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Mod-con boiler right choice for in slab radiant shop with full basement and spancrete floor?

rocket190 Member Posts: 14

I'm looking at building a 40' x 60' insulated shop building in Wisconsin (Approx 8,000 hdd). There will be a full basement under this building and spancrete plank floors. In addition, there will be a loft area of approx 900 square feet that will be open to the main floor. The loft is possible because of 16' sidewalls.The basement will be for storage only, but would want it to stay at least 45 degrees during the winter months.

My plan for the main floor was to use 1/2" pex radiant on 12" centers and the same for the basement. I am a shop rat, but this building will only be used during nights/weekends. The building itself will be very well insulated R50 ceilings, R40 walls (without deducting loss in stud areas) Approx 100 sf of R3 windows, and approx 240 sf of steel insulated garage doors.

Since I need to pour a topping slab over the spancrete and pour a basement floor it seems like radiant is the right choice. All below grade concrete would be R10 EXPS. Basement floor would be 2" EXPS also. I'm not sure if the spancrete main floor would be best insulated with 1" or 2" foam. I'd be okay with two heat zones: 1 for the main floor, and 1 for the basement, but I'm not sure how to heat the loft area. I'd like to utilize this as a "man cave" in the future, but don't think radiant floor heat 8' below will do much to keep this comfortable.

Whatever system I choose, I'd like to have maximum efficiency. It's clean slate, so I can modify as needed. My domestic hot water needs will be very minimal. I may rough in a shower, but will only have intermittent needs for hot water in a utility sink. Hot water use of zero gallons per day is a real possibility for a lot of the days, and even if I did install a shower, I'd say no more than 20 gallons per day of dhw.

Being that it's a shop, and I'm generally active when working, 60-65 is more than enough. It was bitter cold, I would be fine if it dropped below that until temps moderated.

If you had a clean slate system to design, what do you feel is the best option for me? I'd like to keep a moderate budget, but am willing to pay more for a quality long term system.


  • SWEI
    SWEI Member Posts: 7,356

    what kind of temperature do you need on the first floor?  Heating an uninsulated ground floor slab could cover both basement and ground floors.  A few tweaks to perimeter R-values may be necessary to establish balance.
  • rocket190
    rocket190 Member Posts: 14

    Yes, that was a thought also since I don't need high temps. I would like it comfortable for working, but don't have any issues working in my current shop which is thermostat limited to 60 degrees.

    However, if I'm only heating the basement slab, this heat would have to radiate through the 12" spancrete decking, through the topping pour, and up another 8' to get to the loft area. It seems like insulating the first floor spancrete would allow more accurate control of the primary workspace, which i the main floor.

    Would it make sense to run fewer pex loops per square foot in the basement and remote manifold them to the first floor zone? The lower temps from the basement might increase delta T to allow more condensing?

    Definitely not an expert on this--just trying to educate myself, as my experience working with contractors is that they push to sell what is easiest and most profitable for them.
  • SWEI
    SWEI Member Posts: 7,356
    slab heat

    I was suggesting you heat the slab that separates the basement and the first floor.
  • psb75
    psb75 Member Posts: 804
    More respect is due


    I would remind you that a whole lot of contractors post on here, often with very helpful and free information to the general public. Your efforts to acquire information would be more successful if you eliminated self-defeating comments about contractors.
  • paul_79
    paul_79 Member Posts: 91

    the first thing to do is get a heat loss calculation, you then know how many btu to replace on the coldest day. also you will know how much each area will need and the spacing on the radiant loops, sizes ect. The slab in the basement is a easy decision. you have to pour one anyway. install the correct slab and edge insulation. the main span is the same way you have a pour to do. insulate between the basement and the main floor or you will have no control over the basement heat. the loft is easy a couple of lines up to it insulate it also between the main and loft. the mod con is perfect for the high mass low temp situation.
  • rocket190
    rocket190 Member Posts: 14
    Thanks for the help

    Thanks everyone for the suggestions. I should have clarified my comment about the contractors. The people posting on these boards go above and beyond, and their depth of knowledge is amazing. I asked for help on here because I felt more confident in the information offered than what I was receiving locally. I should have said that the contractors "in my area" have been pretty lousy in regards to their knowledge about in-slab radiant. My parents spent around $60,000 for their in-floor setup between initial work and re-work. I am willing to bet there have been at least 75 contractor visits to their home--mostly undoing the original poor install. they finally have a livable system, but it's far from perfect.

