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Radiant heating under 2 1/2″ of flooring possible?

ozguy | Posted in Energy, Heating & Insulation on October 3, 2013 12:30pm

Hey all,

I’ve had a gent named Fred Seton design a radiant system for my house, and I’m wondering how to proceed after finding that my floors in the existing house are really thick, which is putting my plan to use “under sub floor” radiant heating in jeopardy.

I want to implement whole house radiant heating in a gut rehab of my 130 year old frame house, was hoping to keep the existing 2 1/4″ oak floor in place (it’s in great condition in general), which made me think of using “under sub floor” radiant heating. Less efficient, but saves a truckload on new flooring.

Further investigation showed that the 2nd and 3rd floors had a layer of old pine flooring under the Oak flooring, which means that the floors on those levels are over 2 1/2″ thick. Is this too thick for “under sub floor” radiant heating?

One thought is to pull up the Oak floors on the 2nd and 3rd floors to reuse on the first floor, where the Oak floor is only 1/4″ thick and needs replacing. I could then remove the Pine flooring as well, and install a 3/4″ plywood subfloor with channels in it for the radiant plumbing on the 2nd and 3rd floors.

A few questions:

  1. Is the existing 2 1/2″ thick floor to thick for effective under sub floor radiant heating?
  2. Is it possible to pull up the original flooring safely for reuse, in a cost effective manner?
  3. Warmboard is very expensive. Are there cost effect channeled sub floors out there to install the radiant piping in? If no I guess I’ll cut the plywood to install it leaving gaps for the radiant heat every 8″ or so.
  4. Should I use aluminum heat plates for either “under sub floor”, or “in sub floor” radiant heating? Fred Seton says I don’t need it, but with the thicker floors, I’m wondering about it.

Thanks in advance for any advice.

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Replies

  1. calvin | Oct 03, 2013 06:49pm | #1

    oz

    What makes up the over 2-1/2" thickness-the oak is ? thick.  The pine is ? (1-1/2?)

    I'd guess it is too thick for underfloor heat-but a radiant heating specialist would be the one to answer that.

    I had a job of building to spec, a radiant channel system that we then laid a floating Kaars floor over (with mat acceptable for radiant.

    The channels were 6" oc and for that we laid strips of ply over a subfloor.  At the ends (turns) we routed half loops in more strips of ply (laid perpendicular to the previous strips), so the hose could run out-turn-run back on those 6" centers.  (* 8" might be alright for your centers, but again-this layout should be designed with heat loss etc taken into account.  You cannot run full temp heat under many kinds of wood floors, so with the lower temp, closer layout might be needed).

    In the above system, there were aluminum plates that laid over/in the channels we made-the hose snapped into the grooves of the plates.  This was to transfer the heat off to the sides of the hose itself.  Again, this was designed by a radiant specialist.

    NRT Rob-a member here can certainly provide more than my limited experience with radiant, subfloor, transfer plates and hardwood.  I only have hose in slab (and some baths with subfloor, the grooves with hose) and ceramic for finish - experience, for the past 24 yrs in our home.

    I'll try to contact Rob and hope he will reply here.

    1. ozguy | Oct 03, 2013 11:10pm | #2

      2 1/2" is from 3 layers of wood

      Thanks Calvin,

      The 2 1/2" is made up of the original 1" x 8" plank sub floor, a 3/4" thick layer of Pine flooring, and a 3/4" thick layer of Oak.

      The solution I'm thinking of is similar to what you did, with the channeled plywood and aluminum heat transfer plates.

      I wouldn't be concerned about the flooring cupping, if I stick with the floor as it is. But I'll have to be really careful with wood selection and water temps, with the radiant tubing directly underneath new flooring (if I go that route).

      Thanks for your input, and for etting Rob know about this post.

      1. NRTRob | Oct 04, 2013 11:17am | #3

        realistically

        you're about an R1 more than a typical wood floor installation if you install in the joist below.  that's like doing underfloor with carpet... higher water temps, heavy plates, and a need for a careful room by room load calculation.

