Snow melt slab insulation

Dan_32

My business is in the NE. All of our snowmelt installations for driveways, sidewalks and patio decks have included a minimum of 1", 2" preferred, of Extruded Polystyrene foam on the bottom and edges. I recently was listening to a podcast and a contractor that works in the NW stated he does not install any slab insulation on his snowmelt jobs. He felt that the direct slab contact with the ground was more benifical than isolating the slab from the ground with a layer of foam board. Can anyone comment on if the amount of ground heat that rises up from the earth aids in maintaining a higher slab temperature is quantitively more beneficial than having the energy directed upward, not downward, (where it does no good) to melt the snow with an insulated slab application? Thanks in advance for your insight.

Jamie Hall

Kind of depends on how deep — if at all — the ground freezes in the winter. If the ground freezes in the winter, there will be NO heat coming up to melt the snow… at least until the ground starts thawing in the spring!

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1845243#Comment_1845243

That is kind of like saying "What goes into a tee must come out of a tee." Everybody already knows this, but it still needs to be said for some folks to hear it!

GGross

it's not ground heat that rises, heat does not rise, hot air rises. Heat moves to cold, the rate of that heat transfer is partially dictated by the delta T. so while the ground temp of lets say 50f is warmer than the unheated slab temp of lets say 20f, when you put a heated slab into the equation the heat from the tubing approx 110f is greater than the 50f below and will also want to transfer to the ground, wasting your BTUs, the thermal break from the insulation helps reduce that unwanted heat transfer into the ground, and direct the BTUs to where you want it, the slab. So while the contractor in question might feel better about it, in reality he just wants to save a few bucks on install, cost his customers more money, and made up a story about the heat from the ground helping melt the snow to make himself feel better about it. This isn't even taking into account that generally the ground at the level of the tubing is not really 50f either its closer to air temp

ethicalpaul

https://forum.heatinghelp.com/discussion/comment/1845243#Comment_1845243

In the winter there is always heat moving from deep to surface, regardless of freeze state.

There could be 30F ice at the surface even with 0F air temps due to the heat moving from deeper (these numbers are totally fictional, just illustrating the point).

Yes it won't melt the snow, but it could make less of a delta for the heating loop to have to raise.

I don't see what the ground being frozen has to do with how much heat is moving from lower up to the surface.

I don't think this question is easy to answer…it does depend on a lot of variables but it's an interesting thought

Edit: After some more thought I do see why Jamie focused on frozen…the energy to get the phase change

Kaos

The issue is that this "warm" earth is many many feet down. The air around is cooling the soil at the same time, so the soil near the surface is near outdoor air temp. It simply doesn't work as proposed.

There was a similar argument with heated slabs in the old days and people built houses without insulation under the heated slab. The only thing they ended up with is very slow response and huge heating bills.

Do your costumers a favor and always insulate under any heated slab. Snow melt or basement, doesn't matter, it all needs insulation.

hot_rod

There were some studies done on this, maybe with the ASHRAE tech committees. I think some of those were at Robert Beans site, which is under construction now. It showed heat flow from slabs and concrete walls to the surrounding ground. I think the engineer, Dr.Kilkis at Heatway did some modeling years ago also.

The type of soil, moisture content, other variables make it a hard number to pin down exactly.

I did notice durning sunny mild winters the slab and ground would store some energy and melt for awhile. at first snow

The insulated slabs would collect snow instantly without that ground buffer connection from below.

Matt_67

I am not advocating against insulation - however one of my most embarrassing experiences happened at a new build with snowmelt. The GC provided insulation under the area of sidewalk with snowmelt but of course did not insulate under some adjacent walkways. The first fall we had an unusually cold day and night, the next morning just before sunrise a front came through and it got warm and humid. The snowmelt area slab iced over and became one of the most slippery surfaces I’ve ever been on. The adjacent areas were not slippery at all. Standing before the president of this company explaining why his snowmelt equipped concrete was so slippery wasn’t my best experience. We had a tekmar control on it and they decided to enable idle operation after that.

Kaos

What @Matt_67 is reffering to is night time radiative cooling, pretty much the same reason you get frost on a roof. In a climate that would need snow melt, this happens for maybe a couple of weeks out of the year on a clear cold night, the rest of the time a heated slab without insulation is wasting heat any time it runs.

