Construction - Insulating with Rice Hulls - Filling the Wall and Ceiling Cavities

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

This is the fourth post on rice hulls for insulation.  The first was back in 2016, a couple of years after I learned about insulating with hulls.  That post was an attempt to confirm their efficacy and understand the uncommon logistics involved with buying, transporting and getting them into a structure.  Two recent posts set the stage for this post that describes the actual use of the hulls for the wall and ceiling cavities.

Reminder:  click on any picture to enlarge it for better viewing.

Buying the Rice Hulls and the Diatomaceous Earth

It appeared that Riceland Foods, Inc was willing to sell direct (instead of referring to a dealer) only because of the size of our order.  Their hulls come in two configurations -- large bales or 50 lb bags -- with the latter seldom sold to end-users, especially consumers, in truckload quantities.  Consequently, our order triggered a special run that needed to be picked up almost immediately after ordering.  I had been proactively in contact with a freight broker who promptly caught a ride for the shipment and was able to schedule it to arrive on Friday so that we could offload it over the weekend. 

Buying food grade diatomaceous earth (DE) was made easy by a local farm and home store that handled it in 40 lb bags for mixing with livestock feeds.  The DE as an insecticide will not only kill rice weevils but any other insects with exoskeletons (hard shells), apparently for as long as the building exists.

Receiving the Rice Hulls

With a crew of 11 and three pickups, 768
 bags of hulls were moved from the semi-trailer
 to our building site in less than 7 hours.

As mentioned in the previous post, our construction site on a narrow one-way street makes it is nearly impossible to receive shipments from semi-trailers unless the driver is willing to exit by backing for several blocks.  And, if s/he were to wait while the trailer was offloaded, we would need a forklift. Therefore, we received the order in a drop-off trailer on a merchant-friend's parking lot and off-loaded it by breaking down the pallets and handling all 768 bags one at a time. Thankfully, we were blessed with enough volunteers and pickups as well as ideal weather for December in the Midwest to have the trailer unloaded in less than seven hours.

It was a bit of a problem storing the hulls and still having access to the exterior walls for insulating and additional drywalling.  They occupied over half of the space in the garage and most of the non-bedroom, non-bathroom floor space on the first floor.  However, intentional sequencing for blowing the hulls quickly eliminated the bags that were most in the way. 

Blowing the Hulls

We positioned the blower in a central

When the blower was positioned at a central location (arrow),
the hose reached all recesses of the building.  Each bag of hulls
was opened, dumped into the mortar box in front of the blower
 and sprinkled with a cupful of diatomaceous earth.  At the time of
this picture, the bags stored in this area were used first in order to
create working space.

location on the first story from which we could reach all exterior walls, upstairs and down, with the 50 foot 3" diameter blower hose. I handled the business end of the hose, not because it required much skill, but because it was extremely dusty and not something I wanted anyone else to have to deal with. At the blower, a bag was laid in a mortar box and cut open to release the hulls that were compressed and under pressure.  The hulls were then sprinkled with a cupful of diatomaceous earth and scoop-shoveled into the hopper of the blower.  The flow rate was

Friend, Bob, loading the blower hopper with hulls at the
rate of about one bag every five minutes. 

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less than 5 min per bag which we thought originally would be too fast for one person working alone to manage. However, after a little practice with two at the blower, we found that one person could in fact keep up.  The guy(s) working at the blower were already wearing N95 or equivalent masks due to COVID-19 although the amount of dust was minimal.  At the business end of the hose, the dust was so problematic that, in addition to an N95 tight-fitting mask, I wore swimming goggles, long sleeves, tight collar and gloves.

With rice hulls, as opposed to fiberglass or cellulose, the hose clogged more readily, presumably due to their greater density.   We found two maneuvers that eliminated clogging.  One was to fine-tune the flow rate by trial-and-error and the other was to make sure that the hose was kept as straight as possible and, when bent, with curves as sweeping as possible.  And, in addition to these efforts, I needed to be careful that the end of the hose did not bottom out and become blocked by the hulls already in the wall or ceiling.  The good news was that the flow rate was not diminished when the hose was elevated to reach the ceiling of the second story.

Second Thoughts About Diatomaceous Earth (DE)

After a day and a half of blowing rice hulls, we began to wonder whether the dust created by blowing was due to the DE rather than the hulls themselves.  By that time we had finished insulating behind the first course of drywall on the first floor and the interior of the building was already pretty dusty.  It was hard to say whether it was hull dust or DE dust or a combination.

