to insulate under slab or not ?

[mod9]

here in the northwest we don't get really cold and if it does it is only around for maybe 2 weeks. for the most part it stays right around the mid 40's during the winter.(now watch we will get a record amount of snow because I said that) but im thinking that a vapor barrier and rebar will be all that is needed. anyone from the NW out there have a opinion ? or from anywhere else for all that matters.

 

[Sureshot]

I don't have mine insulated and I have floor heat. I am sure in the long run it is better but it is a big expense. If I lived there I doubt I would but I only keep my shop in the 40's here.

 

[Architorture]

Planning to heat the space? If so then insulate, if not then dont

 

[mod9]

im only going to heat it with a pellet stove when ill be working in it. dont plan on heating 24/7

 

[bazzateer]

I'd be inclined to insulate regardless of whether it's going to be heated. As a percentage of the cost of the build it shouldn't be too much and it's easy to do while you've got a big hole in the ground.

 

[NUTTSGT]

I'd be inclined to insulate the slab. It'll help the concrete retain the heat. Once you can retain the heat in the concrete, it'll be the biggest thermal mass in your garage and help it stay warm and reduce you heating costs over the life of the building. Not to mention make your toes happy.

 

[stingry]

im only going to heat it with a pellet stove when ill be working in it. dont plan on heating 24/7

If this is your plan as to heating your shop, then I would say don't insulate the floor. Here is why, and this comes from personal experience and not speculation. This sounds weird, but the ground below your slab is actually a source of heat to be utilized. I live in Western Nebraska, which is in climate zone 4. My shop is 3600 sq ft and is well-insulated, R25 walls and R40 ceiling. I am set up for radiant floor heat but it is not yet hooked up. My slab is insulated with 2" EPS.

Last winter I kept track of slab temperature, the temp just below the 2" EPS insulation and the room temperature. There was no supplemental heat in the building. The temp just below the slab insulation stayed at a constant 47 degrees. The slab also stayed at 47 degrees, dropping a degree or two for a short time if the outside temp dropped to zero or below. The interior air temperature pretty much stayed around 47-50 degrees, warming up slightly during the warmer days from solar gain and dropping on the really cold nights or on really cold days. The coldest it got was 37 deg one night when the outside temp dropped to -20 deg

I believe this is what was happening. The warm earth at 47 deg warmed the slab to the same temp by conduction and then the slab radiated heat to the air. On cold days and nights the air temp fell because the slab could not radiate enough heat to overcome the loss thru the walls, windows, ceiling, etc. During the warmer, sunny days the interior temp would actually raise above the slab temp of 47 deg.

I believe that your money would be best spent on insulation in the building walls and ceiling. Insulating the exterior of the slab down to the base of the footer would be money well spent as the slab exterior to atmosphere interface is a major source of heat loss.


Cheers
Steve

 

[Architorture]

If this is your plan as to heating your shop, then I would say don't insulate the floor. Here is why, and this comes from personal experience and not speculation. This sounds weird, but the ground below your slab is actually a source of heat to be utilized. I live in Western Nebraska, which is in climate zone 4. My shop is 3600 sq ft and is well-insulated, R25 walls and R40 ceiling. I am set up for radiant floor heat but it is not yet hooked up. My slab is insulated with 2" EPS.

Last winter I kept track of slab temperature, the temp just below the 2" EPS insulation and the room temperature. There was no supplemental heat in the building. The temp just below the slab insulation stayed at a constant 47 degrees. The slab also stayed at 47 degrees, dropping a degree or two for a short time if the outside temp dropped to zero or below. The interior air temperature pretty much stayed around 47-50 degrees, warming up slightly during the warmer days from solar gain and dropping on the really cold nights or on really cold days. The coldest it got was 37 deg one night when the outside temp dropped to -20 deg

I believe this is what was happening. The warm earth at 47 deg warmed the slab to the same temp by conduction and then the slab radiated heat to the air. On cold days and nights the air temp fell because the slab could not radiate enough heat to overcome the loss thru the walls, windows, ceiling, etc. During the warmer, sunny days the interior temp would actually raise above the slab temp of 47 deg.

I believe that your money would be best spent on insulation in the building walls and ceiling. Insulating the exterior of the slab down to the base of the footer would be money well spent as the slab exterior to atmosphere interface is a major source of heat loss.


