polyiso floor

[davidj]

Has anyone tried using polyiso on the floor, covering it with concrete backer board?  I'm building an 8x10 shed for my batteries and inverter and it needs to be insulated to stop the batteries freezing (it can drop to 0F around our way).  I was planning on putting down backer board on the floor anyway to reduce the fire risk and throwing 2" of polyiso under it wouldn't be much more work.  And it probably doesn't cost much more than batts and h/w cloth for 80 sq ft.  My big worry is the compressive strength, especially as I'll be storing some heavy gear in the shed at times, e.g. a 150lb generator.

[Don_P]

The foam can handle it if there is something to spread the point loads above, you'll punch through the backerboard unless you put a layer of ply or osb between it and the backerboard.

[JRR]

I would go to the manufacturer's website.  IIRC, the stuff you are going to find "on the shelf" is rated @ 15 psi, or is it psf?  ... better check.  But, for some more money, a stiffer board of 3 times compressive strength is available ... special order.  This is per Pink.

[MountainDon]

Wet cell lead-acid batteries won't freeze unless totally or partially discharged. As the state of charge in a battery decreases, the electrolyte becomes more like water and the freezing temperature increases. The freezing temperature of the electrolyte in a fully charged battery is -92 F.

SPECIFIC                     %STATE OF                  FREEZING
GRAVITY                       CHARGE                      TEMPERATURE
1.280                             100                              -92 F
1.265                              92                               -71 F
1.250                              85                               -62 F
1.200                              62                               -16 F
1.150                              40                                +5 F
1.100                              20                               +19 F

So you may not have as big a worry about the temperatures.   Colder temperatures extend battery life but also reduces available (rated) capacity.

[Don_P]

15 psi, IIRC the denser is 25psi... but hold on, I've fallen for this before under slabs, I used the 15psi when my engineer laughed and asked who owned the tank. 15 psi x 144 si/sf=2160 psf. Not that it means anything but they just checked my squeeze 90 lbs and my thumb pinch, 25 lbs, if that gives something to relate to.

[davidj]

I found a random graph that seemed to indicate that the difference between 32F and 22F might be 10% in battery efficiency.    I'm guessing that's the sort of temp difference that might be expected between half-done insulation and proper insulation in winter at our place.  Given I'm gonna have maybe $3K of batteries, max 50% DOD, then that's 20% of my useful battery capacity, and is worth $300.  Obviously all of these numbers are very rough but it does seem like doing the insulation reasonably well is worth it.

So the two options are:

• Above floor - 2" of polyiso, 1/2" OSB, 1/2" backer board
• Below floor - R25 batts, Tyvek and something to keep the animals out that won't rot near the dirt (hardware cloth? 1/4" backer board?).  Plus some backer board above the sub-floor for fire protection

Note I've only got maybe 8" space underneath the joists and the framing is pressure treated - I don't want to use OSB or ply underneath the batts due to wood cutter ants, snow contact and the like.