Thoughts on true R-value of straw bale?

While reading an article in Mother Earth News about building straw bale in the city, I was doing some additional internet searching and came across an article / perspective I had not heard before regarding straw bale

https://riversonghousewright.wordpress.com/about/18-straw-bale-wall-r-value-controversy/

In the article, the author argues that the methodology used to gauge the true R-value of straw bales has been somewhat controversial and I am curious what Jim and Bob have to say in response to these arguments. Before reading the article I had heard the 50+ R-value and had taken that at face value. After reading the article and more on the actual methodology and tests conducted on straw bale to determine the R-value, I am not longer convinced these (high) numbers are accurate.

Hi Bryan,

The article you referenced has a pretty good summary of the work that went into determining a straw bale wall's R-value, and the exact number may have once been a controversy, with some believing that the walls performed in excess of R-50 while actual test data showed values in the middle R-20s for a given wall thickness (18").

The most important variables determining R-values are the bale’s dry density and orientation in the wall.  The IRC Strawbale Construction Appendix S is silent about whether the straw comes from wheat, rice, barley, or oats, and it probably doesn’t matter so much.  It assumes that if the bales meet minimum dry density (more than 6.5 lbs. per cubic foot) that they have the following R-values:  

Bales laid-flat R-1.55 per inch
Bales on-edge R 1.85 per inch

We’re required to test a representative number of bales to make sure they comply, and I always test a representative number of bales in-situ in the wall before plastering.  The code offers a method for determining dry density--weigh and measure the bale's dimensions, determine moisture content with a meter, subtract the moisture weight, and the ratio of the remaining "dry" bale weight to the bale's volume gives you the pounds/cubic foot.

Given that baling equipment isn’t standardized across North America (or the world!) bale sizes can vary, but the range is between the mid to high R-20s for two-string bales laid flat or on edge, and the high R-20s to mid R-30s for three string bales laid flat or on edge.

That’s pretty good compared with conventional 2 x 6 stud wall construction that has an R-value around R-21, and it exceeds code requirements in most climates in North America.  

But when a straw bale wall assembly needs to do better, colleagues in the North East and Canada have developed a way to boost the assembly R-value by adding another layer of blown-in cellulose to the exterior.  There’s a detail and description in Chapter 2 of the book.  

As we say in Chapter 1 of the book, R-value is only half of the story! Something that conventional wall systems rarely have, and plastered wall assemblies have in abundance, is thermal mass. The bales themselves supply some mass, but the thick interior plasters are a key part of what makes these walls perform as they do, and what makes carefully designed and well-built straw bale buildings comfortable to be in. David Eisenberg, referenced in the article and one of the great proponents of building with straw and a co-author of the strawbale building code, relates a great example of this:  If you take a concrete wall and add a 2 x 6 insulated wall on the inside, you have a wall with the same R-value as if you took that concrete wall and added a 2 x 6 insulated wall to the outside.  The 5 ½” of insulation have the same resistance to heat flow (R-value) whether located on the interior or exterior of the building.  But the buildings will perform very differently because in the second example you have all thermal mass inside the thermal envelope where if functions as a large storage battery, moderating interior temperatures, while in the first example all that thermal mass is out there in the temperature extremes heating up and cooling down, day in and day out.

There are many modern conventional insulations with R-values much higher than straw.  All of them use more energy, sometimes A LOT MORE in their manufacture.  Few of them store carbon. None of them are locally harvested from an annually renewable food crop so they don’t support a local economy. Most of them are toxic in manufacture, and most of them are difficult to dispose of or have no known (current) upcycling or recycling potential.  And I use them judiciously where straw isn’t appropriate—e.g. in ceilings, along foundations, under slabs, or in crawl spaces.

Jim, this is great. Thank you for the thorough reply and for confirming some of the veracity of the article. It's good to know your thoughts. I also appreciate the additional perspective you offer with regard that R-value isn't the whole story. This makes a lot of sense and it seems that the modeling that the building industry uses to gauge effectiveness of a set of climate moderating technologies working in concert is not quite up to the task.

You mentioned the NE in your reply, which is where I live. This is part of the reason that I was considering R-value more in-depth just considering what the code requires here. Here the winters are very cold and not having proper insulation could result in a serious issue so it is understandable that the code would require more stringent (higher) numbers than where I am originally from in the PNW.

