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Community and Q&A

Warmboard floor structure recommendations

ModernHaus | Posted in Energy Efficiency and Durability on

Hi all,

I am in the design process of building a home. For this topic, I’ll stick to flooring. Could you please give me your opinions and real-world experiences? They’re greatly appreciated!

General Info:
– Location: Zone 4 in Northeast.
– Structure: Exterior Wall, Nudura ICF; Interior Wall, 2×6; Floor, Hambro; Roof, Green Roof (Undecided on system).
Floor Heights: Basement, 10′; 1st Floor, 12′; 2nd Floor, 10′.
– Size: 15,000 sq. ft. including basement.
– Additional: I’m using mostly concrete – ICF & Floors – because it’s ‘cheap’ (Minus the ICF, Hambro, etc.) and quite frankly, solid. Also, my intentions are to sell it in a few years (MAYBE!) and I need that ‘different’ factor than other homes on the market. I know concrete floors in my area are overkill, but as I stated, it’s cheap due to my career.

1) 10″ of 3/4″ Stone
2) 2″ Owens Corning FOAMULAR 250 (R-Value of 10)
3) 2″ Owens Corning FOAMULAR 250 (R-Value of 10; Total is 20)
4) Stego Wrap, 15 Mil
5) 6″ Slab; May do 8″
6) Heavy-Duty Landscape Fabric
7) Delta FL Dimple Board
8) 2″ Owens Corning FOAMULAR 250 (R-Value of 10)
9) Homasote Comfort Base
10) Warmboard S
11) GenieMat RST05
12) Carlisle Hardwood & Polished Concrete

I still have to consult an engineer about the FOAMULAR 250; the 400 is double the price. I know the FOAMULAR 250 (Step 8) is not necessary, but my thoughts are that it’ll break the Warmboard from the slab (I’m sure the mass from the 6″/8″ concrete takes a lot of energy to heat). I’m considering less than 2″, too. Has anyone put Poly (Stego Wrap, 10 Mil) under the Dimple Board (Between Steps 5 & 7, possibly taking place of step 6)? And I am aware that there is the Warmboard R, but the thickness of the S will – in my opinion – play nicely with the dimple board/foam. It’ll have that extra rigidity.

1st & 2nd Floor:
1) Hambro Flooring
2) 6″ 4,000 PSI Concrete Slab
3) GenieMat FF25
4) 4″ 4,000 PSI Concrete Slab
5) Stego Wrap, 10 Mil
6) 2″ Owens Corning FOAMULAR 250 (R-Value of 10)
7) Homasote Comfort Base
8) Warmboard S
9) GenieMat RST05
10) Carlisle Hardwood & Polished Concrete

So, 10″ of concrete? Yup. With the 6″+GenieMat+4″ layers, there will be an STC Rating of 70, in addition to the Hambro Open Web, concrete and GenieMat RST05. Overkill? Maybe, but the extra soundproofing of the GM FF25 is $6/sf.

Warmboard Install Instructions:
– “Install a 6- or 10-mil polyethylene vapor retarder directly to the slab, overlapping two feet (2′) at the seams. Next, install a 1/2″ Homasote Comfort Base or Homasote 440 SoundBarrier…”
– “After installation, a minimum of R-19 insulation is required underneath the panels to prevent downward heat loss.”

Obviously concrete has a very low R-Value so 19 would be hard to achieve. What are your thoughts on insulation to meet, or work towards that 19? Again, I’m afraid that the exposed concrete slab will slow down the heating process. Or is this something I should not worry about. My climate might be 75 during the day then 50 at night, so it swings quite a bit.

Thanks so much for your inputs!

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  1. GBA Editor
    Martin Holladay | | #1

    Lots of extraneous details here, but it sounds like you will have a hydronic heating system that circulates hot water through Warmboard floor panels, and the Warmboard panels will be installed above 2 inches of XPS.

    Sure, it will work. Not particularly cheap, but it will work.

    Note that green builders try to avoid the use of XPS, since XPS is manufactured with a blowing agent that has a high global warming potential.

  2. ModernHaus | | #2

    Martin, thank you for pointing that out. I honestly 'forgot' and just priced out XPS because the price was readily available and it is more expensive (So, conservative on pricing). I'll have to find an EPS that comes recommended.

    Could I - or rather, should I - put the Poly under the drain board? I've never seen that done.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Should I put the Poly under the drain board?"

    A. Your original post doesn't mention "drain board," so I have no idea what material you are referring to when you ask your question about "drain board."

    What is drain board? Where is it in your assembly?

  4. Expert Member
    Peter Engle | | #4

    I think the dimple mat is the "drain board." And no, you don't need both. Both are plastic films that act as vapor barriers. The drain board has the added benefits of allowing water to drain away out the edges if drains are provided.

