Design - Rice Hull Insulation

SPOILER ALERT -- Summer, 2024

Do not use rice hulls for insulation.  We did and lived to regret it.  Here's our story.

As a methodical and cautious early adopter, I thoroughly researched rice hulls and thought I understood the risk of rice hull weevil infestation.  My contact person at the mill assured me that weevils cannot survive the parboiling of the whole grain that is done before separating it from the hulls.  In the absence of any information to the contrary, despite considerable research, I decided to take a chance on them.  

My research told me that weevil reproduction required a grain of rice into which an egg is laid and the larva develops until emerging as an adult weevil.  In the process. the rice grain is consumed.  If there were rice grains in the sample bag of hulls sent by the mill, they were so inconspicuous we did not see them.  Consequently, we assumed that, even if a few existed in the insulation, they would soon be consumed by baby weevils until no more grains existed.  Besides we did not see any weevils in the sample, although, in retrospect, they were much tinier than we expected and were simply overlooked. 

My research also revealed that diatomaceous earth is an insecticide for bugs with exoskeletons by finding its way under their skeletons and dehydrating them.  So a cup of diatomaceous earth was mixed with each bag of rice hulls as they were blown into he wall and ceiling cavities, all the while thinking that this extra effort was probably overkill but advisable since the stakes were high and our knowledge so limited. 

Three years later, we are beginning to think that we know more about rice weevils than those speaking and writing about them at the time we did our research.  Clearly they reproduce in the absence of rice grains.  The "hulls" would have had to include a huge amount of grain in order to support such exponential infestation.  Moreover, we are not only seeing a large number of adult weevils outside the confines of the wall, principally on the window sills, we are seeing quite a few viable larva a well.  They are larger than most adult weevils, but since they have no means of locomotion, they must be transported out into the open by adults.

A major oversight on my part is that diatomaceous earth as a backup would prevent an infestation.  However, the adults obviously live long enough to reproduce before dehydrating and the eggs each female is capable of laying numbers in the hundreds.  Therefore, new bugs are developing faster than the old bugs are dehydrating.  I must say that the vast majority of adults that we see are dead, many with their feet in the air.  So it is safe to assume that, no sooner do they emerge into the open, they die from dehydration.  Supposedly most are able to fly but we see very few actually doing so although some of those on the sills flap their wings like they are trying.  But, even when prodded, they are so debilitated as to be incapable of flying.

There will be no solution to the weevil infestation short of opening up the affected walls and ceilings, removing the rice hulls and re-drywalling over conventional insulation.  Fortunately, because we designed the exterior walls for minimal air leakage, the only easy egress for the weevils is through the openings in the sub- window sills through which the hulls were blown into the spaces below the windows.  Currently I am sealing these and watching for any other escape routes.  These efforts will suffice for the limited time we have left (ages 88 and 91) but our heirs will need to take more definitive action.

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                                                ORIGINAL POST

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.
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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.