Construction - Insulating with Rice Hulls - The Planning Stage

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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So far, there are just under 140 individual posts to this blog.   Two posts have attracted far more visitors over the past several years than the others.  Most visited has been the post about the plumbing rough-in under the concrete floor, including the homeruns for the PEX water supply system.  The second-most popular, with about

half as many hits, is the one about rice hull insulation posted back in the spring of 2016 that introduced the unfamiliar concept of insulating with rice hulls.  This post and, perhaps as many as three additional posts, describe how we planned for and bought a semi-trailer-load of rice hulls, the equipment needed to blow them into the wall and ceiling cavities, the atypical sequencing of the drywalling necessary for their use and several other hints and observations that we learned about the hulls.  To be sure, the whole process has been an adventure. 

Insulation Blower

Early on, I learned that the insulation blowers available at the big box stores were incapable of handling rice hulls due to their finer texture and their slightly heavier per-unit weight.  My good friend, Keith, being a master innovator, accepted the blower dilemma as a challenge and began experimenting with non-insulation blowers, such as hand-held and tractor-mounted leaf blowers, before giving up and searching the web for alternatives.  Eventually, he learned about a commercial machine that at least one person had reported using successfully with rice hulls.  The ideal model for our situation, FORCE ONE , was one of several models made by the Intec Corporation located in Frederick, CO.  When we called the company we found that that model had been discontinued but newer models would be equally effective but at a much higher price.  Ultimately, through Keith's effort, we were able to find a used Force One in good condition on Ebay.  

Special Effort to Prevent Leakage of the Hulls

Unlike conventional insulation, rice hulls are small enough

ZIP tape sealing junction
between the electrical box
and the drywall

to escape the wall and ceiling cavities through small openings such as those in and around electrical boxes and around plumbing stub-outs.  So we spent extra time eliminating such openings with spray foam and Zip tape.  Doing so also helped to air-seal the drywall but, as discussed in previous posts a seal at the drywall side of the wall is not especially critical since, on the exterior side of the truss walls, we sealed the joints between plywood sheathing panels with ZIP tape and, on the interior, sprayed foam insulation along the mudsill, in transitions between concrete and stick-built walls and around penetrations for electrical wires and water lines.  The extra sealing on the drywall side however will help to minimize moisture penetration into the wall and ceiling cavities that, in excess, could compromise the R-value of the insulation, encourage mold and, as explained below, compromise the insecticidal effectiveness of the diatomaceous earth that we will be blending into the insulation.

Reduced size cover plate
used as a guide for cutting
the tape back enough to be 
hidden under a normal size
cover plate, shown here before
cutting.

As will be described in a subsequent post on the drywalling phase, using rice hulls for insulation required an atypical sequencing of the drywall on the exterior walls.  We will start with the first course at the floor for the walls and next to the wall on the low side of the cathedral ceilings.  After the hulls are blown behind one tier of drywall, the next tier will be added and filled as well, working up the wall and across the ceiling.

Calibrating the Blower

A sample bag of rice hulls came in handy for validating the efficacy of the blower and preliminary calibration of it  by blowing the hulls back and forth between two appliance boxes that were separated by ~30 feet. It took only a few back-and-forths to determine the best size of the opening in the bottom of the blower hopper for a steady stream of hulls. The exercise also familiarized us with the remote controls on the blower.  We even took one of the boxes to the far corner of the second story to see if elevation slowed the flow of hulls through the 50' hose.  It did not. 

Friends Myron (at the hopper)
and Keith (with remote control
and hose in hand) testing
the blower.  

The sample hulls raised enough dust to warn us that mask-wearing while insulating would probably be necessary irrespective of COVID, particularly, as discussed below, when we include diatomaceous earth with the hulls.

Estimating the Quantity of Hulls

After vigorous stirring of the 50 lb sample bag in an effort to fluff up the hulls, we became skeptical that each bag would yield 7 cu ft when blown into the wall or ceiling as contended by the supplier.  Blowing the hulls back and forth between the boxes did not seem to increase the volume very much, if any, over just stirring.  So  I used 6 cu ft to calculate our needs.