Cheers
Steve

That is without heat so it makes sense to depend on the ground warmth...this would make sense if you were constantly heating the space as well. But based on the way he is planning to heat the space- infrequently- he is going to want to insulate otherwise he will be trying to overcome the lower temps of the ground instead of just the slab.

 

[nonhog]

I'm near you and if I were to do it over again I'd wait till that door shop in Tacoma is giving away cut outs. They are free and would make great (IMO)
under floor insulation. Watch for them on CL.
I have a small heater G73 that helps but never warms the slab up.
I'm pushing 50 and my bones and joints ain't what they use to be. I feel that cold radiating from the floor when I am under a car. I wish I went with in floor heat because I'm always moving around the shop and hate to stop and stand in front of the heater to warm up. Also in a previous shop (So.Hill) I had a pellet stove for heat that took several hours to make a dent in the air temperature. My point is make sure your square footage isn't too much for a pellet stove to keep up with.

I recently sectioned off storage space from work space and the lil' heater is so much more effective. I'll borrow a temp sensor and see if there is any difference in the slab from heated area to not heated. I doubt it.

 

[jabberwoki]

I`m in Puyallup and all I did was 2 layers of 6 mil plastic before the slab then painted my floor with a 2 part epoxy paint when the concrete set. I heat my shop with a wood stove and it`s all good.

 

[xyster101]

I just poured a slab in upstate NY. My father-in law recommended insulating the slab and doing radiant heat, even to keep the shop at 50. His shop has forced air heat, which heats the place up fast, but working on the floor is cold and tough on his joints.

Just buy some double bubble. It is around 50 cents a square foot. basically some silver bubble wrap. I did the 2" foam sheets, but that was $1 per sq foot and was $1000 for the whole thing.
I would do something, even if just to act as a vapor barrier. Your pellet stove will not heat up the slab in the winter at all.

 

[Sureshot]

I would be more inclined to spend the money on doing infloor with solar and try to warm the slab and ground to help carry you through the short mild winter you have.

Check out a great build by Walrus.

http://www.garagejournal.com/forum/showthread.php?t=40482&highlight=started+walrus

 

[stingry]

That is without heat so it makes sense to depend on the ground warmth...this would make sense if you were constantly heating the space as well. But based on the way he is planning to heat the space- infrequently- he is going to want to insulate otherwise he will be trying to overcome the lower temps of the ground instead of just the slab.

I do not believe this to be true. The ground warmth will keep the shop at a fairly even temperature, say 50 degrees or so, and then when he wants to heat the shop, he just has to raise the air temp 10 t0 20 degrees. Heating infrequently will not affect the temperature of the slab. It contains too much mass to be heated by the air temp. I have a small woodshop (approx. 600 sq ft) that is part of the larger shop. I heated it occasionally for 5-8 hours at a time to 60-65 degrees with an electric heater. The slab surface temperature did not noticeably change during these times.

If one was to heat the air for extended amounts of time, then some of the heat would transfer to the slab and some would be lost to the ground due to the heat differential between the slab and the ground. This may or may not be a reason to insulate under the slab.

Cheers
Steve

 

[Autorotica]

Insulating the slab to me also seems counter-intuitive. If the ground where I am is normally 55 degrees, wouldnt I want that energy to be available to my shed?

Chris

 

[Architorture]

I do not believe this to be true. The ground warmth will keep the shop at a fairly even temperature, say 50 degrees or so, and then when he wants to heat the shop, he just has to raise the air temp 10 t0 20 degrees. Heating infrequently will not affect the temperature of the slab. It contains too much mass to be heated by the air temp. I have a small woodshop (approx. 600 sq ft) that is part of the larger shop. I heated it occasionally for 5-8 hours at a time to 60-65 degrees with an electric heater. The slab surface temperature did not noticeably change during these times.

If one was to heat the air for extended amounts of time, then some of the heat would transfer to the slab and some would be lost to the ground due to the heat differential between the slab and the ground. This may or may not be a reason to insulate under the slab.

Cheers
Steve

insulation is not some all powerful thing that heat cannot pass through...it simply slows down the passage of energy. if one side of the insulation is always 50 degrees and the other side is an enclosed and insulated space the ground heat is going to make its way into the building...it isn't going to all go out around the edges. heat will go through the path of least resistance, which will be the insulation and slab in all but the perimeter areas.

if you are heating infrequently you would rather have to deal with the mass represented by the thickness of the slab separated from the ground by insulation as opposed to the mass represented by the slab and earth below combined. because once you want to increase the temperature of the space above the normal temperature of the slab and ground below you are going to be attempting to overcome that mass...and you would always prefer it to be smaller if possible.

over long periods of time, the warmth of the earth will overcome the presence of the insulation so you will benefit from the earth's heat for passive heating...over short periods the insulation will keep the short term active heating from the stove isolated from the ground which is now a big heat sink.