        If you can get above the wood with the radiant you'll trim a lot of water temperature.  the value of that depends on a lot of things, not least of which is what the heat loads are and whether you'll make it or not without supplemental heat in either case.

        load calcs answer all questions.

        1. ozguy | Oct 08, 2013 12:47pm | #5

          Under joist vs over joist.

          Thanks Rob & Harry for your advise.

          I was advised to place a reflective foil insulation 2" under the sub floor if i did an under joist install, and the logic of the heat being forced up makes sense, albeit at the cost of having to using higher water temps (higher energy costs), and slower temp change response due to the thicker flooring mass above.

          Has anyone used the "complete radiant panel"? It seems far more cost effective than warmboard (about $4.50 a square foot), but is still more than ripping plywood into strips and then installing the radiant panels in the grooves.

          Rob, would I need to use heat plates (or some heat reflectors underneath) if I installed the radiant in the ripped plywood above the sub floor?

          Thanks in advance for any input.

  2. HarryLudke | Oct 04, 2013 05:54pm | #4

    I have a 170 year old farm house and about 14 years ago I installed radiant floor heating. Like you, on the first floor I had up to 5"  layers of flooring from many generations living there. I stapled underfloor tubing on the first floor but ran baseboard heating on the second floor. Made for a much simpler installation.

    The problem with really old homes is that the floor joists/beams are very difficult to insulate due to their non standard construction. This is what I did. I stapled tubing between the floor joists following a calculated tubing loop plan. Than I nailed 2x2's along the edge of each floor joist. I than attached 1/2 inch foil faced foam board between the floor joist and nailed to the 2x2. This made for a 2" chase that the tubing can reside in. The whole chase gets warm and no need for plates. The most important part is insulation. I contracted to have the rest of the floor joist bay filled with Open Cell foam insulation. This combination is extremely effective. Heat has no choice but be forced up thru the many layers of floor and the foam insulation seals everything tight!!!

    As with any radient floor heating system, it takes a  longgggg time for the house to heat up. The only tricky time of year is the shoulder season. When the nights are cold and the days are warm with lots of solar radiation, the house can overheat easily. There's not enough thermal mass in the wood to absorb the solar gain. Solution is to just open a window, but I grumble and grouse about loosing my hard earned heat.

    We've loved the radiant floor heating. Extremely comfortable and quiet. would do it again in a heartbeat!!

  3. User avater
    Perry525 | Oct 09, 2013 03:26pm | #6

    UFH

    You can of course proceed with this if you want, but as proposed it won't work. Most of the heat will be lost. You will waste a lot of money over the years.

    A pipe full of hot water looses heat by radiation and length, radiation is over 360 degrees, half of the heat goes down and half goes up.

    Most of the half that goes up, will be lost to conduction within the wood floor its joists, the frame and the outside air. The down half...well that depends on the amount of and type of insulation you place below the hot water pipe.

    If you really want underfloor heating to work, and make the room above comfortable, then you need the hot water pipe above the 3/4" sub floor. Not only that, it needs to be fully floating on a bed of polystyrene that is at least 6" thick. This means....joists, 3/4" plywood, 6 inches of polystyrene, hot pipe, 3/4" OSB t&g, glued boards. The new floor, must not touch or be in contact with any part of your home.

    This will give you a light weight quick response warm floor.

    To make it work, you will need a wireless controlled thermostat, and a motorised valve. The wireless thermostat should be left loose and should sit on a coffee table close to your seat, then you can move it about to get the best result.

    At the very least all the living rooms should be controlled by one thermostat, all the bedrooms by another and the bathroom by another.

    You need to think about what you are doing, if some living rooms are on the sunny side of the home, and the thermostat is placed there, the rooms on the sunny side will get warm and turn the system off. Then rooms on the north side will be too cold.

    If the thermostat is in a room on the north side, it will be comfortable and the rooms on the south side will be too hot on sunny days.

    Having thermostats controlling motorized valves in every room works best. This system saves money as every room will be at the temperature you want.

    1. ozguy | Oct 09, 2013 04:25pm | #7

      Ceiling height an issue

      Thanks for the detailed advice Perry.

