Pole Barn Concrete Slab Insulation and Radiant Head

[Mvpjeff106]

Had a concrete contractor friend come to my property today for a quote on the slab for my 50x40x14 Pole barn. I told him that I was going to do Radiant Heat in the floor and he recommended 2" foamboard around the slab perimeter and 2 feet in from the wall leaving the center of the floor with no insulation. He said that he would take 4x8 sheets of the foam and cut them in half and run them around the perimeter, then fill with stone to level the un-insulated center with the 2" foamboard. He would then put a vapor barrier and wire mesh above that.

Question:

Has anyone heard of only foamboarding the outer 2 feet of the slab and the perimeter? I've seen most posts on here where people insulated every square inch of the foundation. Heat will be on 100% of the time in the winter due to Antique Car Storage and temperature requirements for their storage.

I believe he knows what he's talking about since he does concrete for a living AND is a friend so he's not trying to mis-lead me but I've never heard of this concept of not insulating the whole earth below before.

 

[Bondo]

Has anyone heard of only foamboarding the outer 2 feet of the slab and the perimeter?

Nope,.... Bein' a friend, he may think he's savin' ya money,....

Personally, I think ya need the insulation/ thermal break all the way around 'n under, ALL of the heated concrete,...
Otherwise, there's heat loss,...
Probably less in the center, but heat loss none the less,...

 

[Northislander]

In my area we have a high water table in most area's so we do complete underslab insulation. In some areas with a low water table i have heard of some only insulating the perimeter. This would make the ground your slab is contacting part of the thermal mass of your floor definitely going to take more Btu's to bring up to temperature but depending on your ground conditions it could be feasible. But i would still opt for full underslab insulation

 

[nehog]

Where are you located? That is a factor.

 

[Mvpjeff106]

Located in New Jersey

 

[yeldogt]

The theory is ... it's not needed. If your only goal is to heat the space -- it's not. If your goal is to heat the space and not waste energy -- insulate the whole slab.

The fully insulated slab will direct all the heat up -- it will be more efficient/ faster to respond. Your friend is correct in that the further from the wall the less important the insulation.

A properly working radiant slab will heat the space -- but a portion of that heat is going to be wasted -- no one can tell for sure how much.

I would insulate the whole slab --- 2' in is not enough even over sand in a warmer climate. At the very least to 4' and drop to 1". You want that thermal break directing the heat up.

Levitown PA and Willingboro NJ was built with radiant slabs -- no insulation. They work great with big boilers and cheap NG-- melts all the snow from around the outside too.

 

[goneflyin2002]

No question, insulate the whole thing.

There's no more powerful heat sink than the earth. Your hydronic floor will never be efficient until it's fully insulated with 2" foam underneath and all around.

It won't take long to forget the pain of paying for the insulation, after you start enjoying how well it works, built correctly.

 

[ThumperPilot]

I just read something about your guy's approach today at work. I'll see if I can find it again tomorrow. It was full 2" XPS within so many feet of the edges, then down to 1" for some number of feet, then down to no insulation at all in the "center" of the floor.

I'll see if i can find that info tomorrow and update this thread....

Edit: I couldn't find this information again today, sorry about that. I wish I had saved the site...

 

[pstnbly]

Radiant slabs need to be isolated from the sub-base PERIOD! I don't know who is promoting the voodoo of perimeter insulation, but it is clearly wrong. What you save on insulation now will cost you many times over in fuel. The earth is a giant heat sink, that's how ground source heat pumps work.

 

[mrpizza]

My contractor is using something called "Barrier X-5" for the insulation/vapor barrier under my radiant slab.

 

[kj_mustang]

Slab insulation is just like all insulation jobs. Pay more at time of install for more/better insulation or pay more later in fuel expense over time.

 

[yeldogt]

My contractor is using something called "Barrier X-5" for the insulation/vapor barrier under my radiant slab.

That's a roll product -- about R6.. Not sure of the cost.

 

[yeldogt]

I have a home office in my walkout basement -- house on hill. It's an older house -- so the basement concrete floor is not insulated (wall are) -- the rooms are headed with hydronic panel radiators.

Even on the coldest winter days the floor is not cold ... it's not warm .. but not cold. Part of the floor is tile ... this is where the insulation debate gets started. What's enough and what's the payback.

In northern europe they use an insulation method taking the insulation out past the foundation for slabs to use the earth's heat to eliminate the risk of frost.

