Design - Rice Hull Insulation

Why Rice Hull Insulation?

The short answer is economy.  The rice mills have a hard time disposing of the hulls so they are only too happy to sell them by the truckload cheaply.  In fact, the cost of the hulls is

insignificant compared to trucking costs from the Mississippi delta (Missouri boot-heel, Arkansas, Mississippi or Louisiana) to our St Louis area.  But there are many other advantages of rice hulls over conventional insulation which are covered in this longer-than-usual post.

(From a sustainability perspective, cellulose is the best choice among low-cost conventional insulating materials.  Therefore, it is the one most used in the following paragraphs for comparison to rice hulls.)

Rice Hull Properties

My serendipitous discovery of rice hulls is explained in a previous post --  design evolution - insulation.  My new awareness then lead to the definitive paper on the rice hulls for insulation by Paul Olivier, PhD, "The Rice Hull House".  The information below comes entirely from his paper including the following quote from the opening abstract. 

"The rice hulls are unique within nature.  They contain approximately 20% opaline silica in combination with a large amount of the phenyl propanoid structural polymer called

lignin......Recent ASTM testing.....reveals that rice hulls do not flame or smolder very easily, they are highly resistant to moisture penetration and fungal decomposition, they do not transfer heat very well, they do not smell or emit gases, and they are not corrosive with respect to aluminum, copper or steel.  In their raw and unprocessed state, rice hulls constitute a Class A or Class I insulation material, and therefore, they can be used very economically to insulate the wall, floor and roof cavities....."

Olivier's paper goes on to explain in detail why rice hulls are ideal for insulation. Their R-value compares favorably with cellulose and loose fiberglass at a value greater than R-3 per inch. Their natural fire resistance precludes the addition of large quantities of flame and smolder retardants as with cellulose.  Nor is the addition of anti-fungal agents necessary since the amount of moisture absorbed from the air is very low compared to most organic materials that moisturize in equilibrium with the surrounding humidity.  The high concentration of opaline silica on the outer surface of the hulls makes them very hard but, lignin within the hulls adds flexibility and elasticity, making them far more resistant to settling and compression than cellulose. Also their "tiny tips, edges and hairs interlock........( to produce a) peculiar bonding of rice hulls under mild pressure......(such that) no further settling is possible". According to Olivier, cellulose, can settle "as much as 25%" despite stabilizing additives such as (un-green) polyvinyl acetate or acrylic adhesive.  Finally, "since rice hulls require no shredding, hammer-milling, fluffing, fiberizing, binding or stabilizing, they possess far less embodied energy than even cellulose".  And they are durable enough to be recycled indefinitely.

Freight Costs 
Olivier goes on to analyze the total cost -- hulls plus transportation.  "At an installed density of 9 lbs per cubic foot, one ton of rice hulls will insulate 222 square feet of a 12-inch wall cavity........A standard 53 -foot trailer attains optimal transport efficiency at its maximum legal weight of 24 tons.  If.......we pay an average trucking fee of $1.45 per mile, it would cost approximately $15, $30, $45, $60, $75 and $90 to transport one ton of rice hulls 250, 500, 750 1000, 1250 and 1500 miles respectively.......the (freight) cost per square foot would be $0.07, $0.14, $0.20, $0.27, $0.34 and $0.41 respectively.......Those living less than 200 miles from rice hulls should have a hard time justifying the use of any other type of insulation material".  

He goes on to say that, even if the hulls cost $25 per ton (five times the cost at his location in Louisiana at the time the paper was written),........the purchase price of the rice hulls per square foot of wall insulated is on $0.11.......(when this price for the hulls is added to) the cost of transport over these same distances, we arrive at a total cost (per cubic foot of wall space of) $0.18, $0.25, $0.32, $0.38, $0.45 and $0.52 respectively".  (In this analysis, cubit foot is interchangeable with square foot because the wall thickness is 12".)

Cost Comparison with Cellulose
Olivier says that "cellulose insulation in a dense-pack application (reaches) a density of approximately 3.5 lbs per cubic foot (and) will insulate 571 sq ft of our proposed 12-in thick wall.  At an average delivered price of $540 per ton, cellulose insulation costs roughly $0.95 per sq ft of wall insulated........roughly five times the price of rice hulls transported 250 miles and twice the price of rice hulls transported 1,500 miles".

Real Costs

Rice Hulls:    When I called the rice mill in SE Missouri that was the closest to Collinsville three years ago, the price for the hulls was $94/ton.  When I emailed this figure to Olivier, he assured me that they could be bought for $15/ton.  Indeed, the company that handles the rice hulls for a large rice co-op recently quoted me $15 out of Greenville, MS.  

