Straw Bale and Green Home Building
Construction Questions and Answers

If you have a general straw bale house or green building question that might be of interest to other people, send it to us. Although we can not always answer all questions, we will attempt to post the ones with the greatest appeal. Contact us

Topics:

Hybrid Homes
Windows
Floors
Passive Solar
Roof Overhangs and Earth Plaster
Rubble Trench Foundations
Thermal Mass

STRAW BALE HYBRID
What is a straw bale hybrid home?

A straw bale hybrid home combines a variety of materials with new technology and old technology. Like hybrid cars, the combination of various energy systems and building techniques creates a home that is more efficient than any element on its own. A hybrid home may convert sunlight into electricity, use environmentally-sound materials that are healthy for the homeowner, take advantage of the local climate to heat and cool the home by managing solar energy, and use a wide variety of materials such as metal roofing for rainwater collection, straw bale for insulation, adobe or stone interior walls for thermal mass, and Low-E glass windows to maintain desired interior temperatures. Straw bale alone does not guarantee that you will have the most energy-efficient home possible. Proper window placment, the house's orientation to the sun, and interior thermal mass walls can have as much impact on the home's performance as the straw bale itself. It is the bringing together of the best materials for the job that makes a hybrid home.

WINDOWS
We have enjoyed your "Building With Awareness" video and will be building our own straw bale home later this year.  We were wondering what type of, and where you obtained, the windows for your project.  They are beautiful as well as practical.
—Mona K.

The windows on this straw bale house are double-paned, low-e coated, and filled with argon gas between the panes. The frames are wood on the inside and color-coated aluminum cladding on the exterior. Since this home is almost a mile above sea level, the argon gas will escape due to the low air pressre. The gas, at lower elevations, increases the insulation value of the window. The manufacturer is Pella, although there are many good window manufacturers to choose from. We chose a very high-quality window due to the intense sunlight in this part of the country and the desire to limit maintenace.

FLOORS
Received the dvd yesterday. Watched it last night. Wow. Quite excellent. A quick question—your slab used multiple pours. Can you tell me how you tied the discrete pours together (assuming this is necessary)? Did you use garden edging to contain each pour?
—Mark C., Washington

The slab contains a wire mesh that ties the floor together—even though it was poured in sections. We did not use any flexible board. The concrete was poured with a very stiff mix, so it would hold an edge. There has been no cracking, other than a few minor hairline cracks. The grout is holding up perfectly as well. You might want to use an expansion joint where the slab meets the exterior foundation. The radiant-heated slab floor will expand at a different rate than the unheated rubble trench foundation on the exterior walls. In this area, the grout between the floor and the top of the foundation at door entries has cracked a bit due to this expansion. A flexible grout compound would prevent this.

PASSIVE SOLAR
I am surprised that passive solar only meets 1/2 of your heating needs? I thought it was more than that, but re-watching it (the DVD), it seems like that's what you quote in the beginning, but is that really true?
—Rachel, Colorado

This is a conservative estimate. I did not want to state a higher percentage until I install monitors in the home to have real figures. I am cautious not to overstate anything in a home such as this as overselling can potentially harm the reputation of green building. If you come back two years later and say that the home actually worked even better than planned, that is a positive bonus. Everything is a balance. I could have added additional south-facing windows to increase the solar heat gain in the winter. This would have potentially let more heat into the home in the summer, thus increasing the cooling needs (windows do not insulate as well as walls). In a home such as this—and in this part of the country—an increased cooling load would have meant adding a mechanical cooling system and more PV panels to run the cooler, all adding to higher cost and complexity. An alternative cooling system, such as underground cooling pipes, could also have been used. I decided to go with simplicity—no cooling system whatsoever (other than the thermal mass walls and heavy insulation) in exchange for using a bit more natural gas in the winter as a backup. The backup radiant heating system is not turned on until the outside temperature drops below 30 degrees F. Based on the local New Mexico climate, I considered cooling the more energy-intensive problem to solve. Had I had built the home in a climate with daytime winter highs in the teens, I would have changed the design.

EARTH PLASTERS
AND ROOF OVERHANGS
I have thoroughly enjoyed the Building with Awareness video and now that you have been in the home for a bit, I was wondering how the earth plasters have been holding up on the exterior of the home.  We plan on earth plaster on our strawbale home but would like to put in smaller overhangs on the south side of the house versus 8-10 wraparound porches on the other 3 sides.  I am concerned the short overhangs (2-3 feet) on the southern side of the house might not be adequate to protect the earth plasters from the weather while still providing passive solar heating.  Could you comment?
—Machelle W., Colorado

If you designed your overhangs to permit the winter sun to enter, while cutting out the direct summer sun, your 2-3 foot overhang should work fine for protecting the earth plaster. The exterior earth plasters are holding up extremely well after a few years of exposure to the weather. The roof overhangs of the home in the "Building With Awareness" video are around 18-20".The earth plaster on the portal-side (east side) and the south side of the home (with only the roof overhang) look about as good as the day the finish coat was applied. The west side of the home, which is the least protected due to the high gable end, also looks great and is still very smooth. This side did develop a few minor thermal cracks since the wall will reach over 140 degrees F in the late afternoon in the summer, and then drop to the mid 60's at night. These cracks do not present any problems and can be easily filled, if desired. The north side of the home takes the brunt of the weather. This part of New Mexico receives severe thunder storms in the summer. When they blow-in from the north, the rain pounds the wall from an almost horizontal angle and the roof overhang does not add much protection. This wall is showing some pitting. It is strictly cosmetic. Even this wall will not need a new coat of mud for another 8 years. It would be quite easy to skim-on a new coat strictly for aesthetics (although from 10 feet away, the pits are not noticeable). So, to answer your question, if you use the earth plaster formulas shown in the video that include wheat paste, you should be fine with your overhangs. Remember, the thicker the earth plaster, the more years you can go maintenance-free. The plaster on this home is up to 1.5 inches thick. The earth plasters used in the interior of the home look as good as the day we applied them.

RUBBLE TRENCH FOUNDATION
Awesome video. I loved it and will use it alot during the construction of my own house. Your rubble trench foundation information is just what I needed with the high concrete prices these days. I knew that it was in the code, but trying to convince an inspector that it will work is another thing. On that note, did you have any problems with the Sandoval County inspectors concerning the foundation?
—Sean, New Mexico

In order to get a building permit, the rubble trench foundation required the stamp of an architectural engineer, as the permit department would not accept an architects stamp on that part of the plan. We had no problems at all with the inspectors and actually found them quite friendly and helpful.

These divided light windows
(the window glass is divided
into smaller panes) are
double-paned, low-E glass.
They are both energy-efficicient
and visually pleasing.

The wire mesh holds the
individual pours together and
keeps the floor from cracking.
The red tubes are the PEX
radiant floor heating system.
Heated water flows through
the tubes and puts the heat
where you need it—at your feet.

The large, south-facing windows
supply much of the winter heat
in this straw bale hybrid home.