The slab's center is not going to be 30 degrees in 30 degree weather -- so the heat delta is much different. Also, different is the heat delta of the heated floor to the unheated/ uninsulated earth. The door at 90 degrees and the slab at 50 -- heat flow from warm to cold and flows faster with a greater delta .... It's very hard to determine what the payback will be.

 

[2011cummins]

Ignoring the technical and just looking at cost, you're talking about adding 1656 sq feet for insulation. Assuming $1/sq ft for foam vs $30/yd of stone:

Stone: 46x36' with no insulation, requires 2" of stone. 10.2 yards at $30/yd = $306
Insulation: 46x36" x$1/sqft = $1656

So $1350 additional in materials to have the whole thing insulated. I'm going to assume this is a low cost relative to the rest of the building and it's what I would do.

 

[mrpizza]

I am getting the x5 stuff for 87 cents a square foot. Ill do blueboard around the perimeter.

 

[rburke65]

"Something called x5"?????? Who makes x5.....do we know?

 

[mygarageone]

I am getting the x5 stuff for 87 cents a square foot. Ill do blueboard around the perimeter.

If this stuff has only an r6 factor and if it has an aluminum covering please consider something else .
The concrete will eat the aluminum foil up .

 

[mygarageone]

To the OP , insulate the whole under slab , your contractor is steering you wrong . The ground will **** up the heat like crazy , why are you wanting to waste heat like that ?

 

[mrpizza]

If this stuff has only an r6 factor and if it has an aluminum covering please consider something else .
The concrete will eat the aluminum foil up .

Its a poly vapor barrier on it, no aluminum in there.

 

[chickenwings_85]

I ran it under the entire slab on mine aside from where the car lift is going. I would have done the whole floor but I wanted a little more strength under the lift.

 

[jvitez]

Actually you don't have to insulate under the whole slab. The ground is not an infinite heat sink.

When you first turn on the system in the fall the ground will heat up and absorb more heat than if the whole under-slab was insulated, so yes you'll pay more upon initial start up. But an equilibrium will be reached and no further heat loss will occur into the earth.

In the spring you'll get back the "excess" heat used in the fall, as the slab absorbs heat from the ground meaning your heater will be working less.

Insulating under an entire slab will allow faster interior temperature changes though, but that's not how radiant floor heat works best. It's like geothermal, just set-it-and-forget-it. Insulating the slab edge or stub wall is key, as that has the biggest delta T and therefore the most potential heat loss.

 

[Northislander]

Actually you don't have to insulate under the whole slab. The ground is not an infinite heat sink.

When you first turn on the system in the fall the ground will heat up and absorb more heat than if the whole under-slab was insulated, so yes you'll pay more upon initial start up. But an equilibrium will be reached and no further heat loss will occur into the ground

I disagree unless your targeting very low slab/room temps. There is still going to be a temperature difference from ground temps to slab temp. hence heat loss and depending on water table the water flowing under that slab is going to be taking even more heat away. As much as it looks like water sits stagnant when you dig a hole there is usually always flow. That's why ground loops work in geo thermal installations the flow of ground water is bringing in new heat all the time or in the case of a radiant slab taking it away.

 

[yeldogt]

Actually you don't have to insulate under the whole slab. The ground is not an infinite heat sink.

When you first turn on the system in the fall the ground will heat up and absorb more heat than if the whole under-slab was insulated, so yes you'll pay more upon initial start up. But an equilibrium will be reached and no further heat loss will occur into the earth.

In the spring you'll get back the "excess" heat used in the fall, as the slab absorbs heat from the ground meaning your heater will be working less.

Insulating under an entire slab will allow faster interior temperature changes though, but that's not how radiant floor heat works best. It's like geothermal, just set-it-and-forget-it. Insulating the slab edge or stub wall is key, as that has the biggest delta T and therefore the most potential heat loss.

You can't have an equilibrium with the earth -- as long as the temp of the floor is greater it will move to the colder earth.

It's all a question of payback -- once the floor/tubes are installed and running it's impossible to tell the loss. The system will provide the BTU's required to meet the set room temp.

The benefits of slab insulation are documented ... numerous. Again -- it's a cost return. An easy example to understand would be a large driveway with snow melt -- those with insulation under them perform much better.

 

[rburke65]

I'm nota geologist, but I think you need insulation under the entire floor.