Deck of a walking floor trailer

Freight:  The actual costs for us will be more than Olivier's paper would suggest due to higher freight costs-- the trailer capacity is less and the per-mile rate is higher than his analysis.  The fluffy hulls are hauled in enclosed 53 ft trailers with "walking floors". Instead of a dumping action, the trailer deck has three sets of slats that move in a coordinated manner such that load is conveyed towards the rear of the trailer until it falls out. Unfortunately, the trailers actually hold only +/-18 tons instead of the 24 that Olivier described and, since truckload freight rates are based on mileage, less tonnage means higher trucking cost per ton. Accordingly, It will cost about $3,600 to ship 18 tons the 450 miles to Collinsville or $8/mi instead of the $1.45 that Olivier used for his examples.  I checked with a local trucking company only to find that, since its grain trucks have half the capacity of the walking floor trailers, trucking cost from Greenville would be higher due to having to make two hauls instead of one.  I hope that, when it comes time to buy the hulls, more research and comparison shopping will turn up a source closer to home that will save on transportation.

Total cost:   Our cost for a trailer-load will be 18 tons x $15/ton = $270 plus $3,600 for freight, making a total cost of $3,870.  If, as Olivier says, a ton of hulls will insulate 222 cu ft of wall space, 18 tons will do just under 4,000 cu feet.  If we were to insulate the garage walls with hulls as well as the house, we would need about 2,400 cu ft for the walls and 2,800 for the ceilings or 5,400 altogether.  This means one trailer-load will not meet our needs and supplementing with conventional insulation for the garage will be necessary.

Cost comparison with cellulose:  In 2013, a local insulation company quoted our project when the design was still in flux to the extent that we were at a ceiling thickness of 12" instead of 15" and a wall thickness of 7 1/2" instead of 15".  The quote for dense pack cellulose was $4,700.  Extrapolating, our current design would have been quoted at $6,130 (plus 3 years of inflation).  This amount would be more than a third higher than insulating with hulls.  This amount also figures out to be $1.23 per cu ft which is a little more than 20% higher than Olivier's figure of $0.95.

Weight Factor
Rice hulls at 9 lbs per cu ft are almost three times as heavy as cellulose at 3.5 lbs per cu ft. Although Olivier does not discuss the weight factor, it seems reasonable that half-inch drywall screwed to rafters or joists would not adequately support hulls piled thick enough for a high R-value.  Either the drywall would have to be thicker or applied in layers.  In our case, tongue and groove pine ceilings were already planned before considering rice hull insulation so the weight factor will be moot. However when estimating rice hull insulation, I think the cost of a more robust ceiling should be factored in.

Non-monetary Advantages 
The numerous advantages of rice hulls compared to cellulose are covered above in the paragraph about their properties.  Another advantage they have for our DIY project -- and I consider it to be very important -- is that they can be installed incrementally in conjunction with building the walls and ceilings as opposed to an all-at-once job by an insulation company.  For example, the walls can be filled as the drywall goes up and the cathedral ceilings can be filled as the tongue and groove pine is installed.  With visual access to the cavities, the chances of voids will be minimized.

Our Plan

Our plan is to use a blower to fill the walls and ceilings. However, the blower used for cellulose is not strong enough for hulls so a custom blower will have to be assembled similar to the one shown in Paul Olivier's slide show on the Rice Hull House concept. As to where to store the hulls between delivery and installation, my current thinking is to have them dumped onto the slab floor of the future garage then protect them with a tarp while we rush to get the garage undercover.
_________________

Update November 2020

We are finally in the process of insulating with rice hulls in ways that resemble the plan just described very little.  Here is the link that will bring the reader up to date --  The Planning Stage.  It is the first of several posts explaining the surprises we encountered, like sticker-shock on the price of the hulls, dealing with the rice weevil infestation issue and how incredibly dusty the hulls were coming out of the blower hose, as well as atypical structural modifications and drywalling sequencing that were mandated by the hulls.  And one of the posts parses the advantages and disadvantages of using rice hulls for insulation versus cellulose. 

Construction - The First Retaining Wall

Our plans call for four retaining walls with the first one next to the west concrete wall being the most challenging.  It needed to be 4 - 6 feet high and insulated and waterproofed as part of the insulation/watershed umbrella.  The insulating and waterproofing were a challenge while building the wall was easy, thanks to 9 energetic volunteers.  But that's getting ahead of the story.  (Click on the photos to enlarge them for more detail.)

