are exactly pex outer diameter , so there will be no joints. Also I plan on more tubing than is neccessary so as to heat the water to a lower temperature. Solar heating should be easier to integrate should I get to that point. The water in the tubing will have to provide all the thermal mass as there will be no cement anywhere in sight.

  7. James Stufano | | #12

    thank-you Josh

  8. Nicolas Bertrand | | #13

    I did staple up in my 1984 built house with thin aluminum plates spaced out holding the tubing and then some silver bubble wrap below it. My basement below it is unheated and usually around 55º F in the winter. Above the tubing is 3/4" plywood subfloor and then 3/4" oak hardwood. When it is running, the floors are nice and warm and the house is a steady 70º. My system actually runs off my water heater and does a good job.

    Nick

    1. James Stufano | | #14

      Hi Nick - Couple questions - 1) did you use 1/2 inch Pex 2) What temp do you run your water at thru the Pex 3) How far did you space your Pex tube apart 4) how big and area - are they zoned and how many thermastats

      I am thinking maybe just to ducted mini-splits and later add radiators to bedrooms, bathrooms, etc as needed or underfloor PEX to suppliment mini-split. I'm going to need a boiler and I can add as I see what my experience is.

      1. Nicolas Bertrand | | #16

        It is 1/2" pex, runs at about 140º F and tubes are spaced evenly between 16" spaced floor joists (2 runs per bay). It is all on one zone that is roughly 850 sq. ft for the 1st floor. We have a minisplit for a/c, one head downstairs, one upstairs. They can also do heat if needed, but I've never used them. Upstairs only has electric baseboards for heat. With a 4,000 watt hot tub, our highest electric bill in the winter is $170/month. We are zone 6 on the northern-most border of NY state.

        I recommend a small boiler, I just used our water heater because it was already here and was oversized for our needs. It works fine, but once in a while if it's very cold out you can get a luke-warm shower. Just wait 10 minutes and it's usually good to go.

        Nick

        1. Nils Bird | | #32

          I am trying to design an underfloor heating system which steals heat from an oversized hot water tank. I figure if the space heating draw is much smaller than the hot water demand then a couple of small circulator pumps, one way valves, and thermostats could conceivably keep us comfortable. Room temperature thermostats to turn the ,pump on and water temperature to turn the pumps off. How does your heating system get the heat from the hot water tank?

  9. Hugh Weisman | | #15

    To bad you didn't know about Warmboard before installing the Advantech...it's a 1-1/8" thick subfloor with grooves for Pex built in. There are similar products like SmartTrac and HeatPly that install on top of the subfloor.....Another option is to use ductless as the main heat source but keep your floor warm and comfy with low voltage electric heating using Schulter DitraHeat mats which are made to go directly under tile and stone and double as an uncoupling membrane.

    1. Nicolas Bertrand | | #17

      Can always use the Warmboard-R which is meant to go over existing floors. I am considering using it in my new house after it is all framed up and closed in. We already have the Advantech subfloors in, but haven't yet decided on what type of heat to use.

      Nick

  10. James Stufano | | #18

    I did look at Warmboard but for a single floor 2000 sf house it over $20,000+ not including shipping as I talked to them directly. It seemed like a great product but I couldn't justify the price for me. I was orginally going to use radiators but it seems no one wants to install anything but hot air. If I mentioned mini-split or radiators or PEX and it's like they all went to same marketing school trying to talk me out of everything by saying it's so expensive as they throw out crazy numbers. Talking to a plumber he said he charges $100 an hour and was doing me a favor as most others are $140, then he said he didn't want the job because the ceilings are 12 feet and he didn't want to have a vent pipe that high. I told him there are stairs to the attic and a floor so he didn't need to use a ladder. Then says he wish I told him the ceilings were 12 feet because he wouldn't have wasted his time making the 30 minute trip over. So these are the type of experiences that makes one start to decide to DYI - Thank-you to all that have tried to help me through this puzzle. My last house the HVAC put all ducts in crawlspace and attic. 100% of all ductwork was in unconditioned areas and I really paid for it with high energy bills. So a HVAC person can give bad advise or steet a product so there is a backend bonus from a company. So thank-you for the input.

