Thermal Decoupler


Thermal Decoupler

Our butcher shop is designed to be a series of thermos bottles, nested one within the other like a Russian doll, in order to keep the cold where it should be. Sometimes we have places in the construction where there is a joint between the interior bottle and the next one out. Leaving the conductive concrete in contact between the two bottles would result in an energy bleed lowering the efficiency of our big machine and wasting resources. Thus we place thermal decouplers at these contact points.

Most of the time the thermal decoupler is simply insulation, typically Formula 250 pink foam as well as the recycled styrofoam I’ve been saving for decades. But that is only capable of carrying a load of 25 pounds per square inch. Sometimes we need a lot more strength and sacrifice a little insulation for greater psi.

Thus the thermo decoupler pictured above which is two layers of PVC (kitchen cutting board) with a 1/2″ layer of Formula 250 between them. The PVC has a strength of about 14,000 psi yet is not nearly as conductive as the concrete which has a 3,000 psi strength in most cases and 4,800 psi in the case of this coming pour.

Another trick I’ve used is changing the formula of the concrete by adding more air or sometimes styrofoam beads into the mix to make light weight concrete. This is better insulation than normal concrete and stronger than foam insulation.

The hole in the middle of the thermal decoupler is for the basalt rebar that keeps the pillar centered below the beam. Our upcoming concrete flat ceiling pour is a 36′ span. The main steel reinforced beam running East-West in the reefer coolth attic is designed to handle a 500 year snow storm. Beyond that the glacier will have mounded up high enough that the snow load will move toward self supporting.

Being that I’m of ever the paranoid engineer mindset I added two pillars in the middle of the room that will catch the beam should it deflect that 1/2″ down. Additionally there are steel reinforced concrete rib beams running perpendicular (North-South) to the beam which provide additional support.

I don’t want to read about my roof falling in from snow load. Note that this is actually the ceiling, not the roof, which will be another thermally isolated layer above. The goal being an energy efficient long lasting, strong, low maintenance building that is the shell of our grand machine. The final effect will be an insulation level in excess of R-100.

Outdoors: 33°F/15°F Spots of Sun and Snow, Mostly Overcast
Tiny Cottage: 57°F/54°F Still no heat – First fall fire coming soon

Daily Spark: The Armed Bear Dilemma: In Vermont there is no permit required to carry a concealed weapon. Just assume everyone is armed. The bears are… This raises an interesting logic delemma. Let’s call it the Armed Bear Dilemma. In some states it is illegal to carry a concealed weapon. Of course, committing a crime is also illegal. Ergo one must assume that anyone with an intent to commit a crime is also armed because if they’re already commiting crimes why bother worrying about other silly laws since they obviously intend not to get caught. Since we don’t know who is going to commit the crimes, otherwise they would be preventable, we must assume that everyone is carrying a concealed weapon since anyone might be a criminal. You could be a criminal and packing heat. So could I. This logic leads to the conclusion that bans on concealed weapons create more crimes since then the criminals in those states are better able to assume that their intended victims are less likely to be carrying a concealed weapon. Hey, if I was going to be a criminal carrying a concealed weapon to carry out my crimes you can bet your booties I would rather do it in NY where concealed weapons are banned than in Vermont where any of the citizens might be packing and do shoot back. Statistically, both per capita and in total numbers, Vermont is one of the safest places in the WORLD with the fewest gun deaths yet we have the highest rate of guns and concealed weapons with no bans. California and NY on the other hand have bans on guns yet gun crimes are coming out their wazoo. Funny that. The conclusion is that you can assume the bears are armed and they like to prey on unarmed sheep. Same goes for wolves and mountain lions. Better to be an armed bear than dinner.

About Walter Jeffries

Tinker, Tailor...
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18 Responses to Thermal Decoupler

  1. AliceDewitt says:

    I love the concept of how you’re building everything to be so energy efficient. Thats an amazing R factor. I was wondering why the thermodecupler is clamped together? Innovative clamp there, the pliers with the rubber band!

    • The clamps are holding the units together while the glue cures. The three layers of each thermo decoupler are adhered with can-of-foam. The stuff is incredibly sticky and makes a good adhesive. Will’s the one who put it together and innovated the extra clamp. These are literally made of our old recycled cutting board which was a little warped – thus the number of clamps binding it together.

  2. eggyknap says:

    I know you tend to allow politics in to the blog only rarely (NAIS, for instance). So I hope I’m not banned for saying, as the pro-concealed carry zealot I am, that I loved the Armed Bear “Daily Spark”.

  3. Andrea says:

    I love these little insights and details about your projects, the way that you think things through so carefully and the analogies like the dolls and thermoses that you give which makes it understandable to lay people like myself.

