Under slab insulation

[gogolf0401]

I have a 40'x88' pole barn which will be finished with 5" concrete throughout with radiant heat tubing. Someone I know builds houses for a living (1-2 man show) and never insulates under the slab when installing radiant heat. He claims that if the ground underneath the slab is dry, that the added thermal mass will help heat the building or keep the heat longer. He claims he can have the heating system turned off and the temperature in the house will not drop that much in 12 hours. This is west central Minnesota BTW.
He claims that the powers that be are rethinking this matter, and are starting to think that under slab insulation is not needed when using radiant heat.

I should note that he does use 2" XPS around the perimeter 2' down.

I am skeptical... I still think that the energy needed to heat the earth under the slab and keep it heated would be significant... but I do think that it would increase the thermal mass and probably keep the heat longer... maybe..

Looking to see what you guys think since this would save me like $2500!

 

[Randy in Maine]

Who will be paying for the heat on this project?

You or him? If it is you, insulate it.

 

[Movover]

You will never heat mother earth enough.. You need to insulate, not even a question.

 

[BadgerBoilerMN]

"He" is not too bright. It happens in other states as well.

This comes from the dumb and dumber originating in N. Dakota where some power companies convinced local yocals that they could save a lot of money if they used "off-peek" electricity to heat the slab, and earth beneath overnight and shut it off during the day. They further convinced them that it would be OK to have a 10-20 degree indoor temperature swing in 24 hours.

Worse yet, the people on the Left Coast, decided that it would be a good idea set 6" of sand over XPS and tubing under of over that with concrete over all.

No end to human imagination, or lack of rational thought.

 

[dfiler2]

I know a person who put in radiant electric heat and did not insulate under the slab, in the 80's , wasn't too bad the first year, the second year was wetter and you needed a timing light to read his meter. Insulate under the slab, you won't be sorry.

 

[BadgerBoilerMN]

In a typical year it will take a week to heat the slab, the ground two feet down and finally the building. Once you have it charged it will work as advertised with the building overshooting the stat on and off for a month in the spring.

 

[INTMD8]

and you needed a timing light to read his meter.

LOL

 

[BadgerBoilerMN]

That is funny...if it isn't your meter!

 

[TeachDerf]

We're now building a 28x28 attached to house. We have 20"x 9" footer, 64" concrete block foundation on top of that. We've been given 1,000 ft of plex tubing and a manifold for radiant in-floor; BUT 2" rigid insulation will cost $1,000. We don't plan on heating the garage, (however the option would be there for kids that inherit the place someday). Anyone know of more economical alternative?

 

[914wilhelm]

Look on Craig's list for foam board. I've found good deals.

 

[BadgerBoilerMN]

1" 25# would be sufficient.

 

[jives]

I am not an expert, nor do I play one on TV. This topic is one, however, in which I have invested a lot of time for my own purposes.

The safest bet is to insulate under the slab, particularly if heating the garage on a daily basis. But, given the building design, the heating, the soil conditions, the climate, and more, it is not always that cost effective. See, for example:

http://www.healthyheating.com/Radiant_heating_designs/insulating-underslabs.htm#.VcDAVflKXOs

There is a view that perimeter insulation only (vertical or horizontal), as the OP suggested, is just as good. (Offhand I don't have those links.) Such a system works best when heat is collected during the warm months and stored in the soil under the building. Solar collectors with underground piping are one example.

There is another view, and one discussed on this forum a while back, that in an unheated garage that perimeter insulation only will enable the relatively warm ground in northern climates to help warm a building. The building needs to be insulated, of course (walls, ceiling). Consider a 20 deg air temp but a 45 deg earth under the slab of an insulated building; heat will move from the ground to the interior of the building.

In hot climates the relatively cool ground can be used to cool the building. Insulation under the slab can be used to help control the amount of cooling, but may require more sophistication.

 

[akpolaris]

With all the $$ you are spending to do this why cheapout on a couple of hundred bucks for the foam? Spend the money and do it correctly and insulate under the slab

 

[rburke65]

If ya don't do it now you will never do it. Insulate!

 

[CNGsaves]

Curiosity QUESTION for radiant gurus - - - - - - -
. . . . . . is there any circumstance that area UNDER slab (also under insulation) would be used as a geo-thermal field (put pex BELOW slab)??
(a) capture COLD for A/C purposes in hot climate) . . . or . .
(b) capture HEAT for cold climate)

 

[Elginz]

The insulation would also keep the concrete from sweating on the one or two hot humid days. I would think the ground would keep absorbing the heat and you would loose a lot more than you get back.

 

[Radix2]

Curiosity QUESTION for radiant gurus - - - - - - -
. . . . . . is there any circumstance that area UNDER slab (also under insulation) would be used as a geo-thermal field (put pex BELOW slab)??
(a) capture COLD for A/C purposes in hot climate) . . . or . .
(b) capture HEAT for cold climate)

I can't see any way that would make sense - dumping heat under a building you are trying to cool... Or refrigerating under a building you are trying to heat....

 

[DEnd]

Curiosity QUESTION for radiant gurus - - - - - - -
. . . . . . is there any circumstance that area UNDER slab (also under insulation) would be used as a geo-thermal field (put pex BELOW slab)??
(a) capture COLD for A/C purposes in hot climate) . . . or . .
(b) capture HEAT for cold climate)

Absolutely, though you would want to run well loops, not a field. A field would disturb the ground way too much. You would also likely want to up the insulation under the slab a bit as well, because ground sourcing does have an effect on soil temperatures, though much less with well loops than with fields.

 

[jives]

Here are a couple of sites regarding earth energy storage. The Green Building Advisor discussion is mostly negative toward the idea for small scale uses (e.g., homes), but they are considering active, large scale storage as a primary heat system. Passive low cost earth storage meant to moderate the inside temperature is different.

There are a few places in which small small pre-planned residential communities are using large scale ground-source heat pumps to heat the homes in the winter, and then reverse the process in the summer to store heat in the ground.

http://www.thermaray.com/radiant-fl...arth-thermal-storage-system-slab-heating.html

http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/using-sand-store-solar-energy

http://www.earthsongfarm.com/PAHS.html