  11. Josh Durston | | #22

    Here a thought. A boiler and in floor systems are probably going to be more than $20k (a lot more). Consider adding that to your building envelope, and eliminating the boiler entirely. Spend the money on a insulation, windows, airsealing, and good details, and a properly engineered HVAC design to get the most out of your mini splits. Good doesn't mean complicated (stay away from air zoning systems).

    If you're going to DIY, panel radiators are pretty easy. You can home run pex from rads a lot easier than you can lay out a complex underfloor system.

    If you want some floor warming in the bathrooms you can install a bit of electric, in front of the shower and vanity.

    1. Expert Member
      Malcolm Taylor | | #23

      Josh,

      That's good, sensible advice.

  12. James Stufano | | #24

    josh - what is an example of air zoning systems? For example - Mistsubishi mini split can comes with air handlers like in this link. Does anyone know if a mini-split to an air handler can be configured with a main duct that has two or three zones if duct goes to a couple areas?

    https://www.ecomfort.com/cooling/mitsubishi-mini-split-heat-pumps.html?sort_value=&displaynum=0&stores_id=&spec_options_id%5B155%5D%5B%5D=Air%20Handler&spec_options_id%5B155%5D%5B%5D=Concealed%20Duct&spec_options_id%5B1228%5D%5B%5D=-13

    1. Josh Durston | | #25

      Going with a couple more smaller ducted units is likely to be cheaper than a big centralized airhandler. Since you have much less ductwork, and the equipment cost will be lower. The big Mits AHUs are some of the most expensive ASHPs.

      Something like this (or whatever matches your load calcs).
      https://www.ecomfort.com/Fujitsu-F2L36D18180000/p85047.html

      Zoning may or may not make sense for a variety of reasons. If areas have a similar load profile due to shared exposures and usage patterns then it may make sense to combine them into a zone. Many people over-zone beyond what the equipment is capable of and end up oversized with one mini-split head per bedroom, when they would actually have better comfort and efficiency with a small ducted unit feeding a couple rooms (even if the rooms aren't all used all the time).
      Add-on third party air zoning systems for air handler systems are often a kludge even if they have claimed first party support. Better to use a couple smaller low static ducted units and keep it simple without motorized dampers. There are exceptions to this, but proceed cautiously.

      Damper zoning with residential equipment can be tough unless there is a good feedback loop for duct static/airflow control, and modulating equipment that has a zoning friendly modulation algorithm. Generally I think there are usually better options than a single air handler with dampered zones. Usually using fewer but larger zones, with dedicated equipment and good controls. Perhaps a zone per floor, and separate zones for large great rooms.

      Here is a good article on zoning:
      https://www.energyvanguard.com/blog/duct-design-6-determining-zones

  13. James Stufano | | #26

    I will have to go back an read through the feedback from different posters. If I recall correctly it was suggested to not do multi-head and the other was to make sure I get the ducted units that has a stronger blower. I also would like to tie HRV into same ductwork. The house is a giant rectangle of 84x26 with a full walkout basement. Will have a walkup attic that is foamed under roof deck so I would want to put units in the attic rather than basement incase someone wants to finish it and I don't want to have to duck was I walk around.

    1. Josh Durston | | #28

      1:1 units are definitely preferrable. You get a wider modulation range, and better performance (better HSPF), and even though you have more outdoor units they don't tend to be any more expensive.
      Fujitsu concealed ducted units have best in class static pressure capability, and there is also a more powerful medium static option.
      Since it's new construction you can make sure the ductwork design accommodates the capabilities of the equipment, negating the need for more powerful air handling units.

  14. CarsonB | | #35

    I am planning the same setup With radiant under hardwood oak flooring, but with electric spot radiant. I second Akos’ suggestion. There is likely no ROI for installing the hydronic system if you are only using radiant in spots. The electric wires/matts are cheap and easy to DIY. They also make matts designed to be put under a subfloor, suntouch makes one. Of you float the floor you can install matts right under the boards. I actually looked up using warmboard with electric wires but could find no info. I also thought about making my own DIY subfloor solution using wires and aluminum sheets, but decided it was worth the cost of the suntouch and not risk messing it up. Also, my oak flooring is engineered with 4mm riftsawn oak, from my reasearch this shoild be about the best real flooring you can use for this.

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