In the process of designing a new home. It is in the NY area. I am looking at making it ICF with an unvented spray foam attic with some type of energy efficient window of .25 or less. It is a 2000 sqft ranch with a walk out basement that will be mostly finished. My question; Is it worth the extra cost to do a radiant system on both floors? I was thinking of using warm board on the first floor and in slab in the basement. Some have told me that doing radiant in a super insulated house is a waste of money. Are there any issues such as controlling the temperature. For example, the house becomes overheated because the slab holds the temp. If a system is properly designed is this still a problem. Any feed back will be appreciated.

Radiant slabs and heat pumps of any kind go well together because you can design for low supply temperatures which enhances the heat pump efficiency. Tubing a slab is cheap. As you go "up" the ladder in terms of what you have to do to get the radiant heat, i.e. warmboard, dedicated radiators, etc, the radiant components become more expensive and the supply temps needed to serve them go up.

For example, the house becomes overheated because the slab holds the temp.

Why would it become overheated? That sounds like a control issue.

Did you ever consider an ICF roof. I have seen some examples of complete ICF from foundation up to and including roof. The ones I've seen were in FL and MS, which make really great for hurricane protection, but not sure if you could get enough slope or structure to handle NY snow loads. If you have time before you build, suggest you subscribe to ICF Magazine and follow that for a while.

I have never seen an ICF roof in this area. I have seen them do the gable ends to the roof. It might have something to do with snow load and the insulation needed in the roof. The minimum where I am it is R49 in the roof. To do this with an ICF I don't think would be cost effective. Thanks for the thought.

FYI.   Climatemaster has a pretty thorough geo/radiant application guide  here....

http://www.climatemaster.com/downloads/RP881.pdf

The part with the overheating I agree with. I think that would be a control issue. I had one individual tell me I had a choice of slab sensors or a thermostat sensor up to control the air temp.

Thanks for the info.

If I built a house anywhere north of Interstate 40 you'd have a heckuva time persuading me not to go radiant, with design optimized for lowest reasonable supply temperatures.

Don't get too crazy with building assembly techniques uncommon in your area or you will pay a premium for what might turn out to be substandard construction.

peteinny, There is a potential for an over heating situation that's related to stored heat in the slab and the air tightness of an ICF house. Houses built to very tight, well insulated standards to hold heat in simply take longer to cool down as well. At times of sudden large warming temperature changes and in the spring, it is warm outside and the floor slab is full of heat. At those times, the heat delivered from the slab can be greater than the heat loss of the house and a temperature overshoot may be experienced for a short while. The amount of passive solar entering the house and how it is stored within the building will also factor into the experience. In your case, this phenomena would be most noticeable on the upper floor assuming the walk out basement is the typical 50% below grade and would likely require a little heating still. An insulating foam board such as Warmboard or Becotec allows for a thinner layer of light weight over-pour cement. Less cement mass = fewer Btu's stored in the mass, helping to reduce or shorten the event. Radiant floor water needs to be no higher than 105° F in a house built as you are planning, but the water temperature can be modulated between about 80° F and 105° F. depending upon how cold it is outside. An outdoor reset water temperature control, (ORTC) can automatically provide that temperature adjustment for you by modulating the RFH supply water temp relative to outdoor temp. Use of an ORTC has been shown to save about 7% to 8% in European boiler studies. ORTC control of the water temperature would also help in the spring time as it would be unlikely that the system would still be at 105° F. in the later part of the season and I have seen homes in S. Ontario that need heating at night and some cooling in the day because of solar heat. In my experience, very tight houses still need cooling and dehumidification. The dilemma then becomes how to provide the cooling along with the RFH. The radiant heating water temperatures noted above are ideal for any geothermal heat pump system, in fact they are right in the sweet spot for heat pumps best efficiencies. For cooling however, using chilled water from a heat pump is a little complex particularly from a control standpoint, therefore I would opt for a 2 stage water to air heat pump with an optional on demand 100% hot water heating circuit, as offered by several Canadian and U.S. suppliers. Dedicate the demand hot water to the radiant using a ORTC and a mass tank and the DX air conditioning is built into the heat pump. just add ductwork and a heat pump thermostat – simple. With careful planning and careful implementation, I have found ICF houses with RFH and GSHP systems to be some of the best performing homes I have ever worked on. One other note too, whenever I design mechanical systems for a very tight home such as ICF or full spray foam throughout, I provide the same mechanical ventilation rate as I do for a Certified R2000 home to prevent high humidity problems.

Gerry, Thanks for the lesson. Very informative and seems to sum up a lot of my questions.

Not now for radiant heat.

I am remodeling my country home of 1477 sqft cinder-block on a concrete slab. We went to the stud walls, replaced all plumbing, electric and HVAC.

