Floor heat without under slab insulation

[2drx4]

Building a 40x40 and code requirements mean the perimeter foundation is ICF for 4' below grade on top of a footer.

Can I skip insulating under the slab despite doing in floor heat?

I know some of the winter greenhouses here are being built with a "heat battery" in the floor by filling the foundation area with sand and dumping heat into it starting in the fall, and they've found it helps a lot with regulating the temperature through the winter. Basically building a massive amount of thermal mass. Will that same concept work in this case, or is it just asking for massive inefficiency?

Northern BC location, typical to see -30 to -40ish, but typically it's closer to freezing (no units of temperature because who cares). I plan to have a large forced air unit in one corner to blast after the doors have been opened or if something needs to be dried out, but I prefer the quietness of the in floor and it keeping the slab comfortable otherwise.

 

[PCustoms]

Do you want to heat the ground or your building?

I suspect your building. Insulate and put a nice floor, that will be plenty of thermal mass

 

[wssix99]

Can I skip insulating under the slab despite doing in floor heat?

Yes, but you should do something. This article talks about insulation options. (Instead of insulating the entire slab, you can insulate parts of the perimeter and/or the subgrade around the building.) https://framebuildingnews.com/below-grade-insulation-part-1/

If your slab is within the ICF foundation walls, you may be in situation "b" but the technical references in the article would help one dive into the heat loss and payoff of adding more insulation.

 

[Fav Onefour]

One advantage to forgoing under slab would be summer temps in your building.
I redid my garage shop floor ten years ago. I installed 2" under slab insulation and hydronic radiant heating. It had previously been a regular slab on grade and heat was a simple wall unit.
The space is super nice in winter with warm dry floors. In summer the floor doesn't conduct ground temps into the space. It is warmer in summer when we get long spells of heat. The slab heats up and doesn't cool down as quick.

 

[kabinenroller]

Under slab insulation is a must, perimeter insulation is not overkill. I have a full foundation that is insulated on the outside and inside, the floor has a vapor barrier and insulation under it. My walls are 2x6 with full insulation, the area above the 9’ ceiling has blown in insulation. 2,400 sq ft. Extremely economical to heat, I have an LP wall mounted boiler.
skimping on insulation now will cost you in the future.

 

[PoorUB]

Can I skip insulating under the slab despite doing in floor heat?

Yes you can, but insulate the perimeter well, and I would still put the tubing in the concrete.

I have seen a couple instances where the sand under the slab dries out and doesn't transfer heat. One shop the owner drilled 1/4" holes in the floor in a 4'x4' grid pattern and injects water under the floor to get the sand wet, otherwise his floor heat doesn't work.

 

[2drx4]

I'm just skimming the responses and maybe "ICF" is not a universal abbreviation. Insulated concrete form. They're foam forms that provide R22.5, so there is R22.5 insulation to 4' below grade.

 

[2drx4]

Yes, but you should do something. This article talks about insulation options. (Instead of insulating the entire slab, you can insulate parts of the perimeter and/or the subgrade around the building.) https://framebuildingnews.com/below-grade-insulation-part-1/

If your slab is within the ICF foundation walls, you may be in situation "b" but the technical references in the article would help one dive into the heat loss and payoff of adding more insulation.

I believe their study predates ICFs. I'd functionally be B, if doing what I had described. It is typical here to have the slab within the ICFs, I don't know if they tie it in to them or if it floats, regardless there is continuous foam insulation up to the framing and down to the footing. Reading through it and thinking about it more, it would make more sense to do a 2' leg out around at the minimum, as that wouldn't require any more excavating.

Also I think it's 5' of ICF, my drawing shows 4' with the note of to adjust to code requirement, I believe 5' is requirement here.

 

[2drx4]

Yes you can, but insulate the perimeter well, and I would still put the tubing in the concrete.

I have seen a couple instances where the sand under the slab dries out and doesn't transfer heat. One shop the owner drilled 1/4" holes in the floor in a 4'x4' grid pattern and injects water under the floor to get the sand wet, otherwise his floor heat doesn't work.

R22.5 for the perimeter as drawn, as that's what the ICFs provide.

