Monday, March 29, 2004

The Chicken House Effect

Anyone out there who is or someday may be faced with the prospect of retrofitting insulation into an old house ought to be aware of a phenomenon I have dubbed "The Chicken House Effect" (to distinguish if from the better known Greenhouse Effect, of course). I noted this principle when I was insulating the little chicken house I built to brood the chicks early this spring. I had a constant heat source (250 watt heat lamp) and a thermometer in the house. I insulated it in sections, slipping in rigid styrofoam panels. The insulation was having almost no effect on the temperature inside the house, until I put in the very last panel and there was a sudden quantum leap. I found this curious, so I pondered on it, and realized the principle: When you are progressively insulating a house, there will be frustratingly little effect until you get down to the final bits, then it will seem like all the insulation suddenly starts working simultaneously.

Two lessons I have taken from this:

(1) Take heart, even though it seems like all that nasty filthy irritating work you are doing isn't making a damn bit of difference, just have patience. As you get closer to the end of the job, it will all pay off.

(2) Those last little really difficult and costly bits (the storm windows that each have to be individually custom fit to those damn antique windows; those few square feet between the chimney and the window that you have to insulate by stuffing tiny handfuls of loose fill through a narrow crack; crawling in the dirt under the floor amongst spiders and three generations of defunct wiring and plumbing) are very important, and worth the effort and expense.

My own (full size) house has only recently started to experience the Chicken House Effect, after insulating it in stages over a year and a half and wondering why it was still so damn cold.

When I thought about it, the reasons for The Chicken House Effect were in fact very simple... It follows naturally from the way that R-values average reciprocally. The basic concept, though, is that like water, heat finds the easy way out. If you have a leaky bucket you really have to plug all the leaks before it will hold water. Same for heat leaking out of the house. For R-values, let's say that in a house like mine the uninsulated walls (no sheathing, clapboard you can see daylight through, leaky single pane windows) have an effective R-value of 1, and the insulated walls have an effective R-value of 10 (allowing for windows, studs, etc.). You might think that in a house where half the walls are insulated and half aren't, the effective whole house R-value would be the average of 10 and 1, i.e. 5.5. But it don't work that way, dudes. You gotta turn everything upside down, average that, then turn the average upside down. So, the effective R-value of that whole house is really:

R= 1/{(1/1 + 1/10)/2} = 1.8

So you are still freezing. Picture a barn with only two walls and half a roof; how warm will that be? You don't get up to an R-value of 3 until you are 75% done with the job.. and then you might start to notice a little benefit. When the job is 90% done you have your R-value up to 5... halfway there. So that final 10% of the job will give you as much extra warmth as the first 90% of the work did!


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