spray foam on storage tank

if you look back over some of my posts on our thermal storage tanks, you will see that I posted some numbers on standby losses with 3" of foam. The numbers are pretty impressive, and I personally don't see the need for additional foam, at least in my application where any standby loss during the heating season isn't really lost at all since the tanks are inside. Even in the summer when running the system for DHW, there is no measurable temperature difference in the crawlspace where the tanks are and the rest of the basement.

All that having been said... if you have an extra $100 bucks burning a hole in your pocket... go ahead and put 4" on... take some measurements of standby temperature drop, and post your results back here to share with everyone else. It would be a fun experiment.

cheers

It depends on a couple things. How much heat do you want to leak into your basement? And how big is the tank?
All our tanks are 4.5" on the walls and 3.5-4" on the base and top.

The bigger the surface area, the larger the heat loss.
I would opt for as much insulation as you can afford. I want heat where I need it, which is usually not the basement.

BTW, spray foam should be going for about $1 per board foot. So every three inches thickness that is one square foot would cost $3., four inches would be $4.

Have to agree with Tom on the "as much as you can afford" but I would extend that to the shed insulation as well, put as much as you can afford in those walls, floors & ceilings as well.

Essentially comes down to pay now or pay later (over & over) as every btu you loose/waste you will have to replace, same concept I try & get through to new home buyers you can either pay now or pay over & over for the next 40 - 60 years.

Tough concept to get through to people but it is a real budget saver over a long period of time when people get it.

Say a good insulation job saves you $500 every year compared to a minimum or poor one, over 40 years thats $ 20,000.

For those of us who like to escape our winters, that good insulation job will pay for a vacation every other year for most.

Pretty decent incentive. :coolsmile:

is the spray on isulation vunerable to the elements? I would think it may degrade in sunlight and not fare that well exposed to rain, hail, wind, etc. Check the specs or ask the contractor. You may be able to build a simple lean-to or uninsulated structure to protect it. If I didn't already have so much money wrapped up in this project, I would build a simple small uninsulated building next to my 30x60 just to house the 1000 gal tank and keep the wood dry. However, mine will be inside the building and insulated with just some leftover fiberglass batts. I will probably not use it for DHW unless I add a smaller tank outside someday or spray the inside tank.

Wouldn't Mr. Mouse love to tunnel into that foam for a nice cozy winter hideout? Just a thought.

I would suggest protecting the tank from sun and rain (some kind of simple shelter). Then I would go the extra hundred bucks, since this is outside and your ambient temps are much colder than a basement or crawlspace. The extra inch isn't worth gambling with. Then, after the foam is dry, wrap the tank with reflective insulation (the stuff with reflective foil and the "bubble wrap". This will add a small amount of additional insulation, but more importantly, it will reflect any escaping heat back toward the tank. It will also help keep the whole system dry and critter/bug free.

For a tank outside with no roof or walls to protect it from weather and vermin you could use one of the various compounds meant to cover and weatherproof the exposed insulation board on the outside of foundations. I've usually seen it called "parging compound" or some variant of that. Various mixes of portland cement, fine sand, maybe fibers and an acrylic or epoxy binder in the liquid used to make it a thin paste. Troweled on thin it makes a hard shell that can be painted with exterior paint and holds up to weather for years. Insect proof and I would think rodent proof, too.
The stuff I've seen would need about 2 of the 5-gallon pails to cover the usual 500-gallon cylindrical tank. No idea how much it costs. I've always got it as leftovers from constuction jobs. Then you could build your shed a few years later.

The point of diminishing returns for closed cell foam is in the 4" to 5" range (depending on location). If your tank is going to be outside go with the 4" or as mentioned "what you can afford" keep in mind sunlight will eat the foam up even if there is a roof over it.

How about flow checks and gravity loops? I put a flow check on my supply header and the difference is night/day. If I peel back the insulation I get a reading of 140 before and 75 after.

corbond came up with an "efficiency" rating for different thicknesses of foam. They start by measuring heat flow through bare plywood, and then measure the reduction in heat flow as they add different thicknesses of foam. Here are some numbers based on a 93° temperature differential on a 24 sq-ft test.

no foam - through plywood - 1050 btu/hr flow - 0% efficiency
1" foam plus plywood - 277 btu/hr flow - 74% efficiency
1.5" foam plus plywood - 202 btu/hr flow - 81% efficiency
2" foam plus plywood - 165 btu/hr flow - 84% efficiency
2.5" foam plus plywood - 135 btu/hr flow - 87% efficiency
3" foam plus plywood - 109 btu/ hr flow - 90% efficiency
4" foam plus plywood - 78 btu/hr flow - 92.5% efficiency
5" foam plus plywood - 69 btu/hr flow - 93.4% efficiency
6" foam plus plywood - 60 btu/hr flow - 94.3% efficiency
7" foam plys plywood - 53 btu/hr flow - 95% efficiency

with your tank outside, your could be running a differential of maybe twice what this test was performed under. I believe the relationship to heatloss and temperature differential is linear, so at 180° differential you could see the following:

3" of foam - 218 btu/hr flow through 24 sq feet. multiply by approximately 4 since a 500 gallon propane tank is around 100 square feet of surface area and you get around 875 btu's per hour... or around 21000 btu's per day worst case scenario.