I was planning to put the pex in the concrete as you normally would. It's more of a question of if it makes sense to just omit the foam under the slab, as I don't really see it making a huge difference to where the heat ends up eventually. I know the greenhouse guys wind up putting it deeper, 4' or so, but they also aren't typically pouring a slab across the whole building so you'd want it to be deeper, and they're specifically trying to store heat even if it might not be as efficient since the heat is free in the spring/summer/fall.

Interesting that the sand drying would be an issue, but I could see the greenhouse guys wouldn't run into that with high humidity and no vapour barrier or slab.

 

[2drx4]

One advantage to forgoing under slab would be summer temps in your building.
I redid my garage shop floor ten years ago. I installed 2" under slab insulation and hydronic radiant heating. It had previously been a regular slab on grade and heat was a simple wall unit.
The space is super nice in winter with warm dry floors. In summer the floor doesn't conduct ground temps into the space. It is warmer in summer when we get long spells of heat. The slab heats up and doesn't cool down as quick.

Summer temperature isn't really an huge issue. It just doesn't get hot enough for long enough to matter as long as the building is well insulated from the solar gain. The last shop did have insulation under the slab, I never finished plumbing the floor heat as I wound up burning waste fuel to heat it with a Toyotomi furnace, but it did stay very cold in the summer. It was also shaded, minimal windows, light coloured siding and door, so I can't say what made the difference.

 

[lovetap]

What would be the reason to forgo the underslab insulation?

Personally I would not, and did not, skip insulating the slab. If you end up struggling to heat the unit or are hemorrhaging heat, do you want to have it on your mind that maybe you're just heating the ground? One thing you really can't fix after the fact.

2" foam and vapor barrier. For what's it worth I also I have an ICF perimeter.

We just had a few weeks of almost constant -30 down to -50F and the little radiant floor heat kept the garage toasty and comfortable.

 

[jmdirk]

Sounds like you're fairly convinced on the way you want to go. Not sure anyone is going here is to convince you otherwise.

The apron insulation (horizontal-ish like in (a) in the pictures above are primarily to keep frost from getting in under a slab on grade or shallow foundation in frost prone areas. Perimeter insulation provided by your ICF are good, but will not fully compensate for not insulating the underside of your slab.

Think of it this way. You're going to put a certain amount of heat into the slab. That's going to radiate out from the slab into your shop, into the ground and out the sides. You have ICF, so let's assume that's taken care of. How quickly it radiates into your shop versus the ground is based on the thermal conductivity (Lambda - how easily heat can radiate through a medium - directly influences R value)) of air vs the soil. Air is typically about 0.026 W/mK (or about an R value of about 1). Soils can be anywhere from 0.2 to 2 W/mK (r value of 0.1 - 0.01). Anywhere from 10 to 100 times higher than air. Which means the heat is going to move way more easily into the soil than it will into your shop.

Spending a couple grand to insulate below the slab is going to save you a ton in heating costs, plus you won't need nearly as big of a heater to maintain a comfortable temp in your shop since you won't be losing so much to the ground.

You're saying the greenhouse guys put their pex 4 FEET deep. Sounds to me more like they are planting directly in the soil/sand that they're using for the battery. In which case they are more interested in having warm soil for the plants rather than the ambient temperature in greenhouse itself.

 

[PCustoms]

Sounds like you're fairly convinced on the way you want to go. Not sure anyone is going here is to convince you otherwise.

I'm getting the exact same vibe from OP.

Might as well close the thread, his mind is made up

 

[Rusted Nut]

Not sure what code you are under, but IRC requires continuous insulation under heated slabs. What are you saving by not installing insulation, a couple grand at most?

 

[The Metric System]

I know some of the winter greenhouses here are being built with a "heat battery" in the floor by filling the foundation area with sand and dumping heat into it starting in the fall, and they've found it helps a lot with regulating the temperature through the winter. Basically building a massive amount of thermal mass. Will that same concept work in this case, or is it just asking for massive inefficiency?

I would expect the result to be much less efficient than an insulated slab, with no offsetting practical improvement in building temperature.

The slab inside a typical garage already provides a very large thermal mass on the order of many tons, I would not be inclined to add more unless I had math indicating that it was going to be necessary/beneficial.

In the case you describe I would absolutely insulate the slab.

Why would you want to pay to heat a large object you don't care about?

 

[kbeefy]

Northern BC location, typical to see -30 to -40ish, but typically it's closer to freezing (no units of temperature because who cares).

At -40 it doesn't matter!

