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Sustainable Construction Articles

       

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Arm of the Spiral, © copyrighted material. Please do not reproduce in any form without express, written permission of the author.

       

The Importance of Energy Heel Trusses in Straw Bale Construction

Would you go skiing or ice-skating in subzero weather with the finest down parka and no hat? Or answer me this: for a quick cup of tea, would you put a lid on the pan or leave it off? Whether it’s a human body or a pan of water on the stove, everyone knows most of the heat escapes out the top. But when it comes to straw bale houses, too often builders neglect to insulate the roof sufficiently.

I have a problem with straw bale homes that have a wall package of straw with an R-value of 30 to 40, but then the roof is framed with either a wooden I framed roof or a standard trussed roof that will hold today’s minimum code requirement of R-38, but little more.

As recommended by Amory Lovins, efficiency guru of the Rocky Mountain Institute, it is important to have a roof package of R-60 to R-80 to keep your home warmer in winter and cooler in summer. An energy heel truss allows for the thicker roof insulation necessary when energy costs escalate. Additionally, this system allows for air flow above the attic insulation to carry off moisture migrating from the inside of the house toward the outside air.

This is simple physics. The warmer and moister air inside the house (because of the occupant’s breathing, cooking, showering, and house plant transpiration) is driven through the walls and ceiling, trying to reach an equilibrium with the humidity of usually drier and colder outside air. When this humid inside air travels out through the house envelope and hits cold outside air, the water vapor will hit the dew point. At the dew point, water vapor condenses into liquid water, or if the outdoor temperatures are cold enough, forms ice. This can have negative consequences for any home and especially for straw bale walls in colder climates, for when this air vapor condenses, freezes and continues to condense and freeze over many frigid nights and days, there can be a considerable buildup of ice at the north side of the attic insulation.

When outside temperatures finally warm, this buildup of ice will melt and either run into the north straw wall or drip through the ceiling and into the house. Over time, this buildup of ice or water in the attic insulation and eves can cause a great deal of damage, mildew or rot. I have seen this happen to several houses built in the Gunnison, Colorado area. No air flow was planned to carry off this moisture that forms in the lower attic spaces near the wall perimeter. An energy heel truss will allow for more attic insulation all the way to the wall perimeter, and also allows for ample air flow above the insulation; especially when used with continuous soffit vents, step-down truss blocking and adequate gable vents, ridge vents, or roof vents.energy heel truss diagram

When I do use energy heel trusses on very energy efficient homes, I usually have 24” of cellulose insulation blown into the attic space (R-80). Cellulose is recycled paper that has been treated with a boric acid flame retardants. Recycled cellulose insulation has much lower embodied energy content than does fiberglass. A pound of fiberglass batting has 12,000 Btu of energy per pound involved in its manufacture, vs. 750 Btu for a pound of recycled cellulose insulation. Modern blown cellulose has an R-value of between 3- 3.7 per inch1. Before the cellulose insulation is blown into the attic space, and because the depth of this insulation is so great, I usually frame a cat walk into the attic framing, above where the insulation will go, so that the owner or maintenance worker can move about the attic without destroying or compressing this thick layer of insulation. It is also important to install a polyethylene vapor barrier on the underside of the truss bottom cords before the ceiling sheetrock is hung. The vapor barrier is helpful in keeping down the amount of water vapor that migrates into the attic insulation. The vapor barrier can only stop but a fraction of the water vapor, so the air flow above the insulation is essential.

It is possible to get energy heel scissor trusses for vaulted ceilings, but these trusses typically make it harder to work in the attic area. Rather than using scissor trusses, I build the house with nine foot ceilings -- an elegant alternative to the vaulted look, in my opinion.

Energy heel trusses are great for straw bale walled homes: they increase the energy efficiency, make for a uniform insulation layer throughout the attic and make for an insulated lid that has adequate room for air flow above the insulation. Energy heel trusses should be the roof of choice on a straw bale home.

1Environmental Building News, Volume 4, Number 1, January/February 1995.

A building contractor for over 25 years, Steve Schechter has designed and built numerous straw bale homes and additions over the years, including the first permitted straw bale home in Gunnison County, Colorado in 1995. Slated to achieve another first, Schechter will build two straw bale homes within Gunnison city limits this year, one of them being a load bearing structure.

