Slab heat retention in the winter.

[Hobby_Man22]

I walked into the shop last night around 3am vecaise I was worried it was getting too cold for the boat I didn't winterize. Was going to decide if I needed to drain the manifolds. I think it was 19 degrees at the time and I shot everything inside the shop with an infrared thermotometer. The slab was still 50 degrees! Everything else inside was around 45 degrees and the actual air temperature was about 40 degrees. I'm sure everything was still cooling down slowly, but with a 50 degree slab I figured I was okay. I'm just amazed how much heat a concrete slab can retain. Makes me wonder if its because i have it built up on a 3ft tall pad with 60/40 sand/lean clay. The day before it was 31 outside for about 24hrs. Anyways, I figured if every object inside the shop was still 45 degrees, I wouldn't have to worry about water freezing before the sun came up.

 

[Hobby_Man22]

Are slabs just good at holding in heat? Or is it the pile of dirt I have underneath it.

 

[zmotorsports]

If you look at a lot of the heat loss calculators, there is more heat loss through the concrete than people realize. Yes, they do retain heat longer and will eventually equal ambient, but once they do, they take much longer to bring back up as well.

My 3k square foot shop is divided into an RV storage bay (1k ft/2.) and the working shop (2k ft/2.) whereas the shop is heated and the RV bay is not. The shop stays at 50-degrees overnight and I bump it to 62-degrees when I walk out there each day to work. I do the same as you with an infrared to double check things on occasion, especially in the RV bay because although it is very well insulated, it is not heated so any heat is transferred from the heated portion of the building.

Over the last few days we have had daytime highs of upper 20's to low 30's with overnight lows in the 9-13 degree range. Yesterday I went into the RV bay with my IR gun to double check things and the coach was around 38-41 degrees and the concrete/wall junction at the furthest point away from the shop was 36 degrees.

 

[dcg9381]

The OP is in Texas. Our ground temperatures are pretty high, there really is no frost line in most of the state. Part of my house is underground and the ground definitely keeps that part of the house warmer during our fairly rare cold snaps.

It's been in the low 20s overnight here. My shop air temps have been in the low 40s too. My guess is that it'd take at least 4 x 24 hours of 20 degree weather to get the shop to freeze inside. Foam insulation helps. Big leaky roll up doors don't help.

Throw a 60 watt bulb in the engine compartment of that thing and call it done.

 

[OccupantRJ]

In NC. At 21* outside, my thermostat in the shop with no heat on showed 45 degrees. Shooting the slab with a thermal gun showed 48.6. I do not shut down the water in my shop until around 20* overnight on one night or more. I have this setup on the two outside faucets. There is a foam insulating sleeve inside that runs a foot down underground to our frost line level. I have a south facing non insulated metal garage door, and the inside heat gain from it on a cold sunny day will outpace my heat pump if I have it on. I will have it set on 65 and around noon it will be 67 in the shop due to the heat gain. The dog sits on the concrete apron with his **** pressed against the outside of the door in the sun when he is outside.

 

[PoorUB]

Are slabs just good at holding in heat? Or is it the pile of dirt I have underneath it.

At a company I worked for, HVAC, we built our own shop and put in floor heat. We ran it off a computer and monitored a lot of points, floor temp, water supply and return temps, boiler run times and more. Over the years there were a few times that something failed and nobody would know the heat was off for a couple days. We could go back to the computer and almost to the minute figure out when the system failed. A couple times it failed on Thursday. We worked half days Friday and the shop was closed over the weekend. We ran the shop temp at 65 and Monday morning it might be 55. Fix the problem and in 3-4 hours the place was warming up.

 

[jack stand]

Think of you slab as a flywheel for heat, release and gain.

 

[Hobby_Man22]

Think of you slab as a flywheel for heat, release and gain.

I think it releases much slower than it gains.

 

[jblnut]

It's been chilly here the last few days with highs in the -15f area for a few days and the concrete in my 48x112 cold storage shed is 22f. It's been in that 15-25f area for the few weeks prior to the last few cold days. Slabs hold a lot of heat but will definitely cool off with enough time.

Us wierdos that live in the great frozen North put insulation under the slab to stop the heat from going down into the ground and pex tubes to run warm water through. I'm sure you've seen pictures of our stuff up here.

 

[Hobby_Man22]

The OP is in Texas. Our ground temperatures are pretty high, there really is no frost line in most of the state. Part of my house is underground and the ground definitely keeps that part of the house warmer during our fairly rare cold snaps.

It's been in the low 20s overnight here. My shop air temps have been in the low 40s too. My guess is that it'd take at least 4 x 24 hours of 20 degree weather to get the shop to freeze inside. Foam insulation helps. Big leaky roll up doors don't help.

Throw a 60 watt bulb in the engine compartment of that thing and call it done.

Back in 2021, the boat sat outside when I didn't have a shop. The low was 12 and the high during the day was 17 and this lasted 3 days. I didn't do anything to the boat as far as drying to drain any water out. Luckily no damage occurred. I think we got lucky because it was like 80 degrees in the days leading up to the cold front. Plus it had been a month or so since it was last used, so I think some of the water probably evaporated out leaving room for expansion.

 

[PoorUB]

I think it releases much slower than it gains.

It depends on the temperature differences. Shoving 100F degree water into a 60 degree slab will shove heat into the slab faster than a 60 degree slab releasing BTU back into a 50 degree space.

 

[4x4Pete]

I think it releases much slower than it gains.