 

[Racecarl]

Hey Stingray, greetings from McCook! Anyway, when I built my shop I insulated the inside perimeter of my 24X30 with 2" XPS. The insulation goes down the inner wall 48". I compacted 8" of gravel for the floor sub-base and also laid 4X8 sheets flat around the inner wall, so the first 4' of the floor insulated. I then added 6" of gravel on top of the XPS and compacted this gravel. I then installed a 10 mil vapor barrier. I did not chose to insulate under the entire slab because I want to use the gravel/earth under the center of my slab as a heat bank--once I get this material warmed up, I hope it will allow me to bank some heat for sunless days.

My floor is 6" thick and is reinforced with remesh and rebar. Three circuits of 1/2" pex was placed in the middle of the slab, secured to the mesh/bar reinforcement. The slab was poured and power-troweled, and then control joints were cut in. My walls are 2X6 and have R-19 fiberglass insulation with the same 10 mil vapor barrier as the floor. The ceiling is insulated with almost 24" of blown cellulose, again using the same 10 mil vapor barrier as the floor and walls.

I am going to build solar panels to heat the water circulating in the floor. I have not gotten that job completed, so my heat source is a propane 35,000 btu Mr Heater radient heater. I just heat the shop when I am working, and usually a hour or two is all the heat I need from the radient heater. With no heat the coldest I observed was 37 degrees. In the summer it can be 105 outside and the shop will be around 83 degrees.

In short, I would insulate under the slab. I think I used 22 sheets at $20 per sheet, so I have $440 in slab insulation. I really need to redouble my efforts to get the solar panels up and running so I can fully take advantage of my insulated slab.

 

[stingry]

insulation is not some all powerful thing that heat cannot pass through...it simply slows down the passage of energy. if one side of the insulation is always 50 degrees and the other side is an enclosed and insulated space the ground heat is going to make its way into the building...it isn't going to all go out around the edges. heat will go through the path of least resistance, which will be the insulation and slab in all but the perimeter areas.

I agree with you completely

if you are heating infrequently you would rather have to deal with the mass represented by the thickness of the slab separated from the ground by insulation as opposed to the mass represented by the slab and earth below combined. because once you want to increase the temperature of the space above the normal temperature of the slab and ground below you are going to be attempting to overcome that mass...and you would always prefer it to be smaller if possible.

over long periods of time, the warmth of the earth will overcome the presence of the insulation so you will benefit from the earth's heat for passive heating...over short periods the insulation will keep the short term active heating from the stove isolated from the ground which is now a big heat sink.Here is where we disagree! When heating for short periods of time, the temperature of the slab will not be affected for several reasons: one being that heat does not transfer well between a gas (air) and a solid (slab) and secondly, when heating the air, the heat tends to rise by convection and thus not much heat reaches the slab to warm it. If one was maintaining a higher temp for a long period of time in the shop then the slab would eventually warm and heat would be lost to the ground mass. Then insulation would probably be appropriate. In the OP's case, I believe that heat transfer from the ground to the slab is more important than the reverse.

Cheers
Steve

 

[stingry]

Hey Stingray, greetings from McCook! Anyway, when I built my shop I insulated the inside perimeter of my 24X30 with 2" XPS. The insulation goes down the inner wall 48". I compacted 8" of gravel for the floor sub-base and also laid 4X8 sheets flat around the inner wall, so the first 4' of the floor insulated. I then added 6" of gravel on top of the XPS and compacted this gravel. I then installed a 10 mil vapor barrier. I did not chose to insulate under the entire slab because I want to use the gravel/earth under the center of my slab as a heat bank--once I get this material warmed up, I hope it will allow me to bank some heat for sunless days.

My floor is 6" thick and is reinforced with remesh and rebar. Three circuits of 1/2" pex was placed in the middle of the slab, secured to the mesh/bar reinforcement. The slab was poured and power-troweled, and then control joints were cut in. My walls are 2X6 and have R-19 fiberglass insulation with the same 10 mil vapor barrier as the floor. The ceiling is insulated with almost 24" of blown cellulose, again using the same 10 mil vapor barrier as the floor and walls.