Also without insulation you are heating up a lot more mass, so it will be WAY slower to come up to temp. Worst case is wet sandy soil where you first need to melt the frozen dirt bellow.

GroundUp

I got into a heated domestic dispute with an engineer over this about 10 years ago who insisted we install this snowmelt system in a helipad with no insulation because "heat rises". After days of back and forth, my final rebuttal was "if the ground were a constant 54* below this slab as you say, why are we installing a snowmelt system at all?" That finally made the connection, and we got insulation.

Kaos

The engineer was semi right. With a big enough area like that after you install the insulated slab on top, the ground near the center of the pad will be near seasonal mean temperature. The issue is you can't extract any of that heat as it will quickly cool down the same as the dirt outside the pad.

The nice part is the slab losses in the center are actually lower, so if you want to really make the most of your insulation, insulate more around the perimeter and can skimp at bit near the center. This only applies to large areas though.

Dan_32

Thank you all for your responces. I guess there are a lot of variables to take into consideration. Judging from all of your comments, I think I can come away with slab insulation in snow melt applications is the way to go. Thanks again for all your help. Dan Silvestri

Mark Eatherton

Interesting subject. Wouldn't it be great if we could have "switchable" insulation? Turn it off in the Fall and Spring and On during the Winter? I remember testing a BUNCH of different SIM slab insulation years ago on a long driveway we melted. Bubble/foil/bubble, Insultarp, EPS, XPS, and a fan fold hybrid. We buried sensor directly below the slab, and another set directly below the insulation. In fact I wrote an article about it that is out there on the interweb some place… Anyway, the XPS proved to be the best insulation. But of intersting note, the day I pulled the data loggers, as I was walking along the slab pulling sensors and data loggers, I had my IR thermometer with me, and as I was checking the slab and surrounding soil temperatures, I found that the slab was significantly cooler than the ground upon which it sat. The coolness was from NSR (clear night sky re-radiation from the previous night). I was pulling senors at liek 8:00AM. The warmer surround earth is what it is. Thermal mass momentum. It was a decomposing granite soil.

Regarding NSR, along the west coast, it is a real issue due to humidity from the ocean. Lots of cases of a hoar frost (AKA Rhine frost) that forms on the slabs during NSR. I remember speaking with a university that was having this problem, and they wanted a simple solution. The solution, although simple, is ex$pen$ive… Idle the slab and maintain 35 degrees F. You will still get moisture condensing due to dew point, but it won't freeze. So, maybe instead of a go, no go insulation proposal being randomly applied, it should take into consideration it's location and the potential for a higher than usual dew point, like along the west coast.

The ONLY way to find out what works best (to insulate or not insulate) would be to have 3 identical slabs. One with insulation, one without insulation and one without insulation BUT with Slab Idle control. Running all three will give the results, and determine where your money is best spent. I'm guessing with insulation will show lowest energy consumption. Maybe augment the insulation with calcium chloride to avoid frost formation during certain times of the year?

Maybe require the slab to be tied into the DHW preheat system to act as a non glazed solar collector (300% efficient electrical use with a WSHP connected) that could harvest more energy than is being used in the SIM system??? BTW, cooling the slab during the summer will lessen the chances of stress cracking due to thermal expansion. Another benefit of sucking the FREE heat off the slab. We have the technology…

GGross

I must just be in too cold of a climate to grasp some of this because around here 99% of the residential snowmelt idles below a set air temp because the people getting it installed do not ever want snow or ice on their driveway. I think if I told an installer not to insulate the slab because 2-3 days out of the year when the ground actually isn't frozen but its suddenly snowing, and the sun is out, they could melt snow for an hour or two with the grounds latent heat they would fire me. Spend a little extra during those rare cases where the ground could melt the slab for a short time, or spend more money heating the ground the entire melt season, it seems an easy choice for me, for the end user in my town it was never a question they want no chance of snow or ice 24/7.

jayvath

Jay Vath with tekmar. Like many design questions the answer depends on the local environment conditions. The tekmar facility entrance has a snow melt system fully insulated under the slab. In winter, our region has many temperature swings around the freezing point and the result is that we observe frequent frost events on the slab in the morning. The frost is extremely slippery and the snow melt system is manually started to prevent slips and falls. The tekmar snow and ice sensor is not able to detect frost due to the electrical heater being on when the sensor is dry. This heat prevents the moisture formation on the sensor that is necessary to trigger the automatic snow melt system.