    I am trying to avoid their mistakes and gain knowledge so I can eventually vet a local contractor. I'm considering having somebody like NRT design and spec the system so at least I'll know the design is right. Then I can monitor the install.

    What would be an appropriate charge doing a heat loss calc? And also what would be a fair charge for a system design?
  • rocket190
    rocket190 Member Posts: 14

    Many thank to you contractors that are willing to spend time on sites like these to help the inexperienced learn. Radiant heat and boilers are not common in my area. I don't know the exact specifics, but I'd say 90% of new houses are forced air systems, so when asking about an alternate I was generally getting steered away from a radiant system.

    In addition to continuous foam insulation (R10) on the exterior foundation and building walls I was thinking that it would make sense to "wrap" insulation on the edges of the spancrete before the topping pour. I've attached a detail drawing of how I will install the spancrete. This drawing doesn't show foam insulation, but it would sit on top of the spancrete, then I'd install tubing and reinforcement rod. I believe this would be the most efficient thermal break.
  • SWEI
    SWEI Member Posts: 7,356
    basement slab

    absolutely needs to be insulated.  I'm less convinced that insulation between the basement and first floors is absolutely necessary, at least without seeing the heat loss details.
  • paul_79
    paul_79 Member Posts: 91
    mod con

    when i took the radiant panel association classes from john siegenthaler and others, they emphasized the need to control where the heat output of the panel should go, they gave examples of million dollar homes that had not insulated between floors and then the complaint was the basement was warm even though the tstat was turned way down. radiant heat travels 360 degree, so a large slab main floor that this gentleman is trying to keep 60-65 degrees and the basement at 45 degrees must have insulation or he will have a over heated basement. i have seen it before and it is hard to insulate a finished basement after the fact.
  • SWEI
    SWEI Member Posts: 7,356
    individual control

    is great to have, but a properly tuned outdoor reset curve can work wonders.  Whatever the answer is, it will be determined by a proper heat loss calculation.  If the first floor is subject to solar gain or regular opening of bay doors, it will need both the insulation and the isolation provided by separate loops and controls.  If those conditions do not exist, a single source may be able to heat both.  I can't see this model working with frame construction, but a suspended slab is a different story.  Again, the heat loss will tell...
  • Gordy
    Gordy Member Posts: 9,546
    edited March 2013
    Insulated spancrete

    Yes that's what you are looking. A thermal break between the flexicore as we call it, and the topping slab.

    This will give you faster response, and direct the heat to the space above.

    Just to make sure, you said insulate the edges of the precast? Or do you mean the top? XPS then tubing rebar etc? Which is what I'm thinking you should be doing.

    Have the basement, and main floor separate zones. You never know future plans may change so I would do tubing in the basement floor also. Concrete with out tubing is a shame.

    After re reading original post it sounds like you are on the right path for slab insulation details. A heat loss is necessary to dictate tube spacing needed to heat the zones. You need to know what centers needed size of pex, and flow rates to off set losses.
  • RobG
    RobG Member Posts: 1,850

    Another option, insulate and put tubing in basement slab, if it is a fully buried basement you will probably not have to heat the space to maintain 45 degrees, but its better to be safe than sorry and as others have said, you never know what the future plans for the space may be.

    For the first floor, if you insulated under the spancrete (basement ceiling) you would then have a huge heat sink. It would be slow to respond, but with a properly setup outdoor reset control system comfort would not be an issue. in my opinion. it would be a shame not to take advantage of all that thermal mass you are putting in there. The only problem would be that if your boiler broke down, you may not notice it for a few days :)

    What kind of fuel and boiler were you looking at using?

  • rocket190
    rocket190 Member Posts: 14
    Factors in heat loss

    Thanks again for helping. Regarding the heat loss calcs, it seems like a million different variables are in place. Is a heat loss done for each room in a building or does it look at the entire building shell. When a basement and loft are in play, does the software take these areas into account?

    Even though it will add expense, I will def install tubing in basement and first floor prior to the slab. I understand that I won't be able to quickly warm uo the building with such a large amount of thermal mass, which is why i'm willing to spend a lot for extra insulation up front.

    Regarding my fuel options, I have electric or propane. Natural gas isnt available. I also have free waste oil and wood. I dont think I want the hassle and mess of the alternate fuels, so all signs point to propane.
  • rocket190
    rocket190 Member Posts: 14

    Gordy, in regard to your previous question about the foam, I would place it on both the top and edges of the spancrete. Since I will have a 2x8 sill plate, I will keep the tubing limited to the spancrete area only and will avoide placing any on the edges of the topping pour.
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