I'm in the early planning stages of a 1.5 story with full basement ICF build in Minnesota, the 1st and 2nd floor total ~2,600 sq. ft. and the basement another ~1,000 or so.  We're currently considering the costs/benefits of having radiant floor heating as opposed to forced air.  Given our climate we require cooling of some sort, primarily for dehumidification, so duct work will be present for this.  I've lived with radiant floor heating in the past and really enjoy the 'coziness' of it but understand that given the mass effect of an ICF home I may not necessarily see a real increase in comfort levels by going with radiant floor heat as opposed to forced air.  I'm already spending the money on duct work for cooling (assuming there isn't a different cooling option that fits the bill), is it foolish to go with anything other than forced air heat?  In general, will there be any noticable difference in efficiency when comparing forced air heat to radiant?  If yes, is that difference, in general, large enough to warrant the increase in equipment and labor cost to add radiant?  Also in play is whether or not going with a concrete floor system such as Insul-Deck or Hambro, as opposed to a wood joist system, would make any difference in the given scenario.  All else being equal, what's going to be the most cost effective combination here?

The efficiency all depends on the temperature at the cool side of the heat exchanger. With hot air furnaces that's pretty much known, independent of building construction type and heat load, so you'll be running ~ 95% efficiency whether you live in an R2 cardboard shack or a super-insulated castle, whether it's 25F outside or -25F, the temp on the cool side of the heat exchanger will be governed by the room temp, which if it's keeping up with the heat load is nearly a constant.

With hydronic floor system it depends on many things, which all boil down to how many BTUs per square foot the floor needs to deliver, and how effective the flooring system is at extracting the heat from the water & delivering it to the room. A concrete slab is much more effective than a couple inches of wood and tubes suspended between the joists. Both are radiant floors, but the temperatures required will be quite different- the slab will deliver much cooler water back to the boiler, increasing efficiency. But also, if the walls & windows are all very low-loss, the BTUs/ft numbers go down, and in case of superinsulated buildings (R40+ walls) they go WAY down, to the point where yes, you can get 97% or higher efficiency all of the time, but nobody cares , if the floor surface only needs to be 1-2F above room temp to deliver that heat.

From a cost-effectiveness argument it's impossible to make the case for radiant- the cost of radiation is substantial compared to already-designed-in ducts, and it has to be well designed to beat 95% efficiency. But whether there's enough cush factor to make it worth it depends...

The mass effect of ICFs vs. other mean next to nothing when it comes to peak & average heat loads in MN and whether or not radiant is worth it from a comfort point of view. It's all about the average R-values for the whole wall (including windows & doors). Up to about R25 "whole wall" R (which may take R28+ of insulation if you have much window area) you'll probably be able to appreciate the cush-factor of warm floors on a frigid-day as well as reaping a couple percent more in efficiency. Higher than that, not so much. But hydronic systems typically use only a fraction of the electricity for moving the heat around (pumps can move more heat per watt than air-handlers due to the higher heat-density of water vs. air.)

From a total comfort point of view, if going with a hot air system, going with a right-sized 2-stage system for the actual heat load counts. Oversized units have oversized blowers and move too much air, increasing wind-chill. Two stage systems with multi-speed blowers are quieter, cycle less often, and feel better.

Other factors: It's easier to zone hydronic system into micro-zones that run at different temps than hot air systems. If the zones go TOO small there is can be a negative impact on average efficiency, but with high-mass radiation such as slabs that's rare. Even with low-mass radiation adding buffer tanks to add thermal mass to the system can fix any short-cycling loss issues. With hot air systems adjusting flows at the registers to achieve a room-to-room temperature difference tends to reduce efficiency by increasing backpressure to the air handler, and unbalancing the system.

Well done Dana!

Water is 3500 times more efficient than air at transferring heat (about $50/year for a typical residence). If you have had radiant floor in the past, you won't be happy without it.

