Have been learning about super insulated homes for a while and always have new questions. For the small year round cabin/home I am planning I find that insulated raft foundations seem like one of the most efficient and easy foundation methods(eg. isoquick,warm form). I like radiant heat but would like to learn more and get more feedback on air in concrete systems like Legalett sells. It seems like an interesting alternative to liquid systems but I find little information about similar sysytems. Any suggestions? www.legalett.ca

Honestly, the only in-floor heating I'm familiar with are either liquid or electric systems.

I am not sure what the advantages are over a traditional radiant system. I’d say there are certainly downsides- recovery time is 2-3 degrees per day. That is probably deg C. Based on an 8” slab. At first glance I do not see the appeal to this.

In my opinion, you should put insulation above the concrete. You should then put radiant heating above the subfloor using something like warmboard or thermalboard. This is especially intelligent given that this is a cabin and not permanent home. The warmup times of heating up that slab is huge. You will arrive at your cabin and need to wait 12 hours before you feel the heat as the slab needs to be heated up. Then, you leave the cabin, and that slab will continue heating the cabin 6-8 hours after you left (which means you stored heat in it that you did not need to causing a waste of energy).

IMO, this is why many people see an increase in heating costs when moving to radiant floor heating. That is why IMO it is better to isolate the concrete by putting insulation on top of the concrete. Then radiant heating above the subfloor. This reduces thermal mass, which makes your heating system very responsive. With this system, the cabin will quickly heat up when you need it to and quickly cool down when you don't need it. Saving energy.

Actually, most people who live in reasonably insulated and sealed homes typically see a significant decrease in heating cost when moving to HR floor heating. The building performance remains unchanged, but they typically set their thermostat to a lower temperature because HR floor heating provides the same comfort level at a lower set point. A lower set point translates to lower heat transfer delta T which translates to lower heat loss and associated heating cost. It is true that if you have a building that isn’t insulated or sealed well such as a cabin, you are way better off using a responsive HR emitter such as Warmboard and NOT use a slow responding HR emitter such as a concrete slab. Concrete slab emitters and thermal mass in general are best put to significant advantage in well-insulated and well-sealed buildings that are insensitive to outdoor conditions and which consequently don’t need a responsive HVAC system in order to maintain a uniform comfort level. But none of this is related to the subject of this thread, so not sure why it is was brought up and discussed here.

You can build with a sealed, conditioned crawlspace and then moderately over-heat the crawlspace. No concrete slab or ducts needed. The floor won't be "toasty" warm, but it will be more comfortable than an unheated floor.

Posted By jonr on 10 Nov 2017 02:58 PM
You can build with a sealed, conditioned crawlspace and then moderately over-heat the crawlspace. No concrete slab or ducts needed. The floor won't be "toasty" warm, but it will be more comfortable than an unheated floor.

This seems ridiculously wasteful though.  Open air is a terrible conductor of heat (try putting your hand in an oven (not ON it) at 400 degrees.  Notice how you don't instantly burn?

If you want to heat a floor, use a transfer medium OTHER than open air.  Either hydronic (if going full-floor) or electric (if only doing a few rooms).

As to which is best, need will dictate the situation.

In a retrofit home, you're probably better off with a solution like Warmboard.

In a new home on slab floors, you're probably better off with in-slab.  I generally don't care for wood-over-concrete solutions due to moisture wicking issues.  Even only a well-built, well-planned, well executed concrete install.

If doing a trussed/joist flooring system, Warmboard is probably a better option.

I fully agree with you Dilettante. I would only add that if you are doing new multi-level construction with trussed/joist floor structure, it might be worth considering working the structure a little to allow having a thin gypcrete slab and perhaps reduce the overall cost of getting HR heating. Warmboard is great, but it isn’t cheap.

Posted By sailawayrb on 09 Nov 2017 09:42 PM
Actually, most people who live in reasonably insulated and sealed homes typically see a significant decrease in heating cost when moving to HR floor heating. The building performance remains unchanged, but they typically set their thermostat to a lower temperature because HR floor heating provides the same comfort level at a lower set point. ..........

