Edge loss calc for radiant slab

Timco

Brad...how many pens / pencils do you have in your pocket protector right now? You just threw all that out from memory...didn't you!

Tim

E.T

Edge loss calc for radiant slab

Is there a quick calc for thermal loss through the outside edge of a radiant floor slab?

Brad White

How well

is it insulated? It is all delta-T and R value.

A slab edge with R-5 will emit 0.20 BTUH per SF per degree F. difference. If R-10 (My favorite), will emit 0.10 BTUH per degree F. difference.

If a 6-inch thick slab is held at 85 degrees and the outside edge is exposed to the weather at 10 degrees and with R-10 insulation, it will be:

(85-10) x 0.10 x 0.5 SF = 3.75 BTUH per LF of slab edge. I have usually taken 10.0 to be safe. This also accounts for the fact that there may be tubing there, hard as we would try, and that the slab temperature surface may be 85 but the slab edge may be warmer due to tubing proximity.


If the slab is bounded by a foundation wall of say 8-inch concrete, this will add another whopping 0.64 R value.

If the slab has no insulation at all, take your pick.

This does not include downward nor the desired upward losses of course. Finite element analysis will show a radiating band which will change over time as the earth warms.

E.T

Brad - thanks for the info. Here's the scenario - it's a suspended 8" slab in an apartment bldg. with R-3 curtain walls above and below (no edge insulation, 0°F outside design temp).
Engineer wants to somehow quantify what the loss would be through the slabs edge

Brad White

That is why I used \"10\"

as a BTU factor, Tim! I have ten fingers, ten toes but for some reason cannot count to 21 with my pants on...

Brad White

My word, Eamon....

I have to stand on a small soapbox here...

New building? Radiant sold as a high efficiency feature? No continuous (fly-by) insulation outside the slab edge?

Somewhere, an engineer weeps as a dog barks in the distance...

Where is the architect in all of this, I have to ask? What does the energy code say?

I think it would get back to normal slab output practice.

If a square foot of radiant slab emits say 20 BTUH from an 80 degree slab to a room at 70 degrees F. (10 degree DT slab to ambient) and 40 BTUH from a 90 degree slab, (20 degree DT slab to ambient) then extrapolate that out at 2 BTUH per degree delta-T in rough numbers.

If your slab is holding at 80 degrees and the outdoors is zero and your slab is 8 inches thick, that is almost 107 BTUH per linear foot.

If your system does not have glycol in it, it should. A prolonged power outage or someone filling the system with water and forgetting to warm the slab and keep it so during winter construction, would have a bit of a mess on their hands.

For my $0.02, a continuous "fly-by" insulation system will yield benefits beyond energy alone.