Thought experiment: Floor Heat under-slab insulation requirements

[JGinther]

I was recently informed by local government officials that if I am planning on installing a heated slab outside of a building, new energy code requirements mandate the use of foam 'boards' made of polystyrene to be installed on grade and then the concrete poured on top.

While I do not argue the energy efficiency in the case of building heat, where you are heating the slab to higher temperatures than the natural subsurface ground temperature, I think that the requirement in the case of a car wash where the slab is CONSTANTLY kept at just above freezing merits some actual thought.

My thoughts are that if the subsurface geothermal temperatures are around 40 degrees, that heat energy is moving up and then being sucked away from the colder atmospheric temperatures above the surface (Energy only moves from hot to cold). So that heat energy is slowly going into the slab, then leaving into the air. An underslab insulation board would simply slow down that process. So if the temperature of the heated slab is kept at 34 degrees, and the subsoil is naturally at a higher 40 degrees, there would be no point in installing the insulation board and it would actually work as a detriment. However, if you were heating the slab to 80 degrees, which is far above the earth temperature below, then you would be loosing heat to the earth also, and would then be assisted by the use of insulation to keep the heat where it is useful.

Am I missing something? Also curious if others who have poured on top of insulation have had structural problems with the slab thought to be attributable to the foam board.

Thanks for your thoughts!

 

[MGSMN]

"If" the soil stayed at 40 no additional heat would be needed and no snow/ice would form on the surface concrete. Here the frost line is ~36" so the moisture in the soil can freeze down to that depth. Not using insulation between the heat source and the soil will be very inefficient. Not only will you be heating the concrete you will be trying to heat the soil also. We use 2" foam for our in floor radiant heat using a boiler. I have not seen any structural strength issues using foam.

 

[mjwalsh]

Am I missing something? Also curious if others who have poured on top of insulation have had structural problems with the slab thought to be attributable to the foam board.

Thanks for your thoughts!

Here the frostline is 48". I wonder if MGSMN's "very inefficient" claim would only apply towards the end of a very cold winter???

We did not put foam below when we went over existing concrete. Doing so would have created an elevation problem in our specific situation. It would be nice if those expert- local code administrators would actually show some credible study on the actual "energy savings"! Maybe it is significant enough ... maybe not. In our dog wash we actually do use the actual boiler water ... up to 160° F ... there again we went over the older concrete though. We also have the actual boiler water circulating through a nifty very compact heat exchanger space heater with a built in fan we got from Zoro that we added last fall.

For our deicer snow melt system that my brother-in-law & me put in way back in 1980 ... purchased from Herm Deal ... we keep the glycol temperature closer to 100° F by improvising with control thermostat based equipment.

Hopefully, focussing on improved natural gas recovery in the oil fields will keep the price of it from spiking too much. Sorry ... I did not mean to get "borderline political" .... shame on me!

 

[Waxman]

I put down the blue foam board insulation myself at my wash. The pex tubing was then stapled to the foam. They told me it was to keep the heat moving up toward the concrete surface and keep the tubing from getting cold underneath from the earth. Made sense to me and my floor heat works great.

 

[JGinther]

"If" the soil stayed at 40 no additional heat would be needed and no snow/ice would form on the surface concrete. Here the frost line is ~36" so the moisture in the soil can freeze down to that depth. Not using insulation between the heat source and the soil will be very inefficient. Not only will you be heating the concrete you will be trying to heat the soil also. We use 2" foam for our in floor radiant heat using a boiler. I have not seen any structural strength issues using foam.

Thanks for the response! However, I'm troubled with the science of this logic... The air/atmosphere is what is cooling (removing heat from!) the soil from the top, and that is what causes the 'frost line'. The reason it's not freezing below that frost line is because the ground is heated geothermally, right!? The reason the surface freezes is because the air is so cold that it is absorbing the heat energy from the Earth's surface faster than the geothermal energy can give up... So temperature drops and ice forms. Sooo... If the temp of the slab is maintained at 34 degrees and the soil under it is 40 degrees, what makes the temperature below the slab be colder than that? Keep in mind, were not talking about a 'firing up the boiler when the floor is already frozen' event here. I'm speaking about the long run of maintaining a slab temp constantly at 35 degrees over the course of a winter.

If it was terribly inefficient to run without the insulation under the slab, that would mean soil under the slab would be colder than the slab itself. How would that be? After all, it's the air that is cold, and the air cannot make it below the heated slab which is 34 degrees.

I totally agree that if the slab were to be heated to 50 degrees, or 80 degrees to keep a building warm, then the temperature is too high (higher than that of the subsurface earth) and the energy would then be going down into the soil heating the earth. But that wouldn't be true at 34 degree slab temp. What part am i missing with this?

