# Cold climate perimeter slab insulation



## teejk (Jan 19, 2011)

After the winter we've had, I have to wave the white flag on the question of insulating the outside of the slab on a radiant infloor system. I purposely left the tubing loops 1' away from the edges but it is apparent that heat is still leaching from the slab (I have 16' of no heat and 40' of heated space on the same side of the building) and the snow banks show it. I have 2" closed cell foam under the slab on the entire building.

So I have a question…Does anybody market a foam injection system for the outside perimeter? Or do I have to get the shovel out and add rigid foam? I hate cutting foam!


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## madts (Dec 30, 2011)

My Brother-in-law has a business that shoots foam on roofs. Like the Superdome in NO after Rita. You might want to check out similar in your area.


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## Charlie5791 (Feb 21, 2012)

You've got no empty space into which to spry foam. So…. you're diggin'.
Instead of digging straight down right tight to your slab (you can do that if you want, but generally you'll get stone sliding out and create a void), what we've done to retrofit perimeter insulation is to dig down only to the bottom of the slab , tight to the building. If you have stone, then insulation, then slab, you can go to the bottom of the insulation. The point is to NOT dig down through the stone.

You dig a sort of "V" shape trench, lay your new insulation against the side of the "V" closest to your slab, and then backfill. The insulation can be at a 45 degree angle, with the top of the insulation as tight to the slab as you can manage, and the bottom of the insulation about 2 and a half feet out.

All of the above assumes a floating slab. If you have footers and stem walls, then you can dig straight down easily without disturbing the stone


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## jusfine (May 22, 2010)

I concur with Charlie, you don't have much of a choice I am afraid…

Here in Canada, we call it "outsulation" when it is applied on the exterior of a building (my house has a walk out basement, but the buried portion has 3" outsulation around it).

You will notice an immediate difference in your slab heating!


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## teejk (Jan 19, 2011)

Thanks for the tips so far. It is a "floating slab". Soil conditions here would make any sand/gravel pit owner salivate so the forms were set on grade.

There is no added stone like in a basement foundation footing (by code they had to do that on the house and were laughing all the time…but code also made me install a sump pump in the basement…in 50 years if anybody wants to buy a sump sump that will never have been used, contact my next of kin).

But I will consider that idea of the 45 degree set. I was planning on a foot straight down so the idea of injection came to mind. I know it would be expensive (code here requires 1" foam on the ring joists for new construction now and it ain't cheap.


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## NotToDay (Feb 15, 2013)

Teejk
Looks like your going to have to dig and the more insulation you add the better your slab performance will be 
is your building 56 feet long and do you know what the temperature drop through the tubing circuit is 
you may have a flow problem


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## teejk (Jan 19, 2011)

NotToDay

The in-floor is fine…sized to cover 40×30 space using 5/8" tubing and loops 18" apart. There are 3 loops that return to a Rehau manifold. I was careful to not exceed the max tubing length (300' on 5/8" tubing as I recall) and the math worked out so that no loop exceeds 250'. Rehau manifolds allow for individual flow rates for each loop. I have to fiddle with it some more because I can't get to the optimum DeltaT 20 degree between supply and return because the thermostat shuts down before the return loops come back (a function of having hot supply not far away I guess and maybe loops 24" apart would have been better). I do have 2" foam under the slab and didn't think perimeter insulation was such a big deal especially since I held the loops back 12" from the interior walls (concrete is not supposed to be a great heat conductor and I thought 12" was enough).


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## NotToDay (Feb 15, 2013)

Teejk
It's to late to deal with spacing on the tubing the closer the better 
it sounds like your loosing to many btu's to soon you can keep adjusting flow in different zones to try and keep the boiler on longer 
it's not recommended to let the water temperature go above 140 degrees F 
I stay below 130 with my system
Is your pump staying on all of the time 24/7


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## teejk (Jan 19, 2011)

NotToDay

Heat source is a new super-insulated 65gal LP water heater (it used to be hot water heater but somebody figured out that there was no need to heat hot water) adapted to a closed loop system. I can only get it to 120 degrees before the pressure gets close to the pressure relief trigger and I don't want to take any fluid off because it is 50/50 glycol mix.

Because it uses storage, the heater and the pump run on their own demands…i.e. the circ pump is triggered by the air temperature thermostat demands, the heater runs based on the temperature of the water in the tank (which of course is influenced by sending hot water out and returning it at lower temps). I doubt I'll ever get to that 20F DeltaT factor but that may be a function of using 5/8" tubing and it's longer runs. I have no problems with the comfort levels but the LP bills are high. I was advised to insulate the perimeter to minimize heat leaching from the slab into the soil. I poo-pooed it at the time but now looking at how the snow is melting, I think I was wrong.

The circ pump doesn't run all that often. I maintain the slab at 55-60F rather than bringing it up from 40F (it takes a few days to do that). Nor does the heater fire all that often. I have the manifold set at 3/4 gal per minute …faster flow is supposed to return hotter water on the return side so the heater doesn't fire so often but it will require more circulation time…I might be playing whack a mole and failure to get to that 20F difference may be nothing.


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## NotToDay (Feb 15, 2013)

The 120 degree temperature should work 
My set up id different I run control valves to zones and keep the circulator on always with a master thermostat that controls the boiler 
The water heater works are you using a expansion tank with it 
I am in Pittsburgh and when the temperature gets into single digits I need the 130 degrees I maintain temperature 24/7 no set back


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## teejk (Jan 19, 2011)

Different set-up indeed. Mine was designed by an "old-school" commercial boiler guy (a long time friend of mine). The heater element on my system is the tank that is going to maintain the temperature in the tank regardless of demand…it's new and Federal mandates require any new heater be of the super-insulated variety.

The thermostat connects to a Honeywell control box that activates the circ pump when "a call for heat" comes in. That starts the process of circulating the water through the tubes and like I noted above, cooler fluid returns to the tank and some point it's own temperature setting will cause it to fire to regain the heat loss. Guess what I'm saying is that the circ pump can be running and the heater won't be. And the heater can be running and the circ pump won't be. The manifold controls the flow rate to the tubes and each tube has a separate flow control but my boiler guy tells me that "balancing" means to find a setting that works and set them all the same. I'm still trying to get my mind around why that works…some days I think I understand, other days I start to question!

And yes it uses a standard expansion tank (required on any hydronic heating system I think). At "idle" the pressure in the system might be 16lbs but when fired it goes to 28lbs very quickly if the circ pump isn't running (relief valve is 30 lbs). I could take some fluid out of the system but don't have to if I keep the heater at 120F. I have R19 in the walls and R50 in the ceiling. And have the 2" rigid foam under the slab so even at -20F the building maintains quite well at that heat temperature. It's only based on how the snow has melted that I now know that I can lower the LP bills if I insulate the slab edges (my boiler guy can get one of those "I hate to say I told you so" moments).


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## NotToDay (Feb 15, 2013)

Teejk
It sound like you have a plan 
Boiler guys like to have an I told you so every so often were wrong so much of the time we just figure that the odds are with us 
Good luck


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