 

[UpstateNY]

Actually you don't have to insulate under the whole slab. The ground is not an infinite heat sink.

When you first turn on the system in the fall the ground will heat up and absorb more heat than if the whole under-slab was insulated, so yes you'll pay more upon initial start up. But an equilibrium will be reached and no further heat loss will occur into the earth.

In the spring you'll get back the "excess" heat used in the fall, as the slab absorbs heat from the ground meaning your heater will be working less.

This is wrong on so many levels i don't know where to begin .

My 30 x 40 slab weighs 37,000 lbs. That's what I need to heat to radiate heat into the occupied space, "connecting" that slab to the earth i.e. with no insulation under the slab, increases the size/weight of the heat sink.

Insulate under the whole slab. If anyone informs you otherwise, be very cautious about using them on your project. They may be well intentioned, but they're very mis-informed.

 

[jvitez]

Ok gents, let me clarify. I base my statements on having done much research when building our energy efficient house, what my geothermal HVAC contractor advised, and even what I read here on GJ where Badger from Badger Boiler stated the same. I'm not an engineer, so I won't try and cloud the issue with fancy formulas or other baffle-gab.

I agree insulating under a radiant slab is a good idea. It will allow more responsiveness when changing thermostat setting, and may save heating cost by reducing heat loss. But it depends. If there is a high water table then the ground will act like an infinite heat sink. If not, at what level is the frost line? Here we bury water and sewer lines 8 ft deep, so at 8 feet the ground temp is always above freezing. We have an open loop geothermal system in our house, and our HVAC contractor said the aquifer temp is 43°F in the winter and 53°F in the summer.

The rate of heat loss increases with greater delta T. The greatest heat loss in a heated slab is therefore lateral, through the slab edge and/or stub wall, as it might be -20°F outside and +50°F inside. Hot air rises, but heat from a radiant slab radiates into the conditioned space and down into the the ground.

If a heated slab is uninsulated, the ground will absorb heat, so yes this increases total heat loss. But earth is an insulator unless wet. What about the sod huts made by Ukrainian farmers who broke the land here in the Canuck prairies in the 19th century? What about compressed earth walls for houses, also called rammed earth? I admit not as good an insulator as EPS or XPS, but it's not a major conductor of heat. So the delta T from the slab to the earth is less than the slab to outside air.


Insulating under a slab will reduce this heat loss. But is it cost effective? XPS is expensive. Workshop type garages are generally kept cool, and most people don't live in Canadian or US Siberia. Insulation reduces heat loss but in a non linear fashion. Going from R90 to R100 will not reduce the same amount of heat loss as going from zero to R10. Insulating the slab edge and/or stub wall is a must. Insulating the perimeter of a slab would be better than not doing so. Insulating the center is less important.

If I built a garage would I insulate the slab? Yes, because I live in Canadian Siberia. But R12 walls and an insulated slab may mean more total heat loss than R25 walls and a perimeter-insulated slab. It all depends on the local conditions. Is it cost effective? That depends on the local climate, energy costs, design temp, ceiling and wall design and insulation, etc, etc.

 

[Peter Mc Mahon]

I assume the floor will be heated for the entire heating season?
If the frost never gets in then the ground is reasonably warm.
Around here the basement of a house never gets insulation under the floor, and typically the floor does not have radiant heating in it. Not a big deal at all
So, a 6" thick concrete floor in your shop that is heated to ?degrees is a lot of mass. Under that mass is either a thermal break [ insulation] or earth that is not all that cold. Will you loose a little bit? Yes, but a lot, No.
If you have 'spare' money do it, if not...I wouldn't.

 

[will gilmore]

Radiantec talks about this insulation method on page 6: http://www.radiantec.com/installation-manuals/design-construction-manual/

 

[Radiantec Guy]

In most applications we recommend insulating completely under the slab. Research and studies show that most of the heat loss in a slab is in the outer 8' so some of our customers will insulate the outer edge and 8' in. Most of these people, however, are using either alternative energy such as solar or off-peak electricity where some additional storage is beneficial. We did this in our 12,000 sf office/warehouse but the building is earth bermed and the primary heating source is solar hot water.

My recommendation is to insulate completely under the slab unless there is a good reason not to.

 

[jvitez]

Thank you Radiantec Guy for chiming in, much appreciate your professional knowledge.