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Community and Q&A

Hydronic radiant vs heat pumps

brenton_roubo | Posted in Mechanicals on

Hello, 

I am new but I’m building a new house 2700 sq ft with additional 1200 sq ft basement in Scarborough, Maine (Zone 6) which will have 2 inches of closed cell spray foam on basement walls. The entire envelope has R-6 Zip R sheathing and all exterior walls are 2×6 Planned to be filled with open cell foam, 11 inches of Open cell spray foam in the attic roof. Trusses are 2×8 but I will have strips of 1” XPs foam over the edges with 1.5” wood screwed over that followed by 3/4” strapping which should result in no thermal bridging after spray foam is filled flush to strapping. 

I have two questions. 

first, with the zip-r sheathing, would you still recommend the open cell spray foam or would my money be better spent elsewhere with rocksul and a blower door test.

second, the bigger question.

I have been planning on installing a hydronic radiant system throughout the whole house with a wall hung viessman vitodens 100 propane boiler. My wife would like ac for the humid summer months here in Maine so I’ve been considering adding a 5 zone Fujitsu air sourced heat pump. My question is this, would I be better off going solely with heat pumps for my heat and ac and scratch the radiant and put that money into solar? Or should I install the radiant and have the heat pumps for ac and auxiliary heat? 

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Replies

  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Brenton.

    Because you weren't specific about the roof and venting, I want to point out that when using open cell spray foam in the roof, the assembly should be vented. Closed-cell spray foam is the only reliable option for beneath the roof deck in an unvented assembly. Another reliable way to build an unvented assembly is to install rigid foam insulation above the roof deck, then you can use a number of insulation types below the sheathing. For more on that, check out this article: How to Build an Insulated Cathedral Ceiling.

    When it comes to your wall cavity insulation, you have lots of options. Since the ZIP sheathing should handle the air sealing, you do not need spray foam for that attribute. Whatever you choose, you should still be performing blower door tests. In fact, it's a good idea to do one before you insulate, so you can find and seal leaks.

    Also, keep in mind that R-6 Zip R in your climate is not enough R-value for condensation control on the inside face of the sheathings insulation layer. That would require you to use the R-12 panels on 2x6 walls. So you should probably have some interior vapor control, perhaps a smart vapor retarder. For more on working with ZIP R, read this: Working with ZIP R-sheathing.

    I'll let others with more knowledge of heathing and cooling comment on your last question, except to say that the only way to make that decision right is to have a professional perform Manual J load calculations, and go from there.

  2. Trevor Lambert | | #2

    If you're installing a heat pump for a/c, then a hydronic heating system is a colossal waste of money.

  3. Expert Member
    Akos | | #3

    There are a number of items here.

    Before deciding on how to best heat your house, take a look at your energy costs. In my area of expensive electricity and very expensive propane, a cold climate heat pump with a COP above 2 (good ones will do better than this even bellow 10F) has much lower operating cost.

    Propane is worth it if you power grid is unreliable and you need a backup generator. Even there it is cheaper to run a heat pump for bulk heat and a through the wall propane heater for backup.

    You never design in spray foam into your assembly. It is expensive and can esaly be messed up with a bad install. Think of it as a last resort, you only use it when there is no alternative.

    There are many ways of designing a house without spray foam, even without any rigid insulation, that are more energy efficient and cheaper.

    It is pretty straight forward to detail exterior sheathing as an air barrier, with a tight envelope any of the fluffy insulation products work just as well as SPF. The greenest option is dense pack cellulose, but you might have a hard time finding an installer in your area, so check first.

    In your climate, you can't use open cell foam in an unvented ceiling. The open cell foam allows for moisture transfer and you can end up with rotten roof. You can use a mix of closed cell and open cell foam, take a look at the tables here:

    https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies

    A much better option is to go with either a vented celing with blown in/batt insulation or an unvented celing with exterior rigid insulation.

    Probably the cheapest option (provided your roof line is simple, which it should be if you want your home to be simple to build and energy efficient) is to replace your rafters with TJIs. TJIs have much less thermal briding than dimensional lumber, they are definately a cheap insulation upgrade.

    With 14" TJI, you can staple 1" of plyiso as vent baffle to the bottom of the top flange and fill the cavity with 11.5" of fluffy insulation.

    If you are looking for a lower profile, you can go down to 11 7/8 TJI by going with a layer of rigid insulation on the inside.

    For the heat, I would recommend to stay away from multi splits. To get good efficiency on these, they have to be closely matched to your load, this is not something you can guess at. An incorrectly sized multi split will consume much more power, there has been a number of threads on here with people with this issue.

    Generally, the most efficient setup is two one to one mini splits. Use say a wall/floor mount on the main level (provided your space is open) and a slim ducted unit to feed the bedrooms.

    Hydronic heat is something that will add $25k to $30k to your build. There are much better places to spend that money. Most well insulated houses have such low load and temperature difference (most surfaces are the same temperature) that there is little to no benefit for radiant heat.

