Start with a hunk of concrete that’s insulated on it’s exterior. Could be the domes that I do, or could be ICF’s. Through research and empirically, it’s been shown that the mass has a moderating influence on the inside temp of the structure.
Now let’s modify the structure. Cut a slit in the exterior foam 10′ x 3″. Stick rebar out from there and build a concrete awning. (Or bench, shelf, or whatever you wanna call it.)
We’ve created an area of thermal transfer. No longer is the concrete totally isolated from the exterior.
The question is, how does one measure (or approximate) the effect? In a cold climate, heat stored in the mass will tend to migrate out through this area. In the summer, the heat gained by the exposed area will migrate to the cooler interior mass. What’s the practical effect? What’s the proper way to measure the effect? I’m ultimately trying to compare the energy cost of having the concrete exposed in limited areas vs the cost of having to wrap it with foam insulation.
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What I would do is assume, for a first approximation, that the insulating effect of the added thickness is negligible, since the "fin" is so thin.
So one could figure the heat transfer of a square foot of your concrete wall plus foam insulation (ie, 1 divided by the sum of the two R values), and then figure the heat transfer of a square foot of your concrete wall without the foam (1/R). Then multiply the number of square feet of each by the heat transfer per square foot and add them together to get total heat transfer for the wall.
Divide total square feet by the total heat transfer to get an average R value for the wall.
lets make it harder, as the sun come up the mass heat up, greater, greater till high noon then start the cycle down, at night it start cooling off , till morning and it start heats up again. Put this into a cycle of winter fall summer spring.
I feel( which I know I,m wrong )that the mass heat stored again the cooling will cacel out each other and vice versa with the heat cycle so the interior temp start somewhat( close but no cigars) constant. so without any insulation temp could stay constant depending on size of mass
Now this also have to factor how much heat and how much cool in your neck of woods. In the desert west where it get 110 in day and 30 at night this would work. but on the gulf coast where its 95 day and 90 night, its always hot.
Edited 6/24/2004 4:53 pm ET by BROWNBAGG
I think if you assume something other than a desert (island?) location such that you're heating or cooling exclusively for at least several days at a stretch, then the mass effect can be ignored so long as you figure the heat gain/loss based on the average outside temp.
Even if you do assume a daily cycle that goes between the extremes, the assumptions of my first post are probably still valid, it's just that you need to factor the mass into the absolute cost computations, something you'd have to do for any massive wall design.
"What's the practical effect? What's the proper way to measure the effect? I'm ultimately trying to compare the energy cost of having the concrete exposed in limited areas vs the cost of having to wrap it with foam insulation."
The practical effect is negligible.
One way to measure the effect is to take temperature readings of outside air temp, the inside surface temp at the point of the anomaly and at some other point representative of the undisturbed structure.
To model this situation accurately is not practical. The overly simplistic model suggested earlier would be useless as a practical prediction of anything. An accurate heat transfer model would be very complex and in my opinion, not worth the effort.
Tim
>The practical effect is negligible
That's my belief as the designer. The builder and the owner's brother or father or whatever are putting the idea in his head that it'd be the equiv thermal leak of a big wide ol' hole in the wall. The alternative is kinda expensive and I'd rather see the money go for more relevant items.
Looking for how to demonstrate it to him...
The cost may not be worth calculating, but you will definitely have a cold spot at the base of the "cooling fin". The R value of the concrete is considerably less than 1 right there, similar to a sheet of plywood. If that area is bare concrete on the interior, it will sweat all winter, and even frost up during cold snaps. That's tough on paint, and the drips will streak.
Maybe you could make the "fin" out of a SIP and then stucco it.
Edited 6/26/2004 2:13 am ET by KevinD
cloud... it's 2.5 sf with an R-value of 1... if your design temp is 70 in.... and say 20 out that's a Delta-T of 50
BtuH = Area x D-T x 1/R 2.5 x 50 x 1 = 125 BtuH at design temp... you can determine heating costs by using your Degree Day for your area of construction.. our's is 6000 Degree Days
consensus seems to be that air movement has more of an effect on comfort and heat loss than static loss...
net effect.... find something else to worry about
Mike Smith Rhode Island : Design / Build / Repair / Restore