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Community and Q&A

To radiant or not to radiant…

KRgj5wSAdD | Posted in Energy Efficiency and Durability on

A little background first:

We are in the process of designing our new home in north Central Vermont. We would like to build as energy efficient home as possible, while balancing a bit of a budget tightrope. Our current design provides approximately 1500 sf of living space for our family. Our site offers us a lifetime of good firewood, which we plan to utilize.

We are considering a wall section that includes drywall, 2×6 stud wall with cellulose insulation, plywood sheathing, 2″ of felt-faced polyiso, felt paper, 1x strapping, siding. Our window selection (based on price point at the moment) has been narrowed down to a wood/fiberglass double pane, low-e window. This will be slab-on-grade construction.

Questions (among many others) that are currently plaguing us:

1. Does a radiant system in our slab makes sense? Our living spaces will be oriented to the South/East. I’m concerned that using our wood stove will mean the radiant never even turns on.

2. Would it make any sense to not put radiant into the living spaces that are nearest the woodstove (and also most likely to receive any passive solar gain benefits), and instead put it only in the floors on the North side?

3. How much insulation should we put under the slab in either case? 2″, 4′?…

Please feel free to question any other of our tentative decisions at the moment. We are very excited about building our new home and are finding it very difficult to weed through all of the different options and approaches…

Thank you,

GBA Prime

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  1. GBA Editor
    Martin Holladay | | #1

    You will never get another chance to improve the insulation under your slab. In northern Vermont, I would consider 4 inches of continuous horizontal foam to be the bare minimum, and I would recommend 6 inches if you can afford it.

    If you will be heating mostly with wood, then the purchase of a boiler and an in-floor hydronic system is unnecessary and expensive. If you are on a budget, I would choose a cheaper backup heating system.

  2. KRgj5wSAdD | | #2

    What would you suggest for a cheaper back up heating system?...
    Will the additional foam under the slab help keep the concrete from feeling awful under foot?...

  3. GBA Editor
    Martin Holladay | | #3

    Yes, a well insulated slab will be warmer than a poorly insulated slab. Don't forget to install vertical insulation at the slab perimeter.

    Here's an article you might want to read: it's an article on radiant heating systems by Alex Wilson:

    Either an oil-fired furnace or propane direct-vent space heaters will be cheaper than a boiler with radiant tubing.

  4. Riversong | | #4


    I design and build super-insulated, passive solar homes, with radiant slabs and wood stoves. It actually all works together well if properly designed (Home Power magazine, April/May 2010, published my article "Designing Passive Solar Slabs", which discusses this in detail).

    One of my homes is featured here at GBA:, and the frame system is discussed here:

    No matter how well the slab is insulated, it will always feel cold to the feet if it's not heated, and there is no more comfortable and efficient heat than a radiant floor. The subslab insulation can be reduced if the perimeter of the foundation is also properly insulated. I prefer a shallow, frost-protected grade beam, which minimizes excavation and concrete usage and provides a protected bubble of warm earth under the house which reduces heat loss.

    While a good boiler is a significant investment, it can also provide unlimited domestic hot water more efficiently than most other systems. A small, very well-insulated house can also be hydronically heated with a high-quality water heater (such as Polaris or Pheonix).

    I would encourage you, however, to reconsider your building envelope. Exterior impermeable foam board prevents drying to the exterior, and cellulose insulation performs best when it can breathe and help buffer and redistribute moisture. There are a variety of less expensive wall options that will perform better and make your house healthier for its occupants.

    I live, work and teach sustainable building in North Central VT. I'm in Warren and teach at Yestermorrow. I would be happy to work with you on your project. You can contact me, if you'd like, at HouseWright (at) Ponds-Edge (dot) net.

  5. KRgj5wSAdD | | #5


    I was unable to read the article which your wrote for Home Power, any chance that you could send it to me?
    heidirdavis (at) yahoo (dot) com

    As to our thoughts regarding wall assembly: I was under the impression that while XPS and foil faced polyiso were vapor impermeable, a felt-faced polyiso was more permeable. I believe that 2" would provide the minimum r-value at the exterior required to keep the dew point from ever getting through the rigid to the plywood or inside the cavity. Any moisture within the cavity would then dry to the interior.The drainage plane (felt paper and strapping) would work in keeping moisture out as well. Have I been misdirected?

    Also, I completely understand the enchantment behind why to install radiant, I've been to many homes that used radiant heat and loved the feel underfoot. My concern is that the radiant will never even come on with the use of our woodstove, but is there a way to accomplish both?

  6. KRgj5wSAdD | | #6

    We plan to eventually install a solar DHW system, but would like to use propane as a fuel source for any back up heating and hot water. (Showing my lack of knowledge here:) But can you do both with a direct vent propane space heater?...

  7. GBA Editor
    Martin Holladay | | #7

    A space heater keeps your house warm. To heat domestic hot water, you need a water heater. Propane water heaters are common and relatively inexpensive.

  8. KRgj5wSAdD | | #8

    OK, that makes sense. Can you heat the house and domestic hot water with a water heater?

  9. GBA Editor
    Martin Holladay | | #9

    Yes. Robert already suggested two: the Polaris or the Phoenix. (Those are expensive water heaters, but they are relatively efficient.)