Your post brings to mind a few more questions:

With consideration of the embodied energy of straw vs cellulose, why would one want to bother with the additional "concerns" of straw if cellulose offers a reasonable alternative and is a well understood insulation medium? Maybe this is another way of asking "what makes cellulose 'bad'"?
Does using cellulose on the outside of bales add additional considerations that a builder needs to be concerned with? framing, vapor barriers, etc?
How much thermal mass does the internal plastering add to a wall? I've only ever thought of it as a coating vs something like cob or earth bag where there is a very significant mass but I could be mistaken in this regard.

Hi Bryan,

Nothing makes cellulose bad.  In fact, it has the second only to straw in having the lowest embodied energy of all building materials.  I should explain why that’s important.  Assuming you agree that humans have had something to do with the changing climate, and you agree that we don’t have generations upon generations to resolve the problem before we have passed some point of no return, which is not good for us or the many other species we share the earth with.  We have a decade.  Maybe two.

If you build a Net Zero (energy efficient) house with low embodied energy materials the house begins to make an immediate difference—it starts storing carbon while not using much off-site energy, if any, to heat or cool.  But if you build a Net Zero house with high embodied energy materials like foam, steel, concrete, etc., it might be years before that house begins to make a net contribution because the building materials have taken you into embodied energy debt.  Any house build is going to take some energy, but the fewer steps backwards that you take before moving forward, the better, because your Net Zero house will make a more immediate difference.  That’s why looking at the embodied energy of building materials is important.

I recommend using cellulose in ceilings and wall cavities where straw bale doesn’t make sense (e.g. upper portion of gable walls where bales just don’t easily fit into the triangular space).  I also use rock wool because it has a higher ignition temperature and most of my work is in areas prone to wild fires.

Cellulose doesn’t have the thermal mass of a straw bale, nor is it usually coated with up to 2” of plaster on the interior, so while it’s an incredible insulator, it doesn’t do both.

As for thermal mass, approximately 2” of plaster on the interior walls has about four times thermal mass of ½” of sheetrock.  This provides a broadly distributed heat exchange with the interior air, which moderates the interior temperature fluctuations.  The thicker the plaster (see my recent post about how thick the code allows—primarily an issue in seismic areas), the more mass.  

From an aesthetic perspective, cellulose walls look like most other conventional buildings—painted sheetrock on the interior, some form of siding on the exterior.  Nothing special.  Aesthetics is among the top reasons people are drawn to straw bale building.  They report experiencing a “sensory nourishment” we just don’t get with the industrially flat and angular surfaces and finishes more common in conventional construction.  It may be that the plastered straw bale walls resonate on some archetypal level; or that we appreciate the hand-crafted plastered surfaces, which are rarely sheetrock flat and smooth, so that the lighted wall surface has variety and movement throughout the day.

This is where straw bale and cob can be a good pairing.  Cob is excellent thermal mass, not so good as an insulator.  Having interior cob walls, cob benches, etc., inside the plastered straw bale thermal envelop boosts thermal mass.  Speaking of cob, insulated earth floors can play a role here as well.

For the best information on the extreme cold weather combo wall system pairing straw bale and cellulose, contact Chris Magwood at the Endeavor Center in Ontario, and Jacob Deva Racusin with New Frameworks Natural Building in New Hampshire (both contributed to this book!).  They have developed and refined this combo wall system and can explain much more about it.

Jim,

Thank you again for the excellent information. I didn't realize that the plaster was approximately 2" thick! That is substantial and I can see how that thermal mass would greatly change the equation in a favorable direction for straw bale.

It's funny you mention Jacob because as I was reading your reply I was recalling information from a book I have in my library, The Natural Building Companion, which just so happens to be co-authored by Jacob. In the beginning it talks in great detail about embodied energy and it was through this publication I became aware of some of the embodied energy numbers surrounding specific materials. One example I was surprised to learn was just how toxic burning PVC can be for the environment.

I took a look back in the book and it looks like even though cellulose is a low embodied energy material, straw is roughly 1/8 the embodied energy by comparison so this is another argument in your favor. A chart in the book compares the two with cellulose at 2.12 and straw at 0.24. This is a dramatic difference!

On that note, I can see how there is a reasonable trade off to be made where straw is basically unsuited for the job like gable walls. Preferring rockwool over fiberglass when needed also make sense given its substantially lower embodied energy compared to fiberglass (16.8 vs 28)

Thanks again for the info!

One of the biggest factors in heat loss is air permeability, or air loss.  If you have a high R-value but have a 'leaky' structure, you'll experience most of your heat loss when the warm, moist air escapes.  As well, windows are basically holes in your insulation, even if installed air-tight as the best windows have a very poor R-value.  