Concrete is like that. Slow to warm and slow to cool vs outdoor ambient. See how long it is cold for after you get some warmer weather. Probably similar to what you've just experienced but in reverse.

 

[Hobby_Man22]

Concrete is like that. Slow to warm and slow to cool vs outdoor ambient. See how long it is cold for after you get some warmer weather. Probably similar to what you've just experienced but in reverse.

Yeah it's why they make radiant tubing heat. Doesn't work good in places like tx where it's 30 one day and 80 the next though, but I guess it's good if you can set it and forget it for the whole winter like up north.

 

[ipgenie]

We have radiant heated concrete floors in our house and some evacuated tube solar water collectors/heaters on the garage roof that heat the floor on sunny days.
We've been dipping below zero F here the past few nights and it's been clear and sunny during the day. The solar collectors heat up the concrete enough during the day to keep the house warm from sunset till about 1am. By then it's cooled enough to kick on the boiler system to maintain heat through the night.
The concrete is a great thermal mass but definitely takes time to warm up and cool down a space. I like the warm tile and the quiet operation.

 

[jack stand]

I think it releases much slower than it gains.

This is where under slab and perimeter foam insulation comes into play.

 

[cannuck]

This is where under slab and perimeter foam insulation comes into play.

struggling with how to treat this subject when building on ground that could flood. The gold standard of energy efficient design at our latitude is to excavate to 6 or 8 feet and place an insulation layer and backfill from spoils pile, embedding a grid of plastic tubing as you go. When back to grade, install a cutoff/curtain wall of insulation outside of footings, build the structure and pump solar from roof into the heat sink under the pad. A friend who works with this stuff tells me such a heat sink can support a properly insulated structure (including pad) for months, not days or hours.

Problem for me: you are putting footings on backfill (can be solved by columns on piling to undisturbed soil) and what will happen on years when the surface layer (10' of gravelly till) floods since water will not penetrate the 70' of clay beneath)?

It does say to me that even in moderate climates insulating well under slab could give you a heat sink that a well insulated building could use for several days. Getting really goofy and doing the excavate and store could be a lot better, but not exactly the cheap/easy way out.

 

[Shriner]

For reference, we just had our new radiant heat system up in running earlier this month. I was told numerous times in advance by my buddies who installed it that I will be amazed at how well the slab retains the heat as it is such a large mass. Well, a few days ago it got to -8 degrees. I left the house for work at 8am with the ambient air in the garage at 62 and the boiler off. I returned at 7pm, about 11 hours later, and the ambient temp was still 58 in the garage with the boiler not going on once. It is truly insane. With a forced air furnace in the garage, I would have been in the 40's by that time.

 

[dcg9381]

Problem for me: you are putting footings on backfill (can be solved by columns on piling to undisturbed soil) and what will happen on years when the surface layer (10' of gravelly till) floods since water will not penetrate the 70' of clay beneath)?

This is why I hire a slab engineer. We had to drill 12 x 10' piers on the "natural berm" where I built a garage and use fill that will not wash away.

 

[cannuck]

This is why I hire a slab engineer. We had to drill 12 x 10' piers on the "natural berm" where I built a garage and use fill that will not wash away.

For us flow is not any consideration, but surface/soil water level sure is We are zoned ag right now and could throw anything up, but have to do everything to code to be able to go ag/residential to site house. Will need engineered foundations to get building permit(s) as this area famous for being a bit of a ***** to build on.

 

[4x4Pete]

Yeah it's why they make radiant tubing heat. Doesn't work good in places like tx where it's 30 one day and 80 the next though, but I guess it's good if you can set it and forget it for the whole winter like up north.

Haha. There's never a set it and forget it for the most part. You can get it to work satisfactorily but it isn't perfect. Just like there, if the outdoor temperature swings too quickly you'll find it too warm or too cold for a bit while the concrete climatizes.

 

[kabinenroller]

I have a “set it and forget it” building. 40x60, full foundation to 48”, insulated on both sides of the foundation to above grade, vapor barrier and ridged insulation under the 5” slab, four zones of pex tubing 12” on center, on demand wall mounted LP boiler. I set the thermostat to 60 and never touch it. The building has 9’ side walls and all 2x6 construction, insulated 10x8 over head door, one 36” man door, and everything is over insulated. It was -13 here the other day and has not broke 32 for a while, the building never dips below the set temp and I seldom hear the boiler run. I use my laser thermometer periodically to check the temp of items in the building, everything is within a degree or two of the set temp, even my tools, vehicle, and machines.

 

[Hobby_Man22]

I have a “set it and forget it” building. 40x60, full foundation to 48”, insulated on both sides of the foundation to above grade, vapor barrier and ridged insulation under the 5” slab, four zones of pex tubing 12” on center, on demand wall mounted LP boiler. I set the thermostat to 60 and never touch it. The building has 9’ side walls and all 2x6 construction, insulated 10x8 over head door, one 36” man door, and everything is over insulated. It was -13 here the other day and has not broke 32 for a while, the building never dips below the set temp and I seldom hear the boiler run. I use my laser thermometer periodically to check the temp of items in the building, everything is within a degree or two of the set temp, even my tools, vehicle, and machines.

48" footings?

 

[kabinenroller]

48" footings?

The footing (foundation) walls are 48” they sit on a “mud” slab that is below the frost line. In Wisconsin this is common. I ran the top of the foundation to above grade so the sill plate will never be exposed to ground moisture. The interior slab is isolated from the foundation by ridged insulation, no thermo transfer. (the interior floor “floats” within the foundation)

 

[Hobby_Man22]

Ours are only 24" deep.