    If you go this route, you'll still need a hydronic heat distribution system (baseboard heaters, in-floor tubing, or a hydronic coil in an air handler if you want a hydro-air system.)

  10. wjrobinson | | #10

    Heidi , we all could suggest much for your home.

    But, the best advice you can get is to call Robert.

    You will save in all ways, time, money, and all the unforeseens.

    You are luckily neighbors and what he is an expert at is building exactly what you want to build.

  11. Riversong | | #11


    What's the brand of the felt-face polyiso board you're considering. I can find no perm specs on such a product.

    Foil facings have been traditionally used on polyiso board to retain the low-conductive blowing agents so that the foam will retain its R-value longer. They can degrade over time to as little as R-5.5/inch from their original R-7/inch. My understanding is that the felt-faced boards are designed for roofing systems in which they'll be covered by asphalt or other impermeable layers.

    In a cold climate, like northern VT, the dominant moisture drive is from inside to out. It is only in the summer that the drive reverses, and a rainscreen dramatically limits the reverse solar drive (that's it's function). Exterior insulation limits it even more.

    The theory behind the "warm sheathing" approach is that condensation and moisture accumulation in the sheathing can be limited by keeping it mostly above the dewpoint. The reality is that if any moisture finds its way to the sheathing - from poorly controlled indoor humidity, from leaks in the air barrier, or from exterior leakage - it will not only take months to diffuse to the interior but will be coupled with the warm temperatures that encourage mold and decay. Wet and cold is not a problem. Wet and warm is.

    I've been building durable and healthy homes for 30 years and teach hygro-thermal engineering and building science. Studies of both wall systems and roof systems that included a typical leak event demonstrated that non-breathing structures can take months or years to dry out and are highly vulnerable to mold and decay.

    The Bau Biologie movement from Germany, the first modern healthy house approach, has as a primary principle that a building must be able to breathe, or transpire moisture. All my homes are designed to have exterior skins that are 5x as vapor permeable as the interior to allow drying in the dominant direction.

    With a breathing exterior, a rainscreen is not needed (except in very rainy and windy climates). A rainscreen becomes necessary when applying hygroscopic sidings over a non-breathing foam layer. It's an extra complexity necessitated by an envelope which interrupts the natural flow of moisture.

    As far as balancing wood, solar and radiant floor heat, it requires some occupant participation. You can locate the thermostat far enough away from the woodstove so that it reads the average house temperature rather than the highest (also keep it away from direct sun).

    A woodstove is most efficient and burns cleanest when it burns hot, so it's best not to oversize them and not to shut them down to a slow burn at night. In a very well insulated house, it's also best to surround the woodstove with a lot of thermal mass. That way a small stove can burn hot, and have some of its heat stored in the mass for slow release during the night. This makes it more like a masonry heater.

    If the house has a lot of south glazing, then you have to be careful not to start the woodstove when the sun is about to come out or the space will overheat. If the stove is allowed to burn out before going to bed, then the radiant heat can come on at night and warm the floor, and it will stay warm for several hours after the heat turns off.

    But, obviously, you can't both heat the house 24/7 with wood and expect to have a warm floor. You have to carefully balance the two and balance them both with the available solar heat. When the sun warms the mass floor and the radiant heat is on, there will be less heat transfer from the hydronic tubing to the sun-warmed concrete then there will be to the colder concrete in the north side of the house, so the radiant floor effectively balances the heat throughout the house. You can also install a wider tube spacing on the south side if you want to use mostly sun and wood to heat the house, and put the thermostat in the center or north side.

    A lot depends on your floor plan and house configuration. I'll send you the article.

  12. user-872446 | | #12

    If, per Alex Wilson's article, radiant heating in a concrete slab isn't a good fit for a superinsulated house, what covering for the slab can be recommended that will be inexpensive, durable, not hold dirt (like carpet), and will be comfortable to walk on barefoot?

  13. GBA Editor
    Martin Holladay | | #13

    1. Hardwood.
    2. Softwood.
    3. Area rugs.

  14. Bob Alsop | | #14

    Having been thru this "Vermont Dilemma" myself I can offer my opinion. At the very least, put the sub-slab foam in and add the tubing and pour your slab. Even if you decide to never hook it up it will be there for the future. (Trust'll hook it up.).
    With your supply of available wood why not use a wood boiler to heat your domestic and radiant supply water with a "back-up" source for those times when you are not there (vacation,winter in Florida,etc.)
    There are plenty of available "back-up" options that work well such as split-mini systems, electric boilers, tank-type water heaters, propane wall heaters etc. Mod-Con boilers (gas or oil) are an option, albeit expensive.
    Beware of conventional oil or gas boilers with this kind of system (low return water temp's will kill these units).
    You will enjoy the radiant.

  15. Riversong | | #15

    An alternative to a concrete slab is an earthen floor. They are much "softer" on the feet and joints, don't drain us of electro-magnetic energy, and maintain a high negative ion count which is essential for both physical and emotional health.

    Sand and clay can be mixed with cellulose or straw and compacted in increasingly fine layers, with a top layer containing stone dust, and sealed with any of the natural oils and waxes. They are time-consuming and labor-intensive to create, and they will dent and scratch (but are easily repaired), but they are the greenest of floors and connect us daily to Mother Earth.

  16. 2tePuaao2B | | #16

    Dirt floors don't show the dirt as much either...

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