 

[duneslider]

Just remember that the earth likes to be roughly 50 deg and is a much bigger heat sink than anything you are building. The earth will help cool that slab summer and winter unless you are trying to maintain 50-60. If you are keeping it warmer than 50-60 the earth has the ability to **** it all away. Heat naturally wants to move from hot to cold. If the air temp is above 60 then the heat will want to go down and not up. If I lived in northern BC I would be more concerned about keeping warm in the winter than I am here in Utah where my unheated garage never drops below about 45deg with no ceiling insulation.

 

[finn]

Just remember that the earth likes to be roughly 50 deg and is a much bigger heat sink than anything you are building. The earth will help cool that slab summer and winter unless you are trying to maintain 50-60. If you are keeping it warmer than 50-60 the earth has the ability to **** it all away. Heat naturally wants to move from hot to cold. If the air temp is above 60 then the heat will want to go down and not up. If I lived in northern BC I would be more concerned about keeping warm in the winter than I am here in Utah where my unheated garage never drops below about 45deg with no ceiling insulation.

This, exactly.

If you want to heat the shop, add the floor insulation.

If you only want to keep it above freezing, it won’t mater as much.

Then, again, why take a chance?

 

[Youngandfree]

Saving a little by skipping the insulation will cost you more down the road.

 

[Youngandfree]

R22.5 for the perimeter as drawn, as that's what the ICFs provide.

I was planning to put the pex in the concrete as you normally would. It's more of a question of if it makes sense to just omit the foam under the slab, as I don't really see it making a huge difference to where the heat ends up eventually. I know the greenhouse guys wind up putting it deeper, 4' or so, but they also aren't typically pouring a slab across the whole building so you'd want it to be deeper, and they're specifically trying to store heat even if it might not be as efficient since the heat is free in the spring/summer/fall.

Interesting that the sand drying would be an issue, but I could see the greenhouse guys wouldn't run into that with high humidity and no vapour barrier or slab.

Apples to oranges. Don't skip it

 

[Dagny]

Where you live I would insulate it. though I think with 50 dollar a barrel oil the foam should not be 50 dollars a sheet.

 

[Youngandfree]

Where you live I would insulate it. though I think with 50 dollar a barrel oil the foam should not be 50 dollars a sheet.

It would be nice if pricing was dependant on oil prices.

 

[cannuck]

I think most responders are missing the point of OP's question. It is not about the cost of insulation it is about where the thermal break below the slab should take place. The 4' verticals are there to prevent frost heaving under the footings, and these get augmented by a horizontal near surface to push the local soil to underpad soil thermal break away from the footing. IMHO ICF is NOT the best material as it is styrene foam and will absorb water - I would go PU or polyisocyanate board as not only far better insulation value but should last a lot better. To address his question appropriately think like a cold weather guy and imagine how the "frost line" is determined. In his area it is obviously 4 feet - so with local winter air temps, at 4' down you are not going below 0 deg C/freezing. The soil is not only a huge potential heat sink, it is also not a great conductor. The question is: is it better to let the heat from the slab warm the area of soil beneath it or cut off that flow with an underslab heat brake. IMHO, since the soil beneath is already isolated to protect from frost heave damage so I would tend to let the slab keep it warm (i.e. no insulation). It will definitely cost some energy $$ but is safest for foundation. I having a warm floor and lower energy bill is more important to OP then back to insulation under slab.

BTW: the really serious greenhouse guys put insulation at bottom of pit under slab with verticals pushed out 4' and capped with insulation board backilled up side of beam above footing. They use soil (sand if cheap locally) with a LOT of ABS pipe/tubing buried beneath connected to solar collectors on roof or in yard. A really good installation provides most of the heat needed all winter. There is even a whole community in IIRC Norway built like this with one huge shared heat sink. I have a friend who builds single dwelling thermal reservoirs for domestic space heating from intermitent electric (i.e. solar and/or wind) sources in remote areas. You would be surprise how little (dry) sand it takes to heat a house for a day or two.

 

[PCustoms]

I think most responders are missing the point of OP's question.

Yeah, we're totally missing it:

Can I skip insulating under the slab despite doing in floor heat?

Put the slab insulation directly below.....the SLAB

 

[Dagny]

I would do A and D . Insulation slows heat flow it don't stop it.

