I'm in the northwest area of NJ and I'm getting ready to start building a 40x30 pole barn. I will be heating it, 50 when I'm not in it and 65 when working. A friend suggested I put 2 inches of insulation under the concrete. I'm heating with hot air. Whats your opinions? Walls and ceiling will be spray foam.
Insulation under concrete floor needed?
- Thread starter skippydoo
- Start date
ConCretin
Well-known member
Materials will probably cost you something over $1 per square foot. Based on your description of how you will heat and use the space, it would take a long time to get your money back and I doubt you'd notice any difference in comfort either.
6768rogues
Banned
I am further north than you and I have two buildings with no floor insulation, heated to what you mentioned. No problems and the heat bills are low. If you want some floor insulation, put it vertically at the wall line to below the frost depth. It will keep the floor warmer and it will not let frost go under the building, and the cost will be reasonable. I have seen buildings with insulation under the floor where heat loss through the floor was so little that frost went under the insulation and heaved the floor.
jack stand
Well-known member
If your going with spray foam, a premium insulation job, why would you stop at the slab?
I'd consider at least doing the perimeter 4'.
I'd consider at least doing the perimeter 4'.
I am in Michigan insulated walls and attic, but didn't do the floor and it is cold. I have forced air in the shop and the temperature set at 60 when I am out there, but the concrete floor is cold. If I had to do it over again, for how cheap XPS is I would do the floor. But part of the problem with my shop is the 8" of the exposed concrete foundation perimeter is not insulated below the frost line.
Jay
Jay
rlwhitetr3b
Well-known member
I'm starting on a 24x30 garage on a slab. I asked the concrete guy about insulation. He said he did not think I would notice the difference and would not suggest it unless I was going to have in floor heating.
theoldwizard1
Well-known member
I agree with your original statement. Also insulate the inside of the footing to the the near the frost line.
A vapor barrier is also a good idea. If you do 2 layers of 1" rigid foam, staggering the joints, and seal all of the joints with the proper tape, you can probably skip the vapor barrier.
Yep ! Got to insulate the foundation down to the frost line.
I have actually seen pictures of a garage in MN that had insulation below the floor, no foundation insulation used and the snow was melted back about 1-3' on the outside !!
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RVDan
Well-known member
Sometimes takes forever to pay you back.
Every year for the past 7 years I've made substantial energy saving improvements to my house. Three times the original insulation in the attic. Replaced the original aluminum single pane windows with much better new windows. Weather stripped the doors. Put insulation in walls that was missing insulation. Improved vapour barrier in the rooms that I've changed the drywall in.
The difference in my heating bills is marginal at best, and this year was the most expensive yet despite changing to a programmable thermostat that lowers the temperature while we're sleeping and while we're at work.
The new windows won't get paid for in my lifetime through energy savings.
matt_i
Well-known member
I used to think it wasnt needed, but if you run a simple heat flow calculation with Q =u * (delta T) where the U-factor is 1/R-value
R values, lets say 13 for wall and 1 for concrete (that's from looking it up). Say 60F inside, 20F outside, and 50F in the ground. Substitute your own numbers and run it again.
For a wall you'd get Qwall = 1/13 (60-20) = 40/13 = 3.07
For a floor you'd get Qfloor = 1/1 (60-50) = 10/1 = 10.
This is for 1 square foot of area of each, obviously there are more paths with wall area, but you can see how much more conductive the floor is. The simplicity of this doesn't account for hot air rising, convection, etc, but its definitely something to consider if its a factor of 3 difference.
If I could have done my build over I would have done the under floor insulation. I didn't and thus bought better boots....
R values, lets say 13 for wall and 1 for concrete (that's from looking it up). Say 60F inside, 20F outside, and 50F in the ground. Substitute your own numbers and run it again.
For a wall you'd get Qwall = 1/13 (60-20) = 40/13 = 3.07
For a floor you'd get Qfloor = 1/1 (60-50) = 10/1 = 10.
