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Hydronic Radiant Heat Retrofit on uninsulated concrete slab basement

I am planning on installing a hydronic radiant floor heating in my basement on top of existing concrete slab. The slab is not insulated, and we live in MN where it is quite cold. Currently the slab temp is ~45F.

The ceilings are about 8' tall, and in some areas ~7'3" due to soffits. I don't want to lose much ceiling height to install the system.

I am looking at Warmboard and ThermalBoard. I am wondering what insulation and thermal barrier to place underneath and what experience others have.

I have about 1100sq ft of area needed to be heated. I do have forced air as my "primary" heating system. This is mainly to keep the floors feeling warm and ensuring there is not a chill on your feet.

Comments

  • FranklinDFranklinD Posts: 379Member
    The system would probably heat the floor, but it’ll probably cost more to operate than if the slab was properly insulated. Perhaps it won’t be as bad since it’s a basement and, I assume, significantly below grade, but I don’t know. From everything I’ve read here, I believe insulation is key for slab applications.

    When we poured the slab for my new garage/shop, I used 2” foam board under the center portion and 3” around the footings, sides, and “curb” of the slab. Glad I spent the extra money even though I’m not using the radiant slab this winter.

    Also, in a properly designed radiant slab, the floor doesn’t feel “warm”, it just feels “neutral”, I guess, would be the best way to describe it.
    Ford Master Technician, "Tinkerer of Terror"
    Police & Fire Equipment Lead Mechanic, NW WI
    Lover of Old Homes & Gravity Hot Water Systems
  • GordyGordy Posts: 8,021Member
    One word Roth. Their panels are built with insulation and full aluminum skin. Can go 6" centers. About as minimal build up as you can get
  • GordyGordy Posts: 8,021Member
    edited January 7
    It's not that no insulation won't work, it's about how much energy it will use.

    Plenty of uninsulated older radiant slabs still going. They heat at a price. Some old slab on grades keep the tulips in bloom in the winter from the edge losses from no insulation.
  • Mark EathertonMark Eatherton Posts: 5,840Member
    I'd go radiant walls or ceilings, then use thick pad and carpet on the floor... There are MORE ways of providing "radiant comfort" than just heating the floors. Internal bathroom, absolutely, electric radiant floors, but otherwise, look for other surfaces to radiate.

    ME
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • hot rodhot rod Posts: 8,589Member
    It depends on how you are going to use the space. If you or kids plan on being in the basement, playing on the floor, it would be best to add some heat at the floor. Nothing beats a warm radiant floor for ultimate comfort.

    Without a good thermal break below and around the edges it will have some loss to the ground below.

    If you did a load calc on the space you could put some numbers to what it will take to keep it at a comfortable temperature.

    I like the foam panels that Gordy mentioned, maybe even glue a 1" DowBoard down first, then the Roth panels. They make adhesives specifically for glueing foam to concrete or foam to foam.

    You can put a floating hardwood right over the Roth panels.

    I'm building a camper for my truck, making SIP panels for the floor and walls with these products.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • giceman1337giceman1337 Posts: 41Member
    edited January 7
    We want to have the space available to run around on. We definitely do not want the entire area as carpet. We are planning a laminate plank or LVP in most of the areas. There will be a carpeted den area, and bathroom area as well. The primary purpose of the radiant floor is to make the floor feel warm under the feet rather than cold in a typical basement.

    Here's a rough floor plan. Rooms in Red are to be heated.


    I looked at the Roth panel site to get some information. All of their information appears to be relatively dated (~2009). Is this the latest information? Does anyone have experience working with the Roth panels?

    I'm thinking something like this for a height of 1.5" before flooring:
    0. Slab
    1. Vapor Barrier
    2. 1/2" XPS (R3) or 1/2" Rmax (R3.2)
    3. 1" Roth Panel w/ 1/2" PEX-AL
    4. Underlayment?
    5. Laminate or LVP
  • GordyGordy Posts: 8,021Member
    The Roth panel already has the insulation built in. So delete that build up of insulation.

    Yes 09 not much has changed.
  • giceman1337giceman1337 Posts: 41Member
    I took my IR temp readings of the slab. Here's the temps with no current "heat" in the basement besides whatever vents from the furnace naturally.



    The walls on the Left, Right and Bottom are below grade insulated. The wall along the top is 6" insulated studs (no sheetrock yet).
  • GordyGordy Posts: 8,021Member
    edited January 7
    Seems right. Back in the old days when fuel was cheap, and xps not thought of it was a common practice to hold tubing away from foundation 30" for obvious reasons.

  • hot rodhot rod Posts: 8,589Member
    I have done a few Roth system, the largest was a 4000 sq' home, this is the smallest a 160 sq ft tiny home I built last year.

    I use the 3/8" tube size so it cost you 1/2". Not much insulation under the tube as you can imagine, so that is why I suggested an additional foam layer.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • giceman1337giceman1337 Posts: 41Member
    Thanks for the pictures hot rod, that helps. It's amazing how much the aluminum spreads the heat very evenly.