I am going to build solar panels to heat the water circulating in the floor. I have not gotten that job completed, so my heat source is a propane 35,000 btu Mr Heater radient heater. I just heat the shop when I am working, and usually a hour or two is all the heat I need from the radient heater. With no heat the coldest I observed was 37 degrees. In the summer it can be 105 outside and the shop will be around 83 degrees.
Hi Mc Cook
Almost exactly the temperatures I observed in my shop. I have a farmer friend who built a Morton building, 50 x 80 I believe with 14' sidewalls. Very well insulated and his actually stays warmer than mine in the wintertime. I believe this is because he does not have his slab insulated so more heat can transfer between the ground and the slab and then radiate into the shop



In short, I would insulate under the slab. I think I used 22 sheets at $20 per sheet, so I have $440 in slab insulation. I really need to redouble my efforts to get the solar panels up and running so I can fully take advantage of my insulated slab.

Cheers
Steve

 

[mod9]

well with the mild temps in our winter I guess I will only put a vapor barrier under my concrete. actually 2 layers.

 

[nonhog]

Not sure how this applies, if at all. Tonight I did some temp checking.
Oddly enough my slab @ 5pm was 47*
So I went checking other items while my lil electric G73 heater worked its magic.

My unconditioned atrium floor (slab on grade) was 57* Its surrounded by the entire house, heated of course, also slab on grade.
Kitchen floor 60.5*
House garage floor 51*
Front porch, cement outdoors 41.5*

After 3 hours I went back in the shop and looked around. Slab in heaters path. 53.5* Slab not in heat flow 52.5*
Slab on the unheated side of shop. 48.5*
Sliding door cold side 48.5* warm side 60.0*
66 Volvo in cold storage 47.5*

80 Volvo in heated side: engine block 52.5*
fender 58.5*

Under 80 Volvo slab 50.0* just in front of Volvo "slab" 52.0*

Back wall (where heat is pointed) 65.0* side wall (out of direct heat flow) 63.5*

Not suggesting these results are accurate or mean squat. But I found them interesting and thought I'd share.

Oh the Dog was a few * warmer than the cat. Go figure.

 

[Krodad]

Have my new building up, and while I'm not insulating at least half of it, I did insulate the entire slab with 2" of xps, with vapor retarder (heavy poly) under that.
The number one reason I did this is to control the condensate that typically happens here in Iowa in the spring...we get to that point of slightly warmer days, but cold nights, all with somewhat high humidity, and the floors can get so wet from condensate that you can literally squeegee them off. So, by insulating the floor (correctly...meaning at the slab perimeter as well) this allows the slab to stay closer to the air temp, and not the colder ground temp, so the dew point does not happen in the slab.

Vapor barrier alone won't help with this specific problem, and I lived with it for too long in my last shop and barn, so looking forward to a dry floor this spring!

Good luck with your project.

 

[bazzateer]

Oh the Dog was a few * warmer than the cat. Go figure.

That's easy! Dogs have warm hearts, cats ............... err...........don't

 

[Sureshot]

Not sure how this applies, if at all. Tonight I did some temp checking.
Oddly enough my slab @ 5pm was 47*
So I went checking other items while my lil electric G73 heater worked its magic.

My unconditioned atrium floor (slab on grade) was 57* Its surrounded by the entire house, heated of course, also slab on grade.
Kitchen floor 60.5*
House garage floor 51*
Front porch, cement outdoors 41.5*

After 3 hours I went back in the shop and looked around. Slab in heaters path. 53.5* Slab not in heat flow 52.5*
Slab on the unheated side of shop. 48.5*
Sliding door cold side 48.5* warm side 60.0*
66 Volvo in cold storage 47.5*

80 Volvo in heated side: engine block 52.5*
fender 58.5*

Under 80 Volvo slab 50.0* just in front of Volvo "slab" 52.0*

Back wall (where heat is pointed) 65.0* side wall (out of direct heat flow) 63.5*

Not suggesting these results are accurate or mean squat. But I found them interesting and thought I'd share.

Oh the Dog was a few * warmer than the cat. Go figure.

What was the air temp outside?

 

[nonhog]

What was the air temp outside?

Well it would have been dropping from 1st check to second check.
The max that day was 57, low 48 low 50's at time of checking slab. Honestly don't recall the exact #'s.
I did ask my son but failed to write it down.