Construction - Insulating with Rice Hulls - Cellulose vs. 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

Insulating with rice hulls has been for us a drawn-out process whereby the drywalling has to be staged then filled with hulls incrementally.  Consequently, I will not be blogging on the drywall-insulating process until it is done, probably a couple of months from now.  Meanwhile, this is a good time to pause and compare rice hulls with cellulose, the only other low-cost insulating material that is even close to being as sustainable a rice hulls.

We committed to the use of hulls early on based upon the ridiculously low cost estimates put forth by Olivier in his quintessential article and validated later during a phone conversation with him, at least for those of us living close to the rice belt where freight costs were manageable.  My calculation at the time said that rice hulls would cost about a fourth of the cost of cellulose and about a third of the cost of fiberglass.  That said, their use did give me pause from the very beginning, despite their lure as an  innovative, intriguing  and enticingly sustainable choice for insulating.  If the millers were parboiling rice before separating the grain from

Close-up of rice hulls.

the hulls in those days, I did not pick up on it.  I expected to have to buy raw hulls delivered in bulk via a walking floor trailer (see the 2016 post on rice hulls), which would mean that managing any rice weevils would depend solely on the effectiveness of diatomaceous earth (DE) as an insecticide (DE was a topic in the previous post).  Added to that concern was the complicated logistics of receiving and storing a trailer-load of loose hulls.  As it turns out four years later, rather than being cheaper, the cost of the hulls is slightly higher than cellulose probably would have been and freight costs are much higher than four years ago due to a nation-wide shortage of trucks and truckers.  On the other hand, though, parboiling the rice before milling ostensibly solves the weevil problem and receiving the hulls in bags solves the handling problem.  So I consider the sticker shock as a reasonable trade off for the convenience of having bagged hulls without rice weevils and for the personal experience that allows me to blog on a subject for which there seems to be considerable interest but little precedent (e.g., the 2016 post on rice hulls is the second most visited among the +/-140 posts to this blog so far.)

Following is a comparison of rice hulls and cellulose.

Advantages of Rice Hulls

The advantage of hulls that appeals most to me is the lack of settling once they are blown and, where appropriate (ceilings), also packed to place.  It is comforting to know that the walls, a few of which were nearly 12' high, and ceilings will remain packed tight and maintain the original R-factor for the life of the building.  The second most important advantage would have to be the natural fire resistance of the hulls in thicknesses of 15" in the walls and 18" in the ceilings.  By enveloping the wood structural elements and the electrical system in a flame-retarding and self-extinguishing medium, the shell of the house is virtually fireproof, particularly since the exterior is covered with metal roofing and metal siding.  And the natural resistance to moisture of the hulls does two things, (a) stabilizes the R-factor that would be compromised with moisture-absorbing cellulose and (b) inhibits fungal growth without the use of noxious chemicals.  Another consideration that compliments our green project is the low embodied energy of the hulls, most of which resides in their transportation rather than in the hulls themselves, and the fact that they can be recycled indefinitely.  Still another, rather serendipitous advantage, is the excuse to introduce diatomaceous earth into the wall and ceiling cavities that will linger as an environmentally-friendly insecticide for as long as the building exists.

Disadvantages of Rice Hulls

Aside from sticker shock, the biggest obstacle for the universal use of hulls for insulation is the high freight costs beyond the Midwest.  And there are four other disadvantages.  (1) The need for more robust ceiling construction.   The hulls have roughly three times the weight of cellulose and could cause the drywall to pull free of its screws or perhaps sag between roof trusses, especially where two beveled edges come together at a right angle to the trusses. The way we modified typical ceiling construction for additional support will be explained in a subsequent post on drywalling.  (2) The hassle for a DIYer of buying and selling a blower for a one-off project.  (3) Dealing with the excessive dust raised by the hulls.  Having limited experience with cellulose, I have no idea how dusty it would be compared to the hulls if it were to be blown into the confined spaces of our 15" wall 

Concrete first floor showing the amount of dust accumulation
despite having already been swept twice.

cavities and 18" cathedral ceiling cavities whereby the dust blows directly back into the face of the operator.  I can say with authority that the dust was so thick that it is impossible to see the progress of the filling without stopping the blower or diverting the hose to the next bay temporarily to clear the dust and check progress.  (And, as the nearby picture shows, the amount of dust accumulating on all surfaces was formidable.)  After a while, though, the blowing became so routine, especially for the ceilings, that I worked as much by feel as by sight.  It helped also to limit the amount of space to be filled for both the walls and ceilings to the width of a sheet of drywall, i.e., no more than 4' at a time. (4)  Relatively slow flow rate.  The industrial strength insulation blower that we purchased, was able to push the hulls through 50' of hose without difficulty, requiring +/- 4 min to move one bag of hulls (+/- 6 cu ft), which was about as fast as the second worker could open the bags, add the diatomaceous earth and scoop the hulls into the hopper.  

In the chart above, the rest of the factors that compare cellulose with hulls are essentially a wash.

The next posts will document our practical experiences with the rice hulls.