    Like Martin said, there are many extraneous layers in your proposed stackup.

  5. Expert Member
    Dana Dorsett | | #5

    Note, the XPS is labeled R10, but the fine print of the warranty is only good for 90% of labeled R, so it's really R9 from a design point of view. In reality 2" of continous EPS (R8.4) is good enough for zone isolation between floors. The notion that it takes R19 for isolation is just plain wrong if it's conditioned space above & below.

    It's a lot cheaper to put PEX in the slabs and use them for radiation rather than a layer of insulation + Warmboard. Heating up the slab takes time due to it's thermal mass, but the heat isn't "lost", it's all inside the thermal boundary of the house. Put the insulation under the slab to prevent it from heating the rooms (or dirt) below. At 4" the thermal mass is pretty manageable, easy to design for. At 6" it's harder but still possible. At 10" the radiant becomes too unresponsive.

    The heat load of an ICF house basement is ridiculously low, and the water temp requirements will probably be something like 85-90F. Insulate under the slab to at least R15, but using R3-R4 Roth panels on top of the slab (rather than WarmBoard) is sufficient thermal isolation from the slab, which will be idling along at about room temp 24/365. As stated earlier, the thermal mass of a 6" slab is unwieldy if the heat load changes very quickly or very far, but in a basement the heat loads are tiny, but mostly steady. It doesn't take much R between a 70-75F slab and a 68-75F room for the radiant to be able to track the load changes.

    Roth panels might work just fine on the other floors too- this is isn't a retrofit into a sub-code house- your heat loads are a LOT lower than typical WarmBoard applications. Seriously, run the heat load numbers and in a house with that much concrete model the dynamic mass to get more accurate numbers. When you know the BTU/hr per square foot you'll need out of the radiant the amount of isolation needed from the slab will become more apparent, but I doubt it'll be more than R3. (You'll still need to isolate the slab from rooms below with R8-R10 ceiling insulation.

  6. user-2890856 | | #6

    What is the reasoning for utilizing WB S / R in such a home ? In a home such as this there is no real cost / benefit ratio in doing so , unless you are laundering money .

  7. ModernHaus | | #7

    Martin, Peter - thank you! Apparently my fingers weren't typing what my brain was telling them to. I did in fact mean drain board. I have been doing a ton of reading prior to replying and I have learned a lot!

    Looks like a vapor barrier 'sandwich' is not recommended. Understandable.

    Thank you for the great information, Dana! The reason I am looking at Warmboard is because I really, really do not want to put the heating source in the structure itself. Warmboard is expensive, yes, but it gives me a piece of mind knowing that the flooring could be removed and the subfloor/Warmboard could be repaired without affecting the integrity of the structure. Plus, as we all know, cutting out and patching concrete is no small task.

    Richard, thank you! I just started reading about super-insulated homes and I have read that radiant flooring may be overkill, which is worrisome. I have also read some studies on this topic, and since the home will have 11' ceilings in the basement, 12' ceilings on the first floor and 10' ceilings on the second, with multiple 30' high atriums, radiant flooring may be a good option. Regardless, what I have learned is to go to a qualified professional. The stairs are situated in a way where the change of floor height won't make a difference.

    I'm still stuck on the basement....I have had water issues in the past (Broken pipes, poor construction practices, etc.) and I really do not want to deal with the headache again (Or at least, mitigate the risk). For less than an insurance deductible, I can buy steel floor joists for the basement, essentially creating a space within a space. God forbid something does happen and there is water in the basement, the damage could be upwards of $50/sf by the time you add in the cost of removal, the insulation, the Warmboard and the finished flooring. That doesn't include the cost of furniture, drywall, components, etc.

    Foundation Slab:
    1) 10″ of 3/4″ Stone
    2) 2"-3" EPS
    3) 2"-3" EPS
    4) Stego Wrap, 15 Mil
    5) 6″ Slab

    Basement Flooring:
    1) 3" Air Gap
    2) 10" Steel Joists built on a concrete lip around the perimeter.
    3) AdvanTech (For rigidity)
    3) Subfloor / flooring [Either Warmboard if the analysis recommends it or just a double layer of AdvanTech)

    In the basement will be a wine room with stone cladding (Heavy), an indoor pool/spa, bathroom, storage and other living space, so to me, it does make sense to build the floor up to run piping and all.

    With the basement built up off of the slab, are there any downsides other than the added cost? Noise (When something is dropped on the floor. Also why I'd like a thicker subfloor)? Heat Loss (The slab is insulated, but it's under the 6" pour)?

    Thank you again for all of your help!

    1. Expert Member
      Dana Dorsett | | #9

      At the heat loads you'll see in that house (particularly the basement above an insulated slab) R0th panels (less expensive than WarmBoard) would be more than sufficient.

