I am the leader of a senior project group at WSU. We are designing a small (less than 900 sqft) environmentaly friendly home for the Ogden, UT area. Our goal is to keep the overall building cost under $90,000, so I am trying to decide if radiant cooling is a good idea for us. We are going to be using in floor radiant heat. It is a slab on grade home. Do you guys have any pro's or con's we should consider? Any estimates on cost? 

Thanks so much!

AbbyJayne,

Forgive me, but I'm a little curious of the question in general. If the average temperatures there is 76.5 degrees (summer), spring 50.1 degrees (spring), 53.0 (fall), and 26.4 (winter), why worry about cooling? It seems to me that it would be an added cost that would be used only slightly. The ROI would be very small in my opinion.

renangle

I'd consider mold and condensation. I haven't heard of a successful radiant floor for cooling, despite searching for it. Found some successes with overhead panels.

Jon r
Thanks for the answer! I don’t know if mold would be an issue or not. We have very low humidity here. Do you know at what humidity level I would need to start being concerned with mold?

Around 60% RH at the location where mold might grow (say in carpeting on the floor) - although it can go higher if there is some periodic drying (say at night).

I have no doubt that it would work under some conditions - but I'm not sure about all conditions that you will encounter in Ogden.

The problem with radiant cooling is condensation, so the chilled water temp must be carefully controlled against the indoor humidity. Radiant cooling is better done with chilled beams (office building solution) or something like Planterm (an Italian product with drywall/insulation boards with embedded tubing) that can be used for walls and ceilings. Condensation can still be a problem with those, however.

The sensors and controls to prevent condensation for radiant cooling in a humid climate are $$ and complex, although the humidity in UT it may not be a problem. Your floor finshes should be able to deal with condensation if you go that route.

The kit to make chilled water usually isn't cheap unless you already have groundwater you can use through a heat exchanger. Accordingly, chilled water may not be a cost effective solution.

Bruce

A good question would be how often is the dewpoint above ~50F - a quick look shows that you might be OK most of the time (last month high was 56F but average was 40F). It will depend on how cold you make the floor. You might find it cheaper and easier to put in some efficient inverter based mini-split AC units - a small, super insulated house doesn't justify a very expensive heating/cooling system.

Aside from the technical aspect I wonder how comfortable the house would be, with floors very cold underfoot and surrounding air still warm and humid? Doesn't sound like something my wife would enjoy. :-(

I looked up the climate info for that area of Utah.
I think a well designed house should not need any active cooling.
1. You have a slab, so you have a lot of thermal mass in the design by default.
2. You have very cool nighttime temperatures even in the hottest months.
3. You have generally low humidity.

Design the house with walls around R40, ceiling around r60, and very low infiltration. Use a energy hea,l scissor truss so you have a low cathedral ceiling ~10-11 ft tall. Make sure the windows are shaded in the summer months with proper overhangs. Have a good mechanism for night time heat purge, Think insulated ceiling whole house fan with a smart controller. Make sure that the house has a good exhaust fan in the bathroom for humidity removal. Add some ceiling fans. A highly insulated house should be able to coast through daily warm periods easily. I think you could keep this house at less than 70° degrees all summer with a little planning. I think you biggest problem will be heat gain from occupants and electronics in the house.
 I bet if you work out the heat gain of the house on the hottest day, and calculate the thermal mass of the slab, you could keep the daily temperature fluctuations in the house to 3-5 degrees if it is well really well insulated.

When you get a design, I would love to see it.


Good Luck
Eric Anderson

Thankyou everyone for your responses!!!! This has been so helpful

Eric - EXTRA THANKS! and we should have the basic design finished late Nov. early Dec. I will definitely post it for advice and comments then

Summertime dew points in UT are low, and floor condensation issues simply won't exist, even for a not-superinsulated envelope design. If the building is built so miserably from an insulation & solar gain point of view that needs the floors to be cooler than 60F to keep up with the load it's not much of an "environmentally friendly design". If the floors needed to be 50F or lower it could be pretty uncomfortable for extended time in bare feet as well. With any attention to solar gain issues you would likely be able to keep it well under 78F with a slab no cooler than 65F.

