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radiant slab floor suspended over basement

riverwoman's picture

My husband and I are working on a design for a energy effecient home in NW Arkansas. We have long mild springs and falls with freezing temperatures and snow in the winter and a couple of months of very high temperatures in the summer.The site is heavily wooded with deciduous trees which will be benificial for the shade in summer but since it is on a south facing slope we will be able to take advantage of passive solar heat gain in the winter. We would like a finished concrete slab  floor with radiant heat over a walk out basement. The concrete slab will serve as our passive solar heat storage while the radiant heat will serve as a backup and to even out the temperatures for cloudy days. The house will be around 2500 sq feet including the basement. The basement will serve mostly as storage and as a utility area but will have radiant heat in the slab also. We are planning to use ICFs for the basement walls and SIPs for the roof and possibly the perimeter walls.

My questions are: do we need to insulate the under side of the suspended concrete slab (basement ceiling) to keep the upstairs floor warm since it is over the basement?

What is the best way to heat our slab in our relativly mild climate? Tankless water heater? Hybrid? We have considered an outdoor wood boiler since we have an unlimited supply of wood but they are expensive and we would have to have a backup system for when we are out of town.



Hi JJ, heating passive (post #207069, reply #1 of 7)

Hi JJ,

heating passive solar slabs is a little tricky:  it's in "backup" at night when the sun isn't out, and it can't back down before the sun hits it without making you cold in the morning.  so it will increase the temp overshoot in the room a bit and it does reduce how much energy is stored from the sun, since the slab is already warmed by the radiant.  That said, it's typically only warmed a few degrees so it may not be a very big deal. 

I would strongly recommend the use of floor AND air sensors in this case... you don't want to wait for the slab to get cold before putting heat back into the concrete, or you'll have a lag coming out of solar gain periods where the room is cold and the slab is recharging.  floor sensors prevent that by preventing the slab from getting too cold.

Proper heat source selection depends on the results of your heat load calculations and how much you expect to run it.  If it's rarely going to run, a tank water heater with a heat exchanger may be the way to go.  tankless water heaters almost never are.  

unless your heat load calc is very high I don't expect you'll need high efficiency equipment in this case, but if it is you could consider a modulating/condensing boiler or perhaps a heat pump of some sort.

and yes, you always need to insulate underneath a radiant panel.


Rockport Mechanical

HVAC Design and Installation in beautiful Rockport Maine

Thanks for the suggestions. (post #207069, reply #3 of 7)

Thanks for the suggestions. The floor and air sensors are a good idea. I'll look at the modulating/condensing boiler and heat pumps. We are planning to talk to a energy consultant once our house plans are a little more finished. Finding someone in our area who is skilled in radiant design is the hard part so I am researching on my own as much as possible.

As you know concrete has the (post #207069, reply #2 of 7)

As you know concrete has the ability to collect a lot of heat and to, in effect move the hottest part of the day forwards into late afternoon and evening. This only works when the sun can shine on the concrete and when the concrete/brick/stone is no more than four inches thick. The idea is usually used in glass sided office blocks, it doesn't work very well domestically, unless the thermal store concrete slab is isolated from the main slab. As heat doesn't float to the top of a slab like a pool of water, it dissipates throughout the slab, and the slab if too big, may never get warm.

On the other hand, if the slab is exposed to the sun over several days, it will get hotter day by day and if you do not open the windows overnight to let the slab cool, then it will not work absorbing the next days heat as required, and will allow the home to become too hot for comfort.

A hydronic/ water heated slab is a good idea, provided you know in advance what tomorrows weather will be, slabs slowly absorb heat and are even slower to cool down. You may find the mornings cool and spend a lot of money heating the slab, only to find the sun comes out, the heat in the room very quickly reaches 50F or more and all that heating money is wasted, when the windows and doors are opened while you try to cool down.

Or the day starts sunny and turns cold - and the slab takes forever to heat up, leaving you cold and unhappy.

A better idea, is baseboard heating, that responds quickly to the days requirements.

Having a heated basement to live in is a good idea, but it does usually come with the problem of damp, and even Radon gas poisoning, if you intend to use the basement for day to day living, then you need to protect your family with a Radon gas proof membrane. Basements usually have poor ventilation and Radon gas tends to build up in corners and cupboards where it will affect children and animals.

Check out the Radon gas web site to see if you are building in a high Radon gas area, you do not want lung cancer later in life.

Going back to damp. The best way to deal with this is, to have an open space between the home and the hill. It will rain and surface and underground water will run down the hill and find its way into the basement. However an open space between the home and the wet soil will stop this. You can do different things, a four inch wide gap between the home and the retaining wall holding back the soil will work as a minimum, up to a 30 inch path that will provide easy access to the walls and enable easy cleaning and drainage.

The problem with concrete is that radiation takes place in all directions a polished aluminium reflector will reflect the radiation back into the concrete and help keep the upstairs warm. If you are keeping the basement warm, then heat loss downwards will add to the warmth down there.

A concrete slab will absorb a lot of heat before it starts to heat a room, once warm it is best to maintain a high enough input to keep it and the room warm. A tankless water heater will work perfectly. Using oil or natural gas means you can have full control,, 24/7. A wood boiler requires attention 24/7 and that will become a pain in time, especially when you are older. You will need chain saws, log splitters and somewhere dry to store the wood for a year or so. You may well find that it will put people off from buying your home when the time comes to move.