This gives us a chance to go back with full cavity closed cell foam R23-25 walls and R45 avg + Radiant barrier on the ceiling/roof and our foam contractor says that gives us the performance of R40+ walls and R60+ roof. We are spraying the roof so that the duct work is now inside the conditioned space which reduces any loss or inefficiency of the ducting system (predicted at 8-28%). We taped up the otherwise pretty good Ductboard for a tight system. We will perform a whole house air loss test when we get the house buttoned up but we are already planning a air ex-changer to bring in the proper amount of fresh air.

M J shows our new requirement at about 20,000 btus cooling and 22,400 heat (from a 80,000 furnace and 3 1/2 ton a/c unit which had the coil stolen). We will likely install a Miami Heat Pump single stage 2 ton system EER26/COP 4.1+ open loop.

We considered radiant heat since it is a slab home however we decided that very little would be gained after doing the insulation, tightness of foam and 4+Cop/ 26+ EER properly-sized geoH/P with hot water. We have also went with a 7.6 GPM instant on propane water heater using the tank of preheated water from the Geo/HP during air conditioning months.

We calculate that our savings have gone from almost $4000 per year total energy bill: propane heat/water and electric air conditioning 8 Seer, (lighting, cooking) to about 1/2 because of the change in insulation, about 10% more because of the super tight nature of foam and another 40-50% of that because of the Open Loop Geo system and maybe 10% from upgrading all appliances to EnergyStar. At this point we are expecting about $180 cooling, $200 heating, $300 hot-water, $300 lighting, appliance and cooking. So with only $40 per month heating and air conditioning bill it is hard to see much more savings relative to the cost of installation of a hydroponic system.

Our sub contractors have told us at this point the radiant floor on an existing slab would be expensive with little ROI other than perhaps some comfort. Since we have to tear out all the floors to do this, we opted to wait for later and see if we want it after re-inhabiting the house for a while.

So far we have budgeted :

$7-10k for the total house insulation
$3k for the 2 ton Geothermal heat pump with hot water, and 10kw electric heat option
$1k for the 2nd well, and some plumbing which we are already doing anyway (we already have a primary irrigation well on the property to use as the source well and we can divert much of the water to our 4 acre orchard with 625 trees during 7-8 months of year. If we have water level problems a few years out we can switch to a closed loop system at that time.

Budget on the geo is a little light.
You do not have to tear out slabs for radiant, there are routered OSB sub floor panels (radiant tubes lie in the routered cavities) that could lay right over slab and only cost you 3/4" head room.

Thanks Joe,

I am aware i can do a 3/4"-1" quick set over the existing slab using 1/2" or 3/4" 02 pex. This seems to add about $3k in pex and controls. I was talking about existing Tiling, carpet and wood floor coverings already installed but we are considering tearing it all out and going with a polished concrete floor anyway- as we like the retro look. We could then come back and ad the radiant at that time.

The radiant heat sounds like a very nice feature to any house especially a concrete slab home in extreme winters here in Kansas. I have a PITA girlfriend. My girl friend is giving me fits about the radiant heat as she says I keep the floor heat in the car too high and toast her feet all winter long and she has nightmares of my doing the same in the house. I like the idea as I want to do away with carpeting and just walk on the concrete slab or have a Pergo floor system that is easy to maintain. There is no doubt Radiant heat would add comfort in my opinion. My assumptions of $3k costs are probably light as well: the 2' of O2 pex per each sqft of heated space, manifolds, pump and quickest but making our own spacers system, using radiant tape to insulate the pex from the concrete below it.

We are doing all the procurement and work ourselves so the budget of $3k was the cost of the MHP GeoHP $1899 for 2 ton, $100 for the double insulation quit compressor option, $475 for Hotwater option, $266 shipping. We read that TACO valves fail so much that you should use the irrigation pressure to do the job of the switch/valve. I did not include the hot water costs of buying a spare tank for holding

Posted By Tony_Scarpelli on 12 Mar 2012 02:10 PM
I have a PITA girlfriend.

Better than a PITA wife. 

we figure about $6 sf to rough in hydronic, but it depends on what you are using. Not a good place to skimp.

Make sure you differentiate between high mass radiant heat (in a concrete slab) and other types (like wall or floor fin radiators). In general, a high mass radiator is a negative.

I dunno, in Wichita the cooling season is big enough that the additional thermal mass of a radiant slab may be useful, and the very low water temp requirements for a slab will boost the COP of the geo.

Low mass radiation is great for savings using setback strategies, but those gains can be small compared to being able to heat the place with water below human body temp. If you go with radiant CEILING you can probably get there though.

Wichita's heating design temps of +8F is pretty cool, but the average winter temp is ~30-32F, making for significant peak loads, yet fairly modest average loads, which may also point to being able to utilize the thermal mass of the slab to advantage even during the heating season. Warm floors are nice, and unless you have pretty good slab-edge insulation under that slab a tile or stained concrete floor may feel pretty cool under bare feet. But with radiant ceiling the ceiling warms the concrete a bit too.

In general, using thermal mass as a radiator negates its benefit as thermal mass (averaging out daily highs/lows).