      Before picking a heating solution it always pays to run the load numbers, which in your case would include the thermal mass effects, which reduce both peak and average loads.

  8. GBA Editor
    Martin Holladay | | #8

    You are building a multi-million dollar house, and you have lots of basic design and engineering questions. Rather than designing the building yourself, I suggest that you hire an experienced architect and mechanical engineer.

    1. Expert Member
      Dana Dorsett | | #10

      +1 !!

  9. lance_p | | #11

    In a well sealed and insulated home in a relatively mild climate heated floors are not justifiable; you literally can't keep the floors warm enough to have that "warm floor feeling" without overheating the house. This is the first lesson I learned after joining GBA to inquire about my own build in a much colder Climate Zone 6A, in which I was determined to use heated floors. You can do it, but you won't be able to tell the floors are heated.

    At 15,000 sqft, adding Warmboard will add a small fortune in material costs. Even with your considerable construction budget I would think that money could be much better spent elsewhere. If you're planning to sell after a short while I'm sure that money could be better spent else where, even if it was to just to include a new S-Class with the sale. Now THAT would set you apart!

    1. Expert Member
      Dana Dorsett | | #12

      >In a well sealed and insulated home in a relatively mild climate heated floors are not justifiable; you literally can't keep the floors warm enough to have that "warm floor feeling" without overheating the house.

      That's your judgement, but it's mistaken. Reducing the heated floor area to where the comfort factor is most relevant can raise the floor temps in those areas to heat the place without raising it everywhere. A lesser amount of heated floor area will lower cost and increase comfort.

      This is one more reason why it's important to run the load numbers and focus on what the comfort goals are before specifying the heating solutions.

      The rationale for that much concrete isn't clear either, but if that's what he wants, it's his money. From a green-building perspective concrete has a pretty hefty environmental footprint. If that matters at all it's worth looking into using recycled concrete aggregate (at least in the slabs) &/or fly-ash displacement of the cement fraction in all of the concrete in this building. Fly ash makes the concrete stronger, and produces less heat during curing, lowering the amount of micro (or larger) cracking of the material as it sets up. It's good stuff!

      1. lance_p | | #13

        "A lesser amount of heated floor area will lower cost and increase comfort."

        I'm not sure that is accurate. From a total BTU standpoint, yes, reducing floor area heated allows higher floor temps and reduces costs. From a comfort standpoint it's a risky proposition. You can do this in two ways; 1, only heat certain rooms, or 2, only partially heat the floor in each room.

        1. If you only heat certain rooms you have no control in other rooms. Also, the comfort level is a balance between floor temperature and air temperature. You may end up with excessive floor temperatures in one room in order to keep the air temperature comfortable in non-heated rooms. The "Mean Radiant Temperature" of the rooms will differ greatly from heated to non-heated rooms as well.

        2. If you only heat part of the floor you will only be comfortable standing or sitting where your feet are touching the heated part of the floor. If the air temperature in the room is comfortable for where the floor is not heated, you will be too hot while on a heated section of the floor.

        Running the load numbers is how I came to those conclusions, with help (and foreshadowing) from people here on GBA. At roughly 8 BTU/sqft at my design temperature a heated floor is far from practical. As a two story house I considered not heating the second floor, but with doors closed the bedrooms would become uncomfortably cold. Yes my design has high R value walls and excellent windows, but my design temp is also much lower than it will be for the house proposed here in CZ4.

      2. ModernHaus | | #24

        I'm using concrete because I get the materials at cost and have the labor, so it costs next to nothing. That's the only reason. Unfortunately, money makes the world go round, so if I can save a buck (Within reason), I'm all for it. As the saying goes: if it's free, it's for me :)

  10. STEPHEN SHEEHY | | #14

    Why do does anyone want, let alone need, a 15,000 square foot house? And why come to GBA for advice about it? Seriously, unless you have thirty children, build a smaller house.

    1. Expert Member
      Dana Dorsett | | #15

      >Why do does anyone want, let alone need, a 15,000 square foot house?

      Every empty nester couple needs a cozy weekend getaway place like that where they can have free run of the place without much risk of running into one another. :-)

      There are large families from India with three generations (maybe four?) living under the same roof that can easily use that much space.

      In recent years I've known family of four living in about 12,000 square feet who regularly entertain dozens of people at a time, including hosting whole families of friends from out of state.

      A house that big isn't a necessity for most, but this apparently isn't about living frugally on a social security check.

      1. STEPHEN SHEEHY | | #16

        Dana-this weekend I'm hosting two dozen people for dinner in my 1700 square foot house.

        1. Expert Member
          Dana Dorsett | | #17

          Are they sleeping over for a week?