But eric has it right- designing it with clear wall R values north of R30 it won't need cooling system at all, (beyond what could be handled by a nighttime ventilation approach.) But if it's easy to design in radiant slab-cooling with the same heat pump, it won't hurt.

But if you skip the geo altogether and spent the money on a PassiveHouse-tight superinsulated house with ~ R55-60 clear wall values you wouldn't need a heating "system" either. That first ton or so of geo is wicked expensive to install, and a radiant floor isn't cheap either. With a geo + radiant approach you'd be spending at least 1/4 of your 90K budget on the heating system. If you can reduce your heat loads low enough it makes no sense. A small amount of resistance electric heat, passive winter season solar gain by design, and controlling the ventilation rate on heat recovery ventilation (HRV) system would cost a heluva lot less than geo to install & operate.

Getting high-R on the cheap usually means a double-studwall approach and filling the gap with blown cellulose. Designing the framing interface at the top of the walls to allow a continuous thermal break (with minimal penetrating structural timbers) with the attic insulation can be an important key detail to get right, as well as providing an exterior thermal break at the band joist/foundation sill. You'll need some insulation against heat loss to the subsoil too. The cheapest foam options for foundation & ground insulation will almost always be EPS, but it'll be bulkier than XPS (or iso in non-ground-contact applications.)

For mid-R (R30-40) you can go pretty cheaply with a single 2x6" studwall full of cellulose + exterior foam board. (3" of exterior rigid iso + a 2x6" cellulose studwall makes a fairly compact R40). Use blown fiber insulation rather than batts wherever possible, since real-world performance tracks test-lab performance much more tightly when there are no gaps/voids working against you. Cellulose tends to be cheap, and has lower convection than standard density rock wool or fiberglass. (Some of the noo-improoved fiberglass like Optima or Spider are pretty low-convection too, if you dense-pack them to ~1.8lbs/ft^3 or so, and deliver a slightly higher R/inch compared to cellulose.)

But critical to real performance is to get the air-sealing right- a superinsulated wind tunnel is still expensive to heat & cool.

If I may, the problem is not condensation…condensation is the result.

The problem is dew point. Dew point is a condition which exists at some combination of temperature and moisture.

Remove the moisture and you can cool as low as you like…that is unless you are limited by comfort conditions such as floor surface temperature.

So what is the lowest you should control the floor for comfort?
Answer: 66 deg F with standard footwear…warmer for bare feet and warmer for conductive floors such as tile.

What would the relative humidity have to be before a floor would condense at 66 deg F?
Answer: 67% RH

Is 67% RH good for building materials, dust mites, viruses and bacteria?
Answer: No

What is a reasonable RH for building and health sciences?
Answer 50% RH

Solution: deliver the ventilation air to the space lean enough (dry) so that it can absorb passive (infiltration) and active (occupants et al) moisture loads, so the space dew point remains below the dew point of the cooled surface.

Sensible capacity of the cooled floor: At 66 deg F, the cooled floor can absorb app. 1.2 Btu/hr/sf.

What does this mean?

It means that if you can design your building with sensible long wave radiation loads of less than 13 Btu/hr/sf and control dew points above 66 deg F…a radiant floor cooling system at 66 deg F will provide thermal comfort conditions without condensing whilst delivering on all the energy efficiency benefits.

RB

> Solution: ... so the space dew point remains below the dew point of the cooled surface.

You typically want it even dryer than that. While this would avoid condensation, it wouldn't avoid very high RH at the floor surface - so expect mold to grow in the carpeting.

I agree with the advice to forget geothermal and put the money into air sealing, insulation and proper shading of south facing glazing. Be sure to avoid east and especially west facing glass as much as possible. A dehumidifier might be helpful on those few summer days when RH gets a little high.

Since the topic appears to have reached its usefulness for the original poster (last posted Oct. 04, 2010), I’ve taken the liberty to expand for discussion…read it or bypass it – your choice.
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I don’t believe you would get any argument from the radiant crowd that it takes a certain client to own and operate a home conditioned with radiant cooling. A young family in Atlanta, Georgia with kids in a mass produced carpeted starter home wouldn’t be the ideal client. Chances are it wouldn’t be offered by the builder, nor would the client likely have the budget for the necessary system and controls and even if they did - the condensation risk is inflated due to the odds of windows and doors getting left open which increases proportionally with how many friends the kids have. Good practice would suggest radiant floor cooling would not be an ideal choice for that client in that circumstance in that geography.