 

[Kramerwv]

Sou

Thanks for the response! However, I'm troubled with the science of this logic... The air/atmosphere is what is cooling (removing heat from!) the soil from the top, and that is what causes the 'frost line'. The reason it's not freezing below that frost line is because the ground is heated geothermally, right!? The reason the surface freezes is because the air is so cold that it is absorbing the heat energy from the Earth's surface faster than the geothermal energy can give up... So temperature drops and ice forms. Sooo... If the temp of the slab is maintained at 34 degrees and the soil under it is 40 degrees, what makes the temperature below the slab be colder than that? Keep in mind, were not talking about a 'firing up the boiler when the floor is already frozen' event here. I'm speaking about the long run of maintaining a slab temp constantly at 35 degrees over the course of a winter.

If it was terribly inefficient to run without the insulation under the slab, that would mean soil under the slab would be colder than the slab itself. How would that be? After all, it's the air that is cold, and the air cannot make it below the heated slab which is 34 degrees.

I totally agree that if the slab were to be heated to 50 degrees, or 80 degrees to keep a building warm, then the temperature is too high (higher than that of the subsurface earth) and the energy would then be going down into the soil heating the earth. But that wouldn't be true at 34 degree slab temp. What part am i missing with this?

Sounds like a good experiment for someone in Boulder or Fort Collins. Put those academics to work.

 

[JGinther]

Put those academics to work.

How!?

PS , I may have had success convincing the building department of my logic. They asked for a narrative explaining this logic to be supplemented with the resubmittal and they would consider it. Problem is, it's actually written into the code books.... So I guess I will see how it goes...

 

[MGSMN]

I think I understand what you're getting at. Take a driveway that is 10' x 20' and we want to keep it at 34 and our soil temp below the frost line is 40. To keep the soil between the frost line and the driveway at 40 we assume the earth is capable of transferring that sub soil heat up to the soil the driveway sits on. To assist this we would need an insulated frost wall around the perimeter of the driveway to the depth of the frost line to help reduce heat loss to the soil outside the driveway. Now most of the heat loss would be through the surface area of the driveway. Would trying to maintain the driveway at 34 degrees (via hydronic or electric) be viable without exceeding 40 degrees of input to not exceed the temp of the soil the driveway sits on? If the outside air temp was 34 and no wind, sure. What about at 24, 20 or 10 degree air temps and windy for days at a time? Unlikely, I think the heat loss through convection would be to great for the soil temp and external heat source to overcome. So now we have to increase the BTU to the driveway to maintain 34 degrees but that temperature rise will most likely have to exceed our soil limit of 40 degree to get enough BTU to do the job. At this point are we heating the driveway and the earth? Yes, the soil will act like a heat sink just as the driveway does via conduction. I'm sure there is a lot of math involved to do BTU and loss calculations but that's beyond my interest. If you have very few sub 34 degree days during the winter would it make sense to try to use the soil temp as a heat source? We use 2" foam under our heated areas and I've seen people use 2" foam down the sides of foundation walls also. The outside concrete is on a separate zone/thermostat. We use boilers as I've seen too many electric pads fail in my 32 years of washing and although electric heat may cost less to install it cost more to run. It takes less energy to heat a smaller mass so we heat only the concrete and not the soil beneath so we use insulation.

 

[JGinther]

Would trying to maintain the driveway at 34 degrees (via hydronic or electric) be viable without exceeding 40 degrees of input to not exceed the temp of the soil the driveway sits on? If the outside air temp was 34 and no wind, sure. What about at 24, 20 or 10 degree air temps and windy for days at a time? Unlikely, I think the heat loss through convection would be to great for the soil temp and external heat source to overcome. So now we have to increase the BTU to the driveway to maintain 34 degrees but that temperature rise will most likely have to exceed our soil limit of 40 degree to get enough BTU to do the job.

This is a good point. It brings up the method of determining when floor heat should run. I have seen many 'air stat' types which turn the floor heat on when the air is below freezing, and the returning fluid temperature is what is used to determine if the flame should fire or not. But another method not seen as often around here is to use a 'slab stat' to directly sense the temperature of the slab and use that as a zone control in the design layout. The latter would appear on the surface to be a more direct and efficient design to me because the alternative return fluid aquastat principle isn't directly indicative of the slab temperature itself. I would think that would lower the efficiency of the system quite a bit. I have seen return aquastat type systems turn on and run when the temp falls quickly outside, but the slab is still plenty warm to last a whole day before needing to turn on.

 

[mjwalsh]

I have seen return aquastat type systems turn on and run when the temp falls quickly outside, but the slab is still plenty warm to last a whole day before needing to turn on.

I have observed the same. I wonder if anybody has more than one deicer system & who had the foresight to put a earthstat below the concrete besides just having a comparison slabstat in the concrete on a non insulated slab. Then do the same with the another somehow ... but put a slabstat or earthstat below the insulation boards that were now used. Not sure if that would be totally conclusive but I am thinking it may help clarify how many btus are being lost without the insulation boards by downward migration.

It might also help clarify the potential ... especially early in a milder winter ... how much upward migration occurs. Like JGinther suggested those dormant btus rising would be lost with the insulation barrier on the 34° F system.

To understand the losses & gains from the downward positioned insulation ... the slabstat & earthstat would definitely be more useful than aquastats.