 

[andyvh1959]

Under my 24x28 slab I have the D insulation; 2" pink foamboard under the entire slab and 2" pink foamboard outside of the two rows of 8" block and down to the slab. I put heat tubes onto the foam before the pour. Now I only have to install the boiler for floor heat.

 

[Youngandfree]

I think most responders are missing the point of OP's question. It is not about the cost of insulation it is about where the thermal break below the slab should take place. The 4' verticals are there to prevent frost heaving under the footings, and these get augmented by a horizontal near surface to push the local soil to underpad soil thermal break away from the footing. IMHO ICF is NOT the best material as it is styrene foam and will absorb water - I would go PU or polyisocyanate board as not only far better insulation value but should last a lot better. To address his question appropriately think like a cold weather guy and imagine how the "frost line" is determined. In his area it is obviously 4 feet - so with local winter air temps, at 4' down you are not going below 0 deg C/freezing. The soil is not only a huge potential heat sink, it is also not a great conductor. The question is: is it better to let the heat from the slab warm the area of soil beneath it or cut off that flow with an underslab heat brake. IMHO, since the soil beneath is already isolated to protect from frost heave damage so I would tend to let the slab keep it warm (i.e. no insulation). It will definitely cost some energy $$ but is safest for foundation. I having a warm floor and lower energy bill is more important to OP then back to insulation under slab.

BTW: the really serious greenhouse guys put insulation at bottom of pit under slab with verticals pushed out 4' and capped with insulation board backilled up side of beam above footing. They use soil (sand if cheap locally) with a LOT of ABS pipe/tubing buried beneath connected to solar collectors on roof or in yard. A really good installation provides most of the heat needed all winter. There is even a whole community in IIRC Norway built like this with one huge shared heat sink. I have a friend who builds single dwelling thermal reservoirs for domestic space heating from intermitent electric (i.e. solar and/or wind) sources in remote areas. You would be surprise how little (dry) sand it takes to heat a house for a day or two.

You misread his post. He asked if he can get by SKIPPING the insulation. He didn't ask about where HE should place it. His diagram shows his plan would have the insulation directly below the slab.

 

[2drx4]

Just remember that the earth likes to be roughly 50 deg and is a much bigger heat sink than anything you are building. The earth will help cool that slab summer and winter unless you are trying to maintain 50-60. If you are keeping it warmer than 50-60 the earth has the ability to **** it all away. Heat naturally wants to move from hot to cold. If the air temp is above 60 then the heat will want to go down and not up. If I lived in northern BC I would be more concerned about keeping warm in the winter than I am here in Utah where my unheated garage never drops below about 45deg with no ceiling insulation.

While everyone is arguing about what I meant and if we should lock the thread, I think you wound up getting to my point by accident.

I don't heat the shop to a very high temperature. 15 degrees (60) is normally the maximum, if I was painting I would temporarily bump it above that using the forced air, but I run hot naturally and if I'm wearing coveralls I prefer it to be cold. My assumption at that point is the delta between the shop and the earth at about 5' is not very much, so you're starting to split hairs on how much heat is moving. I'm totally willing to be wrong on that, hence why I commented that doing the "L" horizontal insulation to the footer would make sense versus nothing but the ICFs. But I'm questioning just how much heat is going to soak into the ground and not be recovered versus saturating the mass under the slab and more or less staying there to be slowly released back into the shop.

Most of the slab heated shops I've dealt with have issues with the temperature not regulating as the thermostat that is controlling the heat is sensing the air temperature and not the slab temperature, and will wind up firing the boiler at an extreme when it is a cold snap which results in the slab just being way too hot for when the cold has passed. Now, I don't know if those are insulated slabs or not (again, code requires the perimeter foundation to be insulated, but that is a change from only a decade ago), so maybe their issue is not what I'm perceiving it to be and the slabs aren't insulated, and that is causing the issue.

I guess what I am also not considering is that the glycol will be 40 degrees (105) and that would be the number for the delta to ground temperature, versus the 15 degrees (60) that would be my normal maximum shop temperature. In which case it then means you need insulation somewhere under the slab. Maybe? I have to think about if it works that way or not.

 

[2drx4]

You misread his post. He asked if he can get by SKIPPING the insulation. He didn't ask about where HE should place it. His diagram shows his plan would have the insulation directly below the slab.

What diagram? I didn't post one.

 

[2drx4]

I would do A and D . Insulation slows heat flow it don't stop it.

