Insulation Under Slab?

Don Regan

Heat goes to cold. It's that simple. Where do you want the heat to go? In the ground or in the room? When I studied Radiant in Germany and Austria, they start a 4" of EPS insulation, most were 6". In the USA we argue about just putting it in or not. Maybe are heating costs are still to cheep. It's only going to cost more to heat that space in the future. I would stack the odds in my favor if given the choice.

Rich L.

To insulate or not

I just finished reading a story in the October issue of Engineered Systems magazine about radiant heat being installed in a new 148,000 sq ft Air Transport hanger. The pictures show the tubes tied to the rebar, looks like 12" centers with absolutly no insulation beneath. This building is in the Columbus, OH area. The cost savings of not insulating won't begin to be offset by the higher energy usage, right? This seems really short sighted on someones part or am I missing something here? I personally don't recommend anybody install a system without 2" of pink foam. The days of cheap energy are long gone.

What's your thoughts? Are there applications where you don't insulate between a slab and the ground? Why?

Rich L

jp_2

well??

I too have been thinking about this.

say you build a simple 20x20ft heated slab house during the summer. insulated the perimeter well, down to, say 4ft.

whats the temperature under the middle of the slab in the middle of summer? if you start heating early in the fall, the center slab ground temp will stay the same as summer. if its sand or gravel and in a dry area, why would you need to insulate?

everyone here agees not to do a sand/sandwich floor heat cause the sand conducts heat so poorly.

would the ground temp under the slab ever really change?

Tim_41

slab insulation

I have a 60x40x16 high steel building with a 20x32x8'high apartment within the building. 2 zones of radiant in the slab.
The frost walls are insulated to the footings with 2" foam.
Under the slab, I was told to use the dreaded bubble insulation in a roll.This is before I found the wall. Its also a vapor barrier. Right now, the apartment slab sensor is consistantly at 71 and the room is 68. Its well insulated in the ceilings and walls. The garage slab sensor is showing 64 and the room temp maintains 57 until we get some solar gain. The garage has the standerd metal building insulation. Last winter, I dropped a small temp sensor down a hole to around 4' under the slab. All winter long, it varied from 50 to 48 degrees even during the coldest days. I have a baxi condensing boiler with a LLH. I have the t-stats jumped for constant circ (its a taco SR503 relay) and the heat curve on the boiler set to 5. If I had 2" of foam, I believe the temps would be higher on the slab sensors. But, until I invent a way to get foam under the slab, I'll never know. Sorry for the long post.

Tony_23

My 2 cents

The ground temp may never change, but that is the problem. If the earth below the slab is 50 and the slab is 70, isn't the slab going to lose heat to the earth ? Of course it is. Don't you want that heat to come to the top of the slab and into the space ? Of course you do. To think that the earth is going to warm from your slab in an appreciable amount is ludicrous. You may have insulated the edges down 4 ft, but the earth has a bit more mass than that and will suck up anything you give it

martin

Slab Insulation

Years ago I did a blow-out garage for a client, it was used to house his helecopter. Anyway after we bid it the client wanted to eliminate the slab insulation, said the ground was all sand. Well this guy had a history of suing people so I put in a disclaimer to cover my tail. When we went to do the rough in whats there but the nastiest gumbo clay you could imagine. Needless to say the system worked like crap and I cry every time I drive by. For the cost I don't think I'd leave it out for anything again.

Constantin

Depends on where you insulate...

With such a large building, it may be more economical to insulate little to none in the middle of the slab and then to insulate the heck out of the edges. That way, you add the mass of the ground under the slab to the mass of the slab to make any changes in temperature even slower.

It'll take the heating system more time to come up to temp, but an approach of putting 4"-6" around the last 30' of the perimeter may actually save more energy than if you put 2" down everywhere.

Lastly, consider why they use RFH in these hangers... the idea is to open the doors in the middle of winter, roll in a plane, and still have the place back up to temperature in no time. The mass of the slab helps them do it, just like a giant storage tank.

Rich L.

Slab ins.

I have to agree with Tony. Heat always travels to cold right? If that's so and you're running 80 degree water through those tubes and have reached say 65* in the space, then isn't that heat going to travel down into the 50* ground? Even if you get it "warmed up", say it reaches 80* right under the slab it's still going to be cooler below that and I'd think the ground heat would just continue to drive lower and lower. Maybe that's not the way it would work but you're never going to get the entire earth warmed up to the same 65* you're trying to maintain in the space, especially if it's 0* outside. No question soil conditions will have an affect on it though. Just my thoughts, feel free to educate me!!!

Thanks guys, Rich L

hot_rod

The conductivity of the ground

has a lot to do with that heat travel also. A bed of pea gravel, for instance would conduct a lot less heat than solid Rocky Mountain gravel. A slab on grade would be different then a basement 7 feet deep also.

I have a Ground Temperature Below the Frost Line Table.
KC Missouri shows 57F, Atlanta 65F, Duluth 41F, Key West 78F.

I'm all for underslab insulation, not convinced 2" should be the golden rule for all underslab applications.

It's the edge that is exposed to outdoor design temperatures that REALLY needs the most insulation attention. It's also the toughest area to provide a good r-value on. Slab on grade and pole barns especially. I think that www.energyedge.com product is on the right track for a workable edge insulation product.

Foam needs to be protected from the elements, UV, bugs, etc. Most building codes want at the final grade 6-9" below the plate and rim joist. That leaves a lot of exposed insulation to detail.

ICF homes are the best way to assure that edge insulation. A solid 4" or more of foam with 6-8" of concrete between.

hot rod

jp_2

ground temp under build

i agree Hot rod, but what is the ground temp under a slab built house? 1 yr 2 yrs 3 yrs down the road? in well drained soil(sand/gravel) I'd say it no longer changes, only by house temp?

singh

Insulate.

I just started up a cold slab last night at 7pm.
We put 2 layers for a total of 4" of xps down, then 2" on edge exterior.
This slab heated up to temp by this morning, I was a bit suprised, it being a six inch slab.
If I can, from now on I'll recommend two layers, and in my climate , payback should be short.
The walls are double 2x6 insulated and ceiling is R-50, so maybe that had a little to do with it ; )
I'm afraid that this house with it's polished finished concrete floors won't be warm , unless we overheat the place. But that's a seperate thread already in discussion.

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Unknown

Hot Rod,

the website you mention is a link farm. Do you have another address?

hot_rod

www.energyedgeform.com is

the correct link, Rob

hot rod