Schechter is a board member of the High Country Citizen's Alliance and serves on the Gunnison River Water Conservancy District board. "I believe that it is very important for all citizens to give back to their communities. This is what keeps a democracy vibrant and makes for a progressive and caring community," he writes. Schechter is also the wise-cracking concrete worker in "You Can Do It! -- Part One."

 

 

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by Steven Schechter, Mountain Solar Design, mtnsolar@gunnison.com

 

 

 

 

   

A Builder’s Three Best Friends

In the beginning was the footing. And the footing was square. And the footing was level. And from the footing issued the stem walls, which in their turn were square. And rose they forth from the footing in a manner that was plumb. And the Builder looked upon the foundation of her house and said, “It is good.”

You’ve done the research. You have house plans. You’ve secured a loan and purchased land or a lot. Now you’re standing in your lace-up work boots looking at the house site and wondering how to start. The soundtrack of your life is playing dilemma music, and it’s getting louder. Suddenly there’s a ripping sound like the awful yanking of a needle off a record album and then silence. A voice reminds you: “You’ve done the research. You know how to start. You’re a tad old if you know what a record album is, but that’s not important right now.”

Your first consideration in orienting your house is the sun. As a straw bale home owner/builder, you know how important old Sol is to your project. You’ve got yourself a source of nuclear energy a safe 93 million miles away. Some folks like to align their walls to true north, others like to get a jump on the morning sun by orienting their south wall a few degrees east.

Next, you consider the natural slope of the land and how water will drain away from your site after a heavy downpour. Then, you regard the trees. Must I cut a few down? Can I save that one if I sacrifice optimum solar orientation? All these factors and more you must consider, realizing that there is no single answer, just your informed answer. After enough contemplation, you fire up the rented excavator and commence to clear your site.

The trepidation involved with excavating for your house is short-lived if you’ve got some knowledge on how to proceed. The jitters last only as long as it takes to remove the brush and rocks and start to flatten the mounds. Pretty soon you’re boldly pushing dirt like a badger. All the organic material, roots, and branches have been piled away from the site and everything is relatively flat.

Now you need a coworker and a transit. A transit is essentially a swiveling level on a tripod, equipped with a scope. Once you get the scope leveled in all directions, it provides you an imaginary level plane at eye level. Preferably, you want to set and level your transit at a spot where you can scope every point within your site, but out of the way of your excavator. Then, your coworker, friend, husband or wife will hold a measuring rod on the ground at various spots on your site. You will view this rod through the transit scope, taking readings on the measuring rod (often called the story pole) and determining the high and low spots on your seemingly level site. Bring a notebook to record your numbers. It will take a little thinking and head-scratching to become at ease with the transit. High numbers mean low spots and low numbers mean high spots. Your actual reading isn’t important; it’s the variations from any baseline measurement that you are concerned about. You’ll get the hang of it. Your assistant will mark the ground with a can of spray paint, indicating high and low spots.

This is the time when reality usually sets in that your eyeball can deceive you. A slope or a crooked tree in the background can throw you off dramatically. Do not despair. Realizing early in the game that you must depend on measuring instruments is a good thing.

Now we’re not concerned with perfection at this stage. You are going to end up about eight inches above grade (what you’re creating) with your finished floor height, so a flat surface within two or three inches is just fine. Once you’ve marked the ground and have an idea where you need to scrape and fill, you hop on the excavator once again and have at it. Then it’s back to the transit, repeating this process until you’re satisfied that you’re relatively flat.

For those who feel that budget-mindedness begins now, the trusty water-filled, clear plastic tube makes for an accurate large scale level. It may be a hassle at this stage though, because you’ll have a lot of mounds and valleys to run the tube across, and you have to make sure that you don’t have an airlock in the tube. This device works on the principle that a single body of water seeks its own level, and so if your partner is holding one end of the tube perpendicular to the ground and measuring the water line at 12 inches above the ground and you are too, then the ground is level at those spots. You can move the ends around to all the suspicious areas until you have a feel for the site terrain.

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Using a water-filled tube for finding level. The tube can be any length you want. The ends of the tube can be moved to various spots on the site you are checking for level. If measurements taken with your respective tapes are equal, the ground is level at those two spots.

 

My best recommendation is to find someone who will lend you his transit. Our local rental shop charges 32 bucks a day for one, and since you will need it on an ongoing basis, that could get pricey. Perhaps a builder or farmer friend would lend you his for 50 or so bucks a week.