Which may make sense. To throw 2" insulation 2' around the outsider perimeter of the footings isn't a huge cost ($1500 CAD retail) vs the base cost of $6800 (retail) to do under the slab only. Labour cost is effectively zero since it's already going to be excavated.

 

[2drx4]

I think most responders are missing the point of OP's question. It is not about the cost of insulation it is about where the thermal break below the slab should take place. The 4' verticals are there to prevent frost heaving under the footings, and these get augmented by a horizontal near surface to push the local soil to underpad soil thermal break away from the footing. IMHO ICF is NOT the best material as it is styrene foam and will absorb water - I would go PU or polyisocyanate board as not only far better insulation value but should last a lot better. To address his question appropriately think like a cold weather guy and imagine how the "frost line" is determined. In his area it is obviously 4 feet - so with local winter air temps, at 4' down you are not going below 0 deg C/freezing. The soil is not only a huge potential heat sink, it is also not a great conductor. The question is: is it better to let the heat from the slab warm the area of soil beneath it or cut off that flow with an underslab heat brake. IMHO, since the soil beneath is already isolated to protect from frost heave damage so I would tend to let the slab keep it warm (i.e. no insulation). It will definitely cost some energy $$ but is safest for foundation. I having a warm floor and lower energy bill is more important to OP then back to insulation under slab.

BTW: the really serious greenhouse guys put insulation at bottom of pit under slab with verticals pushed out 4' and capped with insulation board backilled up side of beam above footing. They use soil (sand if cheap locally) with a LOT of ABS pipe/tubing buried beneath connected to solar collectors on roof or in yard. A really good installation provides most of the heat needed all winter. There is even a whole community in IIRC Norway built like this with one huge shared heat sink. I have a friend who builds single dwelling thermal reservoirs for domestic space heating from intermitent electric (i.e. solar and/or wind) sources in remote areas. You would be surprise how little (dry) sand it takes to heat a house for a day or two.

I should probably have responded to you first.

ICFs absorbing water is not an issue I've heard of. They are the defacto standard here now, as the code requirement is R20 for the foundation walls, so by the time you form and pour and then put a foam insulation to meet the R20 requirement around it you're well above the costs of ICF. If it is an issue, it's very curious, as almost all new residential construction here is using them. My house is ICF and it is the most uncomfortably hot basement I have ever encountered, although that probably could be somewhat mitigated with adjusting airflows from the furnace. I don't use the basement and my wife likes it hot, so I've not tried to mess with that.

I had not considered if it is safer for the foundation to omit the foam under the slab. This area is all clay on the surface, but I'm guessing we will hit glacial till around 3-4', but I don't know, I have not done any foundation work on this property or dug any holes. The last property (on the other side of town) was mostly sand/gravel on top of glacial till, so I did not worry about the foundation being damaged by frost. If we do not get out of the clay by the time we are at footer depth, then it certain is much more of a concern.

I think the heat cost question to me still is how much heat are you potentially losing, given the loss is constrained significantly by the R22.5 of the ICFs. The cost to heat up a large chunk of thermal mass, even if it is going to be 250~ cubic yards of material, doesn't seem to me like it would be that relevant as I would be doing it in early fall when the gas rates are lower, and then I'd get some of it back as a gradual tapering of heat in late April/May when I shut the boiler off. Now, I could be totally wrong on that too.

 

[2drx4]

Sounds like you're fairly convinced on the way you want to go. Not sure anyone is going here is to convince you otherwise.

The apron insulation (horizontal-ish like in (a) in the pictures above are primarily to keep frost from getting in under a slab on grade or shallow foundation in frost prone areas. Perimeter insulation provided by your ICF are good, but will not fully compensate for not insulating the underside of your slab.

Think of it this way. You're going to put a certain amount of heat into the slab. That's going to radiate out from the slab into your shop, into the ground and out the sides. You have ICF, so let's assume that's taken care of. How quickly it radiates into your shop versus the ground is based on the thermal conductivity (Lambda - how easily heat can radiate through a medium - directly influences R value)) of air vs the soil. Air is typically about 0.026 W/mK (or about an R value of about 1). Soils can be anywhere from 0.2 to 2 W/mK (r value of 0.1 - 0.01). Anywhere from 10 to 100 times higher than air. Which means the heat is going to move way more easily into the soil than it will into your shop.