My conclusion is that regions with frequent temperature swings around freezing would benefit from coupling the slab to the earth by not insulting under the center the slab. I recommend to always insulate the slab edge and perimeter to minimize heat losses. If the system is installed in a colder environment with fewer freeze/thaw cycles, then frost formation is less of a concern and fully insulating under the slab is beneficial.

Lance

Jay Vath, You got it! We often forget what we want vs what we need can differ in many ways. We say snow melt, not frost melt, or better yet moisture evaporation. We say certain types, under certain conditions and where do we get our knowledge, from experience and that differs between needs, weather, location, area and duty. I could care less for snowmelt if I was closed when it snowed. I have been designing a walk way which starts at grade, but elevates on a curve to a porch level. (Handicap access) So my duty is on demand. But not always planned. The path will be exposed on each side above grade. I considered hydronic vs electric mat. I abhor the hassles of fluid balance. Electric it is. As I only need to clear snow / ice right after most events, and only when handicap people are present, it has been a tossup on response vs maintenance. I have even considered ICF blocks for walls that will extend above the surface to accommodate support of slab and railings. So a 1" in 12" slope, concrete surface, insulated edges, but to insulate the center? Or it may not make a difference. Mid Maryland average stuff, except for the year the bay froze over and thousands of water meters in the ground froze. Frost levels in the shade went 6 feet down. Wet snow, wet ice, that evaporates has a cooling effect regardless of what's beneath it. Wind exacerbates it more. But this is only a problem if we constantly are melting it just below freezing, and it could be lower if we add salt. Would we? In an emergency we might.

Anyway I'll calculate the BTUH and use water sensors, but would another control be needed? Its seems an if this or that occurs I do this but not if something else is present or not.

DCContrarian

The soil under the slab is warmer than the air. When the slab is unheated, insulation under the slab shields it from the relative warmth of the soil and an insulated slab will be colder than an uninsulated one.

When a slab is heated, the insulation means more heat goes up and goes to melting snow rather than warming the soil below.

An insulated slab means there will be days when the snow melting needs to be on, when it wouldn't need to be on with an uninsulated one. But it also means that when the heat is on, it takes less to melt the same amount of snow.

jumper

https://forum.heatinghelp.com/discussion/comment/1845243#Comment_1845243

How low can you go? Trench perimeter below frostline or deeper. Insulate with vertical styrofoam and backfill. Some winters you won't need snowmelt at all. In the seventies there were papers published about such subjects. Vertical insulation can prevent frost heave. Horizontal is more complicated question.

hot_rod

If you can find a way to detail it, the slab edge insulation limits heat loss also. It may be exposed to ambient temperature, always.

jumper

https://forum.heatinghelp.com/discussion/comment/1847074#Comment_1847074

Absolutely. Highly unlikely danger to decks but if there's a window involved…..

Kaos

Cost prohibitive, but sounds like one way to deal with this is to put in two loops. One above the insulation and one bellow. During the shoulder season circulate fluid between the two, during cold heat the top only.

If you want to nerd out and don't mind a bit of controls complexity, you can pull a couple of BTU/sqft of geo energy and use the soil loop to pre-heat the snow melt return before it gets to the boiler.

Martial_7erran

I am BSEE, and I have no experience with underslab snow melters, but I think the idea of burning fossil fuel to heat water/glycol to circulate under pavement to melt snow is extremely wasteful and extravagant. At a minimum, an institution could bury PVC pipe loops in ground to use geothermal heat, and use it to store heat in the summer (to reduce cost of air conditioning in summer) and then pump that heat out of the ground to melt snow. Notwithstanding this, I asked Google Gemini 2.0 Learning to help me construct a python script to Model the heat flux and heat equations for heating under slab for melting snow. In the course of figuring which physical circumstances and values must be considered, it became apparent to me that: If the hydronic system must melt more than ___ inches of snow per day, then the latent heat of underlying soil will not help significantly to melt away that snow. Thus any heat dissipated downward (because of absence of underslab insulation) will be wasted (diverted away from melting the current snow). So, I asked Gemini to write up these conclusions as postulates.

