Polyurethane foam or Blown

[Robby]

I need knowledge. It is soon time to insulate my 1200 gal. stroage tank. It is outside, beside my boiler building. A local foam business will spray tank 8" thick (about R50+). They inform me that if I then paint the foam to prevent UV rays from reaching it it will last indefinitely. They suggested even better is the white roof coating used on RV and mobile roofs. Anyone have info on this?

The other option is build a box and fill with batts or blow in fiberglass. I am concerned that no matter what I do there will eventually be a leak.

Others must have already tried various fixes and found what works and more important, what does not.

Thanks,

Robby

 

[tom in maine]

They are correct. High Density spray foam is used for roofing and to make it last, it is covered with a brush or spray on weathering surface that is usually an acrylic coating. A more viscous version of house paint.

 

[flyingcow]

Sounds similiar to what has been used to insulate potato storage buildings up here. Give a good coat of whatever they recommend and it will a long time. I've seen it put on the outside of quonset huts, and not painted. Surprised me how long it works unprotected.

 

[Robby]

Sounds pretty straightforward. Thanks for info.
Now does anyone hazard a guess, or have knowledge on just how much R value is enough. Water temp up to maybe 190F and air temp down to -25F. This is not usual, but will happen. The foam is expensive and there must be some point where more is not worth the extra cost. The foam company suggested no more than 8" based on what commercial tanks have. They had no idea what was in the tanks or the purpose they were used for.

I now have my solar hot water up and running for summer DHW and am looking forward to the large tank for house heating next winter. I am convinced I can improve effiency of the heating sytem by 20 to 30 %.

 

[DaveBP]

Now does anyone hazard a guess, or have knowledge on just how much R value is enough

You can get a pretty good idea of the heat loss if you know the surface area of your tank. The BTU/hour loss is tank area in square feet times the temp difference between the tank contents and the surrounding air in farenheit degrees divided by the R value of the insulation. There is no 'point' where the diminishing return will announce itself. The advantage just keeps going down until you run out of money; or you decide enough is enough.

If you're handy with spread sheets you could make a graph. Or plot it out by hand on graph paper.

 

[jklingel]

Dave: A picky point. Yes, Q = A (delta T)/R, but delta T is going to be a moving target. Somehow that equation needs to be integrated over time; I am clueless on that, but just using a fixed Delta T will give a high value to the heat loss. Robby: Make sure the foam is kept dry, too, painted or not. Wet, it is between worth less and worthless. How much insulation? Tough, arguable question. For a house, R50 walls is perhaps a bit excessive, but a boiler has a pretty steadily large Delta T. I'd go fairly heavy on the insulation, (R50 to 60-ish) and keep it cheap (ex: fiberglass). If your wood is cheap, an expensive insulation job will take over 20 yrs to repay you. For a rough reference, when I built in 1980 and used R38 walls, it was going to take around 13 yrs to pay for the extra materials over an R19 wall, burning oil. For insulating a boiler and heating w/ wood, is 30-40 yrs a fair guess to recoup? BTW: See "tank insulation" below. Good data published there on actual tank heat loss, etc.

 

[DaveBP]

Trying to calculate the actual total heat loss over time would be way too frustrating for my primitive calculus skills. Too many variables. I would just estimate the worst case with some approximation algebra like I suggested to get an idea of what I might see in the worst conditions.

Watching my tank cool off without adding to my personal comfort would drive me to madness (I don't have that far to go), regardless of any considerations of return on investment period.

Financial concerns might be the first questions we ask ourselves when considering one design element or another, but the emotional aspects of personal satisfaction can't be neglected either. Those are the ones we live with longest.

 

[John D]

I think I would try to use the spray foam,with the tank outside it is subject to to wind.Wind will be the biggest robber of the stored heat.Air infitration into and between the fiberglass insulation eventually,and will draw out the heat irreguardless of the R value. With closed cell spray foam insulation ,this is not an issue,and it cannot happen.I know batts are cheap,but by the time you build a strong,suitable structure,and roof over the tank,you may not be saving much over the spray on,and some roofing paint.

 

[DBoon]

For an outside installation, you should really go with an insulation that is impervious to air infiltration from wind, and won't degrade if it gets wet UNLESS you can build a perfect weathertight box around it (not an easy thing to do).

Heat loss through the insulation can be easily calculated if you make some basic assumptions. First, just calculate heat loss through the insulation for each of the four seasons using average outdoor temperatures. These temperatures might be on your electric bill, or available on-line.

Then, you want to solve for BTUs of heat loss. Units of R-Value are [(square feet*degrees F*hours)/BTU]. Thus, if you know the number of square feet you are trying to insulate, the degree difference between the water in the tank and the outside air, and the number of hours (in this case, in a season), then you can solve for BTU of heat loss for a given R-Value.

If you have a cylindrical tank, you can calculate the surface area if you know the tank diameter and radius (radius = 1/2 the diameter) and length. Surface area is 2*3.14*radius*length +2*3.14*radius*radius. If you had a diameter of 3 feet (1.5 foot radius) and a length of 6 feet, then the surface area is 70.65 square feet.

If you assume that your tank temperature is always 180 degrees F, and the average winter outside air temperature is 20 degrees F, then the average temperature difference is 160 degrees F.

The number of hours in a season is 24 hours/day * 91.5 days/season = ~2200 hours in a season (rounded up)

Then, the BTU loss for the winter period through the insulation is (70.65 square feet * 160 degrees F * 2200 hours) / 50 [(square feet*degrees F*hours)/BTU] = 497376 BTUs, or the equivalent of about 3-1/2 gallons of oil burned at 100% efficiency. Not too bad for the winter season. It works out to about 226 BTUs per hour. Loss through the summer, spring, and fall season could be similarly calculated.

HOWEVER, with superinsulated systems like this, the heat loss by conduction through things like connected pipes, pressure relief valves, etc. is probably much higher. Therefore, you need to use every trick in the book to minimize heat loss through this fittings, especially if they are exposed to the outside air.

Once you know the BTU loss for a given value of insulation, it is pretty easy to calculate your payback period for a given insulation value compared to another insulation value. Or, how quickly the tank will cool off if it is not re-heated.

If the calculations seems a little intimidating to you, post again and I'll help you work through it.

 

[Robby]

I think everyone is right, foam it is. You start figuring plywood and framing for a box, batts, roofing material = foam starts to look better and better.

My tank is 4.5' x10.5'. I approximated the surface area as 140 sq.ft. Not exact because ends are domed, it is a pressure tank. Heat loss from conduction etc. will be minimal, I run 1" copper lines inside 4" PVC and insulate with fiberglass or expanding foam. I have used both, seems to be very effective. Pipe runs are very short distance into boiler building.

Ambient air temps. Huge variance. Can be as cold as -30F. Usually for very short period of time. -10 to +10, fairly often in winter. Down to freezing at night and above during the day, usual in early spring, late fall and winter. Boiler system shut off from June through September. Water storage temp of 180 to 185F down to 135F.