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Community and Q&A

Heating a Three-Story House With ICF Finished Basement

DrunkTrainPolka | Posted in Mechanicals on

i am building a little place (47+ N) on a hillside that is only 1,034 sqft (interior dimensions) but is set up as a tri-level; two stories plus a walk out basement on the south side.  and i say basement because it’s built out of ICFs.  the other two stories are conventional 2″x6″ framing.  all three levels are to be conditioned and function as normal living spaces.

i have already made the commitment of running hydronics to heat the place.  and thought, until my wife led me astray, to use runtal panel/base board radiators.  the place is contemporary and there is nothing like hot water – and going this route seemed like a luxury.

BUT since the place is small, wall space is at a premium even ‘tho the runtals don’t protrude too far.  with that, a thought of running pex with heat dissipating plates crossed our minds…  at this point it would be no different then an old house retrofit.

yeah, i get the conductivity and heat transfer issues of the ‘heat’ having to go through 1-1/2″ of OSB and red maple.  and i get the labor intensity of installing such a thing as well.

however, while i have read tons, in a place this small, would giving up the radiators for this second-thought half-ass ‘infloor heating’ be an ok idea?

as an engineer i am more concerned about the panels transferring too much energy to the exterior walls and outside (as opposed to my wife worried about if a couch will fit or not).

would zig-zagging the pex between my joists on the second and third floor (basement is slab on grade with no pex running inside of it) be a better idea?  i am not concerned about losing heat to anything below because all three levels are conditioned and our staircases are open anyway.

house would be set up with three zones; one for each level.  the ‘main’ heating source will be a morso 1440 woodstove that can handle 1,000 sqft.  however for the next five years at least this will be a second home; we won’t be there all the time so 40F or-so will need to be maintained during the cold months.

thanks in advance!

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  1. Expert Member
    Akos | | #1

    There is nothing wrong with your idea to add in floor heat, provided the ceilings are not finished. Except for the labor, floor heat material cost is significantly less than any decent rads.

    First I would run your house through one of the online load calculators (ie If you are somewhere in zone5-6 area, you'll be well above the heat load requiring floor heat under 100% of the floor. You can strategically put it where you are walking and under entrances/windows. This would significantly reduce the amount you need to put in.

    For simple install I'm a fan of ultrafin, much quicker install and it keeps the pipes away from potential nails through the floor. You do have to run warmer water though it but it is small compromise.

    The best compromise in your case would be to install hydronic baseboards for the basement and bedrooms and do the floor heat only on the main floor and under any upstairs bathrooms. The good thing with ultrafin is that you can run it off the same temperature water as your rads, you don't need a separate low temp loop.

    If you strategically insulate under your main floor, you can also skip most of the basement baseboards and treat it as a radiant ceiling. You don't want to rely on this for 100% of the heat as you have no control over it.

    Also don't go nuts on zoning, try to keep most things on TRVs, much simpler and cheaper. Make sure whatever setup you choose is within the min fire range of your boiler. It is hard to heat a small efficient house with even the smallest boiler, sometimes a high efficiency water heater with a heat exchanger for hydronics is a better fit.

  2. user-2310254 | | #2


    If you are building a tight and energy efficient home, a single split mini might be a cheaper option (and provide AC). You might want to search GBA for threads on the pros and cons of installing in-floor radiant systems. One issue seems to be that efficient envelopes need much less heat than old, leaky homes. In practical terms this seems to lead to a situation where the system is often idling and the floors aren't particularly warm. At least, that is my interpretation of what some posters are suggesting.

    Here is a key point from one article:

    "Why isn’t my floor warm? Homeowners who look forward to walking barefoot over warm floors are often disappointed by homes with in-floor hydronic heating. That’s because these floors are rarely as warm as most homeowners expect.

    Wilson described this problem in his 2002 article. “Heat is transferred from an exposed slab to the [indoor] space at a rate of about 2 Btu/ft2•hr•°F,” Wilson wrote. “In a well-insulated house, this rate of heat flow means that even when it is very cold outside, the slab can only be a few degrees warmer than the rest of the room or the room will keep heating up. For a concrete slab to feel warm, however, it needs to be about 80°F. Thus, for most of the heating season, the greatest feature of radiant-floor heat — a warm floor — won’t occur.”

    In many well-insulated homes, a 'radiant' floor may be maintained at only 75°F — which is less than the temperature of your bare feet."

    You can go to to read the full article.

    1. Jon_R | | #3

      Lots about floor comfort here. Summary, outside of barefoot (eg, a bathroom), it's not a big comfort issue. Better to focus on zoning to get air temp just right.

  3. DrunkTrainPolka | | #4

    thank you akos/steve/jon...

    i am in zone 7A per 2009 IECC... the zip-code for the house is 49930 (i currently live further north in 49913).

    yeah, i have not spend too much time trying to figure out what is needed pex-/plates-wise as far as what is necessary to satisfy the load. with the rads it is a little easier since they would be coming off the shelf. the more i think about this, the more i like a small combo between the two... whatever i do 'tho, i would like it to be low temp even if it means more radiators/pex... but will look into ultrafin.

    what steve mentioned... very interesting. need to read more about it. the house is pretty darn tight. i will see about checking temps against what it would feel like to be barefoot or not. personally i have to wear danskos inside because of my ankles' pronation. so there goes the 'benefit' right? my wife will be surprised about this possibility....

    a minisplit is no-go because of the eye-sore and my ego being bruised by forced-air. ;) and the amount of snow we get, trying to even find a place to locate the outdoor unit would be a challenge - i almost would have to build a platform away from the dwelling to house the unit. and boy do i hate those holes through my walls. as a side note; the house has no plumbing and no electrical (besides a conduit connecting all levels) running inside any of the exterior walls.

    house is open on every level... including the ofuro-tub/toilet/sink on the third level. the only place that could be considered 'enclosed' would be the full bath in the basement which will also house the laundry (mid level has the smallest half-bath per code). i am going with zones to be able to have an order with possibly different temp settings - this, i think, i need when we will be using a wood stove, and at times, not using a wood stove. much of my design is a "2-pipe reverse return system" that came from here (see figure 3-20);

    the hydonic system is already set up for four zones (with a manifold with hydraulic separation); one for DHW and the other three for each the home levels. i will see about some pex running between the joists. and possibly coming up with an above the concrete in-floor option for the enclosed downstairs full bath, that may be just right.

    thank you so much, you have no idea how much these comments help!

  4. Expert Member
    NICK KEENAN | | #5

    Do a Manual J.

    The critical question with in-floor radiant heat is whether you can meet the building heat demand with the square footage available while keeping the floor temperature reasonably low. The amount of heat a floor can provide is directly proportional to the temperature difference between the floor and the air. So if your air is at 72F, a floor at 92F provides twice as much heat as a floor at 82F.

    Other than bathrooms you don't want floors to be noticeably warm. It hurts the feet to stand on them for too long. It's especially true if you have any kind of foot issues, you get swelling and blood pooling if you stand on a warm surface.

    Until you do a Manual J and get your heat need you're shooting in the dark. At the outside you can get about 30 BTU/hr/SF from a floor before it gets too hot. If you use 100% of your floor -- which is never actually possible -- you're looking at 30K BTU/hr. Unless your Manual J comes out significantly below that in-floor isn't going to do the job.

    Have you looked at in-ceiling radiant? This article has detailed instructions. They claim 28 BTU/hr/SF:

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