Truss vs Rafter

Ultimately, energy efficiency is a function of DESIGN

Design is often confused with decoration because the word is often misused in just that way. This is also the word engineers who in are the business of specifying a heating system for new building must use to describe what they do.

That big ugly SUV your neighbor bought when gasoline was 99 cents a gallon will continue to get 8 miles per gallon until it is ground up for scrap.

Houses can be retrofitted for better energy consumption but such a project is very expensive, requiring costly parts like triple-glazed windows and fussy, labor-intensive efforts like re-insulation. Because such projects are so expensive, they almost never pay for themselves in energy cost savings. And so they don’t get done very often and those who would do this sort of work, never get the chance to improve their skills.

Probably the most significant contributor to increased energy consumption in USA over the past 50 years is urban sprawl. This is a problem that no drive to increase energy-efficiency is likely to solve because the only way to increase the energy efficiency of a city once it has been built would be to move buildings around.

Use your Brain

Recognizing that design determines energy efficiency and overall consumption suggests a whole range of policy options that have a far greater chance for success than merely mandating higher standards. Let me try to explain with a really good example.

Several things, actually, if the intent is to insulate the building.

1) There isn’t much space for insulation where the roof and wall intersect.

3) Since typical insulation batts cannot be tapered, the practice is to just force them into place. Unfortunately, crushed insulation loses its effectiveness. So not only is there not much insulation, what insulation there is doesn’t work very well.

4) Heat rises. So the greatest amount of heat in the room just happens to be where the potential for heat loss is also the greatest. This little problem has probably pumped more carbon into the atmosphere than all SUVs combined.

The solution to ALL these problems in astonishing simple because almost no one uses rafters these days. (The replacement is the engineered truss and its use is so widespread that VERY few carpenters even know HOW to cut rafters any longer.) With an engineered truss, there is absolutely no reason why there should even be a rafter / wall intersection.

That simple solution is to DESIGN a space for the insulation to go.

No tapered insulation, and the installation of insulation vents is now optional.

So the labor savings to insulate this way easily make up for the tiny cost increases for custom trusses and finishing a slightly taller exterior wall.

The lessons learned

I have known about this insulation solution for almost 30 years. I have never had a truss builder say anything but “No problem” whenever I have asked if such trusses could be built. Yet I have actually seen this done less than 10 times in my whole life. The question is WHY?

It’s not my problem. Unless an architect or designer specifies this kind of roof / insulation construction, no one considers it. There are a few architects around who understand that insulation systems must be designed from day one, but these people are rare indeed. MOST architects concern themselves with the aesthetics of a building and just assume that the builder will be able to insulate whatever building has been drawn. The builder, in turn, often subcontracts out the insulation work and so remains essentially unconcerned about the labor considerations of the job.

The preservation of archaic traits. Rafters intersect with walls. It has been that way for centuries, why change now?

Who cares? Most homes are built for retail sale. The new owner will live in the new house for less than five years on average. Details like how the roof was insulated will hardly register as a consideration under such circumstances. Such home buyers will be much more worried about the wallpaper in the bathrooms. (sigh)

So a simple, cost-effective way to reduce major amounts of carbon emissions goes unused. This doesn’t require new technology or a major investment and yet it isn’t implemented because of social habits. Yet it precisely this realization that can guide public policy.

Suggestions

Since energy efficiency and pollution are largely a function of design, we could hold our designers to higher standards. For example, architects would not be able to get or renew their licenses unless they could demonstrate an understanding of total insulation systems. Building permits could not be issued unless the insulation details were specified in the plans and met VERY high standards.

Finally, new buildings could not be granted a mortgage unless they met strict energy requirements as indicated by thermal imaging and leak-down tests. The home buyer may only be interested in bathroom wallpaper but society as a whole has a HUGE interest in the structure’s energy consumption and should enforce these interests with lending standards.

Energy efficiency talking points

1) The bigger the energy user, the more important the need for social constraints. It is more important to regulate the energy consumption of cars than of TV sets, houses than cars, or cityscapes than houses. Good city planning is by far the most important element of any drive for higher energy efficiency.

3) Energy efficiency is determined by the laws of nature. Anyone who claims efficiency improvements that cannot be explained using basic physics should be ignored like any other charlatan.

4) Since the laws of nature cannot be altered through human legislation, the social desire for greater energy efficiency can only be assisted through legislation that provides for design and research funding, mandates minimum standards based on what is physically possible, or addresses social issues like land use planning.

5) Design and planning is extremely important. Just remember, once something has been built, it is very difficult, if not impossible, to change the amount of energy it consumes. For example, while it may be possible to mitigate the some of the consequences of sprawling a city over precious surrounding farmland such as with telecommuting, it would be MUCH wiser to avoid such disastrous decisions in the first place.

http://www.elegant-technology.com/kossack_energy_efficient.html