Okay to use 1" polyiso as a insulation base for 50 gallon heat pump water heater?

I am about to install a 50-gallon Rheem heat pump water heater in our garage at our house near Pasadena, CA. (after this installation, our home will be all-electric. yay!)

I watched Bruce Manclark's and Wade Cohn's excellent how-to installation video and noticed the EPS insulation pad. They seem like a good idea. It sounds like they're required by code in WA (and presumably readily available there), but are not easily acquired online or in-person in our area. So here's the question: would it be okay to use 1" polyiso rigid foam (which we have a LOT of scraps of) as an insulation base instead of the EPS? Is there anything special about this EPS?

The garage is concrete slab on grade. It looks like the highest compressive strength version of EPS is about 25 pounds per square inch (PSI)? According to the manufacturer of my polyiso, its rated compressive strength is 20 psi. 

The area of the base of the water heater is about 388 square inches. The empty weight of the water heater is 178 lbs. Fifty gallons of water weighs about 417 lbs. So the full water heater should be something like 595 lbs? Divided by the area of the base (assuming the load is distributed uniformly) gives a load of about 1.5 psi? And polyiso's rated for 20 psi, so -- we're good to go?

Does this sound reasonable? Anything I'm not considering? This will be my first water heater installation. Thanks for your help.

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1st for anyone wondering - do not do this for gas

Sure if you put a drip pan underneath the water heater - so concrete, foam, drip pan, water heater

Are you sure you can lose that inch or two of height though - most of those units are freaking tall already & cause issues just because of that. You may also not get as much benefit from the unit as you may think do to temp cutouts - I will have to pull your temperature numbers but for places like Phoenix the bulk of the time during summer they are on regular heat, not heat pump

Thank you, Sean.

Technically EPS is "structural". PolyIsoCyanurate is more compressible. It also doesn't do well with shear, tension, hence the foil face. There is also a famous story about the failure of the Green Roof on the Ford Factory that substituted PolyIso for EPS. Polyiso is less resilient. An increase of compression force deforms PolyIso, then the same compression force deforms it again. The EPS will deform and then if the same compression force is applied again, the EPS only deforms to the same point it did last time. 

If I was going to put something under it, I would use EPS. If you want a higher r-value, use more EPS. I would probably just put some feet under it, like 2 by 4 sleepers.

Thank you, Mick. Very helpful. Definitely leaning towards EPS or XPS at this point.

Thanks for explaining why EPS works better for this

Personally I would NOT use polyiso in this case.  I've seen it age ungracefully in garages and you have weight ontop of it.  I'd stick with something like an Dow XPS Formula 250 - which I believe has a 250 #/square foot.  I've stacked concrete bags on them over the years and hadn't seen any compression problems on the XPS.  Also put a 1/2" plywood above the foam and then the water capture tray ontop of that ...   the plywood helps spred the weight evenly across the XPS.. even if there are dimples in the bottom of the hotwater heater (dimples for feet).  The XPS may compress to match floor over time.

Thank you, Dennis. I can benefit from your experience. I was considering the plywood to evenly distribute, glad to hear experienced others have had the same impulse. 

I've used the same approach when replacing furnaces and HVAC equipment.  Seems crazy to set duct work onto concrete floors, or set an updraft furnace on a concrete floor with the return vent ducting set on the concrete floor feeding into the side of furnace.  I'm in Washington State and that concrete floor can easily set at 55-65 degrees... 

Not all insulation is the same. For example,

Dow's Super Tuf-R polyiso has a compression strength of  25 psi (3,600 lbs per sq.ft.) and a permeance of <0.03 perm, but IKO's Enerair has a compression strength of 20 psi (2,880 lbs per sq.ft.) and a permeance of >1 perm.

Dow's Styrofaom XPS has a compression strength of 30 psi and a permeance of 1.5 perms

Owen's Corning Foamular 150 XPS has a compression strength of 15 psi and a permeance of 1.5 perm

     and  their Foamular 250 XPS has a compression strength of 25 psi and a permeance of 1.5 perm.

We use Super Tuf-R on roofs, which is an approved application. Personally, I would use a scrap of Tuf-R if that is what I had a scrap of, but maybe with a thin piece of plywood on top to protect from physical damage.

Probably would be fine as you propose (plywood layer over any foam) but I would suggest you check with your local code office to make sure you use foam / assembly that meets life-safety (fire) code.  Not all foams meet this essential safety requirement.

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