I design radiant floor systems with high velocity, split and conventional DX systems for cooling in Minnesota and radiant floors with condensing boilers or condensing water heaters always come first since space "heating" 8000 hrs in Minneapolis, and domestic hot water outweigh the 400hrs of cooling we may need.

interesting read as I too head down that path of radiant, but probably only in our basement.  I want to do passive solar w/ direct gains for the upper floor, and then radiant floors in the basement.  The plan down there would be to have exposed stained/stamped concrete in the "public" areas (game area, tv area etc) with radiant heat.  Then in the 2 bedrooms that would be down there, have carpet.  However I planed on putting 2" of XPS below the entire slab.  Is radiant needed under the carpeted areas?  I would assume it would not be very efficient, and since ductwork for ac would be present anyway, just have those areas heated with forced air.  Will the 2" of foam, the carpet and pad insulate the slab enough to not feel cold? I am assuming it would be.

From a comfort to bare feet point of view R10 would be just fine with/without radiant heat in the floor, but...

It should probably be more, if you're hoping to heat mostly with solar, and it's a high-R house. Look up your approximate subsoil temp from county/state agencies or take a WAG at it here: http://mb-soft.com/solar/soilmap.gif

Once you have as little as R5 under the slab, the effects of soil type on heat loss become a secondary factor- R5 is comparable to or more than the R of the soil between you and the deep subsoil. At R 10 or higher you can pretty much treat the thermal mass of that soil as a constant, with a the interior room temp an deep subsoil temps so as constant, you can reasonably simplify the heat flux through that as that of a constant delta-T against the R value of the slab insulation. If your deep subsoil temp is say, 50F, and your slab is 70-75F (radiant slab), that's a 20 to 25F delta against only R10 with 2" XPS, for a constant heat loss of 2 to 2.5BTU/hr per square foot. Compare that to an R30 wall in an area with an average January temp of 20F outdoors, and a 70F interior, which is only 1.67BTU/hr per square foot. If your walls are R40, it's only 1.25BTU per square foot, and the heat loss out the slab will be nearly 2x that of the above grade walls on a square footage basis. You may want to the R of the slab to make it more in-line with the rest of the place (whatever that is), but R10 would be the absolute minimum for any radiant slab in the upper midwest, and on the low side for any super-insulated house, heated slab or not.

For non-structural slabs you can do just fine insulating with much cheaper type-I (1lb density) EPS rather than 1.5lb XPS, but even type-II (1.5lb) EPS is much cheaper per unit R than XPS. EPS has a higher proportion of closed cells than XPS, but has interstitial spacing between the cells reducing it's performance to about R4/inch @ 75F. (more like R3.6/inch for Type-I, a hair over R4/inch for Type-II.) But with the positive temperature coefficients at lower temp, even Type-I is a good performer against cooler subsoil temps. See: http://www.transconsteel.com/products/ultraframe/docs/Other_Properties_of_EPS.pdf

A 4" layer of Type-I EPS would deliver a true R15 against 50F subsoil under a slab, 4" of Type-II would deliver about R17, both for significantly less money than going with 3" of XPS, for a similar R. At R15 the constant heat loss per square foot out the slab ( 24/365) would be similar to the heat loss of an exterior R30 wall in January for central IA. Is that enough?

Depends on what your goals & budget are, but maybe- it'll lower the AC bills in summer, but adds a constant load during the winter. Still for overall energy efficiency 5-6" of EPS for something around R20 might be better. For TRULY superinsulated houses they go ~2-3x that. The Urbana IL PassiveHouse went with 14" (x4= R56) of type-II under the slab: http://www.e-colab.org/ecolab/SmithHouse.html

If your clear-wall Rs are only R13-R15, (like 2x4 stick framed construction), 2" of XPS would be in keeping with the rest, but if you're over R30 a mere R10 slab looks like a big fat hole in the thermal envelope.