However, mature financially established clients constructing a high performance home located in say Calgary, Alberta finished with low VOC conductive floors such as tile, slate etc are an ideal candidate. What they have going for them is less distractions in managing their property, a low humidity geography, building performance designed to deliver low cooling loads, interior design choices based on IAQ and thermal performance, and the financial ability to invest in the necessary systems and controls.

A risk assessment considering the above circumstances, et al (emphasized) influenced in part by the ratio of moldy buildings conditioned exclusively with air versus buildings conditioned with a hybrid of radiant floor cooling with a dedicated outdoor system with dew point controls and safety limits should be standard practice by the design professional. At the end of the day, 100% of all mold issues in buildings cooled exclusively with air didn’t have radiant floor cooling to blame.

End of message
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Additional info:

In general consideration to orientation of cooling surfaces (ceiling, walls and floors) one should look at the dominant load (heating or cooling), the available surface area and thus required flux, panel/surface characteristics and the available fluid temperatures and of course clients aesthetics requirements.

One design example - all values nominal/appx.:
 
In a high performance slab on grade building with say less than 10 Btu/hr/sf cooling using stained concrete or tiled floor, 6” o.c. tube spacing; the average required fluid temp at maximum load is a nominal 67 deg F. With a controlled room temp of 78 deg F, the corresponding floor surface temp is app. 70 deg F. or only a few degrees warmer than the fluid temperature and within recommended thermal comfort standards. [all values sans discussion on tube placement and diameter which have some modest influence.] A 70 deg F surface dew point corresponds to app. 75 %RH at 78 deg F space temperature.

75% RH as stated earlier doesn’t serve any building or health science needs whereas 50% does (or lower as per comment). At 50% RH and a space temperature of 78 deg F the dew point is app. 58 deg F. leaving a huge safety margin….normally 2 to 3 deg F is adequate. [sans discussion on dehumidification cap and op costs.]

If the load were heating dominated at say 15 Btu/hr/sf heating using the same criteria for cooling the average required fluid temp at maximum load is a nominal 82 deg F. Putting that into perspective is to say the fluid in the floor is app. 16 deg F cooler than blood temperature (sans academic discussions on difference between resting and awake, age, activity, etc.). With a controlled room temp of 70 deg F, the corresponding floor surface temp is app. 78 deg F. also within recommended thermal comfort standards.

67 deg F average fluid temperatures in cooling and 82 deg F average in heating does great things for exergy and energy efficiency regardless of the chosen heating and cooling plant. It is possible to do direct connected earth system with such temperatures (sans discussion on geography, geology, drilling etc.)

The above is an idealized but possible project BUT it all changes if the building sucks, is located in a high humidity region, is operated by inexperienced owners with kids on a soccer team, and where the design is based on reduced radiant surface area in another orientation etc., etc., etc.

Energy consumption for indoor environmental quality problems is a Rubik’s cube with different solutions for different problems.

Does radiant floor cooling solve all problems? No.

Is it a good solution for some applications? Yes.

What about cap and op costs? Among other considerations, it depends on building geometry, investment in the building enclosure and the source of power and fuel.

Expensive vs affordable? All relative...expensive to one is affordable to another.

Skills required? Higher and not always available…this is not to be underemphasized. Packaged solutions from blue-chip companies mitigate this challenge.

(note correction to previous post: missing unit on the heat transfer coefficient, 1.2 Btu.hr/sf s/b Btu/hr/sf deg F)

Note: for those who may read this in the future and are involved in research, ASHRAE T.C. 6.5 maintains a list of research citations on this very topic. To date the inventory of papers exceeds 400 documents several involving field studies of residential buildings. If you want the link to this resource let me know.

http://www.ashrae.org/education/page/2517

Contrary to what you have been told, you are not asking for nor expecting anything out of the ordinary.

Not sure what forum policy is with regards to publishing emails, in any event at the risk of getting hand slapped....send me a private email at [email protected] and I'll point you in the right direction...you have resources available in your neighborhood.