Filling the box with fiberglass is a great idea.

You could also use a large tarp or scrap piece of poly sheeting to wrap around the insulation to keep it tidy.

For the price of even the cheap plywood, you could use 1" thick structural XPS foam to build the box it's R5, so it can only help. The 2" stuff gives R10 and is very stout if you buy it without the cut-lines. If it needs to look nice, put some wallpaper on the foam.

What was it now I think the 5 1/2" nch I used in the basement wall was R25 I forget what I paid for a bag now.

I will be using rigid foam on the new houses walls and 11 inches or R50 worth of fiber insulation for the floors and roof so will have left overs of both kinds to work with

The Rigid would be good with the logo on the inside and foil taped edges.

My fridge is wrapped with layers of the black wood floor insulation the last layer was using the blue type I like the look.Plywood is durable and has a decent enough insulation value costs $50 a sheet for good one side 3/4 inch / 16 sq. ft

I have to edit that ~
Just checked the 3/4" ply would has a .94 R-value , that's no good. Rigid blue is the way to go. For sure for sure.

Yeah, foam is a kick-*** value in a situation like this, you get some structure and some decent R-value too.

They have 2" thick foil-covered stuff at the box-stores around here for around $23 a sheet. The R value is superior with the mylar foil, but it's Expanded Polystyrene (EPS) foam so it's not as strong as that blue or pink Extruded Polystyrene (XPS).

The foil tape is a a good idea for the joints.

The R-30 fiberglass rolls meant for attics are 25' long and do not have a vapor barrier. That's about the best value per square foot and R-value you can get from a hardware store. For someone without a bunch of leftover glass, it would be a great way to get enough to stuff an enclosure or box around a water tank.

I put a piece of foam under my 19 gallon tank.
I don't think it will work so well for a 50 gallon tank, same diameter but weigh almost 3x as much, I think it would flatten the foam unless it was special high density foundation foam.

Its good to talk about this stuff and work out the details , thought it best to double check the psi

Im thinking plywood (again haha) on top of the foam base would work well to spread the 800 lbs or so load from a big 80 gal tank. Although the tank bottom is flat enough to spread the weight good enough I guess.

The math at 24 sq.in works out to 5750 lbs load at 10 psi per sq. inch.

Type I EPS has been tested and found to fall within 10 to 14 psi. The resilience of EPS insulation board provides reasonable absorption of building movement without transferring stress to the interior or exterior finish at the joints

XPS foam has far more dimensional stability than EPS foam. The blue stuff at the local stores is usually 15 PSI structural XPS. I special-ordered 25 PSI for use under my concrete slab foundation and used a leftover 2' x 2' square under my water tank.

80 gallons of water is about 667 lbs. Add the weight of the tank and the heat pump and that's the load on your foam when filled.
If your water tank is 24" diameter and only sits on a 2" wide ring / lip around the circumference, the surface loading of the foam would work out like this:

P = M / ((π * R^2) - (π * r^2))

Where P is pressure, M is mass, R is major radius and r is minor radius

P = 750 lbs / ((π * 12 in ^2) - (π * 8 in ^2))
P = 750 lbs / 251.33 in^2
P = 2.98 PSI

So for a large water tank, you may be in the 3 PSI to 5 PSI ballpark.
10 PSI foam is more than enough to handle that load.

I swear I have nothing against plywood... :thumbup:

I was thinking to myself they use spray foam under houses foundation I should check on the psi of the wall board and yup its tough enough.

You used the XPS 1000 for your foundation or equivalent hey ? I know little about the XPS / EPS but need to learn it.

The main difference is in the manufacturing process used for each foam.

EPS starts with a bunch of tiny polystyrene beads. They pour them into a mold and then heat it all up with steam or hot gas. The beads expand and bond together. That's why most styro-foam packaging and the EPS foam looks like it was made from a million little tiny BB sized beads. It has voids that don't completely close and when you break it, it breaks along those random voids.


XPS foam starts with melted polystyrene. They agitate it and foam it with a gas as it is forced through an extrusion die and cooled. The extrusion die can be of any 2 dimensional shape and the 3rd dimension is a continuous length.

For boards, planks, rounds and other simple shapes, the extrusion process is great. The density of foam can be controlled by foaming it with more or less volume of gas. Also, it is one big contiguous structure of polystyrene with air bubbles.

For complex shapes, the mold filled with beads used in the EPS process is extremely versatile. EPS foam is used in the lost-wax or investment casting process for automotive aluminum parts. You can see the EPS foam bead texture carried through from the foam pattern on plenty of aluminum cylinder heads and engine blocks.

In reality, the two types of foam are almost the opposite structure of each other. XPS (extruded foam) is a big block of polystyrene with air bubbles (tiny spherical voids). EPS (expanded foam) is a bunch of tiny spheres packed closely and expanded until they touch each other and bond together.
(Of course, it is true that each bead has a bunch of little air-pockets inside it from the steam or hot gas).

I think the decision of going with the hot water heaters installation is much clever. If you want to know more about the BTUs of heat pump, then an expert heating system serviceman can give you better knowledge about this. Either you can consult home heating NJ team or else can go with NJR home service. Both are the expert contractors who are well aware of the heating system working and can even help you to choose an efficient water heater.