The next step is to establish a corner and a baseline. Maybe it’s your north side, so you’ve got sufficient setback from the trees or a property line. Maybe it’s the south side, where you know how you’re going to get your solar gain. You and a coworker walk around with a 100-foot measuring tape and get a feel for how the house is going to sit on the site. At last you’ve chosen a corner. You drive your first stake (just so it’s firm, no big deal, because you’ll be pulling it soon enough) and with a compass and the know-how to figure declination from true north, you lay out a wall. Now we’re getting somewhere.

There’s no looking back now. Measure out the length of your first wall, whichever it is, and drive your second stake for corner number two. When I take long measurements or don’t have a solid anchor to hook my tape, I like to “burn a foot” on the measuring tape. That means having a coworker holding what they call the dumb end of your tape at your corner at the 1-foot mark. That way, he is not trying to read a metal hook, but an actual measurement marking on the tape each time. The only thing he has to remember is that he must burn a foot each time you take a measurement, and YOU must add that foot to your measurement at the smart end of the tape. This may sound complicated, but believe me, the reasons for it become as plain as a windy day. The person on the smart end of the tape walks out to the next corner of the house site and takes a reading. Be sure to pull the tape tight. Also make sure your partner likes his reading before you make yours.

Assuming your next wall is a right angle to your first, you will now use good old Pythagorus to lay it out. As you and the Scarecrow in the Wizard of Oz know: the square of the hypotenuse of a right triangle is equal to the sum of the squares of its two sides. Phew! That’s a mouthful, but the simple fact is that if you want a 90 degree corner, then if you mark out three units (inches, feet, miles, etc.,) along one side of the angle, and four units along the other side, then the distance between those marks will be five of the same units. For better accuracy, double each measurement and you get the 6-8-10 rule. Adjust the line of your second wall until the three numbers reconcile, and then mark that spot as accurately as you can in the dirt, or with a temporary stake. From your second corner, you can now extend your second wall line through your mark to the full length of your second wall. Drive your third stake. You now have laid out three corners and two walls that are square to each other.

If you executed layout for your second wall correctly, there should be no need to do it again for the third wall. You’ll be checking it later with another squaring method anyway. Let’s assume your house footprint is a rectangle (it probably isn’t, but once you learn this method, you can stack your rectangles and even add curves and forty-fives if you’re brave and rich). Your third wall should be of equal length and parallel to your first, so if you go back to your first corner, the one you staked out in the beginning, and measure out the same length and direction as your second wall, then you can lay out your third wall. Drive your fourth stake, and now you’ve got a rectangle defined by four stakes at its corners.

Time to double-check. I don’t know to whom credit should go for this method, but it’s a geometrical fact that if you have a right rectangle (four square corners), then the diagonals of that rectangle should be equal. First, make sure that the opposite sides of your rectangle are exactly equal to each other. Refine if necessary. After that, take a reading on one diagonal, and then the other, remembering the “burn a foot” method. It doesn’t matter what the measurement is, just that it’s the same. You should be close, but more than likely you’re not perfect yet.

If you are more than, say, an inch and a half off, you should go back to the 6-8-10 method on your corners to find out where you might have gone wrong. If your diagonals are off by less than that, you’re close enough to lay out string lines. The ultimate goal is to be within one eighth of an inch difference between your two diagonals, which you will be able to do with your string lines.

Now it’s time to drive board and battens. Basically, they are two stakes driven into the ground with a horizontal crosspiece attached between them, forming what looks like a capital H. With your partner, stand on opposite sides along the line defined by two corner stakes, well outside of your house site with room for your excavator and piles of dirt -- you want these board and battens to stay put until your stem walls are completed. Stretch a survey line until you have extended it to where you want to put your boards and battens. Drive your stakes firmly into the ground in positions that put the line of your wall bisecting the horizontal batten. In all, you will be driving eight board and battens around the perimeter of the site.

Stretch survey string tightly so that it lines up with the four stakes of your house corners, tying the string to the horizontal battens. You now have a rectangle in the air above your four corners, but even better, this rectangle is adjustable. With a combination of measuring your diagonals and making refinements on where you wrap your strings at the battens, you can create a nearly perfect rectangle, within an eighth of an inch margin of error. Don’t be afraid to go back to the 6-8-10 method and measuring your wall lengths to make these refinements. Now that your string is up, you can mark it precisely and get an exact measurement for your hypotenuse. When you’ve got an excellent rectangle, mark and notch your battens where the string is located -- that way, you can take down your string lines and reset them in the exact same place.