I have a similar issue with a house I plan on building. It will not be ICF, but will be somewhat close to superinsulated (R-30 clear wall and R-60+ attic with tight airsealing and good windows). I originally wanted radiant heat (probably a warmboard-type application since I'll have a basement), but am a little troubled by the lack of the cush factor of warm floors since they only might be 1-2 degrees warmer than the ambient air temp. Has anyone had experience with radiant in a well insulated, tightly sealed house? How do the floors feel? Warm, cool, neutral? I'm starting to think that fin-tube baseboard heat might be a better alternative, with the money savings going into a better envelope. I want to stay with a hot-water based heating system (wife and I hate forced air with the dryness, cool air blowing on startup, etc.). I also liked the idea of a low temp radiant floor system, as I thought it would be easier to tie into a solar water heater in the future. However, looks like this may not be the best use of the considerable money it would take to put in radiant floor heat. The warm floor factor would be the key benefit of installing radiant.

Also, has anyone had good experience with radiant panel baseboard heating? Seems like a good alternative to radiant floor heat if you want that "radiant" feel. I assume water temps would have to be higher, though, since there's less tubing and no benefit of thermal mass? Any experiences or thoughts, good or bad, would be appreciated.

Radiant ceiling is a very low cost alternative to floors if you are going to have flat ceilings and in superinsulated situations, comfort will be similar.

Radiant panel radiators will typically be better choices than radiant panel baseboard.

that said, my father in law has a superinsulated house over an unheated basement. even with R19 floor joists, they noticed the difference between before and after his radiant retrofit was noticeable. I used to say it wouldn't matter at all, but I'm backing down on that now. Often, in superinsulated houses you can use a very low cost floor install method (say, suspended tubes in the joists), still get low temp operation and save a lot of bucks too.

many options. heat load calculations answer most of the questions.

Rob- how superinsuated is that house? If it's an unheated basement without substantial wall & slab insulation it could be that MOST of the heat loss from the first floor was out the floor if they have R40+ walls and minimal glazing.

In US climate zones 5, 6, & 7 basement slab & basement wall insulation R-values tend to be about the same as the first floor whole-wall R in PassiveHouse designs, dropping to about half the first-floor whole-wall Rs for zones 2 & 3.

it was double wall construction and well built though it did have some significant glass coverage as well.

no doubt being over cold space makes radiant floor more attractive.

If the slab & foundation walls were similarly super-insulated, I suspect there WOULD be no cold space below.

Just by itself the R19 batts under the radiant (even with the radiant off) is enough to make a comfort difference, even with not-so-cold 64-65F basement temps (as I've discovered while retrofitting and re-configuring stuff my antique not-so-superinsulated house.) But at my heat loads the cush-factor goes way up with the radiant is running during 50%+ of design condition loads.

I've considered the same thing for a house that I hope to build. Since I've never had radiant, I don't know what I'm missing. However, the manual J load will be low. The warm floor feel to the feet would not exist. That would be what I really was looking for from radiant floor heating. Therefore, it doesn't make sense to me to have to pay for duct work for AC and not use the same ducts for heating. Also, the installed furnace price appears to be lower than the boiler and plumbing for the radiant heat. With today's variable speed blowers in a properly designed system, I doubt the draft issues and noise issues would be an issue with a forced air furnace.

Now for an off topic observation of your plan. Is there a reason the basement is so much smaller than the 1st and 2nd floor? I suspect your code required foundation depths in Minnesota get you about halfway to a basement. The price delta to go the rest of the way may not be much more.

Also, my first new house was built with a partial basement. That part of the house over the crawl space was the hardest room to maintain comfortable temperatures in. Whether that was due to the crawl space or not, I don't know for sure. However, it was clear the little extra digging and the small amount of materials required to make it full wouldn't have amounted to much. After living there a few years, the extra storage would have come in handy, too.