During the interval for installing the second course of drywall, I did more research on the health risks associated with DE (I was definitely motivated to do so after experiencing mucus-like drainage from my red, itchy eyes for a couple of
days after the first session).  The search yielded enough information to warrant a bit of caution.  The major concerns are pulmonary effects and eye irritation.  The former was a non-issue for us in that the warnings apply to workers who experience long-term exposure such as those mining and processing DE and we were already wearing N95 masks which, according to the online sources, was adequate for DE dust.  The latter concern, eye irritation, was real for me after being at the business end of the hose but not a concern for those working at the hopper.  It motivated me to search for goggles that sealed against the face better than the ones I was using and to consider alternatives to mixing the DE with the hulls before blowing.

In order to decide whether to continue mixing DE with the hulls, we blew a few bags of hulls without the DE to see how dusty they would be compared to rice hulls with DE.  As I had hoped, the amount of dust with or without DE seemed to be a wash.  Since I would be the one at the dusty end of the blower hose and would rather not miss the opportunity to have walls and ceiling laced with a deadly but environmentally-friendly insecticide, I decided to continue with the DE.  By the time the second stage of drywalling was over and we were ready to resume insulating, I had bought tight-fitting swim googles that eliminated most of the eye irritation that I experienced after the first session.  However, the amount of dust at the business end of the hose, even with masking and goggles, made the job extremely unpleasant to say the least.  

Wall Insulation

Yours truly at the business end of the hose

Insulating behind the lower 4' high course of drywall was relatively easy and gave us a chance to practice our technique.  Insulating behind the second course of drywall up to the 8' level was more challenging.  Not only did I have to hassle with a ladder or a mobile scaffold and, despite using a headlight, visibility into the wall cavity was limited by the 4" space between the 2 x 6 tandem top plates.  However, insulating behind the lower course of drywall gave me confidence that gravity would pull the hulls into all of the nooks and grannies up to the bottom of the top plates.  At that point it was necessary to overfill the wall slightly then reach through the opening between the top plates and manually pack the hulls into the corners under the plates. The amount of dust raised due to the proximity of the ceiling was much worse than it had been with the first course.  I was definitely thankful for the mask and swim goggles and amused later to find hulls between all of the multiple layers clothing that I wore against 40 degree temperatures.

We filled the wall cavities brimming full so that there would be no doubt that the junction between walls and ceilings would be filled uninterruptedly when later the insulation would be blown into the space above the first 4' course of ceiling panels.  By the time the walls were filled, not quite half of the original 768 bags of hulls had been consumed.

Getting the Ceiling Ready for Insulation

First floor ceiling showing the temporary strips
supporting the weight of the rice hulls until
 they can be replaced by definitive trim boards.

In the chart comparing rice hulls with cellulose insulation in the previous post, I pointed out that the rice hulls, at nearly three times the weight of cellulose and at a depth of 18", might cause the ceiling drywall to pull loose from its screws if installed directly to the roof trusses unassisted.  So I decided to kill two birds with one stone -- add support while creating an architectural feature that we had been considering in any case.

For all of the ceilings in the main rooms, upstairs and down, the esthetic effect that we had been contemplating was a 4' x 4' grid pattern comprising 1 x 6 trim boards wide enough to cover the beveled edges of adjacent drywall panels.  To that end, we installed the drywall in the usual manner with screw spacing a little closer than normal.   As a temporary support measure, we added 3/4" x 2" strips screwed through the drywall and into the trusses.  They were less than 4' long so as not to interfere with the east-west final trim pieces that will run perpendicular to the trusses, be screwed or nailed to the trusses and cover the seams between drywall panels in lieu of taping.  The plan is to remove the temporary strips when the final longitudinal trim is in place then replace them with the trim pieces that complete the grid pattern.  

Installing the Ceiling Drywall

First floor ceiling bays being filled with rice hulls
as seen through the second floor wall.

For the ceilings on both floors, we hung only one course of drywall next to the wall then insulated it in order to be sure that the junction between walls and ceilings was thoroughly filled and compacted with hulls.  We found that the blower did not shoot the hulls with enough force and sufficient distance for us to insulate with confidence more than one course at once so we stuck with doing each course separately.  The highest part of the first floor cathedral ceiling was filled by reaching through the second story wall, as seen in the nearby photo.  The highest part of the second story ceiling -- the last space to be insulated -- was our biggest challenge due to limited access.  We switched to installing the last course of drywall one 4' x 4' panel at a time starting at the southwest corner and proceeding to the southeast corner.  That way, we were in better position to blend the insulation with that already in the wall despite having to work crossways of the trusses instead with them as was possible with the rest of the ceilings.