Spending a couple grand to insulate below the slab is going to save you a ton in heating costs, plus you won't need nearly as big of a heater to maintain a comfortable temp in your shop since you won't be losing so much to the ground.

You're saying the greenhouse guys put their pex 4 FEET deep. Sounds to me more like they are planting directly in the soil/sand that they're using for the battery. In which case they are more interested in having warm soil for the plants rather than the ambient temperature in greenhouse itself.

The greenhouse guys are just dumping heat into the ground as they are storing it from when it was hot out/they had more than needed solar gain. There is multiple ways to do it, cannuck mentions a similar way using air, but basically the greenhouse and whatever collector system they have will get really hot in the summer, they dump that heat into the ground, and that heat can be slowly released later. No, they're not specifically directly planting into it, they will often have raised beds, tiers, and hanging planters in them as well; the issue is trying to keep the air temperature suitable despite having such poor insulation from all the glass/plastic transparency. The air temperature is key, as they lose any solar gain once the sun is down, and the plants will either enter dormancy or just straight up die from the overnight drop without some means to add energy back into the air. Ground temperature does not swing as fast. Even without adding any sort of air tubes, or having the system recirculate glycol from the ground to a radiator or similar, they will get enough heat radiating off the ground to keep the air temperature suitable overnight. Obviously there is a lot more design and optimization that they may or may not do, but the goal is to allow for the greenhouse to not need active heating (beyond whatever the solar collectors/solar gain provide) until it gets to about -30~ (or whatever, pick a number, but -30 is about the lower limit of what seems to be feasible).

Disclaimer, I don't have one of these greenhouses. They're somewhat expensive to build and require a clear horizon that faces the south. So what I have to say about them is based entirely on my casual reading and having seen a few of them.

 

[cannuck]

I should probably have responded to you first.

ICFs absorbing water is not an issue I've heard of. They are the defacto standard here now, as the code requirement is R20 for the foundation walls, so by the time you form and pour and then put a foam insulation to meet the R20 requirement around it you're well above the costs of ICF. If it is an issue, it's very curious, as almost all new residential construction here is using them. My house is ICF and it is the most uncomfortably hot basement I have ever encountered, although that probably could be somewhat mitigated with adjusting airflows from the furnace. I don't use the basement and my wife likes it hot, so I've not tried to mess with that.

I had not considered if it is safer for the foundation to omit the foam under the slab. This area is all clay on the surface, but I'm guessing we will hit glacial till around 3-4', but I don't know, I have not done any foundation work on this property or dug any holes. The last property (on the other side of town) was mostly sand/gravel on top of glacial till, so I did not worry about the foundation being damaged by frost. If we do not get out of the clay by the time we are at footer depth, then it certain is much more of a concern.

I think the heat cost question to me still is how much heat are you potentially losing, given the loss is constrained significantly by the R22.5 of the ICFs. The cost to heat up a large chunk of thermal mass, even if it is going to be 250~ cubic yards of material, doesn't seem to me like it would be that relevant as I would be doing it in early fall when the gas rates are lower, and then I'd get some of it back as a gradual tapering of heat in late April/May when I shut the boiler off. Now, I could be totally wrong on that too.

I think the first thing you need to do is find a foundation specialist engineer who knows your local conditions and have a little chat. When it comes to modelling thermal profile under the slab, the greenhouse guys are your best source. Helps a lot if you have a university campus close by where you are, since most are pretty good at community support. Another great source of thermal info is the ground heat pump sales and installation people.

My aversion to EPS and ICFs come from a fire my good friend (and car resto partner) owned an insulation business. He had a fire break out and EPS half way across the yard flashed off while PU stacked right against the building was just charred along the edges. Also having a son-in-law who is the hazmat instructor within city FD makes me NEVER want to be in a building with EPS inside. On that count PU is pretty nasty when it does light up, but that risk can be minimized by placing PU on outer face of envelope or best of all cast into concrete walls (tilt up done this way). While ICF people may not mention it, it actually does aborb a lot more water than PU/PIC but I can't honestly tell you the life span of each in wet soil.

 

[Tracs]

If you build without insulation under the slab, be sure to come back here and update us,...... unlike the guy in the other thread who is trying to heat his slab in an uninsulated 40x60 shop in the hopes to have it warm enough to work in, because some people told him it should work.