Good info from all, thanks for the input thus far. If the ICF shell itself doesn't really lend much in terms of thermal mass, am I better off , in regards to efficiency and cost, going with a poured floor deck if we decide to do with radiant? At that point we're comparing radiant with the thermal mass of poured decking (as well as basement slab) combined with a high velocity cooling system against a complete forced air system for both heating and cooling. Going with full fledged forced air we'd likely default to wood joists and decking as a cost savings measure. 'Comfort' and noise-reduction aside, which option is going to be most efficient both in up front install and ongoing operation in this scenario? Should something like Warmboard be under consideration here as well?

The reason for the basement being smaller than the 1st and 2nd floor is due to the footprint of the plan and the attached garage. Going to a full basement, as opposed to the partial, would mean another ~500+ square feet of finished space. Cost is the main consideration here both in terms of finished square feet of the project as well as heating/cooling expense, not to mention having no real need for that much additional space.

just for fun: you could do a radiant heating/cooling system, with just IAQ ventilation and inline dehumidification. muah ha ha ha!

makes the most sense if you're doing geo or using a heat pump... chillers are pricier than regular condensers.

pretty advanced design too... I'm partly being a brat... but we're having fun getting into it.

jboisen: Without knowing the R values, it's hard to even take WAG on what's "right" thing to do.

An ICF home with only 2.5" EPS on both sides isn't exactly a super-performer in a MN winter( but is probably legal from a code-minimum point of view) and the cush-factor of radiant is totally real.

In an ICF home with 6" of EPS on the exterior and 4" on the interior, with minimal glazing & high-performance windows & doors, and 6" of EPS under the basement slab and R60 in the attic, radiant might be a waste.

The more insulation you have on the interior face of the ICF, the less the thermal mass of the walls matter, but the mass of concrete floors will. WHEN the ICF mass matters will be during the shoulder seasons & summer though- in MN the winter outdoor temps are low enough that even the average temp is a real heat load, and the mass in the walls only moderates the average heat loss through the walls. In an ICF with R40+ clear-wall values the windows will dominated the heat load to where the mass in the walls are irrelevant, but the the mass of the floors will still matter. Since that mass is fully inside the thermal envelope and not isolated from it by insulation, it's better able to sink/source heat to the conditioned space.

We can't really advise, if we don't know much about the actual heat load, how many BTUs per square foot the radiant would be sourcing, etc. But if the radiant isn't sourcing much, the comfort difference also isn't much. It's mostly about R values, not thermal mass- give up on that, will you? The thermal mass is secondary effect, and in mid winter a very SMALL secondary effect for you (unless you've worked out a passive solar scheme), yet we're still in the dark as to your insulation levels and the percentage of glazed area on those ICF walls.

Planning on asymmetrical ICF construction, 2.5" interior foam, 4" core, 4" exterior foam. Approximates R-30. Roof will be R-40 via decking flashed with foam and filled with dense cellulose. High performance windows, seriously considering Unilux, Optiwin, and various Canadian fiberglass.

Basement:
Footprint ~ 950 sq. ft.
Wall Area ~ 1,350 sq. ft.
Glazed Area ~ 18 sq. ft.

1st Floor:
Footprint ~ 1,750 sq. ft. (includes ~520 sq. ft. of attached garage)
Wall Area ~ 1,700 sq. ft.
Glazed Area ~ 360 sq. ft. (includes four exterior doors that are full-lite)

2nd Floor:
Footprint ~ 1,350 sq. ft.
Wall Area ~ Zero (roof is pitched so that entire 2nd floor is contained by roof structure, interior knee walls only, no 'exterior' walls except a few small dormers)
Glazed Area ~ 75 sq. ft.
Roof Area ~ 2,700 sq. ft.

Thermal mass is effectively out the window - got it.

"Approximates R30", would be from an annual heating/cooling energy use, and maybe a daily peak, (but not daily average) heat load point of view.