    In my case I'll be raising the temp to maybe 65-67F. We have our forced air system at 67F in the winter on the main level. Most of the area is currently 60F, so I don't have a major temp difference to overcome. I want to ensure I don't sink too much heat through the slab and waste money on heating the foundation.

    My goal was to keep the total height <1.5" and get as much insulation as possible. Obviously not spending the money on insulation and the time for installation is good, but if it will help make the room feel warmer it is probably a good idea.

    In terms insulation on the exterior walls, is it best to have a "void" area of the water (e.g. plywood) and not waste the thermal energy on that area, or apply additional heat to those areas?

  • hot rodhot rod Posts: 8,589Member
    The insulation would be more for reducing the loss downward and lowering the operation cost. With insulation it is always cost vs benefit, and in your case the thickness of the build up.

    I'd guess the largest loss in a basement is any area of the foundation above the grade as that is exposed to the greatest temperature differential. I would look at the rim joist and first few feet of the concrete wall to have a good r-value and eliminate any infiltration. Cans of spray foam are ideal for sealing rims, with batts or pieces of foam board added. Spray the rim seams then add at least a 6" batt in the rim area.

    In your favor is the fact you are not starting from a cold temperature if the FA keeps it 60 in the basement.

    I think ridgid foam comes in 4X8 sheets down to 3/4" the fan fold used under siding maybe 1/4"

    As you can see here not much r value under the Roth in the channel.

    I feel more comfortable with foam over concrete compared to wood, radiant products. Unless it is a PT wood product or a vapor barrier is below the wood. I think most building codes insist on treated wood when concrete contact, just a good practice.

    I used a notched trowel to spread the adhesive and press the Roth onto the adhesive. Even if you had some floating areas the foam would not squeek.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • giceman1337giceman1337 Posts: 41Member
    In terms of computing needed supply for BTUs per SqFt I see that in MN they recommend 50BTU/SqFt for heating. I've seen some graphs of BTU/SqFt with different floor coverings. I'm planning Laminate Planks (R0.2) and possible underlayment (R0.2). On this graph is looks like I would need about 135F water to achieve that. I've seen other places that say only ~20BTU/SqFt is needed, which is obviously a much lower water temp (95F).

    I think even 135F water temp is fine for the PEX and this system, but I was wondering how about to compute the needed heat?
  • GordyGordy Posts: 8,021Member
    edited January 7
    I've thought about the thin areas under the tube areas of the Roth panels. Don't you think that heat gets transferred to the aluminum faster than the downward loss can manage. We are only talking 1/2" areas at 6-12 in centers depending. Floor temps of 60 ish not a huge delta.

    i keep in mind the more you sandwich different components the more out of wack things can become for finish floor.
  • hot rodhot rod Posts: 8,589Member
    edited January 8
    It takes a few steps and some number crunching to get to that answer.

    First a load calc, determine how many BTU/ sq ft. is required in every room. Find a load program that has a basement calculator for most accurate numbers.

    I like the load calc program at www.hydronicpros.com the "Heat Load Pro" software has a free demo, you can buy and download it. It allows you to build up the wall assembly to what you actually have. Best generic program I have used, no sales BS in the results like some of the manufacturers programs..

    https://www.hydronicpros.com/downloads/index.php?id=23


    Once you know the load for each room you can determine how much heat the floor needs to supply. Load divided by available square footage. With a low load like you have, maybe 15 BTU/sq ft and 100F or so supply, just guessing.

    The RPA also has some good non biased design info www.radiantprofessionalalliance.org

    That Roth with the solid aluminum and 6" on center can put out some good heat and nice even temperature spread as seen in my IR pic.



    The better the conduction from tube to floor the lower the required temperature, faster response, and more even temperature spread.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • giceman1337giceman1337 Posts: 41Member
    Thanks for the great info. I noticed you also said you used the 3/8" PEX. Was this as a cost savings and height savings? Do you use PEX with some form of conductive material (e.g. PEX-AL or Roth's DuoPex S5)?

    3/8" limits the maximum run length (200') vs the 1/2" allowing for longer runs (300').

    Also, what is the spacing you typically use, and what are the considerations you use? From your picture I would guess 6" spacing, but I couldn't tell for sure.
  • hot rodhot rod Posts: 8,589Member
    edited January 8
    Correct 200- 225 max length for 3/8 tube. 1/2" can run out to 300'

    3/8 is of course 1/8 smaller diameter then 1/2" so the panel is lower.

    3/8 Roth Panel is 3/4" thick, 1/2" panels are 1" maybe a price difference also?

    Roth Panels are built with grooves 6"on center so you can either use a 6 or 12" layout.

    Regular pex is fine, some of the "A" pex brands Rehau, Uponor and Mr Pex seem to be the most flexible.