Insulating the Concrete House Wall
The first book I bought when contemplating an energy neutral home was "Earth Sheltered

Wall insulation in place

Houses" by Rob Roy.  Through it I learned early on that insulation should be inserted between a retaining wall and a concrete house wall in order to keep the retaining wall from sucking heat from the house.  In our case, I was already planning to insulate the concrete walls of the house that would not protected by the insulation/watershed umbrella. Since the umbrella will dip down behind and go under the retaining wall, the house wall adjacent to the retaining wall will fall outside the umbrella and would need to be insulated.

Cementitious board before adding the lower section

The exact way I insulated the house wall will be covered in detail in another post but suffice to say at this juncture, I used 3 5/8" steel stud track to support 3 1/2" of expanded polystyrene for an R-15 on the outside of the wall.  (The inside of the wall will be insulated in a similar fashion eventually for a total of R-30.)  I then fastened 1/2" old-fashion, heavy, hard-to-cut cementitious board and parged it with top-coat stucco, the latter primarily to cover the junctions between boards and to make the exposed areas of the wall more aesthetic.  Six mil plastic separated the

Wall parged to height of retaining wall;
horizontal insulation in place (part of
which already covered with sand);
vertical insulation supported from
 behind with steel fence posts

galvanized steel from the concrete on one side of the insulation and from the cementitious board on the other side.  Six linear feet of the wall were insulated and covered from the top of the wall down to meet the horizontal insulation already in place over the footing.  

Integrating the Retaining Wall with the Insulation/Watershed Umbrella 
As part of the Annualized GeoSolar system, the insulation/watershed umbrella should extend 20' outward from the house in all directions.  On the west side of the house, it has be convoluted in order to accomodate the retaining wall.  Above the wall it will slope gradually southward

Insulation wired to
steel fence posts

towards the wall then, dip sharply downward behind the wall, go under the wall and finally blend with the umbrella in front of the house. 

I smoothed out the soil under the wall then covered and leveled it with sand to provide a base for a 4' wide wall. Next, I laid down plastic sheeting (6 mil) such that it extended several feet beyond the prospective wall in the up-slope and down-slope directions.  Then came a thin layer of sand over the plastic where the wall would rest followed by two panels of 4' x 8' x 2" insulation board next to the house and another panel of 2" lateral to it such that the insulation under the wall would be 4" thick for the first 8' then 2" thick for the last 8'.  Ideally, the insulation should have extended 4' further to satisfy the 20' width for the umbrella but the original excavation did not accomodate it.  

Three layers of plastic -- one between
insulation and soil contact then two
between insulation and the outside
environment

The vertical insulation was also 4" thick for the first 8' from the house then 2" thick for the last 8'.  To support it, I drove four steel fence posts into the ground behind them and wired the insulation to them.  In retrospect,  all of the insulation should have been the stronger extruded polystyrene (pink) instead of expanded poly (white), especially the vertical pieces, one of which cracked while building the wall and had to be held together by hand until it could be supported by sand in front and dirt behind.

I next liberally covered the horizontal insulation with sand and added two more sheets of 6 mil plastic with a layer of sand between them. Finally, I covered the plastic with a heavy layer of sand into which the stones of the wall could be nestled without damaging the plastic.

Building the Retaining Wall
Three or four years ago, we salvaged foundation stones from a 19th century barn in such

Some of the volunteers at work

quantity that we do not have to use them in a miserly way. Consequently, I decided to make the wall massive enough that the rocks could be laid randomly and still resist the pressure of the backfill behind it.  The final dimensions were roughly 4' wide at the base, 2' wide at the top, 5' high and 16' long.

A view from the loader; step-son Keith
(left) and my good friends Dave and Pat
lifting rocks out of the bucket

The wall came together in less than three hours.  As fast as I could track loader the rocks to the wall site from the rock-pile, the volunteers could set them, including packing sand into the crevices between rocks, laying a sandy base for the next level and compacting the sand with hose water.  Some of the stones weighed quite a bit north of 100 lbs so I was more than grateful for the help. (Parenthetically, the crevices between the rocks will be filled someday with plants native to our area.)

The result

Backfilling Behind the Wall
Backfilling immediately before it rained was critical otherwise water pooling behind the plastic might create sufficient hydraulic pressure to move the wall. I did the backfilling the same day the wall went in. As it was filled, the grades next to the house wall and the rock wall were intentionally tilted to create a swale for carrying runoff safely around the retaining wall.  As the back-fill settles over time, it may have to be tweaked to protect the wall.

Initial backfilling

  
Eventually, the umbrella and a couple of feet of topsoil will be added over the initial backfill after which the rock wall will be protected from runoff permanently. However,  chances are the wall will need a couple of courses of stones added on top to accomodate the horizontal insulation in the umbrella and the topsoil over it.  It also looks as if I should have made the black damp-proofing membrane higher on the concrete wall.  Not to worry, it can be extended later.