    You can get very similar comfort for much less by strategically placing a bit of resistance floor heat in your home. A bit of floor heat in the bathrooms, kitchen area, entrance mud room and under larger windows/doors is cheap and gives you that warm toes feel.

    1. CarsonB | | #6

      Akos,
      do you have a link to your TJI cathedral ceiling assembly? Articles on GBA seem to point towards using exterior rigid foam to reduce thermal bridging instead of using TJIs, though the TJI setup does seem like it may be more cost effective. Would this still result in snow melt at the TJI locations in cold climates? Would there be concerns with long term durability with the TJIs? If moisture was an issue, it would not only compromise the sheathing, but with TJIs it would also be the supports.

      1. Expert Member
        Akos | | #8

        There is no link, couple of articles about it on here:

        https://www.greenbuildingadvisor.com/article/an-insulated-cathedral-ceiling-for-a-european-passivhaus

        The foam strips shown on the TGIs are pointless if you are blowing in insulation or using the correct width batts.

        Typically TGIs have a 3/8" web, so 1/4 the thermal bridging of dimensional lumber. This would bring a typical roof framing factor from 15% down to 4%.

        I wouldn't bet that it completely eliminates striping on the roof, but it would reduce to barely noticeable.

        I'm not sure why moisture would be an issue. This is still a standard vented assembly with a warm side air barrier. Unless you have roof leaks, no moisture should make it there.

        1. CarsonB | | #9

          Great link Akos. According to them though the top was vapor open, is this an issue not being able to dry to the exterior using rigid foam for the vent baffle? Also, from other articles on here when using foam at the top of a ceiling assembly it must be thick enough to avoid condensation. For zone 5 that would be 4" for a roof?

          1. Expert Member
            Akos | | #10

            There are some weird details in that article.

            Keep it simple, use TJIs and build a standard vented roof assembly with a vent baffle.

            This is low labor cost and pretty high performance. If you want super insulated roof, bump up the TJI to 16" but you might need to fur out the bottom for the ceiling to support the additional insulation wight.

            No need for any vapor open special membranes and foams.

    2. Jon R | | #16

      Open cell spray foam is fine (per Lstiburek and code) in unvented roofs.

      "In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation."

  4. brenton_roubo | | #4

    I should have mentioned that I have my close friend is a master plumber, licensed oil/gas tech and installs boilers and heat pumps on a daily basis. He could install the full radiant system and heat pump parts and labor for me for a little under 20k. So I’m getting a massive savings.

    I’ve attached a picture of the house to give an idea of what it looks like. We are currently living above the garage apt.

    1. Expert Member
      Akos | | #7

      From the picture it doesn't look like the attic will be finished? If you are not finishing the attic, there is no point insulating the rafters, insulate the floor. Blown in insulation there is by far the cheapest.

      Even cheap hydronic is not free, you are still estentially installing two complete heating systems.

      There is very little cost between installing AC only or a heat pump, just a bit more hardware cost.

      If your electricity is anything less than $0.20/kWh, propane heat would cost you more, never mind the extra $20k for the install.

      For you walls, I would skip the SPF and insulate with either batts or dense pack. Save the spray foaming for the rim joist and air sealing the attic.

      1. Mike Mills | | #12

        Don't know what part of the country you're located, but in my area propane is ~ 1.50 per gallon. Break -even for electricity with a 2.0 COP is around 12 cent per KWH. Only trouble is - My winter design temp is -26F. Not sure if any heat pump will even work at that temp, much less give you a COP of 2.
        We are building a new house this summer with radiant slab heat. There are many reasons to prefer radiant heat - regardless of cost. We are planning the same as OP, viessman boiler & Fujitsu or Mitsubishi mini- split heat pump. The mini split works great for a little supplemental heat in the shoulder seasons ( We have a Fujitsu mini-split in our current house).

        1. Stephen Sheehy | | #13

          Mike. Where do you live, the North Pole? -26 design temp?

          1. Mike Mills | | #14

            Northern Minnesota

          2. Expert Member
            Akos | | #15

            Just because you have a -26F design temp, it doesn't mean that the system will be operating there all the time. You do need to make sure that you have enough heat at -26F.

            To figure out of this could work, you would have to bin your average data to see what COP you hit for the whole year, most cold climate units will deliver full rated power at 5F with a COP around or a bit above 2.

            My guess is that a heat pump for base load with propane for very cold days would still be significantly cheaper to run than propane only. I wouldn't be surprised that heat pump with electric baseboard assist would also come out ahead on yearly heating costs.

  5. Expert Member
    Malcolm Taylor | | #5

    Brenton,

    A bit too late for your build, but for others starting their own houses, the time to make these decisions is at the design stage. They type of heating and insulation have ramifications on how you build. Once the house is up, your options are often limited by the assemblies chosen.

  6. Brian P | | #11

    "My question is this, would I be better off going solely with heat pumps for my heat and ac and scratch the radiant and put that money into solar?"

    Yes.

    We heat and cool our 1300+ sq ft house (2 story) in the White Mountains with a single heat pump on the first floor. A properly designed/installed heat pump system would be great for the Maine coast.

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