          1. lance_p | | #18

            Excellent points, Dana. To each their own.

            Stephen, we are building a 3400 sqft two story house (+basement) and there will be three of us living there; my Wife, her son (a dependent with Down Syndrome) and myself. We currently live in a very nice 1800 sqft townhome (including the basement family room) which gets us by. We need for nothing.

            Having said that, her son has dreams of having his own apartment (though living on his own will never be possible). My wife loves to entertain, and we can only practically have about 4-5 couples in our small main floor area before there's no room left. She is also from India and would like to have an additional bedroom or two for out-of-town/country friends and family to stay. I love to work on cars, dream of having a small woodworking shop, and have outgrown my modest 10 x 20 single car garage and single driveway. I'm also tired of living in a development where I'm one of very few people to have a nice property... there's an increasing percentage of rentals popping up, and many people here are (I assume) first time homeowners who don't care about caring for their properties.

            Do we need more? No. Could we use more? Yes. Our build will give us the ability to do the things we want to do that our current house does not allow. My Stepson will have a feeling of independence with his own "apartment" (a good size bedroom with walk-in closet and en-suite bathroom), we will have a main floor family room, a separate full size dining room and living room for entertaining crowds at least 3x as large as we can now, four bedrooms (and a fifth when we finish the basement) so friends and family will always be welcome, and I will have a garage to keep our cars out of the winter weather with enough space for a workshop in the back and maybe someday a two post car lift so I can maintain my independence of working on my own cars as I age. Looking forward as WE age, I made sure we had second floor laundry and wide straight staircases for easy implementation of stair lifts. You just never know what curve balls life will throw at you.

            Many people will look at this house and judge us as showoffs or whatever, building way bigger than we need to, but I don't care. It will be ours, it will be built well, have very modest energy bills for its size, and we will (hopefully) be there for a long long time.

    2. Deleted | | #20


    3. ModernHaus | | #21

      Great question! I grew up in a 900 sq. ft. row home with my mother - it was perfect. I lived in a 800 sq. ft. studio apartment in the city for 8 years. I do not mind small spaces, at all. However, with life comes growth. Technically, the above-grade living space is roughly 8,500 sq. ft., which is also huge. However, a wife, four kids, four adults (In laws, etc.), eleven nieces and nephews and 15 other close relatives, it's nice to have some space. Obviously the nieces/nephews/other relatives will not be staying with us 100% of the time, but currently, we see them a 2,500 sq. ft. - it's time to move. The in-laws/parents stay with us 33% of the time; together sometimes. Therefore, it's nice to give them an 'apartment' - sitting area, bedroom, private bath. Between all of this, I think 8,500 sq. ft. will go quickly. So, your 30 children statement was quite accurate ;).

  11. STEPHEN SHEEHY | | #19

    Lance- there's a world of difference between 3400 and 15,000 square feet. And I'm not suggesting that Modern doesn't have the right to build what he wants. But he can't call a 15,000 square foot house green. Your house sounds pretty reasonable.

    1. ModernHaus | | #23

      Yes, a big difference. But as I stated in my previous reply, there are essentially three adult couples living under one roof; at 1,700 sq. ft. per couple, that's a total of 5,100, 3,400 sf left for my family of 6 as well as any relatives/guests. Granted, each couple does not need that space but for out-of-town relatives who stay quite often, I'd say making it feel like 'home' is important. Plus, that doesn't factor in my families sanity!

    2. lance_p | | #25

      I'd like to think it's not too unreasonable. My parents built a 2700 sqft house when I was young (three kids), and it was the "big one at the end of the street" in a small town custom home development. Part of me thinks about that when looking at what we're doing now.

      That was 1985, things have changed a little since then. Looking back, that house had a spacious main floor but a cramped upstairs with three small bedrooms and a reasonable master suite. Our new place will be similar, but scaled up a little so that the bedrooms are all a little bigger. Fortunately our lot is in a neighborhood where larger fancier looking houses exist, so we won't be drawing too much attention once it's built.

      Agreed that a 15,000 sqft house won't be a green house unless there's five families living there full time. But people will build what they want to build, so hopefully the green building community can help point them in the right direction when it comes to choices that impact the energy use and environmental impact of the home.

      Dana's suggestion to look into high fly-ash concrete is interesting, one that I'll now look into for my own foundation. A lot of what needs to be done in green building is simply spreading the word so that builders and homeowners are made aware of the options.

  12. STEPHEN SHEEHY | | #22

    Dana -one advantage of a small house is the they don't sleep over. If I say "you're all welcome to sleep on the ( concrete) floor," I don't get many takers. Since there are airbnbs everywhere, building a bigger house just to accommodate the occasional guest made no sense to us. When our daughter visits, she gets the guest/tv/ office room.

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