The volume of rice hulls that we will need for the exterior walls (15" thick) and the ceilings (18" thick) turns out to be just under 5,000 cu ft.  A 53' tractor-trailer load comprises 768 bags (50 lb each).  The hulls are compressed for bagging such that a bag contains 5 cu ft.  If 768 bags expand to 6 cu ft when blown into the wall and ceiling cavities, the total volume for a truckload would be 4,600 cu ft, slightly less than our needs.  If they expand to 7 cu ft, a truckload might be even slightly more than enough.  The plan is to proceed with a truckload and see how far it goes then, if necessary, decide what to use to finish insulating.  If very little additional insulation is needed, perhaps locally-available cellulose would be the best choice for finishing.  If the amount needed is excessive and the price differential between hulls and cellulose is substantial, it might make sense to pay freight on a few more pallets of hulls.

Receiving and Handling the Shipment of Rice Hulls

In the March 2016 post on rice hulls for insulation, I was unaware that they could be purchased bagged and on pallets (if in fact they were actually available then).  I assumed that they would be delivered in bulk on a walking floor trailer.  Having them bagged, though more expensive, will greatly simplify their handling at every stage -- from truck to blower.

In order to avoid commercial warehousing fees and the inconvenience of off-site storage, it took a bit of head-scratching to figure out the best way to receive a truckload of 48 pallets on a narrow almost-one-way dead-end street in the heart of the hilly Mississippi River bluffs.  Finally, we settled on the following plan:  using a retailer friend's parking lot for a drop-off trailer, pickup trucks to move the pallets from the trailer to the storage sites in the garage and living space of the house under construction and a rental pallet jack.  The pallets will be double-stacked in the trailer.  Two double-stacked pallets weighs 1,600 lbs but we envisioned little difficulty moving them to the back of the trailer with a pallet jack then breaking them down so that bags could be handled individually. 

Rice Weevil Problem?

Also in the 2016 post, I was not yet aware of the potential problem of weevil infestation.  The ensuing years have provided time to research rice weevils.  The available information online is spotty and inconsistent but seems to indicate that, as a minimum, we should add diatomaceous earth (DE) to the hulls as they go into the wall and ceiling cavities.  (DE, also known as silicone dioxide, is the best green insecticide for weevils and most other insects with exoskeletons and works indefinitely as long as it stays dry.)  DE is the fossilized remains of microscopic diatoms that, to paraphrase Wikipedia, were protists, a cellular organism with a nucleus that is not an animal, plant or fungus.  The sharp edges on the fossils kill insects by scratching or piercing their exoskeletons, causing them to dehydrate.  (Check out the Wikipedia link for an electron microscopic image of DE particles.)

The hulls we will use come from parboiled rice.  The Riceland Foods, Inc. representative with whom I had been working, contended that parboiling kills all three forms of the weevil -- eggs, larva and adults. Again paraphrasing Wikipedia, parboiling rice makes it easier to process by hand, boosts its nutritional profile, changes its texture and makes it more resistant to weevils.  However, "resistant" is not total prevention and so far I have not found any studies that say unequivocally that parboiling eliminates weevils.

Diatomaceous Earth

In the absence of definitive information on parboiling and weevils, I decided to add DE to the hulls as we insulated but maybe not as much as would be the case if they were not parboiled.  As of this writing, our best source for the kind of DE that we need is a local farm and home store which stocks it as a livestock supplement.

A quick search online reveals that there are two kinds of DE.  One kind goes by various names -- industrial grade, filter grade, pool grade -- while the other is food grade.  The former is inappropriate for our use because it is heat treated or chemically treated that leaves it ineffective as an insecticide and tends to make it a health hazard, particularly with regard to silicosis, although several sources recommend dust masks when using food grade DE as well, not for fear of silicosis but to prevent airway irritation from its microscopic particle size. The amount of food grade DE recommended as an insecticide in grains for human consumption seems to be one cup per 50 lbs of grain, which is probably overkill for our purposes considering that parboiling probably leaves minimal or no weevils to worry about and the hulls will contain hardly any rice grains that weevils would need for long-term survival.  Nevertheless, we decided to go with 1 cup of DE added to each 50 lb bag of hulls.  It will then be in the wall and ceiling cavities indefinitely to control all types of insects with exoskeletons, not just weevils.  

The next post will chronicle our experience with the hulls from receipt to incrementally insulating with them.