      I see how the heat would radiate out better in the polystyrene, however my ceilings are not high enough to be able to add 6" to the height of the floor (we're trying to keep an 8' ceiling.

      Given that I can't use polystyrene, would the {EX installed in plywood slots with aluminum heat plates (or warmboard competitor) right under the flooring work best?

      Can I use Oak (quarter sawn 3") over that install? I've seen some pretty nice engineered flooring, but would like to be able to have some thickness to sand once or twice (if ever needed), and am looking for thicker engineered wood to suit.

      Your points regarding thermostats are appreciated. I do intend use many zones, so that each bedroom, bathroom, and living area can be controlled separately (within reason). I want to place the thermostat somewhere on a wall as far from a window or door as possible to get the most accurate reading of the rooms temp.

      This way we can not only adjust for the solar thermal load in a granular fashion, but also for each individuals comfort levels.

      1. User avater
        Perry525 | Oct 11, 2013 09:23am | #9

        UFH design

        The basis of all modern insulation is dry air. Some things like Thermos Flasks rely on a vacuum, but a vacuum is not easy to make in the shape of a room.

        Polystyrene is made of thousands of tiny air bubbles, each surrounded by a thin plastic skin.

        Each air bubble is so tiny that the air inside is static - it cannot rotate and enable heat to cross. The plastic skin of each bubble is so thin that very little heat can cross over by conduction. Simply polystyrene is a good insulation because static dry air is a good insulation.

        When you install under floor heating there is always a trade off. The cost of heating the home over the lifetime of the property and how much money can you save by doing the job properly, against how cheap can the job be done. Pay up front for proper insulation or pay over and over - the cost of wasted heat.

        Keep in mind that the pex pipe, will emit its heat, whatever. The question is, where does the heat go? Does it heat the plywood its laying on, joists, frame, crawl space and world outside? Does it heat the room?

        The better installed underfloor heating delivers most of its heat to the room.

        But only you can decide how much waste you want. If an eight foot ceiling is imperative, then go without the insulation and have higher heating bills. I would guess that a ceiling that is 7'6" would hardly be noticed, having lower bills over 50 years and warm feet will probably be noticed more. A room with a lower ceiling is cheaper to heat.

        Why not split the difference and lay the pex over one or two inch thick sheets of polystyrene and lay strips of polystyrene between the pex to support the floor?

        The floor can be made of anything you want it to be, but keep in mind that the wood needs to be tongued and grooved, so that it can be stuck together as one monolithic piece and, it really is essential that the wood floor does not touch any part of your house including the base boards.

        Keeping the wood floor isolated from everything else, means the floor acts as one very large warming plate, keeping the heat in the room. You do not want to let your expensive heat escape into the frame and world outside.

        The reason we have zones is, when one room is cold and it turns the boiler on, the heat goes only to that room, it does not go to the other rooms that are already warm. This is sound economics and enables the system to save money and to pay for itself very quickly.

        Dwelling on you ultimate paragraph, it is often desirable to split the UFH in a room, each part having its own thermostat, so the wife can sit in one part and be warm, while you can sit in a cooler part and be comfortable, no need for the wife to sneak up to the thermostat and turn it up.

  4. User avater
    Mongo | Oct 09, 2013 07:39pm | #8

    My house?

    I'll tell you what I did in my own house when I built it back in '95.

    I have TJI joists spaced 19 3/16" apart.

    I have 3/4" T&G subfloor, 1/2" ply underlayment, then 5/8" brazilian cherry glued down to the 1/2" ply underlayment. Total floor thickness of 1-7/8", roughly R2.

    Underneath the subfloor I have two runs of 1/2" PEX per joist bay, stapled to the subfloor. No aluminum plates. I friction fit 1/2" foil-faced polyiso insulation between the joists, it's tucked right up to the underside of the top flange. About 1-1/2" to 2" below the subfloor. In the basement ceiling (underside of the first floor of the house) I also added R19 FG batts under the polyiso.

    I circulate about 115-120 degree water.

    I live in CT. Cold winters, but not horrific. The house is toasty comfortable. No issues. No long running times. No problems with the wood floors.

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