Taking the first floor, total of 6.5" of Type-II EPS is about R26, add in another ~ R2 for interior finishes+ concreted core + siding and you're at R28. At 1750 total wall area, 360 of which is glazed that's ~ 20% glazing. With ~20% of the wall area glazed, the window losses with EnergyStar minimum performance U0.30 windows the heat loss out the glazing is about twice that of the rest of the wall. If you cut the glazed area to ~10% of the total (or 15%, with higher performance glazing with U values closer to 0.20 or even less, such as some of these: http://www.seriouswindows.com/html/numbers.html ) the wall & glazing heat losses would be roughly equal.

Some really crude "doin' th' math on my fingers" estimates: At say, 15F outside temps you're looking on the order of of 5 BTU per square foot for the radiant to deliver even if it were all bare floor for that 1750. Reduce floor area of the radiant by 25% for furniture closets cabinets you're talking maybe 7BTU for a January average, and 10BTU/ft when it's -15F out.

Which adds up to, yes, radiant floors will make a "cush-factor" difference with your R values & glazed area, in your climate. Were it only 5BTU/ft at the 99% design-condition outdoor temps (which might be averaged down to only the 90-95% level for a peak load by the thermal mass of the wall, since peak conditions are usually only an hour or two in duration, and well below the design-day average) maybe not, since during most waking hours the floors would feel pretty neutral, not warm under foot.

you can reasonably simplify the heat flux through that as that of a constant delta-T against the R value of the slab insulation....the heat loss out the slab will be nearly 2x that of the above grade walls on a square footage basis.

This has been gone over numerous times and all the soil (aka insulation) between the slab and "deep subsoil" makes such a simplification quite inaccurate. It's not like an outside wall where the lost heat can be convected away (unless you have moving groundwater near by).

It's usually cheaper to add R20 of EPS than it is to do the site-engineering to prove that you don't need it. In MN where the frost lines are often below the slab and deep subsoil temps are 40F or less, a bit o' nonlinearity to the model by making the simplifying assumptions may still insert a double digit percentage error, but it'll be low double digits for R10 & above. Soil is not "insulation" just 'cuz it has an R value- that R value is very limited. It has a substantial thermal mass as well.

Soil can only be treated as insulation against the deep subsoil when there is some ridiculously low R value under the slab, and the heat loss out the slab is warming up the thermal mass of the soil, where it eventually finds a seasonal balance lower than the constant that it would otherwise have. With R10+ under the slab the heat flux is low enough that the local warming factors are a secondary phenomenon. Whether you only really needed R17 not, R20 under the slab with a particular soil type isn't really very important, but still a 15-20% error.

With middle-off the road soil types the R-values are on the order of R1 per FOOT. A slab edge only 6' below has but R6 between it and the winter-outdoors average, and WAY less than a year's worth of thermal lag from it's thermal mass. In the center of the slab you have more R from the great outdoors, but the amount of "effective-R" to the subsoil will vary. (And both average soil moisture and distance to the ground table matter.)

But it's only a significant issue when analysizing high-R structures. For code-minimum buildings the heat loss out an insulated slab 5' below grad is "in the noise."

Radiant floors are not necessarily warm (comfort being defined as the lack of discomfort) so if you have windows your floor will likely be 66F or thereabouts. Cold to most. With outdoor reset nearly any building will benefit from radiant floors.

Though there can be quite dramatic savings in commercial applications, residential radiant floor heating is and always has been, a luxury.

As for insulation, most of Minnesota has a ground temperature of 47F. R10 is plenty under the slab and R20 at the edge to be reasonable.

Basements are a cultural thing (I never saw one while working in New Mexico for 3 years) and are cheap space if you need it. I have always contended that most basement are a complete waste of space and resources. As in New Mexico and many parts of Canada and Alaska where permafrost is the norm, frost protected shallow foundations (FPS) are the intelligent choice.

Nothing could be tighter or more efficient for tubing, insulation and long-term comfort.