Where is the slab edge

Ken

on slab edge, and the last thread was becoming cumbersome.

One subject the PE and I differ on is the significance of lack of slab edge insulation. He has come up with a number for the amount of BTUs lost, and then compared this to what would have been lost if R5 insulation had been used. He writes the difference is "minimal" and not worth trying to remedy.

I have run the numbers using John Siegenthaler's formula (pg 22 Modern Hydronic Heating) and get a heat loss factor about twice as big as the PE did.

However, in this case, i don't feel the formulas are relevant. My floor is poured tight to the footings, and on the outside the patio slab is also poured tight to the footings. This creates a solid connection of concrete and turns my patio slab into a radiant panel. Last winter when the plumber brought an infrared thermometer, I checked the patio slab. It was +6 F next to the building and slowly reduced to 0 about 5' away and remained at 0 to the perimeter. The thermostat had only been on for about 4 hours, after a setback to allow for testing. (The infrared gun was needed because the plumber didn't keep any record of which loop end went where. Balancing, what's that?)

My reason for doubting the formula is because it uses the linear distance of the exposed slab edge, whereas in my case, both the top and bottom of the heated patio slab have become the exposed edge, and that is a considerably greater area. Sitting at a desk near the outside wall, you certainly notice the heat isn't warming the inside floor.

I know the formulas are at best only estimates, but is there any support out there for my contention that it may greatly underestimate the heat loss in this situation? I know a few months ago hotrod was installing sensors under his shop outdoor slab for research purposes, but I assume he must have had a thermal break.

Thank you all for your comments on slab edge insulation, as well as Bill's concerns about the installation drawbacks.

Ken

hr

Go to

www.beaverplastics.com Click on library, click on Radiant insulation topic. Be patiant it takes a long time to load. Print it and pass it on to unbelievers! Excellent info here and colorful graphs from this Canadian company.

This should help.

hot rod

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Bill Clinton

a thought

Apropos the combined loop on the previous post: Is there more than one loop total in the area? That is; in the area where the thermostat is, is there anything more than the single loop that serves two areas? If so, perhaps the additional loops could be throttled to reduce flow and therefore output, pushing a balance toward the underheating area. You would have to throttle to about 85% closed to have much effect. Just a thought.

Bill

Chris Maderia

Edge Insulation

Ken,

I have recently desgined 2 jobs that were giving to me late in the construction phase. No edge insulation was used and in both cases the heat loss DOUBLED. In the first case, with insulation heat loss was 35,000 w/o it went to 70,000. In the second case, heat loss went from 8,600 to 17,000. Now you tell me that's minimal. Let's think about this. There is 139,000 btu's in a gallon of oil in case #2 you would waste approximately 18 gallons per every 275 gallons you purchased. In case #1 you would waste approx 69 gallons per every 275 gallons you purchased. I would say that's extreme.