7" of foam - 106 btu/hr flow through 24 sq feet. multiply by approximately 4 since a 500 gallon propane tank is around 100 square feet of surface area and you get around 424 btu's per hour... or around 10000 btu's per day worst case scenario.

going from 3 to 7 inches of foam worst case scenario will save 10,000 Btus per day at an additional cost of $400 for the foam. without going into calculating degree days etc etc... assume average savings for the entire heating season is around a million btus (assume 10,000 btus per day for 180 days and divide by two... roughly). To put that into perspective, that's roughly 1/20th of a cord of wood. if a cord of wood costs $150, that's a savings of $7.50 per year... which will take 53 years to recoup the investment.

I am sure there are other ways to look at this. But this is my personal take on it.

edit to note that the transfer of btu's through plywood is different than through steel..., though with 3 or more inches of foam, my guess is the difference would be negligible... though I have been wrong before.

cheers

Just a note... I checked my numbers on what I was seeing for standby loss on our two 500 gallon tanks... it's not TOO far off from the scenario I played out above. (depending of course on what you consider "too far off") Understand that there is a certain amount of unmeasured heat loss from our piping on top of the tanks even when they are in standby. I was seeing what appeared to be about 1500 btu's per hour through the foam (and piping) on two tanks... so 750 for one tank, but at a pretty low differential between the ambient air in the crawlspace and the water in the tanks. You might see 3 times this in an outdoor setup because of the cold air temps, which turns your $7.50 per year savings into $22.50 and an 18 year payback.

just thinking out loud...

cheers.

...also think out loud....

If Pikers numbers are accurate, and I'm not doubting they are, I'd tell the spray foam guy 4" just to be sure I got a full 3" everywhere and not worry a minute after the project is completed.

ken999 said:

...also think out loud....

If Pikers numbers are accurate, and I'm not doubting they are, I'd tell the spray foam guy 4" just to be sure I got a full 3" everywhere and not worry a minute after the project is completed.

Click to expand...

This is probably a pretty safe bet. I checked to see how consistent the thickness was on the foam in a couple areas on our tanks. It was pretty good actually, though with the tanks being close together, there's quite a mass of foam built up between them on top... not so much underneath. There are a few spots with less than 3" for sure... but also quite a few spots that are heavier than 3".

cheers

Been missing this thread.
If you stop and think about a house for a moment, we aspire to R-40 for a living space insulation value.
As Piker said, the tank is apt to be 180F, which can be up to 3 times the heat load of an equivalent living space.
The piping that comes off the tank will become nice radiators with flowing hot fluid inside of them, there
is enhanced heat loss there. Get all the piping hit with at least 4" of foam.
4" is R-24-28, depending on who you believe.
Being outdoors, this is a big consideration. None of that heat loss is usable.

Please enclose the tank with pressure treated lumber to try to keep the critters away from the insulation. They will burrow into anything that they have
easy access to. PT lumber will help dissuade them from getting to the foam what is going to be a very comfortable living space.
I would insulate the enclosure also.

If you want some heat in the garage, I would hang with that. You can always add extra insulation over that with larger diameter Armaflex.
That piping is in a heated space and is indoors, more or less.

sootsucka said:

Thanks again everyone. I was planning on building a simple shed around the tank after it is foamed. I already have a shed around it but the foam guy said it would be easier to spray without it. The tank is wrap with 3 layers of the foil bubble wrap right now and them I have a bunch of rigid foam boards around that. There is 3 or 4 layer of foam board that is 2" thick. I just have way too much heat loss the way it is right now. When I charge the tank to 200 degrees the heat loss over night is around 15 degrees over 8 hours, the reason I want to foam it. I tried sealing all the gaps to slow the heat loss but no such luck. I was going to fill in the voids with cellulose but I believe it would be around the same price after pricing out the foam. I guess maybe 4" of foam, some bubble foil around that and then the foam board around the shed walls would work. Anyone know if the heat loss on say 6" of foam would be less than the other setup because it would be worth it in the long run.

Click to expand...

If you're building a shed around it why use spray foam? We housed our Garn in an ISO shipping container which has an 8'x8' square interior. The Garn exterior is 6' in dia., leaving a minimum of 12" between the Garn and container walls. We used $275 worth of fiberglass insulation that varies in thickness from 12" to 24". That's R38 to R72. Parasitic loss during January is only one degree per day. The fiberglass is the lose hand placed type (similar to blown in but much denser) and has a temp rating of 300*. For the size of tank you have I would imagine the cost to insulate to R50 would be around $100.

critters love to tunnel into foam. Some of my spray foam in my shop extended under the roof overhang. Birds have built nests in it. inside the mice ansd packrats tunnel into it. Time to upgrade the shop cat!

Most foam tanks have the coating mentioned above. Here are some large solar tanks on a solar job we supplied. these were built by Lochinvar and were outside in the Mississippi weather for months before being installed. the coating weatherproofs and protects the foam.

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Birds and bugs in foam.

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