The Energy Adviser

[John Raabe]

A public electric utility in Washington state (Clark PUD) has steadly assembled a great deal of good information on energy-efficient home construction over the years.

http://www.clarkpublicutilities.com/Residential/TheEnergyAdviser

This is good information for just about any region of the country.

[JohninVT(Guest)]

While it isn't intended, this shows how misleading insulaion R values are.  Doubling the nominal R value of a ceiling increases the ceiling heat loss efficiency by only 28%...not by 100%.  Since 15% of a given houses's heat loss is through the ceiling/attic and you're increasing that 15% by 28%.....You've only netted a 3-4% increase in efficiency overall.

Say you're building a small 1000-1200 sq.ft home and your roof is 1000 sq. ft.  If you used urethane foam panels you'd need to buy an additional 30 sheets to double the R value.  Your cost would be roughly 1100 bucks to net a 3-4% increase in efficiency.  If you burned 5 dollars worth of oil a day every day all year, you'd only save $73 a year.  It would take 15 years to break even.  Realistically, you'd never break even.  You'd be better off upgrading your heating source to make it more efficient than trying to super insulate the house.      

[Amanda_931]

If you're burning buffalo chips in Manitoba--or wood from your own kind of marginal woodlot in the U.P. it might be worth the extra.  Or if there were a major depression at the same time that oil went up a couple of hundred per cent.

And there's definitely something to be said for solar and thermal mass type gains--if you've done the smart things about shading don't have--as I have in a couple of rented places--heavy duty solar heating on summer afternoons.

Lots of things do not increase or decrease in a linear fashion.  Whether all natural or man-made like seismic measurments.  It might well be that "comfort" is one of them--that, say, cold walls or cold ceilings lead you to increase room temperature more to a higher temperature than you would if they were warm.

[bartholomew]

If this is the article you're talking about... //www.clarkpublicutilities.com/Residential/TheEnergyAdviser/Archives2001/2-01-3 ...the example given is for an increase in attic insulation from R-30 to R-49, only a 63% gain. The article claims that will reduce conductive heat loss by 28%, which seems to be a mistake. Heat loss is inversely related to insulation R-value. If there is 1.63 times as much insulation then the heat loss will be 1/1.63 times as much, or 61%, so the improvement is 39% not 28. That improves the economic analysis a little as the improvement for the house as a whole goes to 39%*15% = 6%.

The reason people often focus on adding insulation to the attic is that it is easy to do and there is usually lots of room to pile on loose fill cellulose or unroll layers of fibreglass. Both are cheap, much cheaper than urethane foam board. It really only makes sense to even consider foam board where there isn't room for anything else... the walls and maybe the attic eaves.

One shortcoming of talking about R-value is that it only considers conductive heat loss. It doesn't take into account air movement through or around the insulation. In most cases, the cheapest way to reduce heat loss is actually not by adding insulation but by sealing off all the gaps and cracks which allow heated air to escape.

In any case, crunching the numbers to determine the financial payback makes a lot of sense to me.