Are you still with me? We’re all square, now we just have to talk about plumb. The next step is to use white lime as a ground marker and create a line on the ground directly below your strings, which should be only about 12 to 16 inches above grade, by the way. As you pour your lime, shaking the lime from a coffee can, you can pull up your original stakes. You’ve just created excavating lines. It’s time to wind up your survey string and set it aside. You’re about to make a mess. You may wonder at this point why you created such a precise outline of your house, only to remove strings and dig rough trenches. Well, all this prep work means that once you’ve excavated and the dust clears, you can put those strings back up and project plumb lines down to the bottom of your footing trench -tres magnifique! Just don’t bump those board and battens or you’ll have to start all over!

Below frost line for your area, you will pour a concrete footing on which to build your stem wall. Now I had to form both sides of my footing, having been forbidden by local code to use one wall of the trench as a footing form. Therefore, to provide enough space to swing a hammer, my trenches were four feet wide! You may be able to just pour directly into a neatly dug trench in your neck of the woods. Whatever the case, just remember that your stem wall will describe the outline of your house, while the footing should be wider, so that the stem wall can be roughly centered on it.

As I said, when you get through excavating, it will look as though a giant prairie dog hit the site. You may even run out of places to pile your dirt. Whatever you do, try to leave the space for your survey string to go back up -- you’ll have to clear it one way or the other.

After all that digging, it’s time to put your strings back up. You can rest assured that amid all those mounds of dirt, you’ve got a darn near perfect rectangle hovering over your trenches. All you need to do now is project your corners straight down to mark the corners of your house in the trench. How to do that? Why, with a plumb, Bob ... I mean a plumb bob. Plumb is just the opposite of level, it means straight up and down. You’ve seen plumb bobs before -- they are simply weights with a string tied on one end and a point on the other. Gravity does the rest, telling you exactly where straight down is. Hold the plumb bob so that it touches the crossed lines of your survey string. Allow the plumb bob to stop swinging and then mark and stake the spot in the trench that is plumb from your suspended corner. Do all four corners that way and then do the math to add the proper exterior lip of your footing.

Keep that plumb bob with you, it may become one of your best friends. Check and double check your work around the perimeter of the house. It is now time to make sure your trenches are level, but of course, you already know how to do that with the transit. Have some 3/4 inch road base on hand to fill and compact the low spots and follow all local building codes (you won’t regret conforming, in this case).

If any or all of this has been absorbed by your gray matter, you should be able to find and make level, plumb and square from here on up to your roof. Even when you build your cabinets, you’ll find much of the same principles apply. Be diligent and precise, but remember that perfection doesn’t exist here on Earth.

Above all, have fun.

 

 

 

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by John T. Rehorn, john@armofthespiral.com(he's enjoying one of those famous pretzels on the Staten Island Ferry.)

 

 

 

   

"You Can Do It!" DVD Provides Inspiration, Perspective for the Owner/Builder

If you are looking for a Spielberg film, then Arm of the Spiral's "You Can Do It!" may not be it. Maybe check out "Building with Awareness" -- much less 'hand held' filming.

If you are looking for a life metaphor, an inspirational film, in a 'Rocky' sort of way, to motivate you into 'now I want to go build one myself', dispelling fears and offering hope and inspiration, this may be the straw bale film for you.

John is an owner/builder with little experience but the tenacity to step into the abyss of building his own home and the faith to know that there is help out there to guide him, that he will sprout wings and soar through the process...and so can you! John, wife Kathy and brother Ron are building the straw home while living life. There are some volunteers that usually find their way onto straw bale projects. Within the film there will be a day of bale stacking interrupted by a pan to a snake close by, or knee deep powder skiing, kayaking, a lizard, the aftermath of a micro burst that snapped a tree (bales still up), deer, robin eggs and don't forget Aunt Mabels Marvelous Pickled Beets.

There is this upbeat feeling about John and Ron as they go about the daily chore (especially while lifting the 75 lb. bales) that lends itself, as is often heard, to laughing and jocularity. John offers reassurance to the 'do it yourselfer' that building yourself is not as scary as you believe and not only doable but rewarding in ways hard to explain. Check it out!

 

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by Mark Schueneman, Director Colorado Straw Bale Association, 2005-2008

   

 


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