No insulation at the center

Unknown

But you said it much better. That's almost bumper sticker material ;)

Dave_4

Under the radiant slab

I have been studying up in prep for doing RFH in a slab, and wonder about the value of doing no insulation at the center area, the area bounded by doing 2" of rigid foam board for only 4 feet or 8 feet of the perimeter.

The underlying concept here is that you want to insulate the slab to prevent heat loss at the perimeter, but at the center, you really want to get some semi-permanent heat into the ground to help warm things. A heat sink.

What say you?

Brad White_9

Canaries, Parakeets and Crickets all agree....

Cheap! Cheap! Cheap!

Ahem....

The earth is on average 50-55 degrees below frostline and about that year-round. (It mirrors the average annual above-ground temperature incidentally.)

That said, while it is a heat sink as you said, the cost of charging the heat sink is on your dime (or the dime of whomever pays the bill).

The perimeter is a great source of heat loss for sure. But that does not mean you should ignore the bottom of the boat.

Insulation is fuel you pay for once as someone once said in this forum.

jp_2

-27F

with that kind of temps to deal with and a walk out basement, 2 inchs is probably not enough. better hit the books again on heatloss calcs.

no point in preheating a slab a week before 'that' weekend you want to use it.

Unknown

Very large slabs on good sites can sometimes skip the center insulation, as the heat load there is minimal.

But I only see this in commercial applications.. very large slabs.

I would never recommend playing with this in residential. You really don't want to warm the ground. You want to warm the house.

ALH_4

thermal break

At least put something everywhere. Thicker is better, but the savings from the second inch of insulation is far less than that from the first. Insulate everywhere with extruded foam min 1" thickness. There are diminishing returnd for additional thickness. 2" is not twice as good as 1". Ideally the insulation would be 2" everywhere. I see little benefit from more.

Brad White_9

I agree with you there, Rob

for the simple premise that a slab that large has an interior space with no heat loss to speak of. Self-defining. But a house? Never would do that.

jp_2

footing?

what about the footing? if you are using ICF, are you insulating the footing(outside surface) around the walk out part? if yes then i'd agree more with andrew that 2 inch is fine.

PM_2

No insulation below grade

In Seigenthallers book they give no credit for under slab insulation, just perimeter insulation. The equasion gives a fixed R value of something like 40. I can see arguments both ways, any comments?

Unknown

First, that makes a serious number of assumptions about the conductivity of the ground.

Second, you can adjust for a known R-value.

Brad White_9

That is right

If the earth is wet, for conductivity, take your pick. Wet clay or matrix of water in earth will suck heat faster than you can make it. I say, define your thermal envelope. To not do that is hubris, that you can somehow predict and control the exterior.

Unknown

that is an excellent philosophy!

Dave_4

Howzis look?

Brad White

Actually Rob

I was merely reflecting and expanding on yours

Darrell

I read something somewhere once where somebody had done a study...no underslab insulation and they had perceptible delta T at twenty feet...that's alot of heat sink! One inch brought the Perceptible Delta T up to something like 7 feet. One and a half brought it to 5 feet, and two inch came to 4 feet. My numbers are close...but you get the idea. Remember, this is a vertical measurement...not a horizontal measurement. The rules are different for edge insulation...the Delta T is way bigger on edge insulation, so the insulation is more effective/important. Somebody smarter than me can put all the right words and numbers on it!

I do know of a job where they didn't insulate under the slab...a big shop...and now the creek out back never freezes until it gets to 30 below...talk about heatloss! How do you factor that into the boiler sizing?!