If you have a tight house, it will perform better than a house with more insulation but drafts.  With a tight house and a heat recovery unit, you'll have great performance but can suffer from poor air quality.  Straw bale structures don't have the hazardous off-gassing of man-made materials, so it's an excellent choice.

Timothy is absolutely correct! I'm lucky to be working in an area with will over sixty permitted straw bale residences--all built over the past thirty years, most of them in the last ten or fifteen years.  Since much of my work involves remodeling these older buildings I get to poke around inside the walls, see how the buildings have weathered, ask original and new owners about performance and energy costs, and have learned quite a lot.  

First, there are plenty of energy in-efficient straw bale buildings out there!  Just having plastered straw bale walls doesn't make the building efficient, even when the bales have been stacked tightly and the plaster applied well. What saves these structures from being uncomfortable is their incredible thermal mass--they may use more energy than necessary to heat and cool than if they had been built the way we know how to today, but the thermal mass delays abrupt temperature swings so they are, despite being full of holes, usually quite comfortable--just not energy efficient.  Conversely, there are also plenty of super air tight energy efficient buildings out there as well.

Second, there are things a homeowner can do to improve the efficiency of an inefficient straw bale building, mostly by filling these gaps and cracks.  Look at the joints between ceilings and straw bale walls, around windows and doors, and any plumbing or electrical outlets in the straw bale walls. If the gaps are large, you can use foam backer rod, followed by caulk (colored to match, more-or-less, the finish plaster or adjacent materials.  If the gaps and cracks are small a bead of caulk or other bits of insulation stuffed into the space can seal it. You can also use gap filling expanding foam in many places.  With clay plasters we can often wet and re-tool the area around the gap to close it off once the caulk or insulation has been installed. For electrical boxes, turn off power to the circuit, open the box covers, and if you can see straw through the little holes in the back of the box, air can come and go.  If the straw you see is discolored (grey or black), that might be the moisture from the escaping through the wall, deposited in the wall on its way through.  Openings like these account for five times more moisture in the wall than the highly vapor permeable plasters, and climates with very cold winters that water vapor can condense as it nears the exterior wall...so best to keep it out of the wall! caulk the holes and install outlet and switch plate cover gaskets.

Third, if building new, think about stopping air from passing through the wall as you build.  Straw Bale Building Details: An Illustrated Guide for Design and Construction is loaded with details showing easy things you can do that cost next-to-nothing.  One example: a single or double layer of building paper (I use 60 lb. Grade D because that's what I have around for separating wood from lime plaster) folded into the straw-bale wall's ceiling wall joint will be lapped by plaster on the wall, and lapped by sheetrock on the ceiling.  Should the plaster shrink a small gap might open there, but it's "backed" by the paper air barrier.  We do the same thing where partition walls about the straw bales.

I recommend getting a blower door test once the house is completed.  A competently applied interior and exterior plaster is an effective air barrier, as are well flashed and sealed doors and windows.  Anywhere plumbing enters or exists the building, or where wood stoves or range exhaust vents exit are potential leak areas.  Seal them up!

One way to learn how effectively you have sealed gaps and holes is to have a blower door test done on the house.  This test simulates the conditions of a windy day when air might be drawn (forced) out of your living envelop by the difference in air pressure between the inside and outside.   The test doesn’t tell you how much mechanical ventilation the house should have—only how much the building envelope leaks. The lower the score, the less leakage.
You may be wondering, but won't an air-tight house be dangerous.  Yes, if there isn’t some kind of ventilation system.  A lot of building scientists believe that all residences should have automated mechanical ventilation systems that exhaust stale, moist air and bring in fresh, filtered air all while exchanging heat so there’s no net energy loss in the building.  Some builders are aiming for a Passive House Air Change per Hour (ACH) of .6 or better, which absolutely requires a heat-recovery ventilation system.  But a lot of people just don’t like the idea of living in a nearly sealed envelope, and prefer to open and close windows to keep fresh air flowing in—knowing that some energy is lost out the window.  You can decide where on this scale you want to be, but know for certain that you don’t want a really leaky straw bale house!

I'll add two additional points to this conversation:
1.My understanding is that above the level of R-30 insulation, in most US climates, increasing the amount of insulation has increasingly diminishing returns. You have slowed the heat flow down so much that further slowing is not especially important.  In walls, that is;  Roofs,  because of their solar exposure and interior heat rising, can still profit from higher insulation levels.
2. Studies of thermal mass in passive solar buildings decades ago showed that on a daily basis, only the outer 2 inches of mass walls participated in the thermal flywheel effect we talk so much about. So installing  an extra thick plaster was as effective as putting in a mass wall., and usually less expensive.