Can you provide a link to an article on-line to support this claim?

I've heard this claim many times over, just never knew it's source.

One study in Canada says there really is no difference to speak of.

Canada - Study

I picked that link above up from this article below - see the Con's and Myth's section - middle of page.

In-Floor Heating Worth It?

To the original question/topic.

Large churches often use below slab air supplies in floor vents below windows and at the perimeter, the return air is whole house plenum often behind the altar.

Sometimes mold can grow in the below grade ducts be it warm air supporting growth or A/C condensation, not much of an issue if always working and on.  These systems are used because they are quiet compared to roof top unit (RTU) force air systems. 

If this underground duct-work finds it's self unused for long with no air flowing though it, moisture and mold can build up.

 Low moisture air flowing though these ducts is our friend, abandonment and neglect is not.

Not suited for part-time occupancy.

My comment comes from personal experience living with HR heated floors for many years and from comments from many other people we have designed or built HR heated floors who have lived with them for many years. There is also a common sense scientific element to this as well. The closer your body is to a radiant heat source, the warmer and more comfortable you feel. I have read about this too, but I tend to place more weight on facts and data that I personally gather and personally verify...especially these days where lies and opinions trump facts and data...

I am also interested in the Legalett heated floor system in a FPSF for a small 20x40 home, to be well insulated. In Minnesota.
It is a closed loop, like hydronic, but circulates heated air not water.
Does anyone have experience with this brand, or other air-based heating systems?

Water has about 3200 times the volumetric heat capacity of air. So you have to move a lot of air compared to a small amount of water to heat with air. Moving air also creates air pressure differentials in the building which increases building heat loss because of the associated increased air infiltration. Hydronic systems don’t affect building air pressure.

Thanks for your reply about volumetric heat capacity of water. I’ve wondered about how effective air heat may be, and I’m delighted to learn more about this kind of a system. I’m not sure why it has anything to do with “air pressure differentials” or “infiltration” though. This air heat system is Closed. It circulates warm air in a closed loop. It is not blowing air through the floor and then out registers.

I don't have any first hand experience with this system. I assume it has a massive blower that is located in the building. So it is getting air and pressurizing duct work. I have yet to see duct work that doesn't leak. Mainly, I can't imagine why you would want to use inefficient air to convey heat when you can use water.

I don’t want to use something inefficient. I want to learn about different ways of doing things and make the best decision for my circumstances.

I made my own conversion on my house 17years ago --routed the chip board floors --lined with foil ,fitted pipes then covered with 8mm ply to put strength back infloor -insulated under the suspended floor
floor can change temp in under 1 hour --so was ideal solution for my climate where one day can be 10-15degrees different than next day.
these days as you have been suggested you can buy simple insulation panels with pipe runs already cut out --dead simple and very responsive --so yes insulate above concrete raft -If you were an infirm person who never moves out of the house then maybe concrete slab with pipes in it to get big thermal store so once its warm very little costs to keep it there --both ideal for air source heat pumps or if you get lot of clear days in winter solar thermal --very popular in the alps where lots of clear bright days in winter and coupled with large pir insulated storage water tank -usually made at gable end of house full height to get good thermal separation of hot and cold water with solar input coil at bottom--only running costs is a domestic central heating pump --which could be run of PV if out in the woods?

Yes that system does work well. My daughter built a photography studio used the Legalett system 30x40 footprint with 12’ sidewalls. Consists of 2 layers of 3” foam prefabricated exterior curbs and protective edge of metal on top edge. Monolithic design 8” of concrete with air tubes consisting of 4” pvc in and out of a central heating exchange totally enclosed with the heating boiler being the domestic water heater. That building has 8 loops in and out and uses a small fan and radiator to transfer the heat. Cost for the total system less concrete was about 25,000. It also has to be installed by someone certified by the company otherwise they will not warranty the installation or any part of it. It’s expensive but it works as it is in the Rochester Minnesota area and is going on winter 2 this season. I personally helped the certified guy with the installation and my daughter loves the warm floor in winter with young infants and baby’s during those sessions.