This is for 1 square foot of area of each, obviously there are more paths with wall area, but you can see how much more conductive the floor is. The simplicity of this doesn't account for hot air rising, convection, etc, but its definitely something to consider if its a factor of 3 difference.
If I could have done my build over I would have done the under floor insulation. I didn't and thus bought better boots....
RWorth
Well-known member
Skippy, I was going to insulate mine, however I dropped the ball and the guy that did my floor came a week early. However I'm kind of glad he did. In the summer now my garage stays cool as long as I keep the garage door closed, the only drawback I have is IF i open the garage door, and the slider on the other side of the shop and get cross ventilation when it is humid, the floor gets wet, real wet. So I can't open both doors for any length of time.
bmwpowere36m3
Well-known member
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Biggest heat loss in a structure is almost always infiltration. Roll-up doors are so pretty common in shops/garages and last I checked their "seal" is pretty poor.
Insulation under the slab isn't so much to slow the flow of heat out of the structure... but to "redirect" in-floor heat into the structure.
Insulation under the slab isn't so much to slow the flow of heat out of the structure... but to "redirect" in-floor heat into the structure.
rusty1
Well-known member
...probly won't have to heat over 50* ever, that's just rite for workin...
jack stand
Well-known member
AMCguy
Well-known member
If you're going to rule out an upgrade like this based on the savings in heating cost taking a long time or even forever to pay for it, you're missing out on the comfort and increased quality of life that is realized immediately and forever. Surely that is worth the extra cost.
Glenn
Glenn
gnpenning
Well-known member
^^^^. This. Cost savings is great, but comfort is everything. My home is ICF with radiant heat. Not my first radiant heat home either. But by far my most comfortable and efficient home. Didn't do a good job insulating the slab or thermal breaks on the first. Never again. It makes a huge difference.
I'm on jobs all the time without thermal breaks and everyone pulls heat to the outside. Study how A/C works.
Even with forced air I would do thermal breaks and at least the first 4' inside the slab. Even using a inch or two under the slab will help. Personally having done it I would do the whole slab again.
Don't buy insulation from the box stores. We have several places that sell drywall/insulation and they beat the box stores prices easily on everything.
It takes allot of Btu to bring everything up to temperature. Even more when you are heating the great outdoors.
BruceMc
Well-known member
This /\
This is one of those times where you won't get a second chance to do it right.
RWorth
Well-known member
The cost wouldn't be an issue for me, however I'm 100% serious when I say my shop is really cool in the summer. And that is quality of life, and comfort.
I guess it depends on what is more important, summer or winter comfort.
This, my shop is heavily insulated. It may not payback in heat savings but it sure does in comfort. It's a joy in winter . Anywhere you go in there is warm. It's worth doing right in cold cruel north.
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myredracer
Well-known member
The closed cell rigid foam isn't exactly cheap. I got ours for our 1800 sq ft shop & garage from a building material recycler for a fraction of new. Had a few chunks and holes in the 2'x4' pieces but just ran them through a table saw. Getting the fill material nice 'n flat was a big chore. I used sand and ran a large plate compactor over it which made it hard as concrete. Sand compacts a lot and need to do it in multiple lifts.
I previously built another shop around the same floor area and didn't insulate it and highly regretted it afterwards. I think you will regret not installing it too. I remember sitting some boxes of car parts on the floor for a long time and when I picked them up they had obvious signs of condensation on the bottoms and the parts fell right out.
I previously built another shop around the same floor area and didn't insulate it and highly regretted it afterwards. I think you will regret not installing it too. I remember sitting some boxes of car parts on the floor for a long time and when I picked them up they had obvious signs of condensation on the bottoms and the parts fell right out.
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6768rogues
Banned
It is not too late. You can excavate around the building and put insulation in line with the exterior walls to below frost depth, keeping the floor and soil below it isolated from the outside ground. I don't think it would be worth it, though, as the floor will still feel cold unless you heat it up to body temperature.