    Warm pex is flexible pex, 3/8 is very easy to work.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GordyGordy Posts: 8,021Member
    I noticed you have forced air. What will be your heat source for the radiant, or is it hydro air?
  • giceman1337giceman1337 Posts: 41Member
    I do like the idea of using the 3/8" PEX for cost and ease of working, as well as saving some height with the Roth boards. I wanted to ensure I would be able to push adequate heat and flow through the smaller tubes. I've also wondered if there is benefit in the PEX-AL and other "radiant" PEX tubing. Is this more Oxygen barrier, or does it actually help extract more heat into the floor?

    I should also mention that I am planning on running tubing under the subfloor for the main level to be able to supply heat on that floor as well. About 80% of that floor is hard-wood, so I won't get the "warmth" of a tile, but at least the wood won't be cold. My goal on the main-level is reduce the usage of the forced-air to reduce noise and dust during the winter. I'll still need forced air usage as my sole heat source for the 2nd story.

    The planned heat source for the hydronic floor will be either a boiler or water heater. I have a 15 year old electric water heater that I want to replace with gas. I'd love to get a super efficient 50 gallon natural gas water heater that I can use for both domestic and hydronic. Any pro/con list of using a water heater for both supplies?
  • hot rodhot rod Posts: 8,589Member
    I don't think the PAP adds much more transfer compared to regular pex. The PAP often takes different fittings also, regular pex can be crimped or expanded.

    Bradford White has a tank typesetter with a coil for radiant.
    HTP makes some great high efficiency tank type
    heaters.

    http://www.htproducts.com/versahydro.html

    For the upstairs use aluminum transfer plates if you can to get the best transfer and allow the lowest water temperature.
    www.radiantengineering.com is a great brand of transfer plates.

    The up stairs, like the downstairs should have a heatload calculation.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • giceman1337giceman1337 Posts: 41Member
    Thanks hot rod. When you say "regular PEX" are you talking about the same PEX as domestic hot water, or one with an oxygen barrier?
  • giceman1337giceman1337 Posts: 41Member
    Has anybody had direct experience with the Roth panel installation and subsequent usage of the flooring on top? I will be installing a pool table in one area, and a home gym in the other. The Roth panel manual says 60PSI compressive load. There is also the aluminum that will spread some of the load as well as the flooring. However, I'm only planning laminate or vinyl plank directly on the Roth panels. I don't want to install any plywood layer on top to conserve as much vertical height as possible. Has anyone ever had problems with the Roth panels "smashing" if impacted or from heavy objects?
  • hot rodhot rod Posts: 8,589Member
    I think the point load of the pool table legs would be a bit too much for the Roth without an overpayment.

    The type of legs and weight of the table would need to be examined. Most tables have adjustable legs for leveling with a small contact point.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • giceman1337giceman1337 Posts: 41Member
    My table legs are approximately 4"x4" flat wood surface. The leveling is done on the slab. Best information is my table is ~750lbs. So with 16 in^2 per leg * 4 legs, that would be <12 PSI.
  • GordyGordy Posts: 8,021Member
    If you know exactly where the pool table is going. Lay it out, and when laying Roth panels you can carve the foam out at the table leg locations, and substitute same thickness treated plywood,xps, or other to beef it up.
  • giceman1337giceman1337 Posts: 41Member
    Thanks Gordy, that's totally obvious, but didn't think of it.
  • GordyGordy Posts: 8,021Member
    I'm sure there is hard plastic that will work if you google. Your only hurdle is landing on tube lay out. Not a big deal shift one way or the other a tad.
  • giceman1337giceman1337 Posts: 41Member
    Was wondering if anyone has experience with the finished flooring directly on the Roth panels. I'm wondering if in carpet you would notice the void around the PEX notch.
  • hot rodhot rod Posts: 8,589Member

    Was wondering if anyone has experience with the finished flooring directly on the Roth panels. I'm wondering if in carpet you would notice the void around the PEX notch.

    I have put 3/4 T&G hardwood directly on Roth, and some 3/8 bamboo flooring.

    I doubt carpet and pad would work over Roth. Or even plain throw rugs.

    Use that thin, less than 1/4", maybe 5 or 6mm mahogany underlayment that you see at lumber and box stores. It's inexpensive and you will not notice any r-value. It may in fact help spread the heat output.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GordyGordy Posts: 8,021Member
    Just remember the carpet, and pad you choose should be low rvalue radiant friendly.

    Carpet over radiant floors is like the sun on a cloudy day :)
  • giceman1337giceman1337 Posts: 41Member
    The vast majority of my rooms (1000ft^2) will be laminate planks or vinyl planks. 1 rooms (200ft^2) will be carpet because it will have a home theater in it. I don't need much heater in the theater room, but I will be putting the radiant flooring in there just so it adds some warmth.
  • GordyGordy Posts: 8,021Member
    Whew! :) , and understand the acoustics.
  • giceman1337giceman1337 Posts: 41Member
    Use that thin, less than 1/4", maybe 5 or 6mm mahogany underlayment that you see at lumber and box stores. It's inexpensive and you will not notice any r-value. It may in fact help spread the heat output.

    Thanks hot rod, that's a great idea. I'll check that out. I think that will help smooth out the surface, and will help distribute some of the point loads without adding a lot of height and has minimal R-value.
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