  4. andy says:

    Guessing you know that there are higher density foam boards. We regularly use a 60 psi version for installations under green roofs. I think the R value is similar to the lower density types.

    Enjoy seeing your use of concrete in an arched form to create very strong ceilings and roofs with the various vaults and arches.

    • Yes, I had wanted to get Formula 1000 which is 100 psi. They use it for airport runways so I figure it would be strong enough for our VTOL. But it doesn’t seem to be readily available locally – a limitation of living in a rural area – so I made some engineering design changes to accommodate the use of the Formula 250 which is what I can get easily around here. I am considering the F1000 for some future stuff if I can line it up. One issue is we needed this delivered nearly 30′ up in the air. Hauling lots of materials up the building by hand is a drag. So the lack of the F1000 made me get a little more creative in my structural engineering so the F250 could do the job other than these key points.

      I like the arches too. Some of my early designs had the final roof be an arch however last year I changed that to a flat one since it gave me more options for using the roof for some interesting future functions like solar arrays for electric and hot water. This in turn led me to having to rise to the challenge of designing large expanses of flat roof, something I have never done. It was quite interesting.

  5. Charles says:

    I have a friend who is in the foam business. They formulate and sell foam products for all applications and ship worldwide.

    If you should need a contact in that business to work with for any type foam applications let me know and I will provide you with his contact information.

    I enjoy reading your blog posts of the construction process. I have been in the construction industry all my life and can appreciate what you are doing.

  6. Bob says:

    So, this is only load bearing if the roof sags? If I understand this correctly the Formula 250 would in that case crush, transferring the weight to the pillar. I’m not sure what the role of the PVC is.

    • Correct, the pillars and the thermal decouplers only become load bearing if and when the roof sags – unlikely but I like to plan for just incases. If the roof sagged, which it shouldn’t, then the F250 would crush and the load would transfer through the PVC to the two pillars. The F250 is a strong insulator and will likely never be crushed so its purpose is to keep the roof from coming in contact with the pillars and thus prevent the conduction of heat out of the reefer. If the F250 crushes the two layers of PVC are secondary insulators, they are far less conductive than the concrete but not nearly as insulative as the F250 pink foam insulation. The job of the PVC is to take the load while reducing thermal conductivity. It’s a thermal decoupler between the roof, actually the inner ceiling, which will run at about 50°F to 55°F naturally, and the pillar which will be much colder than that.

  7. Rick says:

    So you have about 1800 pounds of pink insulation strength and about a million pounds of PVC insulation strength. I think that should hold you pretty well. If the roof is sagging more than that then we have bigger worries like global ice age! Unless your mastodons are stomping on the roof!

  8. I think I’m safe from the mastodons, or rather woolly mammoth traffic.

  9. Bob P. says:

    I especially admire the attention to insulation details that you put into your construction, Walter. This is an investment that will pay off handsomely in future energy savings. I have seen all to many facilities built in this industry which had insufficient insulation or were completely lacking. The owners just pump their money into the environment which is sure to hurt their long term viability. By spending a bit more now you are going to save tremendously for decades to come.

  10. David B. says:

    I like the de-coupler. When pouring a footing for your concrete walls do you put pink foam below them as well, and if so… do you use the F250?

    • The building has a split foundation. The history of this is that we build on a pair of existing foundations which used to be our hay shed. That foundation is tied to the bedrock. That foundation is actually two separate foundations which are stable as I measured them over 20 years ago and then remeasured them after we removed the hay shed. We poured a bond foundation to join the old foundations into one solid structure. On top of that foundation we laid a pad of insulation that isolates the building from the earth since the earth is not the correct temperature although close. You can see that in this post. On top of that we poured the structural building and within that we poured the thermally isolated reefer two thirds of the building which is kept cold. Within that is the Brine, then Cooler, then Freezer – successive thermos bottles like Russian Dolls. That layer of Formula 250 that separates the sub-foundation from the structural building is designed to hold over four million pounds of weight. Our building will weigh in at less than one million pounds so we have over a four fold safety factor. There is also a safety on that. By doing the isolation as a large pad rather than under the footers we gained more structural strength and spread the weight out over a much larger area than is done with common foundation work. An air gap is implemented under the sub-0°C sections to prevent frost heaves – this is part of the building’s lungs which I’ve mentioned here and here. The lungs also serve to drain condensation and keep the insulation in the reefer section dry – three of the main failure modes of commercial scale refrigeration systems.

  11. Kevin says:

    Walter – are you sure those cutting boards are PVC? They look like the polyethylene cutting boards I have. You could check with PVC cement. Not that it really matters, properties are close enough for the application, although PE has a lower modulus.

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