SIPS are a good idea, but you need to understand humidity, condensation, water vapor and how water vapor can destroy SIPS panels. Unfortunately, over the year the heat from the sun, the cold and the wind cause homes to move, this movement will lead to joins opening, water vapor getting into the joints and mold and wood rot.
You need someone who understands the need to keep the warm wet air inside your home away from SIPS panels. This is best achieved by fitting water vapor proof plastic sheets, carefully joined and sealed over the frame to stop the water vapor from entering any open joints.

Thank you. We have thought (post #207069, reply #4 of 7)

Thank you. We have thought about radon and plan to vent beneath the basement slab. Even though we are not in a high radon area there have been homes here that have tested above safe levels. Better to err on the side of caution and from what I've read it is fairly simple to add during the building process. 

I haven't found any information of a combination of radiant floors in conjunction with passive solar. My thought is that the hydronic system would spread the heat through out the slab on sunny days. We would keep the slab at a constant temperature in the winter with the hot water heater and a gas fire place for  instant heat.

We are still weighing the pros and cons of SIPs but whatever we decide to use we will of course want to use proper vapor barriers. 

A passive radon vent takes (post #207069, reply #5 of 7)

A passive radon vent takes care of the radon problem (essentially a  vertical pipe penetrating the roof that is joined and sealed to the vapor barier under the basement slab - air within the pipe is warmed by the house and wants to rise creating a slight vacuum on the underside of the vapor barrier allowing natually occuring gases including radon to vent harmlessly to the exterior.)

A designer will be able to show the added cost of a full concrete slab, so be prepared for a few extra grand in framing to support it.  Along those lines and depending on your spans and local suppliers you may run into a good use for a prestressed concrete slab - they can span a fairly long distance and there is an additional slab poured on top of the prestressed portion which could handle the radiant.   I like how it brings in even more mass to your heat sink.

I think too much fluff is written about solar heat sinks and how they should be at certain temps at certain times of the day.  I'd want as much mass as possible.  Extra solar gain simply means less energy needs to be put into the slab to keep everything at a comfortable even temperature.   I don't want my house temps going up and down.   As long as it's within the building envelope and you don't mind keeping the house at a constant temp then there is no downside to added mass.

I've worked on a number of full ICF houses and there is a huge advantage to the added mass in the walls over SIPS.   If SIPS are used they have to be installed and detailed well, and exterior upkeep is very important, but that's not big deal if you like to keep up on things.   I think you'd have more bang for the buck with ICF walls over SIPS, and more bang for the buck with trusses over SIP roof if the trusses are built to allow extra insulation (R60-R80 cellulose).

As for what you heat the radiant system with, long term durability is possible with a number of different approaches from conventional water heaters to tankless to boilers.   Modulating condensing boilers are probably as good as you'll get efficency ways, but it comes at a steep price.  Super insulated houses don't need much to keep them warm so you'll see many people use a tank type water heater just fine, but you can plumb in a number of heat sources and only use the one you want at the time. 

Beware that most/many designers will try to sell you on something other than a full slab with wild claims they are better for radiant heating, but there is a lot of salesmanship selling many of these systems and I haven't seen anything that convinces me they are better than a good old slab with pex tubing.    If hydronic systems aren't common in your area look to an area where they are for advice.  This isn't rocket science and knowing what consistently works well in the real world is what you want to focus on, not someone overthinking the system.

What a fun project - let us know how things go as you move forward!

If I could edit my location it would say I'm now in Reno :-)

          gotta be careful (post #207069, reply #7 of 7)


  gotta be careful idahodon.  ICF mass may be particularly helpful in your area, but in more mild climates the mass isn't really that helpful.  

Mass also isn't why slabs are good at radiant.  it's conductivity.  the best radiant emitter is an uncovered slab (polished concrete floor).  you can't beat not having much R between your pipes and your room.  but once you put finish floors on, pipe in concrete isn't much better than pipe in highly conductive panels.  and in some cases, like lightweight concrete, they are worse.  so if the concrete is going down anyway, great, use it.  if not, it's not necessary to get good results from a radiant itself itself.   If your loads are low, you can do almost anything and get good results.  the higher your loads get the more it matters.


Passive solar of course needs mass to soak up the energy.  even more importantly, it needs timed or automatic shades or other methods of sealing off the heat loss at night to help the day gains stay gains.

  On the other hand, in a more mild climate like arkansas, lots of mass in your heating emitter can cause a problem with temperature swings.  you are starting from "off" more often, which has a lag time.  You have the solar gain issue, which will raise internal temps quickly in many cases.  certainly you should at least be using floor sensors in these cases.  at the end of the day though the mass you want for passive solar is not a real benefit to the radiant emitter and if ideal results are desired, you wouldn't heat the floor mass directly in that case.  Radiant ceilings may, in fact, be a better choice.  Not too much more expensive and more responsive, still low temp, and not reducing your solar gain storage by much.  they do require flat ceilings though.

I won't take offense at your comments about designers "overthinking" systems.  I will say though that I have seen a fair number of high mass systems in tight envelopes fail to generate comfortable results because no one paid attention to the details.  Good design will typically justify its cost in better results.  Such as determining with MATH what heat source you should be using, instead of just guessing at random from a few comments on a web forum.  That one decision alone can pay for a design in either fuel savings or first cost savings, whichever made sense to go for for a particular home.

Also:  ICF doesn't mean efficient.  I've seen plenty of ICF and SIP homes that were more glass than ICF or SIP.  Home design is more than a wall construction method.


Rockport Mechanical

HVAC Design and Installation in beautiful Rockport Maine

Thank you for your imput. (post #207069, reply #6 of 7)

Thank you for your imput.