Here are a series of informative postulates, based on the considerations in the Master Prompt and the physics of snowmelt systems:

Postulates on Snowmelt System Design and Insulation:

1. Snowfall Threshold for Geothermal Insignificance: If a snowmelt system is designed to melt more than approximately 1-2 inches of snow per hour (or accumulate more than 6-12 inches of snow in a typical storm), the latent heat contribution from the underlying soil will be negligible compared to the required melting capacity. In such scenarios, any heat dissipated downwards into the ground will be largely wasted energy, diverted from the primary task of melting snow at the surface. (Rationale: High snowfall rates demand rapid and high heat flux for melting, which geothermal heat cannot provide effectively. Hydronic system must be the primary heat source.)
2. Insulation Priority in High Snowfall Climates: In regions with average winter snowfalls exceeding [quantify based on climate data, e.g., 50 inches total snowfall per season], and/or frequent heavy snowfall events (e.g., storms depositing >6 inches at a time), under-slab insulation is highly recommended and likely to be economically beneficial. The energy saved by preventing downward heat loss will significantly outweigh any potential benefit from utilizing geothermal heat gain, especially during and immediately after snowfall events. (Rationale: High snowfall means the system will be operating frequently and at higher output. Insulation becomes crucial for efficiency.)
3. Limited Value of Geothermal Heat for Rapid Snow Removal: Geothermal heat flux is a relatively low-intensity, continuous heat source. It is insufficient to provide rapid snow removal during or immediately following snowfall, which is often the primary goal of a snowmelt system for safety and convenience. Relying on geothermal heat for snowmelt will result in slow response times and potentially inadequate snow clearing, especially in moderate to heavy snowfall events. (Rationale: Geothermal heat is too slow to be effective for rapid melting. Hydronic system is needed for timely performance.)
4. Frost and Ice Mitigation vs. Heavy Snowmelt: While geothermal heat might offer a marginal benefit in mitigating light frost or very thin ice layers over extended periods in shoulder seasons (fall/spring), its contribution to melting significant snow accumulations is minimal. Snowmelt systems are primarily designed for managing substantial snow loads, not just frost prevention. Optimizing for frost mitigation at the expense of snowmelt efficiency is generally a poor design trade-off in snowy climates. (Rationale: Frost mitigation is a secondary concern compared to snowmelt. Don't compromise snowmelt performance for marginal frost benefits.)
5. Insulation as a Baseline, Geothermal as a Marginal Consideration: For any snowmelt system design in a climate with measurable snowfall, under-slab insulation should be considered the baseline best practice. The potential for geothermal heat gain should only be considered as a marginal factor, and its benefits, if any, must be carefully weighed against the guaranteed energy savings provided by insulation. In most practical scenarios, the consistent energy savings from insulation will dominate over the unreliable and limited contribution of geothermal heat. (Rationale: Insulation is consistently beneficial, geothermal is uncertain and limited. Prioritize the reliable benefit.)
6. Edge Insulation is Always Critical: Regardless of the decision on under-slab insulation, edge insulation around the perimeter of the snowmelt slab is always essential. Heat loss to the ambient air at the slab edges is a significant source of energy waste, and edge insulation provides a universally beneficial measure to improve system efficiency in all climates and snowfall conditions. (Rationale: Edge losses are always present and significant. Edge insulation is a no-brainer.)
7. Location-Specific Analysis is Key, but Insulation Remains Robust: While the optimal design may vary slightly based on specific location and climate details (e.g., soil type, ground temperature, wind exposure), the fundamental principle remains: in climates requiring snowmelt, insulation generally leads to net energy savings and improved system performance. Detailed location-specific analysis should primarily focus on optimizing insulation levels and snowmelt control strategies, rather than debating the basic necessity of insulation itself. (Rationale: Location matters for optimization, not for the fundamental need for insulation in snowmelt applications.)
8. Snowfall Intensity vs. Total Seasonal Snowfall: Consider both the total seasonal snowfall and the typical snowfall intensity (snowfall rate, amount per storm) when evaluating the need for insulation. Regions with lower total snowfall but frequent, intense snowstorms might benefit even more from insulation than regions with higher total snowfall spread out over many light events. Intense storms necessitate high hydronic output, making insulation more valuable. (Rationale: Intensity of snowfall events influences the immediate heat demand and thus the value of insulation.)

These postulates aim to translate the complex heat transfer considerations into practical design guidelines, emphasizing the importance of insulation for efficient and effective snowmelt systems, especially in climates with significant snowfall. They directly address the original forum question by highlighting the limited practical benefit of relying on geothermal heat for snowmelt compared to the tangible advantages of insulation.