 

[OneEyedMan]

We have a 40x80 shop that was done without anything under the slab and heat pex in the slab. It takes all the heat to overcome that cheap out from thirty years ago. As my plumber friend relayed to me “heat radiates in all directions evenly” and I never seem to get a benefit from the heat I’ve sent down.

If you’re still on the fence, just put down insulation. Now is always the cheapest time to insulate.

 

[finn]

It’s not uncommon to regulate slab temperature rather than air temperature if having hot feet is your issue.

The controller I bought twenty five years ago showed two options: first, using a typical wall thermostat that measures air temperature rather, and second, an optional sensor that can be buried in the concrete directly or in a sleeve inserted in a drilled hole. You can also attach the sensor directly to the floor in the case of staple up PEX.

The probe came with the Danfoss thermostat included with the control panel setup I bought.

I don’t remember how to program (I use that term loosely) the thermostat to go the floor regulation rather than the air temperature. My Danfross thermostat died eventually (started stuttering or dithering) so I deleted it and converted to a Taco controller and now common Nest thermostat. The Danfross system was mechanical with DIP switches, . it pre-dates electronic thermostats.

Simply setting up a system like this negates the hand wringing about whether to insulate under the slab and letting the earth regulate slab temperature.

Hot slabs are a fairly common complaint and issue for some people, though, if you dig deep enough (no pun intended). Even with outside air correction that many modern boilers have, the slow response of in floor radiant can cause them to not be in sync on days with wide temperature swings commonly seen in the shoulder season.

In the house I deal with this by turning down the thermostat and using the mini split in the “shoulder seasons”. In the shop I set the boiler to 45 degrees and modulate temperature with a separate 60k hanging heater.

 

[Kezorm]

My assumption at that point is the delta between the shop and the earth at about 5' is not very much, so you're starting to split hairs on how much heat is moving.

Ground temperature at 6' deep generally stabilizes around the average annual air temperature for the location. Depends on exact location, but I expect typical "Northern BC" averages < 40degF. If you're keeping air temp at 60F, then slab is probably 70F or more. Point is, the ground is still a massive heat sink, regardless of any vertical insulation in the ICFs. My opinion, 4" insulation directly under slab.

 

[Bert_]

Just remember that the earth likes to be roughly 50 deg and is a much bigger heat sink than anything you are building. The earth will help cool that slab summer and winter unless you are trying to maintain 50-60. If you are keeping it warmer than 50-60 the earth has the ability to **** it all away. Heat naturally wants to move from hot to cold. If the air temp is above 60 then the heat will want to go down and not up. If I lived in northern BC I would be more concerned about keeping warm in the winter than I am here in Utah where my unheated garage never drops below about 45deg with no ceiling insulation.

Well put.

A heated slab will have to be warmer than air temperature too. Anything other than a heated slab will benefit from the ground temperature.

 

[2drx4]

Just remember that the earth likes to be roughly 50 deg and is a much bigger heat sink than anything you are building. The earth will help cool that slab summer and winter unless you are trying to maintain 50-60. If you are keeping it warmer than 50-60 the earth has the ability to **** it all away. Heat naturally wants to move from hot to cold. If the air temp is above 60 then the heat will want to go down and not up. If I lived in northern BC I would be more concerned about keeping warm in the winter than I am here in Utah where my unheated garage never drops below about 45deg with no ceiling insulation.

Here you lose most of your heat through the bay doors as even the "premium" ones tend to be R18. There is some higher R value ones available now, but it's still a lie as that is the panel R value, every seam is much less, any fastener in it creates a thermal path, and they will leak some at the perimeter no matter what you do. Compared with 2x8 wall giving you R28 and not being as bad for the thermal paths, plus near zero air leakage with modern vapour barriers/tape/sealant. Windows are the other big culprit, bay doors with them being even worse. Ceiling/attic insulation is easy to push to R40, or whatever you feel like.

 

[2drx4]

Ground temperature at 6' deep generally stabilizes around the average annual air temperature for the location. Depends on exact location, but I expect typical "Northern BC" averages < 40degF. If you're keeping air temp at 60F, then slab is probably 70F or more. Point is, the ground is still a massive heat sink, regardless of any vertical insulation in the ICFs. My opinion, 4" insulation directly under slab.

You made me look it up and the average is 39.7 F.